Test equipment “workshop/demo”

The aim of this workshop/demo is to demonstrate the use of several items of test equipment. The

emphasis will be on the use of the oscilloscope and how it can be used in conjunction with other test

equipment as an aid to diagnosing faults in faulty equipment.

The following test equipment will be demonstrated.

SIGNAL SOURCES

Fig 1 - Blackstar Jupiter 500 500kHz

Function Generator.

Fig 2 - Home Brew Sine Wave Signal Generator

Fig 3 - Farnell SSG520 10MHz to 520MHz RF Signal generator

SIGNAL MEASUREMENTS

Fig 4 - Fluke 79 Digital

Multimeter.

Fig 5 - DT830B Digital

Multimeter.

Fig 6 - TMK500 Analogue

Multimeter

Fig 7 - Blackstar Meteor 600 Frequency Counter. Fig 8 - Telequipment D61a

20MHz Oscilloscope.

Fig 9 - Trio CS5270 100MHz Oscilloscope.

OTHER ITEMS

Fig 10 - Home Brew Power Supply For Demo Circuits. Fig 11 - Tuner/Amplifier

Fig 12 - Demo Board 1 – Voltage

Measurements

Fig 13 - Demo Board 2 – Amplifier and 555

Oscillator

In addition I’ll be using a DVD player, TV monitor and an Aurora standards converter.

SIGNAL SOURCES

Blackstar Function Generator (Fig 1)

This is a low frequency function generator that can generate sine, square and triangle waveforms

from 0.1Hz to over 500kHz. It is based on the infamous 8038 waveform generator IC, which

generates square and triangle waveforms and uses wave shaping to generate a passable sine wave

from the triangle wave. The frequency is set by a combination of push buttons and a rotary switch

for the coarse frequency setting and a rotary control for the fine frequency setting.

There are three outputs, a high level signal, a low level signal, 20db down on the high level output,

and a TTL square wave output. The high and low level output amplitude is adjustable as is the dc

level.

The TTL output is useful for connecting to a frequency counter to allow the exact frequency to be

set or for connecting to an oscilloscope trigger input. This will be demonstrated later.

Sine Wave Signal Generator (Fig 2)

This is a home brew audio signal generator obtained from a forum member and is based on the

Wien Bridge oscillator with thermistor amplitude stabilisation with a range of 15Hz to 170kHz in 4

ranges. I’ll show the effect of the thermistor later.

Farnell Signal Generator (Fig 3)

This is a synthesised RF signal generator covering 10MHz to 520MHz. It uses a combination of 8

voltage controlled oscillators, a programmable digital divider, a phase detector and an accurate

crystal oscillator to cover the range in 100Hz steps. The output level is controllable from 0dbm

down to -110dbm with the output attenuator, into a load of 50Ω. The output can be modulated with

an internally generated sine wave of 400Hz or 1kHz or an external signal. Either Amplitude

Modulation (AM) or Frequency Modulation (FM) can be used.

SIGNAL MEASUREMENTS

Multimeters (Fig 4, 5, 6)

Multimeters come in two flavours, analogue and digital. Each has its advantages and disadvantages

and every one has their own preference. They are normally capable of measuring Resistance, DC

voltage and current, AC voltage. Many can also measure AC current. By the use of range switching

the full scale reading on the meter can be set from 1v to 1000v on the voltage ranges, 50μA to 10A

on the current ranges and 200Ω to 20MΩ on the resistance ranges.

A typical example of an analogue meter is the AVO 8. On the DC voltage ranges a typical

sensitivity is 20kΩ/volt, equivalent to a 50μA meter. On the 1V range this is equivalent to placing a

20kΩ resistance across the measurement points. On the 1000V range this rises to 20MΩ. On the AC

ranges the sensitivity is usually lower. The analogue meter I’ll be using, the TMK500, is slightly

unusual as it has a sensitivity of 30kΩ/volt equivalent to a 33μA meter.

The AVO can be considered as an industry standard meter and many service manuals have voltage

readings based on use of an AVO.

Digital meters, such as the Fluke 79, have a constant input impedance of 10MΩ on the voltage

ranges and therefore present the same load to the circuit being measured on the 1v range as the

1000V range.

Note that some of the cheaper digital multimeters have an input impedance of 1MΩ.

The digital meter has a better resolution than the analogue meter being able to measure down to

typically 1mv on the 1v range, which can be difficult for an analogue meter.

Analogue meters, on the other hand, are better for seeing slowly changing levels such as when

peaking tuned circuits.

However some digital multimeters, such as the Fluke 79, have a bar graph as well as the digital

display, which can be used to emulate an analogue meter.

As far as I know all analogue multimeters have manual range switching as do many digital ones but

autoranging is common on digital multimeters. In these you just select voltage, current or resistance

and the meter will select the appropriate range.

Additionally digital multimeters can incorporate many other features such as capacitance

measurement, frequency measurement, diode forward voltage measurement and an audible

continuity tester. Some also incorporate transistor testers.

It should be noted that the AC ranges on most multimeters assume the voltage or current being

measured is a sine wave, any other waveform will give an inaccurate reading. Some digital meters

offer RMS measurement where the meter will measure the RMS value of the signal irrespective of

the waveform. Be aware that when measuring AC signals the accuracy may only be specified over a

limited frequency range.

Whatever multimeter you chose you’ll need test leads to connect to the circuit under test. These are

usually supplied with the meter. They need to be rated to cope with the maximum ranges of the

meter e.g 1000V and 10A. Leads supplied with meters from the major manufacturers will usually

be rated to match the meter but be aware that some of the leads supplied with the cheaper meters

may not be rated to the maximum voltage or current.

A multimeter is the one piece of essential test equipment for repair and servicing radio and TV

equipment.

Frequency Counter (Fig 7)

This frequency counter can be used to measure both low frequency and high frequency signals up to

a specified 600MHz although I’ve checked it with a 1GHz signal generator and it will actually

measure up to 850MHz. It has two inputs, one for low frequency signals up to 10MHz and a high

frequency input for signals up to 600MHz and a switch to select the frequency range. The low

frequency input has a variable sensitivity control and a low pass filter to remove any higher

frequency interference, >50kHz, when measuring low frequency signals.

To measure the frequency it counts the number of cycles over a fixed period. This period depends

on the frequency being measured and the accuracy required. The longer the period the greater the

accuracy but the longer it will take to make the measurement.

Some frequency counters have other functions such as period measurement. This measures the

period of the incoming signal and can be useful for measuring low frequencies although it can

involve a little maths to calculate the frequency.

The Fluke 79 multimeter also has frequency measurement up to approximately 500kHz. It measures

low frequencies such as the mains 50Hz to 2 decimal places. I believe this uses period measurement

and calculates the frequency mathematically as to measure to that sort of resolution implies a

measurement time of 100s.

OSCILLOSCOPES

Now the scopes. I’ll go into these in greater detail later when I demonstrate them but here’s a brief

description of them.

Telequipment Oscilloscope (Fig 8)

This is a basic 20MHz dual channel scope, it has two inputs (Y) with variable sensitivity and the

ability to position the trace anywhere on the screen. It has a single variable time base (X) and a

trigger circuit with various triggering options.

Trio 100MHz Oscilloscope (Fig 9)

This is a 100MHz dual channel scope and has all the features of the basic scope with a few

additional advanced features. It has onscreen readout of the Y amplifier and timebase settings, a

dual time base and more trigger options when compared to the Telequipment scope.

OTHER ITEMS

Power supply (Fig 10)

No piece of electronic equipment can run without a power supply of some sort. They all need some

form of power to function, which can range from a single battery to a multiple output switch mode

power supply such as those used in PCs.

This power supply is one I built back in 1985. It provides between 7.5V and 17V at up to 2.5A and

is actually based one I used in a talk on power supplies I gave to a local radio club up in North

Wales.

I’ll be using it to power the demo boards and the home brew sine wave generator.

Demo Boards (Fig 12, 13)

These are two boards, which will be used to help demonstrate some of the test equipment. One

board will be used to show the effect of making measurements with a multimeter and the other will

be used in the oscilloscope demonstration.

Aurora Converter

I’m sure many of you will be familiar with this item, which converts 625 line TV to 405 lines for

use with 405 line TVs. I’ll be using it as part of the advanced scope measurements.

Tuner Amplifier (Fig 11)

This is a Jasonkits AM/FM tuner dating from around 1957. It was obtained from a forum member

and brought back to life over a couple of evenings. I added an audio power amplifier plus switching

to allow it to be used as a mono audio amplifier.

For this demo it will be used to monitor the audio signals and later to show the passage of the RF

signal through a typical radio.

Where To Get Test Equipment

To equip yourself with test equipment can cost a fortune if all items are bought new but with care

bargains can be had.

I bought my Fluke, the cheap digital multimeter, the analogue meter, the frequency counter and

function generator new but the other items were acquired secondhand.

If you work, or know someone who works, in the electronics industry, or work for a company

which uses test equipment such as scopes, multimeters etc it’s worth keeping an eye out to see if

any equipment is being disposed of. I acquired my RF signal generator from a company I worked

for back in the mid 90s. It had a fault and when I enquired, the accounts department effectively

wrote it off as it was about 13 years old and had been replaced by a newer model. I had to get it

officially taken off the company system and get a form signed by management but I got it, with the

manual, for nothing. The fault took about an hour to find and fix and it’s been working faultlessly

ever since.

The 100MHz scope was bought from the company I now work for when they decided to dispose of

a number of older scopes.

Other sources are secondhand equipment dealers, ebay, friends, but remember it’s not necessary to

buy all items at once. Spread your acquisitions over time but if the opportunity for a bargain comes

up take it. I got the analogue multimeter in 1968 and the 100MHz scope in 2012.

If there is one piece of test equipment you must have it’s a multi meter. Get this before any other

test equipment. Go for the best you can afford analogue or digital whatever your preference. And

get decent test leads to go with it.

The Oscilloscope

The two oscilloscopes to be demonstrated are the basic Telequipment D61a and the more complex

Trio CS5270. Although the Trio looks more complicated than the Telequipment both have the same

common controls but most of the demos will be done with the Trio.

I’ll describe and demonstrate the basic functions, which are common to both scopes and then

demonstrate some of the more advanced features.

An oscilloscope can be considered as a device for showing how a voltage varies with time. It’s like

drawing a graph where the Vertical (Y) axis represents the voltage being measured and the

Horizontal (X) axis represents time. The screen is equivalent to the graph paper and is usually

marked with a graticule just like the grid on graph paper.

There are three basic sections to an oscilloscope

1. The Timebase

2. The Y amplifier or amplifiers

3. The Trigger circuit

The Timebase

Position

Horiz

speed

Fig 14 - D61a Timebase controls Fig 15 - CS5270 Timebase controls

This section controls the speed at which the trace

moves across the screen. The speed is set by the

time/division control in 1 – 2 - 5 steps. The D61a

has a range of between 0.5s/div to 0.5μs/div

whereas the Trio has a range of 0.5s/div to

50ns/div due to having a higher Y bandwidth.

0.5μs represents one cycle of a 2MHz sine wave

and 50ns one cycle of a 20MHz sine wave.

The X position control is used to position the trace

horizontally. This can be used to move the trace to

align with the graticule to make measurements.

Fig 16 - Horizontal trace

The Y Amplifier

CH1 CH2

Position

Sensitivity

Ac/dc/gnd

Switches

Input

connectors CH1 CH2

Fig 17 - D61a Vertical controls Fig 18 - CS5270 Vertical controls

This is where the signal to be measured is connected. As the signal amplitude can vary the Y

amplifier has a switched sensitivity. This is usually calibrated in volts per division. The D61a Y

sensitivity ranges from 10mV/div to 5V/div and the Trio sensitivity ranges from 1mV/div to

5V/div. The sensitivity is adjusted in 1 – 2 – 5 steps. The Trio also has a variable setting allowing

fine control of the Y sensitivity but the volts per division will not be calibrated. With 8 divisions on

the screen graticule the maximum signal that can be displayed is 40V peak to peak.

The Y position is used to move the trace up and down the screen.

AC/Gnd/DC Switch.

With the switch in the DC position the Y amplifier will respond to the dc component of the input

signal.

In the AC position a capacitor is connected in series with the input, which removes the dc

component of the signal. This is useful when measuring the signal on, for example, the collector of

a transistor or the anode of a valve. The DC component of the signal is usually significantly higher

than the AC component and if the switch is set to DC and the sensitivity is set to show a trace on

the screen the actual AC signal can be too small to be seen and make measurements. The value of

this will be demonstrated later.

In the ground position the input signal is disconnected and the Y amplifier input is grounded. This

allows the ground reference to be established on the screen.

Scopes are usually quoted as having a bandwidth e.g. the D61a has a bandwidth of 20MHz and the

Trio has a bandwidth of 100MHz. This is the 6db bandwidth* of the Y amplifier i.e the frequency at

which the amplitude of the signal displayed has fallen by 50%. Many higher bandwidth scopes have

a bandwidth limit switch, which reduces the bandwidth of the Y amplifier to 20MHz. This may

seem odd but reducing the bandwidth can reduce the high frequency noise on lower frequency

signals making measurements easier. It is also an industry standard to make some measurements,

such as ripple voltage measurements on switch mode power supplies with a bandwidth of 20MHz.

* 6db is for voltage measurements and is equivalent to 3db for power measurements.

Y Inputs

Most modern scopes are dual trace and have two identical inputs. How is this achieved?

Early dual beam scopes used tubes with two electron guns and two sets of Y deflection plates and a

single set of X plates. These tubes were expensive and the separate traces did not always cover the

full screen.

All modern analogue dual trace scopes use a single beam tube and electronically switch, or chop,

the outputs of the two Y amplifiers to the Y plates. The frequency of the switching signal is

important as it can affect the display.

There are two settings for the beam switching, Alternate and Chop.

When set to Alternate the switching frequency is synchronised with the timebase so that on

alternate sweeps the display alternates between the Y1 and Y2 input channels.

On the Chop setting the switching frequency is

non synchronous with the timebase and is set at

a much higher frequency compared to the

timebase speed. The rapid switching between

the two channels gives the impression of two

traces.

Chop is more suited to low timebase speeds

and Alternate is more suited to faster speeds.

On the Trio the switching is controlled by a

switch on the front panel, whereas on the

Telequipment it is automatically set by the

timebase setting. If the wrong setting is made

the display may show the chopping signal (Fig

19).

Scope Probes

The standard input connector for the Y amplifier is a BNC connector and the input impedance of

the Y amplifier of most scopes is usually quoted as 1MΩ with a parallel capacitance of 30pf. This

can affect the circuit being tested. It also means you need to make up a lead with a BNC connector

to actually make connection with the circuit under test. This can add extra capacitance and affect

the higher frequencies so the standard method of making connection is to use a x10 scope probe.

This increases the input impedance to 10 MΩ with a parallel capacitance of 3pf, which reduces the

loading of the scope on the circuit. The disadvantage is that is attenuates the signal by a factor of

10. So you have to multiply the readings of the Y amplifier by 10. Some scopes with on screen

readouts have settings to change the indicated Y sensitivity and some can do this automatically but

require the use of special probes with an extra pin.

Adding a scope probe does raise another issue. The drawing below (Fig 20) shows the equivalent

circuit of the scope probe and the scope input. There are two time constants one formed by the

probe resistance (R1) and its associated capacitance (C1) and the other formed by the scope input

resistance (R2) and capacitance (Cin). Provided these time constants are equal the frequency

response of the probe and scope combination will be flat. As we well know this will almost

certainly not be the case so C1 in the probe is made variable to ensure the time constants can be

adjusted to be the same.

Fig 19 – Effect of incorrect chop / alternate

setting.

Most scopes have a square

wave probe calibration

output. The probe is

connected to the calibration

output and the trimmer

adjusted until the scope

shows a square wave.

If the trimmer is adjusted

incorrectly the calibration

waveform will look like Fig

21 or Fig 22. The trimmer

should be adjusted so the

waveform looks like Fig 23.

Fig 21 - Ct too low Fig 22 - Ct too high Fig 23 - Ct correct

If the probe is set as shown in Fig 21 the HF response will be too high. If set as shown in Fig 22 the

LF response will be too low.

In theory the probe compensation should be re-adjusted each time the probe is connected to the

scope or swapped between inputs but in practice once set up for a particular scope no adjustment is

usually necessary. However the compensation should be checked and adjusted before making any

critical measurements.

Now we’ve sorted the scope controls and set the scope probes up, we’ll feed some signals into the

scope.

The following pictures show signal of approximately 1kHz from the sine wave signal generator and

sine wave, triangle wave and square wave signals, again of approximately 1kHz, from the function

generator.

Fig 20 - Equivalent circuit of Scope probe and Scope input

Scope probe

Ground clip

Probe tip

R1

C1 Cin

R2

Oscilloscope input

Fig 24 - 1kHz Sine Wave from Signal

Generator

CH1 – 0.2V/div. Timebase – 0.2ms/div

Fig 25 - 1kHz Sine Wave from Function

Generator

CH1 – 0.2V/div. Timebase – 0.2ms/div

Fig 26 - 1kHz Triangle Wave from Function

Generator

CH1 – 0.2V/div. Timebase – 0.2ms/div

Fig 27 - 1kHz Square Wave from Function

Generator

CH1 – 0.2V/div. Timebase – 0.2ms/div

Compare the sinewaves from the sinewave generator and the function generator. Notice the small

blip at the top of the sinewave from the function generator. This because the function generator

generates its sinewave by shaping the triangle wave. As you can see it’s not quite perfect but it’s

adequate for most purposes. However for measuring the performance of audio amplifiers the

dedicated sinewave generator is the preferred option as the sine wave is purer.

The square wave looks a bit odd as you cannot see the transition between the high and low levels.

This because the rise and fall times are too fast for the scope to respond too.

Trigger circuit

Source

Level

Mode

Slope

Fig 28 - D61a Trigger controls Fig 29 - CS5270 Trigger controls

So far we have seen the X timebase and the Y amplifiers and can display a waveform on the screen.

However the waveform is moving as the timebase is not always synchronised with the waveform

being measured i.e. the signal on the Y input is at a different level each time the timebase starts its

sweep.

This is fine for showing a waveform or impressing people in a Sci-fi film but it is virtually

impossible to make meaningful measurements.

This is where the trigger circuit comes into play. It ensures the timebase always starts at the same

point in a repetitive waveform. It does this by preventing the timebase from starting until the Y

signal has passed a preset trigger level.

The trigger section has the following basic controls.

Trigger source

This selects the source the timebase is to be synchronised to. It’s usually one of the Y inputs but it

can also be set to trigger from an external input. This can be useful if the amplitude of the Y signal

is variable or occurs intermittently. There needs to be a suitable signal available to act as a trigger

but this is usually not a great problem.

Often the trigger source includes a line setting. This will synchronise the timebase with the AC

mains supply.

Mode

The basic settings here are Auto and Normal. In Auto mode the timebase will run continuously and

display a trace even if there is no signal. In Normal mode the time base is inhibited until there is a

signal present on the selected trigger source.

In addition many scopes will have a single sweep setting. When triggered the timebase will only

sweep once and will need to be reset before it can run again.

Level

This is a variable control, which will set the point on the source waveform that the timebase will be

triggered. Both positive and negative values can be set.

If the trigger mode is set to Auto the timebase will run unsynchronised to the source signal whereas

in Normal mode the timebase will be inhibited until the level control is adjusted so that the trigger

level falls within the range of the input signal.

Slope

The slope determines whether the timebase is triggered on the rising or falling edge of the source

waveform.

The following plots show the effect of the level and slope controls.

Fig 30 - Trigger level set to positive value Fig 31 - Trigger level set to negative value

Fig 32 - Trigger slope set to positive slope Fig 33 - Trigger slope set to negative slope

Earlier I mentioned the sinewave generator uses a thermistor to stabilise the amplitude. This gives

rise to a characteristic of the Wien bridge oscillator. As you change the frequency the amplitude

will vary. Why is this the case?

The Wien bridge relies on a non inverting amplifier with a carefully controlled gain to generate the

pure sinewave. To do this the gain is set by a feedback path which includes an NTC thermistor. As

the amplitude of the signal increases more power is dissipated in the thermistor reducing its

resistance, the amplifier gain and hence the amplitude. This continues until the amplitude has

stabilised.

Changing the frequency by altering the resistive component of the bridge, the conditions in the

feedback path are changed and the gain, and hence the output amplitude, will change. It will take a

short time before the gain can be stabilised by the thermistor, during which time the amplitude will

vary.

Use Of Test Equipment

Before I demonstrate the use of the oscilloscope and other equipment with the demo board I’ll

briefly talk about the effect of circuit tolerances and the effect on a circuit of making measurements.

All electronic components have a tolerance in their values. The most common examples are

resistors and capacitors where you’ll see the values quoted as say 10kΩ ± 5% or 100uF ± 20%.

When components such as resistors are manufactured no two parts will come out with the exactly

the same value even though they have been made from exactly the same material and have been

through the same processes. There will be a slight variation in the values. In the early days

manufacturers used to check the values and place them in various bins according to the measured

value. Those that were closest to the required value, within 5%, were then marked as 5% parts (gold

band) those that were within 10% were marked as 10% parts (silver band) and the rest that were

within 20% of the required value had no band. The 5% and 10% parts were sold at a premium. That

is why if you measured a new 20% resistor it would be between 10% and 20% of the marked value.

As manufacturing processes improved components such as resistors could be made to much closer

tolerances and in many cases the actual values could be trimmed to within 1% or better of the

required values. These days 1% and 2% resistors are readily available and are relatively cheap.

For most manufacturers of valve consumer electronics 20% resistors were perfectly adequate and

more importantly were cheaper than the closer tolerance parts, which were only used in cases where

the value was critical. An example of this is the biasing of the typical class B transistor output stage

of a transistor radio where 5% resistors were used, all the other resistors being 10% or 20%.

The effect of the tolerances can be shown by examining a simple resistive divider across a fixed

voltage from a 12V regulator.

The voltage at the junction Vj is

12 * R1

(R1+R2)

Fig 34 – 12v regulator and potential divider.

If we set both resistors to be the same value the voltage at the junction will be 6V won’t it. Well it is

until we take into account the resistor tolerances.

If we assume both resistors are 10kΩ then the worst case values for each resistor are.

Tolerance 20% 10% 5% 2% 1%

Max 12kΩ 11kΩ 10.5kΩ 10.2kΩ 10.1kΩ

Min 8kΩ 9kΩ 9.5kΩ 9.8kΩ 9.9kΩ

How will this affect the voltage at the junction?

R1

R2

Vj

12V regulator

+12V

Taking the worst case conditions for the resistor values, with one being the maximum value and the

other being the minimum value, the junction voltages are.

Tolerance 20% 10% 5% 2% 1%

Max 7.20V 6.60V 6.30V 6.12V 6.06V

Min 4.80V 5.40V 5.70V 5.88V 5.94V

The nominal voltage is 6V in all cases but this shows that if the divider is built with 20% resistors

the voltage at the junction can be any where between 4.8V and 7.2V and still be within

specification. The tighter the resistor tolerance the closer to the required 6V the voltage will be.

Now lets add in another factor, the tolerance of the voltage regulator powering this circuit.

Typically these have a tolerance of ±4%. For a 7812, 12V regulator, this will result in the output

voltage being between 11.5V and 12.5V.

Adding the regulator tolerance into the circuit the resulting voltages at the junction are thus.

Tolerance 20% 10% 5% 2% 1%

Max 7.49V 6.86V 6.55V 6.36V 6.30V

Min 4.61V 5.18V 5.47V 5.64V 5.70V

So we have moved from a nominal voltage of 6V to a voltage, which can be up to 1.5V from the

nominal value and still be in specification.

The Effect Of Measuring The Voltage.

When we come to measure the voltage at the junction the most common method is to use a

multimeter. What effect, if any, will this have on the reading?

The voltage at the junction Vj is now

12 * R1a

(R1a+R2)

Where R1a is the parallel combination of

R1 and the voltmeter resistance.

Fig 35 – The effect of the voltmeter on the potential divider.

The actual voltage measured will depend on the values of the resistors used and the meter being

used to measure it. Lets take 3 different meters and examine the effect of each one on the circuit.

Meter 1 - Typical analogue meter 20kΩ/v on the 10v range presents a resistance of 200kΩ

to the circuit being tested.

Meter 2 - Typical cheaper digital multimeter. Presents a resistance of 1MΩ to the circuit

being tested irrespective of the range setting.

R1

R2

Voltmeter

Vj

Meter 3 - Typical digital multimeter. Presents a resistance of 10MΩ to the circuit being

tested irrespective of the range setting.

The voltage is measured across the lower resistor and the meter will reduce the effective value of

this resistor. The effective value will depend on the value of the resistor. The following table shows

the effect for values of 1kΩ, 10kΩ, 100kΩ and 1MΩ.

1kΩ 10kΩ 100kΩ 1MΩ

20kΩ/v meter (10v range) - 200kΩ 0.995kΩ 9.524kΩ 66.667kΩ 166.667kΩ

DMM - 1MΩ 0.999k 9.901k 90.909k 500.000kΩ

DMM - 10MΩ 1.000kΩ 9.990kΩ 99.010kΩ 909.091kΩ

It can be seen that the higher the values of resistor in the divider the greater the effect of the meter

resistance.

If we assume the junction voltage is 6V, the voltages actually measured by the different meters are.

1kΩ 10kΩ 100kΩ 1MΩ

20kΩ/v meter (10V range) - 200kΩ 5.985V 5.854V 4.800V 1.714V

DMM - 1MΩ 5.997V 5.970V 5.714V 4.000V

DMM - 10MΩ 6.000V 5.997V 5.970V 5.714V

As would be expected the meter with the highest resistance, the 10MΩ DMM has the measured

value closest to the actual value as it presents the lowest load to the circuit.

This demonstrates that when measuring any signal the effect of the measuring instrument must be

taken into account. This is especially important in high impedance circuits or RF circuits where the

capacitance of the measurement probes can affect the circuit operation.

The tables do not take into account the tolerance and measurement accuracy of the meter itself.

Analogue meters are typically ± 2% (AVO 8) and digital meters are typically 0.3% ±1digit (Fluke

79). This will affect the actual reading taken.

Many service manuals for older equipment will have voltages specified for various test points.

When measured with a modern DMM the measured value will often differ from the specified value

because of the different loading of a DMM compared to the meter used to measure the voltages

originally, usually an AVO. It’s also important to note that the service manual will often specify

which range the meter was set to. A 20kΩ/V meter will present a resistance of 200kΩ to the circuit

on the 10V range but on the 100V range the resistance will be 2MΩ which can result in apparently

different voltages being measured.

Whenever you measure a voltage, current or check a waveform you need to ask the question is it

what I was expecting?

If you measure the voltage on the collector or anode of a small signal amplifier what level of

voltage would you expect? To coin a phrase is it in the right ballpark. If you have service data with

expected voltages and it says for example the collector voltage is 9V and you measure 8.7V would

you worry? In that case no but if it’s 0V or close to the supply voltage then yes something is wrong

and needs further investigation.

It’s always worth measuring the supply voltages first as if these are wrong then all the other

voltages will be wrong. In many cases incorrect supply voltages will be the cause or a significant

contributor to the fault.

Demo board 1

Fig 36 - Demo board 1 circuit. 12v regulator and potential dividers.

This board has a 12V regulator and four potential dividers each one comprising two equal value

resistors. The resistor values are 1k, 10k, 100k and 1M all 1% components.

I’ll measure the voltage at the centre point of each potential divider with each meter and compare

the results.

Firstly measure the 12V supply on the demo board.

Fluke 79 DT830B TMK500

Range Auto range 20v range 25v range

+12V supply 12.13V 12.12V 11.6V

Next measure the voltages at the centre points and compare with the theoretical value of 6v.

Fluke 79 DT830B TMK500

Range Auto range 20V range 10V range

1k 6.06V 6.05V 5.9V

10k 6.07V 6.03V 5.8V

100k 6.02V 5.76V 5.05V

1M 5.79V 4.04V 2.2V

Note that the TMK500 being 30k/V represents a resistance of 300k on the 10V range. This changes

the theoretical value to 2.25V instead of 1.714V

This demonstrates the effect of the resistance of the meter on the measurements and shows that in

high impedance circuits the measured voltage may not be as expected. One example of such a

circuit is the screen pin of the DAF91/96 of a battery valve radio. These are usually in the megohm

range so the measured value will be lower than expected. The service manual for the Bush BAC31

gives the DAF91 screen voltage as 22V when measured using an AVO 7 on the 1000V range but

with a note stating it is subject to wide variation.

78M12 +12v

1k

1k

10k

10k

100k

100k

1M

1M

More advanced use of the Scope

We’ve seen the basic operation of scope with the signals from the sine wave and function

generators with a single trace. Now we’ll move on to show how the two traces can be used to show

the relationship between any two signals.

First here’s the output of the RF signal generator, Fig 37, connected to the scope with the timebase

setting unchanged (0.2ms/div) from the previous signal inputs. The trace is just a blur and no detail

of the signal can be made out. The only thing we can say is that there is signal of some sort there,

with an amplitude of approximately 0.45V pk-pk.

The signal generator output is actually set to 10.7MHz although you wouldn’t know it.

Fig 37 - 10.7MHz signal with the timebase

setting unchanged (0.2ms/div)

Fig 38 - Time base setting changed to 50ns/div

To see more detail of the signal change the time base setting to 50ns/div as shown in Figure 38. We

can now it’s actually a sine wave and by measuring the period of one cycle we can estimate it’s

frequency. Looking at Fig 38 we can see that one cycle takes approximately 100ns, two divisions,

so the frequency is approximately 10MHz. The signal generator is actually set to 10.7MHz so that’s

not a bad estimate.

Now let’s add some modulation to the signal. Good old Amplitude Modulation (AM) although

10.7MHz would normally have frequency modulation as it’s the standard IF frequency for FM

tuners.

Fig 39 - 10.7MHz with 1kHz amplitude

modulation.

Fig 40 - Showing the modulated RF with

the modulating waveform.

Fig 39 shows the RF signal and Fig 40 shows the RF signal and the relationship between this and

the modulating signal. You can see that the amplitude of the RF varies with the amplitude of the

modulating signal. If the minimum value amplitude of the RF signal falls to zero at the peak of the

modulating signal then it is 100% modulation. In practice this is rarely the case as the signal can

become distorted as I’ll show later.

By rectifying and filtering the RF signal, the original modulation can be recovered. It’s a very

simple process and has been at the heart of Long Wave, Medium Wave and Short Wave

broadcasting for almost 100 years.

I would have shown frequency modulation but with a deviation of 75kHz the difference in the

period of the 10.7MHz signal is approximately 700ps, which is not noticeable on this scope.

Amplitude modulation is far easier to show.

For this demo I actually set the scope to trigger on the modulating waveform as it shows the

relationship much more clearly.

Now we’ll move on to a simple audio amplifier circuit

Demo board 2

Demo board 2 has a regulator to power the amplifier and 555 oscillator circuits with a 12V

regulated supply. This is also similar to the regulator that powers Demo board 1.

Fig 41 - Demo board 2 12v regulator circuit.

The input is fed from an external supply via a diode. This prevents damage in the event of the input

connections being reversed but does incur an extra 0.7v drop. An optional input capacitor, C, can be

fitted if the connections to the external supply are long. A typical value for this capacitor would be

47μF to 220 μF. The output is decoupled with a 1μF capacitor which helps to reduce any noise on

the output. Optionally a 0.1μF ceramic capacitor can be fitted across the output. The LED across the

output is used to indicate the power supply is on.

There is a minimum input voltage as the 78 series of regulators require a minimum voltage between

the in out and output terminals to operate correctly. Typically this is 3v. Adding this to the output

voltage and the diode volt drop means a minimum input voltage of around 15.7v.

78M12

1 μ

+12V

1k

Input

16 to 20V

1N4001

C

1) Audio Amplifier.

Fig 42 – Audio amplifier circuit

Before demonstrating the amplifier I’ll briefly go through the design process I used. Several

approximations were used to speed up the design process.

The requirement was for an audio amplifier with a gain of 5 with an input of up to 100mV pk –pk

(35mV rms) and powered by a 12V supply.

Firstly set the collector voltage of TR1. It should be set to a value that will allow the collector

voltage to swing over the expected output voltage range. In this case the range is 0.5V peak to peak.

I chose a target voltage of 9V.

Next set the collector current. 1mA is a reasonable value. From this the value of R3 can be

calculated. 3v across R3 at 1mA = 3kΩ. Although this is a preferred value in the E24 series I chose

a value of 3.3kΩ, which is in the E12 series and is a more common value. This will give a collector

voltage of 8.7V, which is close enough to the target voltage.

Next is the value of the emitter resistor R4. The gain of the circuit is approximately equal to R3/R4,

therefore if the gain is 5 R4 should be 660Ω. The nearest preferred value is 680Ω giving a gain of

4.85, within 5% of the required value.

This will set the emitter voltage as the current through R4 is approximately equal to the collector

current, assuming the gain of the transistor is high. With a collector current of 1mA this gives the

emitter voltage as 0.68V.

The typical base-emitter voltage of a silicon transistor is approximately 0.65V to 0.7V giving the

base voltage as approximately 1.35V.

Now for the base bias resistors, R1 & R2. The current through these resistors should be

approximately an order of magnitude greater than the transistor base current. A typical current gain

(HFE) for the BC108 is 200 giving a base current of approximately 5μA for a collector current of

1mA. A current of 100μA is a reasonable value to choose giving the calculated values for R1 and

R2 as 106.5kΩ and 13.5kΩ. The nearest preferred values (E12 series) are 100kΩ and 12kΩ.

That’s the basic design of the dc conditions for the amplifier. Now for the ac side of the amplifier.

Input

R1

100k

R2

12k

C1

0.47μ

R4

680

C2

S1 10μ

R3

3.3k

R5

110

+12V

R6

10k

C3

10μ

Output

TR1 & TR2

BC108

TR1

TR2

We need to capacitively couple the signal into the amplifier to prevent any dc levels on the input

affecting the biasing of TR1. This capacitor will affect the low frequency response of the amplifier.

The LF –3db point of the amplifier is given approximately by

F = 1 / 2πC1.Rin

Where Rin is the parallel combination of R1, R2.

With C1 set to 0.47 μF and Rin approximately 10kΩ the –3db point is approximately 34Hz.

The high frequency –3db point is determined by the circuit capacitance in conjunction with the

various impedances in the circuit.

We’ll measure the high frequency –3db point during the demo.

I’ve added a couple of additional items to the circuit, one is an emitter follower to buffer the output

from the amplifier and the second is a switched capacitor and resistor, C2 and R5, across the emitter

resistor. This allows the gain to be increased from x5 to approximately x20.

The emitter follower, TR2, has a low output impedance and a high input impedance which reduces

the loading effect on the collector resistor of TR1 from the input impedance of the following

amplifier. If this impedance is relatively low, typically 10k, the collector load will be reduced to

approximately 2.4k as the input impedance is effectively in parallel with the collector resistor and

will reduce the gain of the amplifier to approximately 3.6. Adding the emitter follower minimises

the effect of the following amplifier input impedance on the gain.

The output from the emitter follower is capable of driving into the input impedance of most audio

amplifiers. To keep the low frequency response a 10μF coupling capacitor is used.

Checking the circuit

We’ve been through the theoretical side of the circuit now the practical side.

Demo board 2 has the circuit built up. Firstly I’ll measure the voltages on the emitter, base and

collector and compare them with the theoretical values then measure the gain and frequency

response.

Measurement

point

Theoretical

value Measured value

TR1 emitter 0.68V 0.485V

TR1 base 1.35V 1.135V

TR1 collector 8.7V 9.84V

The question is. Are these values acceptable?

The collector current calculated from these measurements is approx 0.65mA, which explains why

the collector voltage is higher than expected. This will reduce the maximum output voltage range,

as will be seen in the next section. However this is not a critical application so these values can be

considered acceptable. Had the application been more critical the values would have to be reviewed

and if necessary changed.

Measuring the gain

The signal generator is connected to the

input with the frequency set to 1kHz with

an amplitude of 400mV peak to peak. Note

that the Channel 1 (input) setting is set to

20mV/div but as a x10 scope probe is

being used the effective setting is

200mV/division.

Channel 2 (output) is set to 0.1V/div with a

x10 probe giving the effective setting as

1V/div.

The gain is simply Output voltage/Input

voltage, which is approximately

1.8V/0.4V

or a gain of 4.5

Also notice that the output is inverted compared to the input so strictly speaking the gain is – 4.5.

This is within 7% of the calculated value of 4.85 or in db –0.7db. Comparing it with the original

requirement for a gain of 5 it’s –0.9db.

Again the question is. Is this acceptable?

As with the voltage measurements it depends in the application. This is not a critical application so

the slightly lower gain is acceptable. I would doubt if you would be able to hear the difference

between a gain of 4.5 and 5.

Had the gain been critical, the circuit could be modified to include a preset gain control but note

that in commercial designs preset controls are discouraged unless absolutely necessary as it can be

costly to adjust the preset on the production line.

Frequency Response

To check the frequency response we need a reference level against which to measure the

frequencies, high and low, at which the output level falls to half the reference level or –3db. For an

audio amplifier the usual convention is to set the reference level at 1kHz.

We’ll set the frequency to 1kHz, and set the amplitude to give a peak to peak value of 4 divisions

on the scope. Then adjust the frequency up and down until the amplitude on the scope is 2 divisions

and note the frequency at which this happens.

To measure this I had to change from the

original signal generator to a high frequency

one. The output levels were set to the same

level as the original generator. The output remained constant well into the MHz region falling off

above 10MHz finally falling to half the 1kHz level at approximately 40MHz.

Fig 43 - Input and output signals

Upper trace – Input signal 20mV/div

Lower trace – Output signal 100mV/div

Low frequency 20Hz

High frequency Approx 40MHz

This shows that even though it was intended to be an audio amplifier it still has significant gain well

into the VHF region. The transistor used is a silicon planar transistor (BC108) typically used as an

audio amplifier and checking the data sheet shows that it will have gain at over 100MHz. This can

make it susceptible to RF interference.

Gain increase checks

Next I’ll set the gain to approximately 20 by closing S1. Note that the dc conditions are still the

same but from the AC point of view the emitter resistance, or strictly speaking the emitter

impedance has been reduced.

Fig 44 shows the input and output signals. The

output signal is about the same level as that in

Fig 43 (2V peak to peak) but the input signal is

now only 100mV peak to peak.

Note that the input trace in Fig 44 is noisy. This

is because to get the same output signal with the

higher gain the input signal has had to be

reduced but to get a viewable trace on the scope

the input sensitivity has had to be increased

which inevitably reduces the signal to noise

ratio. If the noise is too intrusive it may be better

to remove the x10 probe and connect the scope

input directly to the measurement point but be

careful that the lower input impedance and the

higher capacitance do not affect the circuit.

What happens if we increase the input signal

level? If you monitor the output of this circuit

with a power amplifier and speaker, then

increase the input there comes a point where the

tone of the sound changes significantly. If we

now monitor the output from the demo board

amplifier we’ll see a significant change in the

signal.

The waveforms are shown in Fig 45. The input

has the familiar sine wave shape but the output

does not look like a sine wave. What has

happened?

The input has increased to 400mV peak to peak

so with a gain of 20 the output should be 8V

peak to peak. This is ± 4V about the mean collector voltage of TR1, which is a nominal 9.8V. The

collector is thus trying to swing from 4.7V to 13.8V. The 4.7V is no problem as can be seen in Fig

45 but the collector voltage cannot go higher than the supply voltage so is limited to a swing of

4.7V to 12V. Hence the distortion.

The original requirement was for an output level of 0.5v peak to peak so trying to get 16 times the

output voltage is actually operating it outside its specification. However looking at Fig 44 it can be

seen that it will actually give an output of at least 1v peak to peak so there is a plenty of margin

should the input signal level be greater than the original requirement.

Fig 44 – Input and output signals - S1 closed

Upper trace – Input signal 50mV/div

Lower trace – Output signal 1V/div

Fig 45 – As Fig 44 but with the input

increased to 400mV peak to peak.

2) 555 Oscillator.

Fig 44 - 555 oscillator circuit

The 555, introduced in 1972, is probably one of the best known ICs of all time. It can be configured

as either a monostable timer or an oscillator and due to its internal configuration the timings are

independent of the supply voltage. For more information see the datasheet which is readily

available on the internet.

For this demonstration it is configured as an oscillator. Capacitor C1 is charged from the +12V

supply via R1, R2 and R3. During the charging period the output is high. When the voltage on the

capacitor reaches 2/3 of the supply an internal comparator (pin 6) pulls the discharge pin (pin7) low

which starts to discharge C1 through R3 and sets the output low. When the voltage has fallen to 1/3

of the supply the discharge pin and the output are both set high by the trigger pin (pin 2). The cycle

then repeats.

Two observations can be made on the timings

1. The on and off times can never be the same as the charging resistor is always higher than the

discharge resistor. It cannot therefore produce a 1:1 mark-space ratio square wave.

2. By changing the value of the charging resistor the on time and hence the frequency of the

output can be varied.

The frequency of the oscillator is

F = 1.44 / (R1+R2 +2R3).C

Where F is the frequency in Hz, C is in Farads and the resistances are in ohms.

Typically the value of R3 is > 1k. With the bipolar version of the 555 it is advisable to restrict the

maximum value of the timing resistors to approximately 1MΩ as the bias currents on the timing

pins (2,6 & 7) can become significant with higher value resistors and affect the timings. Similarly it

555

R4

10k

4 8

3

1 5

7

2

6

R3

10k

R1

100k

R2

10k

C1

0.047μ

C2

100μ

C3

0.01μ

S1

R5

2.2k

C4

10μ

Output

+12V

is not advisable to use very low values of capacitor for the timing capacitor as the circuit stray

capacitance can adversely affect the timings.

The values shown in the circuit above were chosen to give a maximum frequency, with R1 set to its

minimum value, of 1kHz. Using the formula these values give a frequency of 1021Hz. With R1 set

to its maximum value the calculated frequency is 236Hz. We’ll see how accurate these frequencies

are in practice.

Capacitor C3 decouples the internal divider chain for the comparators. Pin 4 is the reset pin,

connecting this pin to 0v prevents the operating and sets the output low. R4 ensures this pin is held

high so that the 555 can operate as intended.

Because of a design flaw in the design of the IC where both transistors in the output stage can be

switched on briefly and a large current “spike” is taken from the supply when the output stage

switches, it is important to fit a supply decoupling capacitor close to the IC to prevent interference

to other circuits on the same supply. This is more important when the dual 555, the 556, is used and

lack of decoupling can cause unwanted triggering between the two timer/oscillators.

This only occurs with the original bipolar version, the later CMOS version does not have this

problem.

We’ll now look at the waveforms in the circuit monitoring the output on pin 3 and the voltage on

the timing capacitor C1.

Fig 45 - Left

Upper trace. (5V/div). 0v level.

Lower trace (5V/div). Output.

Fig 46 & 47 - Below

Upper trace (5V/div). C1 voltage.

Lower trace (5V/div). Output.

Left R1 Minimum. Frequency = 1058Hz.

Right R1 Maximum Frequency = 242Hz.

The lower traces show the square wave output. You can see that the mark-space ratio is not 1:1

because of the different charge and discharge resistor values. The low time is set by the discharge

resistor R3 and the high time set by R1, R2 and R3. By increasing the value of R1 the on time is

increased while the off time remains the same.

The upper trace shows the voltage on the timing capacitor. You can see that it is oscillating between

4V and 8V or between ⅓ and ⅔ of the supply voltage.

There is a LED fitted across the output which will light up when the circuit is oscillating. As R1 is

varied, changing the frequency and also the mark-space ratio, its brightness will vary increasing as

the value of R1 increases. By changing the timing capacitor to 100uF you will be able to see the

LED flash to show the mark-space ratio.

The measured frequencies are both well within 5% of the theoretical values which, taking into

account the component tolerances, is perfectly acceptable.

Dual Timebase

As I mentioned earlier the Trio has a dual timebase. You may ask how can you have two timebases

in a scope and what would you use them for. The answer is to examine parts of a waveform in

greater detail.

The Trio has two time base speed controls A and B. So far we have only used the A timebase.

Suppose we have a video

waveform showing colour

bars (Fig 49). The

waveform that produces

this test pattern is shown

in Fig 50.

Because of the timebase

settings (the A timebase)

it is not possible to see a

great deal of detail of an

individual line. What we

need is a method of

expanding one TV line

from the waveform on the

screen.

Fig 49 – Colour Bars Fig 50 – Colour bar video waveform

B Timebase delay

Horizontal Mode

setting

B Timebase speed

A Timebase speed

Fig 48 – Dual Timebase controls

This is where the B timebase comes in. The Horizontal mode switch has 4 settings A, ALT, B and

XY. We’ll come to the XY setting later. Until now this switch has been in the A position, if we

move it to the ALT position part of the trace is highlighted and a second trace appears. This second

trace represents the highlighted part of the trace as shown in Fig 51. The position of this highlighted

section can be moved by adjusting the two Delay position controls (Coarse and Fine) and the width

of the highlight can be adjusted by adjusting the B Timebase speed setting.

Fig 51 – Horizontal Mode switch in ALT

position

Fig 52 – Horizontal Mode switch in B position

Moving the switch to the B position shows just the highlighted section of the original trace.

Note that the scope on screen display shows the timebase settings for both timebases and the delay

for the B timebase from the trigger point.

Before moving on Fig 53 shows the components of 625 line PAL colour signal.

Chroma (colour) information

Colour burst

Line sync pulse

Line period 64μs

Fig 53 – 625 line PAL Colour TV line

One of the characteristics of a colour signal (PAL or NTSC) is the burst, which is used to

synchronise the demodulators in the decoder. Pre DSO there was always a burst even if the

programme was in black and white.

Fig 54 shows the colour bar waveform and the output from the Aurora standards converter, which is

converting the 625 line PAL colour bars into a 405 line black and white signal.

It also shows the field sync pulse for the 625 line input and the 405 line output and shows the major

differences between the two systems, the lack of equalising pulses in the 405 line system and the

presence of the colour burst in the 625 line system.

Upper trace - 405 line output

Lower trace - 625 line input

Field sync pulse

Equalising pulses

Fig 54 - Input and output of Aurora standards converter.

X-Y setting

This is the fourth position of the horizontal mode switch. In this position the input to the X

amplifier is switched from the timebase to the output from the Channel 2 input. You may ask what

can this be used for but it can be very useful.

Previously we saw the modulated output from the RF signal generator with the modulating signal.

If we feed the modulating signal into the X input and the modulated output into the Y input we can

see how linear the modulator is, as the scope trace will show the RF output corresponding to each

point on the modulating signal.

Fig 55 – Modulation set to 80% Fig 56 – Modulation set to 100%

Fig 55 shows the output with the modulation set to approximately 80%. You can see the envelope is

straight with no kinks or distortions. However if we increase the modulation to 100% (Fig 56) the

right hand side is not straight indicating that the modulator is not linear at high modulation levels.

The classic use for the X-Y mode is for Lissajous figures. These can be used to compare two

frequencies but it can be very difficult to use but can look impressive. Just right for a Sci-Fi film

where the scientist looks at the rotating figure on the screen and predicts there are 20,000 alien

space craft approaching Earth.

Figures 59 to 61 show the outputs of both the function generator and the sinewave generator fed

into the X and Y inputs with the X input frequency fixed at approximately 1kHz.

Fig 59 - Frequency ratio 1:1 Fig 60 - Frequency ratio 2:1

Fig 61 - Frequency ratio 3:1 Fig 62 - Frequency ratio <1:1

Fig 62 shows what happens when the frequencies are close. The theory of using Lissajous figures to

measure frequencies is simple but in practice it is not easy. It’s much easier and quicker to connect

a frequency counter and measure the frequency directly. The fact that the frequency control on both

generators is a standard pot makes setting a precise frequency difficult. Fitting a slow motion drive

to the control would make it easier to set the frequency.

Stereo signal indicator

One other use for the X-Y mode is to detect stereo audio signals. If the same signal is fed into both

the X and Y inputs the resulting plot will be a straight diagonal line. If the signals fed into the X and

Y inputs are different the plot will deviate from the diagonal line.

Fig 57 – Mono signal Fig 58 - Stereo signal

We now move to the final section Signals in an AM radio

The radio is a superhet, where the signal frequency is mixed with a local oscillator to produce a

fixed intermediate frequency, which is amplified and demodulated.

Firstly we’ll look at the local oscillator.

Fig 59 - Tuned to Pulse 2 1278kHz Fig 60 - Tuned to Absolute Radio 1215kHz

Figs 59 and 60 show the local oscillator signal on both the oscilloscope and the frequency counter.

The local oscillator signal looks similar but the frequency counter shows the difference between the

two stations. From this you should be able to work out what the IF frequency is (470kHz).

We have to be careful when measuring the local oscillator. These measurements were taken using a

x10 probe. Earlier I mentioned you need to be aware of the effect of the measuring equipment on

the circuit under test. This is a good example because although the probe has a high resistance

(10MΩ) it has a small capacitance ( approx 3pF) as well which will affect the oscillator circuit. To

monitor the local oscillator frequency there are several alternative points in the circuit where the

probe can be connected as shown in Fig 61.

What effect will connecting the probe to each of these test points have? Connecting the probe will

add capacitance at each test point and adding capacitance to an oscillator will shift the operating

frequency.

Adding the capacitance at

test points 1 and 3 decreases

the frequency by

approximately 5kHz but the

additional capacitance at

test point 2 has a greater

effect decreasing the

frequency by 75kHz.

When I tried this it actually

retuned it to a different

station shifting from Pulse 2

to an off tune Absolute

Radio.

Connecting the probe to

other points in the circuit, such as the input of the IF amplifier, has a different effect. The input of

the IF amplifier has a tuned circuit tuned to the IF frequency, adding the extra capacitance detunes

the IF secondary which has the effect of reducing the signal level.

The final pictures show the demodulator

operation with one probe connected to the

output of the IF transformer and the other

probe monitoring the demodulated audio. Fig

62 shows the relevant points on the circuit and

Fig 63 and 64 show the IF and audio signals.

The IF signal doesn’t look like the AM signal

seen in Fig 40 because the demodulator diode

affects the positive part of the signal when it

conducts.

Note the incomplete trace in fig 63 and 64 is

due to the camera shutter speed and is not a

fault with the scope.

Fig 63 – IF and demodulated audio Fig 64 – Audio shifted up to show relation to IF

Test point 1 - Anode of

oscillator triode.

Test point 2 – Across tuned

circuit.

Test point 3 - Across

padder capacitor.

Fig 61 – Local oscillator circuit showing alternative test points

IF signal (upper trace)

Audio signal (lower trace)

Fig 62 – AM demodulator test points

That ends the demonstration and I hope you now have a better idea of how an oscilloscope can be

used to show what is happening in a circuit. It is invaluable for tracing instability in say an IF

amplifier or distortion in an audio amplifier and is an essential part of professional electronics

design work.

I hope also that you’ve seen how test equipment can affect the circuit under test and why you need

to assess each measurement you make as it will almost certainly not be exactly the same as the

voltage or waveform in the service manual. With time and practice will come the experience so you

can say with confidence “That voltage is OK”.

And Finally

A little bit about myself.

I have been interested in anything electrical or electronic from an early age and was very keen to

join the radio club at school, which I did when I was almost 13. It was there that I first came across

an oscilloscope. As I progressed through school I used to supplement my pocket money by

repairing the odd radio and TV for family and friends. I had decided that although I liked fixing

radios & TVs I would prefer to actually design electronic equipment so when I left school I went to

university and after 3 years of hard work and generally having a good time I left with a degree in

Electronic Engineering and started attempting to make a living at this electronics game. I started

working in Stevenage but left to return to university in an attempt to gain a further degree and ended

up working in the university Marine Science department designing, building and maintaining data

logging equipment for use on oceanographic cruises, learning a lot about designing equipment for

use by people who just wanted the data and the electronics was just the means to get and record the

data. After nearly 6 years I left to return to industry and ended up in Leicester where over the next

16 years I had 4 jobs, was made redundant twice, got married and started a family. Back in 2000 I

was made an offer I couldn’t refuse so moved up to Yorkshire working for a smallish company in

the TV set top box design business where I’m now working on failure analysis. Over my working

life I’ve worked in the defence industry, in a university environment, for a fire alarm company

whose products, including one I designed, are fitted in the building I now work in, and in the

consumer electronics industry.

One thing I have found is you never stop learning, when I started work in electronics it was right at

the end of the valve era and small scale ICs and discrete transistors were the order of the day. Over

the intervening years the use of more and more complex ICs has increased to the extent that

virtually everything these days has a micro controller in it. A TV set top box these days is basically

a single processor with built in peripheral interfaces, a bit of memory and suitable drivers to

interface to the outside world plus a lot, and I do mean a lot, of software. It hasn’t always been easy

to keep up with developments but going back to some of the simpler forms of domestic electronics

in the form of valve radios and TVs is one way of keeping sane in this modern throwaway world.

K.W. November 2012