F. Hueber

An external timebase calibrator is a useful accessory forany oscilloscope it provides precise, visible time markerson the scopes horizontal sweep. Basically the circuit is a

pulse generator with accuratetime intervals between pulsesi.e. the pulse repetition fre-quency. If the pulse width ismade relatively small comparedto the repetition rate and thepulse edges are steep then theoutput signal will look like aseries of illuminated dots. Thesecan be conveniently used tomeasure time periods on thescreen just as you use the grad-uation marks on a ruler to mea-sure length.

The circuit diagram shown inFigure 1 uses five commonlyavailable ICs (excluding the

power supply). A 1 MHz crystal provides an accurate timebase for the oscillator circuit built around IC1A. Resistor R3governs the switching threshold while trimmer C1 altersthe loading on the crystal and allows its frequency to be

SUMMER CIRCUITSCOLLECTION

64 Elektor Electronics 7-8/2001

032

IC2.A

CT=0

DIV2

DIV5

CTR

CT

5

7

6

31 +

4 +

2

0

2

IC2.B

CT=0

DIV2

DIV5

CTR

CT

11

9

10

1315 +

12 +

14

0

2

IC3.A

CT=0

DIV2

DIV5

CTR

CT

5

7

6

31 +

4 +

2

0

2

IC3.B

CT=0

DIV2

DIV5

CTR

CT

11

9

10

1315 +

12 +

14

0

2

IC4.A

CT=0

DIV2

DIV5

CTR

CT

5

7

6

31 +

4 +

2

0

2

IC4.B

CT=0

DIV2

DIV5

CTR

CT

11

9

10

1315 +

12 +

14

0

2

1

23

IC1.A

&4

56

IC1.B

&

9

108

IC1.C

&

12

1311

IC1.D

&

1

4

3

2

56

R3

10M

R1

68

0Ω

R2

5k

6

R5

6k

8

R6

68

0Ω

R7

22

0Ω

R8

C2

33p

C1

100p

S1.A

S1.B

1

432 5

6

X1

1MHz

C7

10n

C6

1n

C8

22n

C4

33p

C3

10p

C5

100p

+5V

S2

S3

74121

IC5

RCX

10

CX

11

63

4

15

&1

R

+5V

K2

7805

IC6

D1

1N4001C10

220µ25V

C18

100n

C11

100n

C12

47µ16V

IC1

14

7

C17

100nIC5

14

7

C16

100nIC2

16

8

C13

100nIC3

16

8

C14

100nIC4

16

8

C15

100n

+5V

IC1 = 74HC00 IC2 = 74HC390IC3 = 74HC390IC4 = 74HC390

994092 - 11

*

zie tekst*see text*voir texte*siehe Text*

S1: 1 = 100 ms2 = 10 ms3 = 1 ms4 = 100 µs5 = 10 µs6 = 1 µs

Economical Timebase Calibrator

1

‘pulled‘ slightly which is neces-sary when calibrating the circuit.IC1B buffers the oscillator fromthe rest of the circuitry and R1cleans up the square wave out-put by reducing any overshooton the clock edges. The outputsignal is connected to five cas-caded decade counters type74HC390 (IC2 to IC4A) eachcounter divides its input fre-quency by 10. Switch S1Aselects one of the frequencies ortime intervals from 1 MHz (1 µS)to 10 Hz (100 ms) to route it to apulse generator formed by IC5.The second half of counter IC4is used to provide a divide-by-two function, this can bebypassed by switch S2. In totalthis gives 12 possible pulse rep-etition rates from 1 µS to 200 ms.

The output timing pulse isgenerated by IC5. This is a stan-dard TTL monostable type 74121. Standard TTL devices canbe interfaced directly with HC devices without any prob-lem. The output pulse width of the monostable is a functionof the resistor/capacitor value at pins 10 and 11. As the rep-etition rate is changed by switching the counter outputs withS1A so the second half of the switch (S1B) also switches dif-ferent R-C components to the monostable. This ensures thatthe marker pulses shown on the oscilloscope screen will bethe correct width for each selected range. The output stageof a standard TTL IC does not drive symmetrically so IC1Dis used as a buffer to give a better output performance.Switch S3 allows the polarity of the output pulse to beswitched and resistor R7 provides short circuit protectionfor the output buffer. Unfortunately in combination with thecapacitance of the output lead, this resistor also forms a lowpass filter that has the effect of rounding off the sharp edgesof the output signal. Socket K2 is used for connection of anexternal 9 V mains unit to power the circuit and IC6 regu-lates this to 5 V for use on board. Current consumption isonly a few milliamps so a heatsink is unnecessary.

Fitting the PCB into a case is greatly simplified bymounting the single-sided PCB directly to the back of thefront panel switches.

Mounting the components on the board is begun by firstsoldering the six wires bridges and the smaller compo-nents to the board. It’s worth taking a little care here toensure that the polarised capacitors and diode are cor-rectly fitted. This design will produce RF interference so itis advisable to fit the unit inside a metal case or at least ascreened plastic case, the screen or case should be con-nected to the power supply ground.

To test the circuit, first check that 5 V is available from

the power supply. Next, connect a frequency counter toresistor R1 and adjust trimmer C1 until 1.000 MHz isachieved. If there is insufficient adjustment in C1 then try adifferent value for C2. If you do not have access to a fre-quency counter then just set the trimmer to mid-position orreplace it with a 56 pF fixed capacitor.

The output of the calibrator can be connected to thescope input channel via a short length of 50-Ω coax cable.An output series resistor (R8) is used to dampen ringing onthe output pulses introduced by the cable capacitance. R8can be fitted directly to the output BNC socket and its valuewill be in the range of 220 Ω to 470 Ω.

The best output pulses will be produced by hooking thetip of a 10x scope probe directly on the output pin of the

SUMMER CIRCUITSCOLLECTION

657-8/2001 Elektor Electronics

994092-1(C) ELEKTORC1

C2

C3

C4

C5C6

C7

C8

C10

C11

C12

C13

C14

C15

C16

C17

C18D2

F

H1 H2

H3H4

IC1

IC2

IC3

IC4

IC5

IC6K2

R1

R2

R3

R5

R6

R7

R8

S1

S2 S3

X1

994092-1

+

0

T

994092-1(C) ELEKTOR

COMPONENTS LIST

Resistors:R1,R6 = 680ΩR2 = 5kΩ6R3 = 10MΩR5 = 6kΩ8R7 = 220ΩR8 = *

Capacitors:C1 = 100pF trimmerC2,C4 = 33pFC3 = 10 pFC5 = 100pFC6 = 1nFC7 = 10nFC8 = 22nFC10 = 220µF 25V radial

C11,C13-C18 = 100nFC12 = 47µF 16V radial

Semiconductors:D1 = 1N4001IC1 = 74HC00IC2,IC3,IC4 = 74HC390IC5 = 74121IC6 = 7805

Miscellaneous:S1 = rotary switch, 2 poles,

⋅6 contactsX1 = 1MHz quartz crystal S2,S3 = toggle

switch,1⋅change-overcontact

K2 = 2-way PCB terminalblock, lead pitch 5mm

2

BNC connector, most scope probes will be able to managethis without any problem. A useful addition to the frontpanel next to the BNC output would be a solder/test pointconnected to the circuit earth. This provides a convenientparking spot for the scope probe earth clip.

To check the horizontal timebase of an oscilloscope firstmake sure that and variable time base controls are set tothe ‘calibrate‘ position then select a sweep speed so thateach output pulse corresponds to one square of the screen

graticule. Use the horizontal position adjustment to placethe pulses exactly under the graticule lines Check carefullythat the pulses occur exactly at each graticule line inter-section across the full width of the screen. This will notalways be the case with budget priced oscilloscopes!

If you have a two-channel scope it is also possible to usethe calibrator to perform quick and easy frequency mea-surements so that in many cases you will not need a fre-quency counter at all. First of all connect the signal to bemeasured to channel A of the scope input and the calibra-tor output to channel-B input. Adjust the scope timebasegenerator so that one whole period of the unknown fre-quency is displayed on the screen. With the scope triggermode set to ’alternating’ adjust the vertical positions of thechannels until they are superimposed and the edge of oneof the pulses coincides exactly with a point on the channelA waveform (see (1) in Figure 3). Now to find the fre-quency just count the number of pulses that occur until thechannel-A waveform has completed one complete period(2). In the screen shot shown here there are 12.3 intervals of1.0 µs therefore the frequency is given by

f = 1/ 12.3×10-6 s = 81.3008 kHz.

These are only two applications of this versatile circuit, nodoubt you will find many more.

(994092-1)

SUMMER CIRCUITSCOLLECTION

66 Elektor Electronics 7-8/2001

3

P. Lay

Whenever an antediluvian electric door-bell is used in an apartment building, therain of sparks that is generated when theWagnerian hammer pounds against the‘sounding body’ infests the bell networkwith interference pulses. These can sig-nificantly disturb electronic doorbells, oreven cause them to give up the ghost. Ifyou cannot convince your neighbour toconvert to something more modern, or atleast to build in a noise suppression net-work, you can use the electronic doorbell described here,since it is immune to EMD.This circuit is based on a simple multivibrator stage towhich a loudspeaker is connected. Capacitor C4 providesdc isolation between the multivibrator and the loudspeaker(8 Ω, 0.25 W). The frequency is determined by the RC net-works R2/C2 and R3/C3; it lies at around 0.7 RC = 2 kHz.The multivibrator stage receives its supply voltage from the

bell transformer. For this purpose, the ac voltage must berectified by D3-D6, and Zener diode D7 prevents the volt-age from rising above approximately 18 V. EMD immunityis provided by the lowpass network R5/C1. The bell canalso be silenced using switch S2. In this case, the only thingthat happens when someone presses on the bell button isthat D1 lights up.

(014005-1)

033R1

1k

R2

22

k

R3

22

k

R4

1k

R6

2k

7 R5

680Ω

Tr1

D1

D2

1N4004

S1 S2

D6

D4

D3

D5

1N4004

D7

18V1W

C1

2200µ 25V

4x

T1

BC547

T2

BC547

C2

33n

C3

33n

D8

1N4004

D9

1N40048Ω

LS1

250mW

C4

1µ25V

014005 - 11

rotrood

redrouge

EMD-immune Electronic Doorbell