Am radio receiver

AM-RADIO RECEIVER

Radio receiver is an electronic equipment which pick ups the desired signal, reject the unwanted signal and demodulate the carrier signal to get back the original modulating signal.

• Select desired signal and reject unwanted signal

• Amplify the R.F. signal

• Demodulate the selected signal

Function of Radio-receiver

Classification of radio receivers

Radio receivers are classified according to the type of traffic they are designed to handle.

• A.M. broadcast receivers• F.M. broadcast receivers• T.V. receivers• Radar receivers

Types of AM-Receiver 1.TRF-Receiver 2.Superhetrodyne AM-Receiver

TRF (Tuned Radio frequency) receiver

Drawbacks of TRF Receiver

• TRF receivers are simple to design and allow the broadcast frequency 535 KHz to 1640 KHz. But at the higher frequency, it produces difficulty in design.

• Problem of instability• It has poor audio quality.• B.W vary with tunning• Insufficient adjacent channel rejection.

• The super heterodyne receiver was invented by Edwin H. Armstrong and is still almost universally used.

• A super heterodyne receiver is characterized by one or more stages of RF amplification and the RF stage may be tuned or broadband.

The Super heterodyne Receiver

Super heterodyne Radio Receiver• The shortcomings of the TRF receiver are overcome by the invention of the super heterodyne receiver.• A super heterodyne receiver converts all incoming radio frequency (RF) signals to a lower frequency known as an intermediate frequency (IF).

• The input filter and RF stage are referred to as the Front End of a receiver

• The mixer combines the signal frequency with a sine-wave signal generated by a local oscillator creating an intermediate frequency

Super heterodyne Radio receiver with waveform at various points

SHRR with center freq.& B.W of each block

• The output of mixer will produce sum and difference frequencies:

fo + fs and fo – fs • For all incoming fs IF is constant and constant

frequency difference is maintained between local oscillator and RF circuit through capacitor tuning in which all capacitors are connected together and operated by one control nob. It is called ganged tuning.

• AGC or AVC (Automatic Volume Control) is a system by means of which the overall gain of radio receiver is varied automatically with the variations in the strength of received signals, to maintain the output constant.

• AFC circuit is used to adjust and stabilize the frequency of local oscillator.

Advantages of SHRR

No variation in bandwidth.it remains constant over the entire freq.range. High selectivity & sensitivity High adjacent channel rejection

Receiver characteristics of the radio

1.Selectivity : Radio receiver should have good selectivity and selectivity of radio receiver is its ability to reject the unwanted signals.

• Selectivity is obtained by using tuned circuits, which are tuned to desired frequency. The quality factor of these LC circuits determines the selectivity. It is given by, Q=XL/R

For better selectivity ‘Q’ should be high.

2.Sensitivity Ability to amplify weak signals. Broadcast receivers/ radio

receivers should have reasonably high sensitivity so that it may have good response to the desired signal but should not have excessively high sensitivity otherwise it will pick up all undesired noise signals. It is function of receiver gain and measures in decibels/uv.

3.Fidelity

1. radio receiver should have high fidelity or accuracy without introducing any distortion.

2. it is determined by the high frequency response. therefore it should have high frequency response over entire audio frequency range.

3. Eg. In an A.M. broadcast the maximum audio frequency is 5 KHz hence receiver with good fidelity must produce entire frequency up to 5KHz.

The fidelity is the ability of receiver to reproduce all modulating frequency. equally.

Fidelity curve

4.Image frequency & its rejection

• Definition: In radio reception using heterodyning in the tuning process, an undesired input frequency that is capable of producing the same intermediate frequency (IF) that the desired input frequency produces.

• It is given by signal frequency plus twice the intermediate frequency

fsi = fs + 2fi• In AM broadcast receiver the local oscillator freq.is higher than

the signal freq.by intermediate freq.IF f0 = fs +IF or IF= (f0-fs)------------------------------------------------1 Mixer o/p is SUM/DIFF.of two freq. Now assume oscillator freq.f0 & unwanted signal at freq.of Fsi =f0 +IF reaches to i/p of mixer along with desired signal freq.fs

Then mixer o/p consist of four freq.component.like fsi,f0,(fsi+f0) & (fsi-fo)1.fsi=f0+IF 2.f0 3.(2f0 +IF) & 4.IF

Fourth point (fsi-f0) = IF this component will also amplified by the IF amplifier along with the desired signal fs

This will create interference because both signal corresponds to carrier freq.fs & fsi will tuned at same time.

This unwanted signal at freq.fsi is known as image freq.& it is said to be the image of signal

The relation between fs & fsi isImage frequency = fsi =fs+2IF --------------------------------------------2

• The rejection of an image frequency by a single tuned circuit is the ratio of the gain at the signal frequency to the gain at the image frequency. It is given by,

IFRR (ά ) = √1+Q2ᵨ2

Where, ρ=(fsi / fs) - (fs / fsi)

• IFRR of two tuned ckt.= ά1. ά2 it should be high.

• Image freq. problem can be avoided by highly selective RF Amplifier. or don't allow that freq. to a particular station which is image freq. of other station

5.Double spotting

In this, receiver picks up same signal at two nearby point on receiver dial it is called double spotting.

if incoming signal is 1000khz & it is strong signal.if we tune the local ostr.at 1455khz.we will get 455khz at the o/p of mixer i.e.IF freq.

Similarly if we tuned the local osltr.at 545khz.then also difference will be 455khz.so the signal of 1000khz will appear at 1455khz.& 545khz. On the receiver dial.& it is double spotting. it can be avoided using high selective RF-Amplifier.

Its draw back is it will mask the weak signal

5.B.W.Improvement

To reduce the effect of thermal noise in the performance of the receiver, it is necessary to reduce the B.W because noise is proportional to square root of B.W.

Reduction in B.W is therefore equivalent to reduction in the noise figure of receiver.

Noise reduction ratio is achieved by reduction in B.W is known as the B.W. improvement

B.W improv (B.I) =RF.B.W/IF.B.W

Noise figure improvement = 10Log10 B.I

6.Insertion loss

• The insertion loss is the ratio of o/p power of filter (Pout) to the i/p power (Pin) for signal freq.lying within pass band of filter.

• Insertion loss (I.L) = 10Log10 (Pout/Pin)

7.Dynamic Range

Dynamic range is the I/P Power range over which the receiver is useful.It is in decibel. Highest range is about 100 dB

AM-RECEIVER CIRCUITS RF Amplifier using BJT

A radio receiver having RF-Section to tune the signal from antenna There may not be RF-Amplifier.but if RF-Amplifier is used with RF-

tuned ckt.then the performance of receiver is superior.

Advantages of RF-Amplifier1.Greater gain i.e better sensitivity.2.Improved image freq. rejection.3.Improved signal to noise ratio.4.Improved rejection of adjacent unwanted channel.i.e better

sensitivity.5.Better coupling to antenna./Good impedance matching.6.Prevention of reradiation of local oscillator through the antenna of

the receiver.

2.MIXER or FREQ.CHANGERThe mixer is basically a nonlinear device with two set of inputs at different frequency & one set of o/p.

In square law mixer, square law device is used. as mixer.It is device having squared relation between i/p & o/p.Let two i/p applied to square law device are

Vi = V1Sinω1t & Vosc = V2Sin ω2t --------------------1

O/P of mixer is square of sum of two i/p .i.eVout = (Vi +Vosc )2 = Vi 2 +Vosc 2 +2 Vi.Vosc

Put the value of Vi & Vso in above eqn.

Vout = V1 2 Sin 2 ω1t + V2 2 Sin 2 ω2t + 2 V1V2Sinω1t . V2Sin ω2t .

We know Sin 2ω1t =1-Cos2ω1t/2 & 2Sinω1t.Sin ω2t =Cos(ω2-ω1)t – Cos(ω2+ω1)t

Vout = V1 2[1-Cos2ω1t]/2 + V1V2[Cos(ω2-ω1)t – Cos(ω2+ω1)t + V2 2[1-cos2ω2t]/2 Vout = V1 2 /2 - V1 2 Cos2ω1t/2 + V1V2Cos(ω2-ω1)t - V1V2 Cos(ω2+ω1)t + V2 2 /2 - V2 2 Cos2ω2t/2----2

So mixer o/p is consist of SUM & Difference (ω2+ω1) & (ω2+ω1)Freq. component.thus square law device acts as Mixer

Doide ,transistors & FET are square law device

Self Excited Mixer

Same ckt. acts as Mixer as well as Local oscillator. this Mixer is preferred over the separately excited mixer.

The same ckt. oscillates, the transconductance of the transistor is varied nonlinearly at local oscillator rate.

This variable .gm. Is used by the transistor to amplify the RF-Signal.

Thus two different signal at different freq.are applied to nonlinear resistance. required IF component produce at the o/p

Tracking The receiver has a number of tunable ckts.like RF-amlifier,Local

osctr. & Mixer. All these ckt.must tuned correctly if any station to be tuned. due

to this reason the capacitor in the various ckt. ganged together. due to this arrangement it is possible to use only one tuning control to vary the tuning capacitors simultaneously.

The local oscillator freq.(f0) is precisely adjusted to a value which is above the signal freq.(fs) by IF i.e f0 = fs + IF.

If tuning is not done properly then freq. difference is not correct.(f0-fs).this type of error is known as “tracking error”

Due to this error the station will appear away from their correct position on the freq. dial of the receiver.

Tracking is a process in which the local oscillator freq.follows

or track the signal freq.to have a correct freq.difference.

Tracking is mainly classified in to

1.Two point tracking a. Padder tracking b. Trimmer tracking

2.Three point tracking

Padder tracking : A small variable capacitor Cp called padder capacitor is connected in

series with oscillator coil as shown below. due to series connection of Cp & Cosc the equivalent capacitance is less than osctr. capacitance so freq. is increased making the tracking error is positive.

The padder capacitor is so adjusted to have zero tracking error on two extreme points on freq. dial as shown below

TRIMMER TRACKING

A small variable capacitor Ctr is connected in parallel with oscillator capacitor Cosc.

due to parallel combination the equivalent capacitance is greater than oscillator capacitance.

This will decreases the oscillator freq.so it make negative error.

The trimmer is so adjusted to get zero error at two point on the freq. dial.

Trimmer tracking & error in trimmer tracking

Three point tracking

Procedure to calculate Padder & Trimmer capacitor

• Step-1 to calculate f0min,fomax : It can be calculate from fsmin,fsmax & IF

• Step-2 Calculate the ratio Csmax/Csmin Csmax/Csmin = [famax/fsmin]2 -------------------------------1

we know Csmax=Cosc max & Csmin = Coscmin

• Step-3 Calculate effective capacitance of oscillator .Co It is the series combination of padder cap.Cp & Oscillator cap.Cosc (Cosc = Cs )

Co = [Cp xCs]/[Cp+Cs] ----------------------2

• Step-4 Calculate the ratio of Comax/Comin

From eqn.2 Comax/Comin = [Csmax.Cp/Csmax+Cp] / [Csmin.Cp/Csmin+Cp]

Comax/Comin = Csmax[Csmin+Cp] / Csmin[Csmax+Cp] ----------3

Also Comax/Comin = [fomax/fomin]2 --------------------------------------------4

Step-5 to calculate oscillator coil

As we know fo =1/2л√Losc.CsLosc = 1/(2л)2(fomin)2 Comax ------------------------------------------5

Losc = 1/(2л)2(fomax)2 Comin -----------------------------------------6

Comin = [Cp xCsmin]/[Cp+Csmin]

Comax = [Cp xCsmax]/[Cp+Csmax]Trimmer value can be calculated asCo = [Cosc +Ctr] or = [Cs+Ctr] -------------------------------------7

LOCAL OSCILLATOR The local oscillators are Armstrong, Hartley & Colpitts

oscillator. Having high freq. stability. Oscillator freq.(f0)should be greater than signal freq.

Why ? For medium wave band the freq.range is from 540 Khz

to 1650Khz. Assuming IF = 455Khz. the local oscillator freq.will be in the range of 995-

2105khz. f0 max/fomin = 2105/995 = 2.2 -------------------------------1 If local oscillator freq.is designed to be lower than the

signal freq. in the range 85Khz-1195 Khz.this gives freq.ratio

F0max/fomin = 1195/85 =14 -------------------------2 The normal tuning capacitor has capacitance ratio Csmax/Csmin = 10 this capacitance ratio will easily gives a freq.ratio of

3.2 which is greater than 2.2

The freq. ratio 14 using tuning capacitance could not be achieved.

Hence local oscillator freq. must be greater than signal freq. tracking error is also reduced

IF & IF-Amplifiers

The choice of IF depends on following factor1.IF should not be too high as it will result in poor selectivity

& therefore poor adjacent channel rejection. difficult to obtain high Q.at high freq.

2.Also tracking problem is increases.3.IF freq.is lowered then IFRR is poorer.4.The IF should not fall in the tuning range of receiver.

instability occures & heterodyne whistles will heard. so tuning is impossible.

5. suitable IF range is 438-465 khz. with 455khz best.

IF Amplifiers : IF amplifier will decides sensitivity, selectivity of the receiver.

It is fixed freq.amplifier.it is supposed to select the desired freq.& reject the unwanted freq. to obtain higher gain two stage IF amplifier is used in radio receiver.

IF amplifier operates over fixed freq. band. hence it is easy to design.

It should satisfy following condition1.It should be stable.2.It should not radiate the EM radiation.

Multistage is inductively coupled & double tunned ckt.is used. 1. To obtain sharper freq. response. 2.It is possible to increase B.W.3.The freq.response falls very sharp as compared to single

tunned amplifier.this improves selectivity.4. Impedance matching is possible.

Frequency Response of double tuned amplifier

Freq.response depends on coefficient of coupling between L1 & L2

In double tuned load is transformer. primary & secondary forms separate resonance ckt.

Both resonance to same freq. the coefficient of coupling(K) may be tight, critical or loose coupling.

Critical coupling (Kc) =1/√Q1.Q2 --------------------------------------1

Q1 =quality factor of primaryQ2=Q.F of secondary

AM DETECTOR/ENVELOPE DETECTOR

It produces an O/P signal that follows the envelope of the I/P AM Signal exactly. It is used in all commercial AM Radio receivers.

I/P,O/P Waveform of Envelope detector Capacitor charges through the series resistance Rs &D, when diode is ON.& it discharges through R when diode is off. Charging time constant Rs.C. should be short as compared to carrier period 1/fc Rs.C <<1/fc

discharging time constant R.C. should be long enough so that cap. should discharge slowly through load .R.

1/fc <<R.C << 1/fm

DISTORTION IN DIODE DETECTOR 1.Diagonal peak clipping

If R.C time constant is too long then diagonal clipping distortion is occurred. at point tA. discharging time is slow & i/p voltage to diode is less than the cap. voltage so diode becomes reverse biased & stop conduction. So from t A.a to t B o/p follows the discharge path of cap. instead of modulating signal. so diagonal part of mod. signal will be clipped & it is called diagonal peak clipping.

At this time diode c/n will be zero.so to avoide diagonal clipping diode c/n should not be zero.for that modulation index must be less than 1

2.Negative peak clipping

To reduce the distortion it is necessary to choose the RC time constant appropriately.

• O/P of demodulator is weak & it is amplified by amplifier .coupled through coupling capacitor. it blocks the DC & pass the ac.so it gets charged .its voltage is Ec. So due to this voltage min. voltage is generated across R i.e

• Vmin = [R/(R+R1)]X Ec ---------------------------------------------1• but Ec = 1• So it will be Vmin =[R/(R+R1)].

• When i/p voltage is less than Vmin diode becomes reverse bias .o/p produced due to voltage Vmin.from tA to tB.

• -ve peak o/p of mod.signal is clipped hence it is known as –ve peak clipping.

PRACTICAL DIODE DETECTOR

Direction of diode has been reversed, negative envelope will demodulated. due to this negative AGC voltage will be developed. R1,R2 provides a series DC path.

• R1-C1 forms low pass filter used to remove the RF ripple presented in detected o/p.C2 is used as coupling capacitor. which avoids DC-component to reach the R4.Across the R4 we get demod. o/p. this signal is applied to AF-Amplifier.

• The R3-C3 forms LPF it allows DC component from through it & used as AGC voltage.

• This AGC voltage applied to the RF,IF amplifier to control their gain. automatically.

• This AGC voltage is proportional to the signal strength, stronger the AM signal higher the AGC voltage

SYNCHRONOUS DETECTION OF AMI/P modulated signal is multiplied with a locally generated phase & freq. synchronous carrier at the receiver .the product is passed through a LPF to pass only the mod.signal

COMPARISION OF SYNC.DETECTOR Vs ENVELOPE DETECTOR

SR.NO PARAMETER SYNCH.DETECTOR ENV.DETECTOR

1 Locally generated Carrier Used Not req.

2 Types of error Freq.& phase error Envelope distortion & diagonal clipping

3 Operating principle Multiply AM-signal with pilot carrier & pass thr.LPF

Rectify & pass thr.LPF

4 Complexity High Less

5 Synch. with TRX. Necessary N.R

6 Used for DSB-SC,SSB,VSB DSB-FC

Automatic Gain Control

If the receiver gain is const. then receiver o/p will fluctuate according to the strength of i/p signal.& this is not desirable .

AGC is used to adjust the receiver gain automatically so as to keep the receiver O/P constant irrespective of the strength of I/P Signal.

Simple AGC• It is a system which will change the overall gain of receiver

automatically.• It maintain constant gain even when strength of i/p is changed.• In AGC bias voltage derived from detector o/p .• The AGC biased ( -ve bias) given to RF,IF & Mixer block its

transconductance changes & gain controls.

When i/p signal is strong –ve bias increases so automatically gain controlled.

If i/p is weak then –ve bias is less automatically gain increases so that o/p level maintain constant.

Delayed AGCFor weak signal also sound level at o/p also decreases which is not desirable & it can be avoided by delayed AGC .

AGC Ckt. will in operation if sound level at o/p (d.c level) is above a certain voltage .V.

Advantages of delayed AGC

• Gain is not reduced for weak signal

• Gain is reduced for strong signal.

• The chs. Is closed to ideal AGC Chs.

• The delayed AGC Chs. Is adjustable.

SQUELCH CIRCUIT It is also called as Muting ckt.or quiting ckt.It is used to disconnect the noise. In absence of signal. In absence of signal, AGC will be zero & squelch ckt. will cut of the audio amplifier so that noise should not pass through the amplifier/speakers.

MORSE CODE

SSB RECEIVER (This ckt.is std.double conversion RX.)

The first IF is at 2.2MHZ & second IF is at 200KHZ.

Two filter follow the second IF amplifier. one for each sideband. Each filter has sideband 2.7 KHZ.

The appropriate sideband is selected by Switch.& apply it to SSB Demod. after demod.it apply to audio amplifier.

Oscillator freq. remains stable due to freq. synthesizer.

ISB RECEIVER

• It is again double conversion SHRR up to low freq. IF stage. two independent sideband separate with mechanical filter.

• As it is DSB-SC, no carrier is transmitted ,AGC is obtained rectifying part of the combined audio signal.

• First IF is very high 40.455 MHZ this will provide excellent image freq. rejection.

• Due to presence of freq. synthesizer freq. stability of this RX.is very high.

VSB RECEIVERReceived Modulated VSB is given to the i/p of product modulator, where it is multiply by locally generated synch.carrier

• O/P of product modulator is m(t) = S(t)xEc.Cos(2лfct)• Take F.T.on both side we get• M(f) =S(f)*[1/2.∂(f+fc) + 1/2. ∂(f-fc)]• M(f) = ½ S(f+fc) + ½ S(f-fc) ---------------------------------1 But S(f) =Ec/2 [X(f-fc)+X(f+fc)].H(f)So S(f+fc) = Ec/2 .[X(f)+X(f+2fc)].H(f+fc) S(f-fc) = Ec/2.[X(f-2fc)+X(f)].H(f-fc). Put this value in eqn.1

M(f) = Ec/4. [X(f-2fc).H(f-fc)+X(f+2fc).H(f+fc)] (first term) + Ec/4. X(f).[H(f-fc)+H(f+fc) (second term)----------------------2

First term represents VSB with carrier of 2fc.i.e.eliminated by filter.

Second term represents spectrum of demodulated VSB.Vo(f) = Ec/4. X(f).[H(f-fc)+H(f+fc)

Spectrum of VSB Receiver