2. AM-RADIO RECEIVER
Radio receiver is an electronic equipment which
picks up the desired signal, reject the unwanted
signal and demodulate the carrier signal to get back
the original modulating signal.
3. Function of Radio-receiver
• Select desired signal and reject unwanted
signal
• Amplify the R.F. signal
• Demodulate the selected signal
4. 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
5. Types of AM-Receiver
1.TRF-Receiver 2.Superhetrodyne AM-Receiver
TRF (Tuned Radio frequency) receiver
6. 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 tuning.
7. Super heterodyne Radio Receiver
A super heterodyne receiver converts all incoming radio frequency (RF) signals to a
lower frequency known as an intermediate frequency (IF).
10. Advantages of SHRR
No variation in bandwidth. It remains constant
over the entire frequency range.
High selectivity & sensitivity
High adjacent channel rejection
11. Receiver characteristics of the radio
1.Selectivity :
Radio receiver should have good selectivity and selectivity of
radio receiver is its ability to reject unwanted signals.
12. 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.
13. 3.Fidelity
The fidelity is the ability of receiver to reproduce all modulating
frequency equally.
1. Radio receiver should have high fidelity or accuracy without
introducing any distortion.
2. 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.
14. 4.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.
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
15. 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
16. Advantages of RF-Amplifier
1.Greater gain i.e better sensitivity.
2.Improved signal to noise ratio.
3.Improved rejection of adjacent unwanted channel.i.e better
sensitivity.
4.Better coupling to antenna/Good impedance matching
17. 2.MIXER or FREQ.CHANGER
The mixer is basically a nonlinear device with two set of inputs at
different frequency & one set of o/p.
18. 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.e
Vout = (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
19. 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)
Frequency component. Thus square law device acts as Mixer.
20. IF & IF-Amplifiers
• Intermediate Frequency
– Sum or difference in the output of a mixer that enters the IF
stage.
• IF amplifiers
– One or more stage(s).
– Provide most gain and selectivity.
– IF is much lower than RF easier to design and good
sensitivity is easier to obtain with tuned circuit.
– IF amplifier will decide sensitivity, selectivity of the receiver.
Suitable IF range is 438-465 khz. with 455khz best.
21. 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.
22. 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.
