1. Radio Receivers
Definition:
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.

2. Functions of radio receivers
Select desired signal and reject unwanted signal
Amplify the R.F. signal
Demodulate the selected signal

3. 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

4. Characteristics of radio receivers
Sensitivity
Selectivity
fidelity

5. Selectivity:-
Radio receiver should have good selectivity and selectivity of radio receiver is its ability to differentiate desired signal from unwanted signals.

6. 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.

7. 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.
Fidelity:-
1. radio receiver should have high fidelity or accuracy.
2. it is determined by the high frequency response. Therefore it should have high frequency response over entire audio frequency range.
3. Ex. In an A.M. broadcast the maximum audio frequency is 5 KHz hence receiver with good fidelity must produce entire frequency up to 5KHz.

8. TRF (Tuned Radio frequency) receiver
amplifier
detector F.
amplifier
Modulating signal

9. 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.
It has poor audio quality.

10. Super Heterodyne 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). RF
amplifier
Local
oscillator
mixer IF
detector AF
Modulating signal
Ganged tuning fs fo
IF=fo- fs

11. This IF is constant, for AM receivers it is 456 to 465 KHz and for FM receivers it is 9 MHz to 12 MHz.
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.

12. Automatic Frequency Control (AFC) and Automatic Gain Control (AGC) in Superheterodyne Receiver
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.

13. Image Frequency
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

14. Image Frequency Rejection
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, =
1+Q2 ρ2
Where,
ρ=(fsi / fs) - (fs / fsi)

15. Choice of IF
Very high IF will result in poor selectivity and poor adjacent channel rejection
A high value of IF will result in tracking difficulties
At low values of IF image frequency rejection is poor. Also the selectivity will be too sharp that cut off the sidebands

16. Tracking
The process of tuning circuit to get the desired output is called tracking.
Any error that exists in the frequency difference will result in an incorrect frequency being fed to the IF amplifier. Such errors are known as tracking errors.
To avoid tracking errors ganged capacitors are used.

17. Double Spotting
This phenomenon of Double spotting occurs at higher frequencies due to poor front end selectivity of receivers.
In this, receiver picks up same station at two nearby frequencies or points on the receiver dial.

18. It is possible for receivers without an RF amplifier stage to receive two different stations at the same point of the dial. In a receiver that has no RF stage, the signal on the antenna is fed through a tuned circuit directly to the signal grid of the converter tube. Assuming an intermediate frequency of 455 kHz, the local oscillator will track at a frequency of 455 kHz higher than the incoming signal. For example, suppose the receiver is tuned to pick up a signal on a frequency of 600 kHz. The local oscillator will be operating at a frequency of 1,055 kHz. The received and local oscillator signals are mixed, or heterodyned, in the converter stage and one of the frequencies resulting from this mixing action is the difference between the two signals, or 455 kHz, the I-F frequency. This I-F frequency is then amplified in the I-F stages and sent on to the detector and audio stages. Any signal at a frequency of 455 kHz that appears on the plate of the converter circuit will be accepted by the I-F amplifier and passed on. So on a receiver with no RF amplifier, the input to the converter is rather broadly tuned and some signals other than the desired signal will get through to the grid of the converter tube. Normally these other signals will mix with the local oscillator signal and produce frequencies that are outside the bandpass of the 455 kHz I-F amplifier and will be rejected. However, if there is a station operating on a frequency of 1,510 kHz, and this signal passes through the rather broad tuned input circuit and appears on the grid of the converter tube, it too will mix with the local oscillator and produce a frequency of 455 kHz (1, ,055 = 455). This signal will also be accepted by the I-F amplifier stage and passed on, thus both signals will be heard in the output of the receive. So any station is likely to experience interference from another station that happens to be on a frequency which is higher than that of the desired station by twice the I-F frequency. This is known as image-frequency interference. An RF amplifier stage ahead of the converter stage provides enough selectivity to reduce the image-frequency response by rejecting these unwanted signals and adds to the sensitivity of the receiver.