FM RECEIVERS FM receiver also operates on super-heterodyning principle. Operating frequencies in FM are much higher than AM. FM receivers need circuits like limiter and de- emphasis. FM demodulators are different from AM detectors.
RF amplifier MixerIF amplifier Limiter FM detector
R F Amplifier :- Improves signal to noise ratio Matches receiver input impedance to antenna impedance. Mixer :- Similar to AM receiver, down convert received signal to IF Tuning range is less than that in AM broadcasting (88MHz to 108 MHz) IF produced at output of mixer is 10.7MHz.
I F Amplifier :- In case of FM receivers IF and bandwidth required are much higher. IF is 10.7MHz Bandwidth is 200kHz this large BW will provide low gain per stage. So more IF amplifiers are required (and are cascaded).
Amplitude limiter stage :- Transmitted FM wave has constant amplitude, but unwanted noise and signal added to it change its amplitude. This has to be removed before demodulation process otherwise distortion appears in demodulated signal. As demodulators react to amplitude changes as well as frequency changes. Limiter removes all unwanted amplitude variations
De-emphasis :- Artificial boosting given to higher modulating frequencies in process of pre-emphasis is nullified De-emphasis circuit is used after FM demodulator
AM RECEIVERS Vs FM RECEIVERS AM ReceiverFM Receiver AM: 540 kHz – 1600 kHzFM: 88 MHz – 108 MHz AM detector basically envelope detector FM demodulator basically a frequency to amplitude converter No limiter and de-emphasis stage limiter and de-emphasis stage needed IF 455 kHzIF 10.7MHz For AM radio, each station occupies a maximum bandwidth of 10 kHz For FM radio, each station occupies a maximum bandwidth of 200 kHz AM: Bt = 2WFM: Bt = 2(D + 1)W (Carsons Rule) Input signal is AM waveInput Signal is FM wave
AUTOMATIC GAIN CONTROL Signals receiving at receiver input are not of same strength Signals of strong station are strong and from weak stations are weak If receiver gain constant then receiver o/p will fluctuate proportional to i/p signal strength which is not desired So AGC adjust receiver gain automatically to have constant o/p irrespective of i/p strength
Types of AGC Simple AGC Delayed AGC Ideal AGC No AGC
SIMPLE AGC It will change overall gain of a receiver automatically Thus keep receiver o/p constant even when i/p signal strength changes Receiver gain is automatically reduced as i/p signal becomes stronger
Advantages Simplicity Low cost Disadvantages Weak signals are also attenuated Applications Low cost domestic radio receivers
IDEAL AGC For the portion OA i.e for weak signal no AGC is applied After point A AGC is applied, keeping receiver o/p constant Thus there is no gain reduction for weak signals
DELAYED AGC In this case AGC bias is not applied until i/p signal strength reaches a predetermined level point B After this point is reached AGC bias is applied like simple AGC but more strongly Thus reducing receiver gain for weak signals is avoided
Operation Here an adjustable positive bias is applied to cathode of AGC diode Anode of diode is connected to o/p of last IF amplifier through coupling capacitor Cc R3C3 filter, filters out high frequency appearing at anode of AGC diode When i/p signal is weak, then anode of AGC diode is at less potential than cathode. So it is off
So AGC o/p is zero When i/p signal is strong then AGC diode is forward biased AGC o/p will be constant (Vb + Vd) Thus delayed AGC reduces gain for strong signal and not for weak signals We can say characteristics of delayed AGC is similar to ideal AGC But here we can adjust point B,by adjusting delay adjust potentiometer
Advantages Weak signals are not attenuated Disadvantages Complex than simple AGC Applications High quality communication receivers
Reason for using double radio receiver When choosing the intermediate frequency for a superheterodyne radio receiver there is a trade-off to be made between the advantages of using a low frequency IF or a high frequency one High frequency IF: The use of a high frequency IF means that the difference between the wanted frequency and the unwanted image is much greater and it is easier to achieve high levels of performance because the front end filtering is able to provide high levels of rejection.
Low frequency IF: The advantage of choosing a lower frequency IF is that the filters that provide the adjacent channel rejection are lower in frequency. The use of a low frequency IF enables the performance to be high, while keeping the cost low. Accordingly there are two conflicting requirements which cannot be easily satisfied using a single intermediate frequency. The solution is to use a double conversion superheterodyne topology to provide a means of satisfying both requirements
For good image rejection, relatively high IF is desired. However, for a high gain selective amplifiers that are stable, a low IF is necessary. The solution for above constrain is to use 2 intermediate frequencies, i.e. by using double conversion AM receiver. The 1st IF is a relatively high frequency for good image rejection. The 2nd IF is a relatively low frequency for good selectivity and easy amplification.
WHY NAMED SO? RF signal is down-converted into two IF Higher IF and Lower IF Therefore named Double conversion receiver
INTERMEDIATE FREQUENCIES 1 st IF = f0 – fs 1 st IF = [( )]MHz - [(2-16)]MHz 1 st IF = 1.7 MHz 2 nd IF = f0 – fs 2 nd IF = (1.7)MHz - (1.5)MHz 2 nd IF = 200 KHz
OTHER BLOCKS Delayed AGC BFO ( Beat Frequency oscillator ) Squelch Circuit ( Muting Circuit )
DELAYED AGC The disadvantage of automatic gain control, attenuating even the weak signal, is overcome by the use of delayed automatic gain control. This type of system develops no AGC feedback until an established received signal strength is attained. For signals weaker than this value, no AGC is developed. For sufficiently strong signals, the delayed AGC circuit operates essentially the same as ordinary AGC. The circuit uses two separate diodes; one is the detector diode and the other the AGC diode.
The AGC diode is connected to the primary of the last IF transformer and the detector diode to its secondary. A positive bias is applied to the cathode of the AGC diode. This keeps it from conducting until a prearranged signal level has been reached. The adjust delay control allows manual control of the AGC diode bias. Manual control allows you to select the signal level at which AGC is applied.
BEAT FREQUENCY OSCILLATOR The beat-frequency oscillator (BFO) is necessary when you want to receive CW signals. The action of the RF amplifier, mixer, local oscillator, and IF amplifier is the same for both CW and AM; but the CW signal reaches the detector as a single frequency signal with no sideband components. To produce an AF output, you must heterodyne (beat) any CW signal with an RF signal of the proper frequency. This separate signal is obtained from an oscillator known as a beat-frequency oscillator.
If the intermediate frequency is 455 kilohertz and the BFO is tuned to 456 kilohertz or 454 kilohertz, the difference frequency of 1 kilohertz is heard in the output. Generally, you will tune the BFO from the front panel of a receiver. When you vary the BFO control, you are varying the output frequency of the BFO and will hear changes in the tone of the output audio signal. The sensitivity of a receiver is maximum when no signal is being received.
This condition occurs, for example, when a receiver is being tuned between stations. At this time background noise is picked up by the antenna, and you will hear noise greatly amplified. This noise is highly annoying and occurs because receiver gain is maximum without a signal. You can often overcome this problem by using a circuit called a SQUELCH, NOISE SILENCER, NOISE SUPPRESSOR, or NOISE LIMITER. All of these noise type circuits just clip the peaks of the noise spikes. Squelch will actually eliminate noise.
A squelch circuit is a circuit used to turn off the audio of a receiver amplifier when no RF signal is being received. Without a squelch circuit, such absence of received signal will be heard on the receiver as an annoying background noise. In Figure, the automatic gain control (AGC) circuit, which is used to adjust the gain of the receiver based on the strength of the received signal, outputs a DC voltage that is proportional to the received signal's amplitude.
When carrier is absent i.e there is no signal present at i/p, receiver produces loud noise This is due to delayed AGC that disappears in absence of i/p signal, increasing gain of IF and RF amplifiers These amplifiers amplifies noise present at their i/ps Muting circuit avoid this type of condition In absence of i/p signal AGC will be zero and squelch circuit will cut-off 1 st audio amplifier so tha no noise can pass through loud speaker
Thus, in the absence of a received signal, the output of the AGC's DC amplifier is a very low DC voltage that's fed into the base of Q1. This low base voltage causes Q1 to turn off, resulting in the base of Q2 being pulled up 'high' through R1. This turns on Q2.
When there's a received signal, Q1 gets a high base voltage from the AGC amplifier, turning it on. Q2's base is pulled 'low' by the conducting Q1, causing Q2 to turn off. With Q2 'off', the audio signal from Q3's collector is readily passed on to the audio power amplifier.
Features Fine tuning Variable senstivity Variable selectivity Noise limiting Better image frequency rejection Better adjacent channel rejection
TRF RECEIVERSUPERHETERODYNECOMMUNICATION RXR No frequency conversion Single Frequency conversion Double Frequency conversion No IF frequencyDownconvert RF signal to lower IF frequency Downconvert RF signal to two IF frequency(higher than lower) Instability, variation in BW and poor selectivity due to high frequencies No instability, variation in BW and poor selectivity as IF introduced. Difficult to design tunable RF stages. Main amplifixcation takes place at IF Main amplifixcation takes place at IF stages Rarely usedMostly used No AGCAGC introducedAGC,BFO an Squelch circuit