1. FM RECEIVER AND DETECTION

2. BASIC FM DEMODULATORS
The FM discriminator (detector) extracts the intelligence that has modulated onto the carrier via frequency variations.
FM detection is two stage process:
FM to AM conversion
AM to AF conversion using diode detector t
y(t)
vFM(t)

3. FOSTER-SEELEY DISCRIMINATOR

4. FOSTER-SEELEY DISCRIMINATOR

5. Circuit Operation at Resonance

6. Circuit Operation above Resonance
When a series- tuned circuit operates at a frequency above resonance, the inductive reactance of the coil increases and the capacitive reactance of the capacitor decreases
Above resonance, the tank circuit acts like an inductor Secondary current lags the primary tank voltage
The voltage developed across R1 is greater than the voltage developed across R2, the output voltage is positive

7. Circuit Operation below Resonance
Below resonance the tank acts like a capacitor and the secondary current leads primary tank voltage
The voltage drop across R4 is larger than that across R3 and the output across both is negative.

8. FOSTER-SEELEY DISCRIMINATOR
Calculation of VL3
Calculation of Secondary voltage V2 and Vab
Voltage across diodes D1 and D2

9. Case2 : fin>fc
Case3 : fin<fc

10. Ratio Detector
By making a few changes in the Foster-Seely discriminator, it is possible to have a demodulator circuit which has built in capability to handle the amplitude changes of the input FM signal
This obviates the need for an amplitude limiter
This resulting circuit is called the ratio detector
The same vector diagram of Foster Seeley discriminator applies for ratio detector

11. RATIO DETECTOR

12. Block Diagram of FM Transmitter with pre-emphasis

13. HETERODYNE METHOD OF FREQUENCY UP-CONVERSION

14. MULTIPLICATION METHOD OF UP-CONVERSION

15. 540KHz to 1640KHz
Ganged tuning

16. Problems of Tuned Radio Frequency (TRF) Receiver
Instability- Overall gain of RF amplifiers is very very high so a very small f/b from o/p to i/p with correct phase can initiate oscillations . Due to stray capacitance at high freq.
Variation in BW- For 535KHz – 1640KHz Range BW=10KHz
for fc= Q=fr/BW=535/10=53.5
for fc= Q=164
But max value of Q is 120 so BW=fr/Q=1640/120=13.7K
So receiver picks adjacent channels.
3. Insufficient Selectivity- Due to variable BW selectivity of TRF receiver is poor.

17. Superheterodyne Receivers
Superheterodyne receivers convert all incoming signals to a lower frequency, known as the intermediate frequency (IF), at which a single set of amplifiers is used to provide a fixed level of sensitivity and selectivity.
Gain and selectivity are obtained in the IF amplifiers.
The key circuit is the mixer, which acts like a simple amplitude modulator to produce sum and difference frequencies.
The incoming signal is mixed with a local oscillator signal.

18. fIF is a fixed value (typically 455-kHz for AM radio). fo is tuned
Super heterodyne principle f0
f0+fc
f0-fc
If=f0-fc
If is Intermediate Freq
F0>fc
MIXER Fc
AM Wave Fo
Oscillator
fIF is a fixed value (typically 455-kHz for AM radio).
fo is tuned

19. AGC = Automatic Gain Control
Superheterodyne Receiver
AGC = Automatic Gain Control
AGC samples the signal strength at the detector and then feeds back a control signal to adjust the gain of the earlier stages
AGC keeps audio volume fairly constant if signals level vary

20. IF = LO - Input RF
540KHz to 1640KHz
455KHz

21. Image Frequency= The image frequency fi is a potentially interfering RF signal that is spaced 2 times the IF above or below the desired frequency fs.
Which image that occurs depends upon whether the local oscillator frequency fo is above or below the signal frequency. The mixing process creates sum and difference frequencies for the desired signal (680 kHz). It also creates sum and difference frequencies
for the undesired signal (1590 kHz).
=455 kHz
= 455 kHz
Fsi=fs+2IF
1135
1590
680

22. IF Amplifiers
The primary objective in the design of an IF stage is to obtain good selectivity.
Choice of IF=
High IF – Poor selectivity and poor adjacent channel rejection tracking problem increases.
Very Low If –Image frequency rejection is poor and sharp selectivity cut the side bands.
If must not fall in tuning range of receiver.
So IF is selected as 455KHz.

23. Local Oscillator
What should be the frequency of the local oscillator used for translation from RF to IF?
fLO = fc + fIF (up-conversion)
or fLO = fc - fIF (down-conversion)
Tuning ratio = fLO, max / fLO, min
Up-Conversion: ( ) / ( ) ≈ 2
Down-Conversion: (1600–455) / (530–455) ≈ 12
Easier to design oscillator with small tuning ratio.

26. Block Diagram of FM Receiver

27. FM Receivers

29. FM Capture effect Capture effect
In telecommunication, the capture effect, or FM capture effect, is a phenomenon, associated with FM reception, in which only the stronger of two signals at or near the same frequency will be demodulated
The capture effect is defined as the complete suppression of the weaker signal occurs at the receiver limiter, if it has one, where it is not amplified, but attenuated. When both signals are nearly equal in strength, or are fading independently, the receiver may switch from one to the other.

31. 540KHz to 1640KHz
Ganged tuning

32. Problems of Tuned Radio Frequency (TRF) Receiver
Instability- Overall gain of RF amplifiers is very very high so a very small f/b from o/p to i/p with correct phase can initiate oscillations . Due to stray capacitance at high freq.
Variation in BW- For 535KHz – 1640KHz Range BW=10KHz
for fc= Q=fr/BW=535/10=53.5
for fc= Q=164
But max value of Q is 120 so BW=fr/Q=1640/120=13.7K
So receiver picks adjacent channels.
3. Insufficient Selectivity- Due to variable BW selectivity of TRF receiver is poor.

33. Superheterodyne Receivers
Superheterodyne receivers convert all incoming signals to a lower frequency, known as the intermediate frequency (IF), at which a single set of amplifiers is used to provide a fixed level of sensitivity and selectivity.
Gain and selectivity are obtained in the IF amplifiers.
The key circuit is the mixer, which acts like a simple amplitude modulator to produce sum and difference frequencies.
The incoming signal is mixed with a local oscillator signal.

34. Super heterodyne principle
F0>fc f0
f0+fc
f0-fc
If=f0-fc
If is Intermediate Freq
MIXER Fc
AM Wave Fo
Oscillator
fIF is a fixed value (typically 455-kHz for AM radio).
fo is

35. AGC = Automatic Gain Control
Superheterodyne Receiver
AGC = Automatic Gain Control
AGC samples the signal strength at the detector and then feeds back a control signal to adjust the gain of the earlier stages
AGC keeps audio volume fairly constant if signals level vary

36. IF = LO - Input RF
540KHz to 1640KHz
455KHz

39. Image Frequency= The image frequency fi is a potentially interfering RF signal that is spaced 2 times the IF above or below the desired frequency fs.
Which image that occurs depends upon whether the local oscillator frequency fo is above or below the signal frequency. The mixing process creates sum and difference frequencies for the desired signal (680 kHz). It also creates sum and difference frequencies
for the undesired signal (1590 kHz).
=455 kHz
= 455 kHz
Fsi=fs+2IF
1135
1590
680

40. Image Frequency rejection Ratio
Where Q= Loaded Q of the tuned ckt.
If two tuned ckts are there then

41. Double spotting= Same station gets picked up at two diff.
nearby points, on the receiver dial.
This can be reduced by increasing front end selectivity.
Fo1=1955K
Fo2=1045K
Radio Dial 590K *IF 1500K

43. Tracking= In tuning process the Local oscillator freq tracks the station freq to have a correct diff of IF(455KHz).
If diff freq is not correct the error is called as Tracking error.
3 Point Tracking=

44. Padder tracking Trimmer tracking
950
1500
Trimmer tracking
The Cp in series with Cosc decreases total capacitance and increases Freq. in +ve direction making tracking error zero.
The Ct in parallel with Cosc increases total capacitance and decreases Freq. in -ve direction

45. Diode Detector

46. Two distortions of envelope Detector
1. When RC time constant of load is too long.

47. Modulation index at the o/p side of detector is higher than i/p side.
Zm=Diode load impedance at audio freq
Rc=Dc diode load resistance

48. Practical Diode Detector
Negative envelope will be demodulated and a –ve AGC will developed.
R1-C1 LPF removes RF ripple.
C2 prevents dc o/p to reach to volume control.
R3-C3 LPF removes AF from demodulated o/p.

49. Types of AGC used in receivers
Simple AGC- Receiver gain is automatically adjusted by –ve AGC. Gain is reduced for weak signals also.
Ideal AGC- Gain is not reduced for weak signals only for very strong signals. Then O/p is constant.
Delayed AGC- AGC bias is provided after a level of signal. Used in High quality receivers.