Angle Modulation Reception & FM Stereo
FM Receivers
- Double conversion superheterodyne receivers
Preselector –rejects image frequency
Limiter – final IF amplifier
Frequency discriminator – extracts information from FM wave
For FM broadcast:
❖ First IF – 10.7MHz (for good image-frequency rejection)
❖ Second IF – 455 kHz (for high gain)
FM Demodulators
A. Tuned-Circuit Frequency Discriminators
- convert FM to AM and then demodulate AM envelope with conventional peak
detectors
- requires 1800 phase inverter, adder circuit and frequency-dependent circuits
a. Slope Detector
- Converts FM to AM
- Less linear output
b. Balanced Slope Detector - two-single-ended slope detector
connected in parallel and fed
1800 out of phase
- La, Ca, Lb, Cb, - perform FM to
AM conversion
- D1, C1, R1, D2, C2, R2 - remove info
from AM envelope
- more linear output
- Lacks limiting circuit
c. Foster-Seeley Discriminator
- Similar to balanced slope detector
d. Ratio Detector
- Immune to amplitude variations
B. Phase Locked-Loop Demodulators
- requires no tuned circuits
- automatically compensates for changes in the carrier frequency due to
instability in the transmit oscillator
C. Quadrature FM Demodulator
- sometimes called a coincidence detector
- extracts the original information signal from the composite IF waveform by
multiplying two quadrature (900 out of phase) signals
FM Noise Suppression
- suppresses noise in the form of amplitude variations
- uses limiter circuit
1. Amplitude Limiters & FM Thresholding
Threshold, quieting or capture level – prescribed minimum input level
- Noise limiters can improve S/N by 20dB
FM thresholding – also known as FM quieting or FM capture effect
- improvement in the S/N ratio
3 criteria before FM thresholding can occur:
1. S/N prediction S/N >= 10 dB or greater.
2. IF signal must be sufficiently amplified to overdrive the limiter.
3. Signal must have a modulation index >= 1.
2. Limiter Circuits
3. FM Capture Effect
- if two stations are received simultaneously at the same or nearly the same
frequency, the receiver locks onto the stronger station while suppressing
the weaker station
capture ratio – minimum dB difference in signal strength between 2 received signals
= 1 dB 🡪 high-quality FM receivers
double limiting – two limiter stages
triple limiting – three limiter stages
Problem: For an FM receiver with a bandwidth B =200 kHz, a power noise figure NF=8dB,
and an input noise temperature T = 100 K, determine the minimum receive
carrier power necessary to achieve a postdetection S/N of 37 dB. Use the
receiver block diagram below as the receiver model and the FM thresholding curve
shown for m=1.
Frequency Vs Phase Modulation
PM FM
Advantages Better S/N Can be demodulated using
noncoherent demod which are less
expensive
Does not require preemphasis circuit VCOs can produce high-frequency
deviation and high m
Can use crystal oscillator because
modulation is performed in a separate
circuit from carrier osc.
Disadvantages Requires coherent demodulation Lower S/N than PM
Cannot use crystal oscillator
FM STEREO BROADCASTING
Monophonic – single 50-Hz to 15 kHz audio channel made up the entire voice
and music information frequency spectrum
- can separate the high frequency signal (tweeters) and low frequency
signal (woofers) but impossible to separate sound spatially (no
directivity to the sound)
- before 1951 FM transmission
Stereophonic – information signal is spatially divided into two 50-Hz to 15 kHz
audio channels (left & right channels)
- separate music or sound by tonal quality such as percussion, strings,
horns and so on
Notes:
- Stereophonic receivers can receive monophonic signals
- Monophonic receivers can also receive stereophonic signals
- 40dB separation between the two channels
SCA – Subsidiary Communications
Authorizations
- subcarrier transmission during
1955
- used to broadcast uninterrupted
music to private subscribers
such as dept stores, restaurants,
etc.. Equipped with special SCA
receivers
- subcarrier ranged from 25 kHz to
75 kHz and later standardized at
67 kHz
Maximum frequency deviation for stereo FM = 75 kHz
SCA = 10% of 75 kHz = 7.5 kHz
19-kHz stereo pilot = 10% of 75 kHz = 7.5 kHz
L+R transmission = 60 kHz
FM Transmission
L+R and L-R channel interleaving
Fm Stereo Reception
Fm Stereo Reception
Large-Scale Integration Stereo Demodulator
TWO-WAY MOBILE COMMUNICATIONS SERVICES
Types of 2-way radio communication systems:
1. Two-way mobile radio
• Half-duplex, one to many radio comm with no dial tone
a. Class D citizens band (CB) radio – 26.96 to 27.41 MHz
b. Amateur (ham) radio = 1.8 MHz to 300 MHz
c. Aeronautical Broadcasting Service (ABS) – 2.8MHz to 457 MHz
d. Private land mobile radio services
• Public safety radio
• Special emergency radio
• Industrial radio
2. Mobile-telephone services
• Analog cellular radio
• Digital cellular radio
• Personal communications satellite service (PCSS)
TWO-WAY FM RADIO COMMUNICATIONS
- Uses the following bands:
❖ 132 MHz to 174 MHz
❖ 450 MHz to 470 MHz
❖ 806 MHz to 947 MHz
- Has frequency deviation = 5 kHz and fm=3 kHz
- Deviation ratio = 1.67
- Maximum Bessel bandwidth = 24 kHz
- Half-duplex
Two-way FM Radio Transmitter
Two-Way FM Radio Receiver
Squelch Circuit
