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FM Modulator and Demodulator

- FM modulator
- Direct FM
- Indirect FM
- FM demodulator
- Direct: use frequency discriminator (frequency-voltage converter)
- Ratio detector
- Zero crossing detector
- Indirect: using PLL
- Superheterodyne receiver
- FM broadcasting and Satellite radio

FM Direct Modulator

- Direct FM
- Carrier frequency is directly varied by the message through voltage-controlled oscillator (VCO)
- VCO: output frequency changes linearly with input voltage
- A simple VCO: implemented by variable capacitor
- Capacitor Microphone FM generator

FM Direct Modulator cont.

- Direct method is simple, low cost, but lack of high stability & accuracy, low power application, unstable at the carrier frequency
- Modern VCOs are usually implemented as PLL IC
- Why VCO generates FM signal?

Indirect FM

- Generate NBFM first, then NBFM is frequency multiplied for targeted Δf.
- Good for the requirement of stable carrier frequency
- Commercial-level FM broadcasting equipment all use indirect FM
- A typical indirect FM implementation: Armstrong FM
- Block diagram of indirect FM

Indirect FM cont.

- Then, apply frequency multiplier to magnify β
- Instantaneous frequency is multiplied by n
- So do carrier frequency, Δf, and β
- What about bandwidth?

A simple electronic implementation of frequency multiplier

30 MHz output. X3 (x5)

C1:100pF, L1:2.7μH. D:1N914

L2:.22μH, L3:1.8μH, L4:330μH

C2:120pF, C3:10pF.

Armstrong FM Modulator

- Invented by E. Armstrong, an indirect FM
- A popular implementation of commercial level FM
- Parameter: message W=15 kHz, FM s(t): Δf=74.65 kHz.
- Can you find the Δf at (a)-(d)?

FM Demodulator

- Four primary methods
- Differentiator with envelope detector/Slope detector
- FM to AM conversion
- Phase-shift discriminator/Ratio detector
- Approximates the differentiator
- Zero-crossing detector
- Frequency feedback
- Phase lock loops (PLL)

FM Slope Demodulator

- Principle: use slope detector (slope circuit) as frequency discriminator, which implements frequency to voltage conversion (FVC)
- Slope circuit: output voltage is proportional to the input frequency. Example: filters, differentiator

FM Slope Demodulator cont.

- Block diagram of direct method (slope detector = slope circuit + envelope detector)

so(t) linear with m(t)

Hard Limiter

- A device that imposes hard limiting on a signal and contains a filter that suppresses the unwanted products (harmonics) of the limiting process.
- Input Signal
- Output of hard limiter
- Bandpass filter
- Remove the amplitude variations

Ratio Detector

- Foster-Seeley/phase shift discriminator
- uses a double-tuned transformer to convert the instantaneous frequency variations of the FM input signal to instantaneous amplitude variations. These amplitude variations are rectified to provide a DC output voltage which varies in amplitude and polarity with the input signal frequency.
- Example
- Ratio detector
- Modified Foster-Seeley discriminator, not response to AM, but 50%

FM Demodulator PLL

- Phase-locked loop (PLL)
- A closed-loop feedback control circuit, make a signal in fixed phase (and frequency) relation to a reference signal
- Track frequency (or phase) variation of inputs
- Or, change frequency (or phase) according to inputs
- PLL can be used for both FM modulator and demodulator
- Just as Balanced Modulator IC can be used for most amplitude modulations and demodulations

PLL FM

- Remember the following relations
- Si=Acos(wct+1(t)), Sv=Avcos(wct+c(t))
- Sp=0.5AAv[sin(2wct+1+c)+sin(1-c)]
- So=0.5AAvsin(1-c)=AAv(1-c)
- Section 2.14

Phase-Locked Loop Demodulator

(a) Block diagram for a PLL FM demodulator; (b) PLL FM demodulator using the XR-2212 PLL

32-38

Nonlinear

Channel (device)

vi(t)

v0(t)

Nonlinear Effects in FM Systems

1.Strong nonlinearity, e.g., square-law modulators ,

hard limiter, frequency multipliers.

2.Weak nonlinearity, e.g., imperfections

Nonlinear input-output relation

An FM system is extremely sensitive to phase nonlinearity.

Common types of source: AM-to-PM conversion

52

Superheterodyne Receiver

- Radio receiver’s main function
- Demodulation get message signal
- Carrier frequency tuning select station
- Filtering remove noise/interference
- Amplification combat transmission power loss
- Superheterodyne receiver
- Heterodyne: mixing two signals for new frequency
- Superheterodyne receiver: heterodyne RF signals with local tuner, convert to common IF
- Invented by E. Armstrong in 1918.
- AM: RF 0.535MHz-1.605 MHz, Midband 0.455MHz
- FM: RF 88M-108MHz, Midband 10.7MHz

Advantage of superheterodyne receiver

- A signal block (of circuit) can hardly achieve all: selectivity, signal quality, and power amplification
- Superheterodyne receiver deals them with different blocks
- RF blocks: selectivity only
- IF blocks: filter for high signal quality, and amplification, use circuits that work in only a constant IF, not a large band

FM Broadcasting

- The frequency of an FM broadcast station is usually an exact multiple of 100 kHz from 87.5 to 108.5 MHz . In most of the Americas and Caribbean only odd multiples are used.
- fm=15KHz, f=75KHz, =5, B=2(fm+f)=180kHz
- Pre-emphasis and de-emphasis
- Random noise has a 'triangular' spectral distribution in an FM system, with the effect that noise occurs predominantly at the highest frequencies within the baseband. This can be offset, to a limited extent, by boosting the high frequencies before transmission and reducing them by a corresponding amount in the receiver.

Fc=19KHz.(a) Multiplexer in transmitter of FM stereo. (b) Demultiplexer in receiver of FM stereo.

FM Stereo Multiplexing

Backward compatible

For non-stereo receiver

TV FM broadcasting

- fm=15KHz, f=25KHz, =5/3, B=2(fm+f)=80kHz
- Center fc+4.5MHz

Satellite Radio

- WorldSpace outside US, XM Radio and Sirius in North America

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