CHAPTER 5
Frequency Modulation Circuits
1. The component most widely used for FM or PM is the varactor diode or voltage variable capacitor (VVC).
2. A VVC is a specially designed silicon junction diode optimized for large capacitive variations.
3. A reverse-biased junction diode will act as a small capacitor where the depletion region is the dielectric.
4. The capacitance of a varactor is inversely proportional to the reverse-biased voltage amplitude.
5. The most common frequency modulators use a varactor to vary the frequency of an LC circuit or crystal in accordance with the modulating signal.
6. A reactance modulator is an amplifier that is made to appear inductive or capacitive by phase shift. It is used to produce wide deviation direct FM.
7. Crystal oscillators are preferred for their frequency stability over LC oscillators, but only very small frequency deviation is possible with crystal oscillators.
8. An IC VCO produces excellent deviation FM at frequencies below 1 MHz.
9. In a phase modulator, the carrier is shifted in phase in accordance with the modulating signal. This produces indirect FM.
10. One of the best phase modulators is a parallel tuned circuit controlled by a varactor.
11. Most phase modulators produce very small amounts of frequency deviation.
12. Frequency deviation and carrier frequency can be increased by passing them through a frequency multiplier.
13. One of the oldest and best frequency demodulators is the Foster-Seeley discriminator that is a phase detector whose output voltage increases or decreases with phase changes produced by input frequency deviation.
14. A Foster-Seeley discriminator is sensitive to input amplitude variations and, therefore, must be preceded by a limiter.
15. A variation of the Foster-Seeley discriminator is the ratio detector widely used in older TV receiver designs. A primary advantage of the ratio detector is that no limiter is needed.
16. A pulse-averaging discriminator converts an FM signal into a square wave of identical frequency variation using a zero crossing detector, comparator, or limiter circuit. This circuit triggers a one shot that produces pulses that are averaged in a lowpass filter to reproduce the original modulating signal.
17. A quadrature detector uses a unique phase shift circuit to provide quadrature (90°) FM input signals to a phase detector. The phase detector produces a different pulse width for different phase shifts. These pulse width variations are averaged in a low-pass filter to recover the modulating signal.
18. Quadrature detectors are available in IC form and are one of the most widely used TV audio demodulators.
19. A differential peak detector is an IC FM demodulator that uses a differential amplifier and capacitive storage peak detectors plus tuned circuits to translate frequency variations into voltage variations.
20. A phase-locked loop (PLL) is a feedback control circuit made up of a phase detector, voltage controlled oscillator (VCO), and low-pass filter. The phase detector compares an input signal to the VCO signal and produces an output that is filtered by a low-pass filter into an error signal that controls the VCO frequency.
21. The PLL is synchronized or locked when the input and VCO frequencies are equal. Input frequency changes cause a phase or frequency shift which, in turn, produces an error signal that forces the VCO to track the input and reduce their difference to zero.
22. The range of frequencies over which a PLL will track an input is called the lock range. If the input strays outside, the lock range, the PLL will go out of lock and the VCO will operate at its free-running frequency.
23. The capture range of a PLL is that narrow band of frequencies over which a PLL will recognize and lock onto an input signal. The capture range is narrower than the lock range and it makes the PLL look like a bandpass filter.
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