Principles of Electronic Communication Systems Second Edition Louis Frenzel © 2002 The McGraw-Hill Companies

Principles of Electronic Communication Systems Second Edition Chapter 6 FM Circuits ©2003 The McGraw-Hill Companies

Topics Covered in Chapter 6 There are many circuits used to produce FM and PM signals. There are two types of frequency modulator circuits, direct circuits and phase modulation circuits. Frequency Modulators Phase Modulators Frequency Demodulators

Frequency Modulators A frequency modulator is a circuit that varies carrier frequency in accordance with the modulating signal. The carrier is generated by LC or crystal oscillator circuits. In LC oscillators, the carrier frequency can be changed by varying either the inductance or capacitance. In crystal oscillators, the frequency is fixed by the crystal. A varactor is a variable capacitance diode used to change oscillator frequencies.

Varactor Operation A junction diode is created when P- and N-type semiconductors are formed during the manufacturing process. A depletion region, where there are no free carriers, is formed. This region acts like a thin insulator that prevents current from flowing through the device. A forward bias will cause the diode to conduct. A reverse bias will prevent current flow.

Varactor Operation (continued) A reverse-biased diode acts like a small capacitor. The P- and N-type materials act as the two plates of the capacitor. The depletion region acts as the dielectric material. The width of the depletion layer determines the width of the dielectric and, therefore the amount of capacitance. All diodes exhibit variable capacitance. Varactors are designed to optimize this characteristic.

Varactor Diode Schematic Symbol

Varactor Modulator Circuit

Varactor Modulator The capacitance of varactor diode D1 and L1 form the parallel tuned circuit of the oscillator. The value of C1 is made very large so its reactance is very low. C1 connects the tuned circuit to the oscillator and blocks the DC bias on the base of Q1. The values of L1 and D1 fix the center carrier frequency. The modulating signal varies the effective voltage applied to D1 and its capacitance varies.

Frequency-Modulating a Crystal Oscillator Crystal oscillators provide highly accurate frequencies and their stability is superior to LC oscillators. The frequency of a crystal oscillator can be varied by changing the value of capacitance in series or parallel with the crystal. By making the series capacitance a varactor diode, frequency modulation can be achieved. The modulating signal is applied to the varactor diode which changes the oscillator frequency.

Frequency Modulation of a Crystal Oscillator

By definition… Varactors are made with a wide range of capacitance values, most units having a nominal capacitance in the 1- to 200-pF range. A frequency multiplier circuit is one whose output frequency is some integer multiple of the input frequency. A frequency multiplier that multiplies a frequency by two is called a doubler. A frequency multiplier that multiplies a frequency by three is called a tripler.

Frequency Multipliers

Voltage-Controlled Oscillators Oscillators whose frequencies are controlled by an external input voltage are generally referred to as voltage-controlled oscillators (VCOs). Voltage-controlled crystal oscillators are generally referred to as VXOs. VCOs are primarily used in FM. VCOs are also used in voltage-to-frequency conversion applications.

Reactance Modulator A reactance modulator is a circuit that uses a transistor amplifier that acts like either a variable capacitor or an inductor. When the circuit is connected across the tuned circuit of an oscillator, the oscillator frequency can be varied by applying the modulating signal to the amplifier. Reactance modulators can produce frequency deviation over a wide range. Reactance modulators are highly linear.

Reactance Modulator Circuit

Phase Modulators Most modern FM transmitters use some form of phase modulation (PM) to produce indirect FM. In PM the carrier oscillator can be optimized for frequency accuracy and stability. Crystal oscillators or crystal-controlled frequency synthesizers can be used to set the carrier frequency accurately and maintain stability. The output of the carrier oscillator is fed to a phase modulator where the phase shift is made to vary in accordance with the modulating signal.

Varactor Phase Modulators A simple phase-shift circuit can be used as a phase modulator if the resistance or capacitance can be made to vary with the modulating signal. A varactor can be used to vary capacitance and achieve phase shift modulation.

Varactor Phase Modulator Circuit

Transistor Phase Modulator A transistor can be used as a variable resistor to create a phase modulator. A standard common emitter class A amplifier biased into the linear region is used in PM. The transistor from collector to ground acts like a resistor. The transistor’s resistance forms part of the phase shifting circuit.

Transistor Phase Shifter Circuit

FET Phase Modulator An improved phase modulator uses a phase shifter made up of a capacitor and a variable resistance of a field-effect transistor (FET).

FET Phase Modulator Circuit

Tuned-Circuit Phase Modulator Most phase modulators are capable of producing a small amount of phase shift. The limited phase shift, therefore, produces a limited frequency shift. Phase and frequency shift can be increased by using a parallel tuned circuit. At resonance, a parallel resonant circuit acts like a large resistor. Off resonance, the circuit acts inductively or capacitively and produces a phase shift.

Frequency Demodulators Any circuit that will convert a frequency variation in the carrier back into a proportional voltage variation can be used to demodulate or detect FM signals. Circuits used to recover the original modulating signal from an FM transmission are called: Demodulators Detectors Discriminators

Frequency Demodulator Types Slope detector Foster-Seeley discriminator Pulse-averaging discriminator Quadrature detector Phase-locked loop

Slope Detector The slope detector makes use of a tuned circuit and a diode detector to convert frequency variations into voltage variations.

Slope Detector Circuit

Foster-Seeley Discriminator Foster-Seeley discriminators are one of the earliest FM demodulators. Foster-Seeley discriminators are typically found in older equipment. Discriminator circuits are sensitive to variations in amplitude.

Pulse-Averaging Discriminators A pulse-averaging discriminator uses a zero crossing detector, a one shot multivibrator and a low-pass filter in order to recover the original modulating signal. The pulse-averaging discriminator is a very high-quality frequency demodulator. Originally this discriminator was limited to expensive telemetry and industrial control applications. With availability of low-cost ICs, this discriminator is used in many electronic products.

Pulse-Averaging Discriminator

Quadrature Detector The quadrature detector is probably the single most widely used FM demodulator. The quadrature detector is primarily used in TV demodulation. This detector is used in some FM radio stations. The quadrature detector uses a phase-shift circuit to produce a phase shift of 90 degrees at the unmodulated carrier frequency.

Quadrature FM Detector

Phase-Locked Loop A phase-locked loop (PLL) is a frequency- or phase-sensitive feedback control circuit used in frequency demodulation, frequency synthesizers, and various filtering and signal-detection applications. PLLs have three basic elements. They are: Phase detector Low-pass filter Voltage-controlled oscillator

Phase-Locked Loop – Block Diagram