CommunicationElectronics Principles & Applications Chapter 5 Frequency Modulation Circuits

subtopics Frequency modulators:  VVC, frequency modulating a crystal oscillator, reactance modulator, voltage controlled oscillator Phase modulators:  Basic Phase shift cct, practical PM, Using tuned cct for PM Frequency demodulators:  Foster Seeley discriminator, Ratio detector, Pulse averaging Discriminator, Quadrature Detector, Differential Peak Detector, PLL demodulator

DIRECT FREQUENCY MODULATORS Direct frequency modulators vary the frequency of a carrier oscillator. A voltage variable capacitor (VVC) or varactor is widely used to vary the frequency of an LC or crystal oscillator. Older FM transmitters used a reactance modulator to vary the carrier frequency.

VOLTAGE VARIABLE CAPACITOR (VVC) A VVC or varactor is a silicon diode that when reversed biased acts as a variable capacitor. Varying the reverse bias voltage changes the capacitance. Increasing the voltage decreases the capacitance and visa versa. The useable capacitance range is from about 5 pF to 100 pF.

+V RFC Modulating signal in FM output Crystal FM WITH A VVC VVC

2. The divider provides reverse bias for the VVC. 3. When the modulating signal goes positive, the bias increases and the capacitance drops. FM WITH A VVC 1. This circuit is a crystal-controlled oscillator. 4. The crystal and oscillator are pulled higher in frequency. 5. When the modulating signal goes negative, the frequency decreases.

PM MODULATORS A PM modulator generates indirect FM. A fixed carrier frequency from a crystal oscillator is passed through a phase shifter whose phase shift is determined by the modulating signal. A transistor can be used as a variable resistor or a VVC can be used as a variable capacitor to implement a phase modulator.

+V DD RFC 2 Output PHASE MODULATOR RFC 1 Modulating signal Crystal- controlled carrier oscillator Q1Q1 R2R2 R1R1 C3C3 C2C2 C1C1

FREQUENCY DEMODULATORS A demodulator converts the carrier frequency variations into the original modulating signal. There are six common FM demodulators: –Foster Seeley Discriminator –Ratio Detector –Pulse Averaging Discriminator –Quadrature Detector –Differential Peak Detector –Phase-Locked Loop (PLL) The quadrature detector and PLL are the most common.

Output QUADRATURE DETECTOR R C1C1 Phase detector C C2C2 L FM input 90  B 0  A Pulse averaging low-pass filter

QUADRATURE DETECTOR OPERATION A phase shift circuit, usually a parallel tuned circuit and a capacitor, produces a 90º phase shift at the carrier frequency. The varying carrier frequency is compared to the phase shifter output in a phase detector circuit. The phase detector produces a pulse width modulated output that is averaged in a low pass filter to recover the original modulation.

Error Amplifier Phase detector LPF VCO PHASE-LOCKED LOOP In VCO The VCO locks onto the input signal.

PLL OPERATION A PLL is a feedback control circuit that compares an input signal to the output of a voltage controlled oscillator (VCO) and adjusts the VCO frequency if it differs from the input signal. If the VCO and input frequencies are different, the phase detector produces an error signal that is filtered into a DC voltage that controls the VCO frequency.

Error Amplifier Phase detector LPF VCO In THE OUTPUT VOLTAGE VERSUS THE INPUT FREQUENCY Out Output voltage Input frequency Lock range

PLL DEMODULATOR If the input to a PLL is an FM signal, the VCO will track or follow the input. The low pass filter output will be the original modulating signal. The PLL demodulator is the best FM detector as it serves as a filter for good selectivity and noise reduction.

Error Amplifier Phase detector LPF VCO In PLL PROVIDES FM DETECTION Out