LM565 PLL FSK Demodulator Experiment

Study Notes

FSK Modulation Overview

Frequency-shift keying (FSK) modulation shifts the output between two frequencies referred to as mark and space frequencies.
Commonly utilized for the transmission of Teletype information.
FSK standards have progressed over the years to enhance performance.
Frequency gap in FSK signals is typically 200 Hz.

FSK Modulation Implementation

Frequencies of 1070 Hz and 1270 Hz represent space and mark, respectively.
A voltage-controlled oscillator (VCO) efficiently generates these frequencies.
Practical FSK modulators, such as the LM566, convert digital signals into analog signals using specific control voltages to produce designated output frequencies.
Proper tuning of components such as R10, Cs, and Vin is crucial to achieving desired frequencies around 1072 Hz and 1272 Hz.
A fourth-order low-pass filter is used to eliminate high-frequency harmonics from the output of the LM566, yielding a clean FSK signal.

Antenna Transmission

If FSK modulated signals are to be transmitted, a mixer is necessary to shift the signal into the RF frequency range.

Components and Their Functions

Q1 acts as a NOT gate; it produces high output when its input is low, controlling Q2 accordingly.
LM566 serves as a VCO, generating two distinct frequencies based on an external control voltage to represent digital data in FSK modulation.
VR1 and VR2 are adjustable components that fine-tune the carrier frequency and frequency deviation for efficient data transmission.

FSK Demodulation

LM565 PLL: Used for FSK demodulation below 500 kHz; involves a phase detector that functions as a double balanced modulator.
The free-running frequency (fo) is influenced by timing components connected to the LM565, specifically resistors and capacitors at pins 5 and 6.
Lock Range: The frequency range within which the PLL maintains lock with incoming signals, determined by the VR and Cz components.
Capture Range: Frequency range where the PLL can achieve phase lock with an input signal.

Component Functions in LM565

The free-running frequency is calculated using the formula: ( f_{\text{osc}} = \frac{1}{2.197 \times R \times C} ), where R and C are external timing components.
A741 Operational Amplifier: Functions as a comparator in the FSK demodulation circuit, comparing the VCO output with a reference voltage to generate a digital output.
Pin 6 of LM565: Serves as the timing capacitor input; connected to an external capacitor which helps adjust the timing and frequency characteristics of the VCO.

Description

This quiz covers the experiment using the LM565 PLL as an FSK demodulator, below 500 kHz. It includes details about the phase detector, VCO, and power supply connections. Test your knowledge of PLL circuits and frequency shift keying demodulation.

LM565 PLL FSK Demodulator Experiment

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Questions and Answers

The multistage low-pass filter is used to remove low-frequency noise from the VCO output.

PWM modulation converts a digital signal into an analog signal.

The LM555 timer is used as a voltage comparator.

A monostable multivibrator is used to generate the PWM signal.

The purpose of PWM modulation is to convert a digital signal into an analog signal.

FSK demodulation process can be improved by providing a noisy input to the comparator.

The LM565 output may contain unwanted low-frequency components or noise.

The low-pass filter attenuates low-frequency components while passing high-frequency components unchanged.

PWM modulation involves varying the frequency of the pulses based on the audio signal amplitude.

The LM555 timer is illustrated in Fig. 11-4.