Deviation Meter

Posted on Feb 7, 2014

A deviation monitor can be made by connecting some form of frequency modulation (FM) detector to an AC voltmeter and calibrating the meter in units of frequency deviation. One method of detecting or demodulating the FM signal is to use a phase locked loop (PLL). A voltage controlled oscillator (VCO) is locked to the FM signal frequency by comparin

Deviation Meter
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g its output with the FM signal in a phase comparator which generates a correction voltage to control the frequency of the VCO. This voltage is a function of the signal frequency and hence is a demodulated version of the signal frequency. Conventional FM demodulators, such as the frequency discriminator, require precise tuning, that is, they are fixed frequency devices. Tuning can only be achieved by varying the frequency of a heterodyne oscillator. The advantage of the PLL is that it can be used to detect FM signals, without retuning, over a frequency range equal to the capture range of the loop. When heterodyning for a different frequency range, only a fixed frequency heterodyne oscillator is required. The following text describes a deviation monitor which has been constructed using a phase locked loop as an FM detector. Circuit detail is shown in Figure 1. The essential circuit blocks are a heterodyne oscillator, frequency mixer, phase locked loop and a metering circuit calibrated in frequency deviation. The heterodyne circuit is fixed for the 2 metre band but the monitor could be used on other bands with other appropriate heterodyne circuit design. The VCO in the phase locked loop is set for a free running frequency of 1 MHz. The approximate capture range is 500 kHz, hence the VCO will lock to any frequency in the range of 750 to 1250 kHz. Equipped with the crystal specified, a frequency of 145. 3 MHz is generated and...

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