Automatic frequency control which monitors the frequency of a voltage controlled oscillator in a PLL demodulator
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United States of America Patent
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Feb 11, 1997
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N/A
app pub date -
Mar 29, 1994
filing date -
Mar 29, 1994
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Abstract
An automatic frequency control circuit measures the deviation in frequency of a selected signal from a reference frequency and when the deviation exceeds a selected threshold which is variable, steps the frequency of the selected signal back to the proper frequency. The selected signal is typically an intermediate frequency (IF) signal in a radio receiver. For detection of frequency deviation, an exclusive OR (EX-OR) circuit receives a reference signal having a frequency proportional to a desired IF carrier frequency and an input signal having a frequency proportional to an actual IF frequency. A divider coupled to a voltage controlled oscillator (VCO) used in demodulation of the IF signal provides the input signal. An output signal of the EX-OR circuit includes a component with a frequency proportional to the difference between the actual and desired IF frequencies. A low pass filter isolates that component, and a charge pump generates a control voltage proportional to the frequency of that component. A comparison of the control voltage and the threshold determines whether the actual IF frequency should be changed.
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