US 4,316,108 A · Tracking filter for FM threshold extension

Abstract

A tracking filter suitable for use in the IF stage of FM receiver circuitry incorporates a bandpass filter the components of which yield a characteristic that permits the filter to provide substantially the entirety of a required 90.degree. phase shift for phase detection control circuitry that rapidly actively tunes capacitive components of the filter without imparting unacceptable group delay behavior to the configuration. The filter is a two-pole pair, single zero at the orgin, bandpass filter inserted in the signal path between a pair of signal power splitters. Part of the input signal is applied from the first splitter to one input of a phase comparator the other input of which is derived from the output signal via the second power splitter. The output of the phase comparator is applied through a broadband control amplifier to actively adjust the center frequency of the bandpass filter. Advantageously, this configuration enables the filter to have its center frequency effectively coincide with the instantaneous frequency of the deviating input signal, whereby an increase in performance over conventional filter approaches is obtained.

Claims16

Dependent claims

A tracking filter circuit comprising:

a bandpass filter the center frequency of which is adjustable to follow the frequency of an input signal applied thereto and including means for providing a prescribed phase shift to an input signal applied thereto;
a phase comparator, coupled to the input and output of said bandpass filter, for comparing the phase between said input signal and the output of said bandpass filter and for producing a control signal representative of the phase difference between said input signal and the output of said bandpass filter;
and
means, coupled between said phase comparator and said bandpass filter, for applying said control signal to said filter to adjust the center frequency of said bandpass filter to follow the frequency of said input signal;
and wherein
said phase shift providing means operates such that, in response to a change in the frequency of said input signal, said phase comparator produces a control signal for changing the center frequency of said bandpass filter, and said prescribed phase shift provided by said bandpass filter at the center frequency to which said bandpass filter was previously tuned is the same as that at the changed center frequency to which said bandpass filter is newly tuned.

3 more elements

A tracking filter comprising:

an input;
an output;
a bandpass filter the center frequency of which is adjustable to follow the frequency of an input signal applied thereto, said bandpass filter being coupled between said input and said output and including means for providing a phase shift from said input through said bandpass filter to said output of effectively ninety degrees;
first means having first and second input terminals for comparing the phase between signals applied to said first and second input terminals and producing a control signal representative of the phase difference between said signals;
second means, coupled between said first means and said bandpass filter, for applying a control signal from said first means to said bandpass filter so as to cause the center frequency of said bandpass filter to follow the frequency of an input signal applied thereto;
and wherein
said input is coupled to the first input terminal of said first means and said output is coupled to the second input terminal of said first means and the effective phase shift from said input through said bandpass filter to the second input of said first means is ninety degrees, and
said phase shift providing means operates such that in response to a change in the frequency of said input signal, said first means produces a control signal for changing the center frequency of said bandpass filter, and said ninety degrees phase shift provided by said bandpass filter at the center frequency to which said bandpass filter was previously tuned remains unchanged at the changed center frequency to which said bandpass filter is newly tuned.

5 more elements

Dependencies are inferred from the claim text.

Tracking filter for FM threshold extension

Life of this patent

Tracking filter for FM threshold extension

Life of this patent

Abstract

Claims16