Circuit for reducing offset error in FM detection

Claim 2

A circuit for reducing offset errors in a switched RF signal formed of a reference signal and a modulated signal which switched signal is subsequently subject to frequency detection, characterized by means (24) disposed prior to the detection for inverting the phase of the switched signal at a selected rate, to define after detection a demodulated signal containing alternating offset errors; and means (40) for integrating the demodulated signal subsequent to the detection to average the alternating offset errors.

Claim 3

A circuit according to claim 1 wherein the means for inverting include: a phase inverter (26); and control means (52, 58, 28) coupled to the phase inverter for supplying thereto timing pulses at the selected rate.

Claim 4

A circuit according to claim 2 including: means (54, 56) for providing a horizontal line related timing signal; and said control means being responsive to the providing means for supplying the timing pulses to the phase inverter on alternate pairs of horizontal lines.

Claim 5

A circuit according to any of claims 1 to 3 wherein the integrating means include: sample/hold means (42,44) operatively coupled to the FM detector means for holding successive samples of the demodulated signal derived from inverted and non-inverted portions respectively of the switched signal; and integrating means (40) coupled to the sample/hold means to produce in response to the successive samples an output signal with minimal offset error.

Claim 6

A circuit according to claim 4 wherein the sample/hold means is responsive to the control means to operate at half the horizontal line frequency.

Claim 7

An automatic frequency control circuit comprising a modulator (14) for providing an RF signal modulated by a video signal, a source (20) of a reference RF signal, means (16) for producing a combined signal by alternately switching between the modulated and reference signals, an FM detector (30) for the combined signal, and means responsive to the output of the FM detector to provide an error signal for controlling the modulator, characterized by a phase inverter (26) operable in response to timing signals to invert the phase of the combined.signal such that the said output contains alternating offset errors and means (42, 44, 40) for averaging the output signal to cancel the said errors.

Claim 8

An automatic frequency control circuit comprising a modulator (14) for providing an RF signal modulated by a video signal, a source (20) of a reference RF signal, means (16) for producing a combined signal by alternately switching between the modulated and reference signals, an FM detector (30) for the combined signal, and means responsive to the output of the FM detector to provide an error signal for controlling the modulator, characterized by a phase inverter (26) operable in response to timing signals to invert the phase of the combined.signal such that the said output contains alternating offset errors and means (42, 44, 40) for averaging the output signal to cancel the said errors.

Claim 9

A circuit according to claim 6 in which the arrangement is such that the frequency of phase inversion is performed at half the frequency of the said switching.

Claim 10

A circuit according to claim 7 in which the frequency of the said switching is half a horizontal line rate.

Claim 11

A circuit according to any of claims 6 to 8 in which the reference signal represents a blanking level.

Claim 12

A circuit according to any of claims 6 to 9 in which the means for averaging is operable to take successive samples of the said output, said samples containing offset errors of alternating polarity, and includes an integrator responsive to successive pairs of samples to produce an averaged output signal in which the alternating offset errors cancel each other.