Self-correcting afc system

Claim 2

An automatic frequency control system for controlling the frequency of a local oscillator, comprising: a source of input signal; discriminator means connected to said local oscillator and tuned to generate an output signal desirably varying about a reference level as the frequency of said input signal varies about a reference frequency, said discriminator means including a variable element tunable by a correction signal to change the level of said output signal for a given frequency of said input signal; input means for coupling to said discriminator means an input signal from said source having a frequency related to the frequency of said local oscillator; output means for coupling the output signal of said discriminator means to said local oscillator; correction means for producing a correction signal when the output signal of said discriminator means is different than said reference level for input signals having said reference frequency; and retuning means coupling said correction signal to said variable element to retune said discriminator means.

Claim 3

The automatic frequency control system of claim 1 wherein said discriminator means includes inductive means and capacitive means tuned to said reference frequency, said variable element comprises a reactive element forming a part of said inductive and capacitive means, and said correction means produces a DC voltage, corresponding to said correction signal, for tuning said reactive element.

Claim 4

The automatic frequency control system of claim 1 wherein said retuning means includes an analog memory for storing said correction signal during a recurring time period.

Claim 5

The automatic frequency control system of claim 3 wherein said retuning means includes gate meAns actuable to divert the output signal of said discriminator means from said local oscillator to said analog memory, and program means for actuating said gate means.

Claim 6

The automatic frequency control system of claim 1 wherein said correction means includes a reference oscillator for generating a reference signal at said reference frequency, and gate means actuable for coupling said reference oscillator to said discriminator means to cause said reference signal to form said input signal, whereby the output signal of said discriminator means when said gate means is actuated corresponds to said correction signal.

Claim 7

The automatic frequency control system of claim 5 wherein said reference oscillator comprises an oscillator controlled by a piezoelectric crystal tuned to said reference frequency.

Claim 8

In a receiver having local oscillator means coupled to mixer means in order to produce an intermediate frequency signal, an automatic frequency control system, comprising: discriminator means tuned to generate an output signal varying about a first level as the frequency of an input signal varies about said intermediate frequency; input means for coupling the intermediate frequency signal from said mixer means to said discriminator means; output means for coupling the output signal of said discriminator means to said local oscillator means to control the frequency of said intermediate frequency signal; reference oscillator means for generating a reference signal at said intermediate frequency; gate means actuable for coupling said reference oscillator means to said discriminator means in place of said input means to cause said reference signal to form said input signal; and correction means responsive to the output signal of said discriminator means when said gate means is actuated for retuning said automatic frequency control system to cause said mixer means to produce said intermediate frequency signal.

Claim 9

In a receiver having local oscillator means coupled to mixer means in order to produce an intermediate frequency signal, an automatic frequency control system, comprising: discriminator means tuned to generate an output signal varying about a first level as the frequency of an input signal varies about said intermediate frequency; input means for coupling the intermediate frequency signal from said mixer means to said discriminator means; output means for coupling the output signal of said discriminator means to said local oscillator means to control the frequency of said intermediate frequency signal; reference oscillator means for generating a reference signal at said intermediate frequency; gate means actuable for coupling said reference oscillator means to said discriminator means in place of said input means to cause said reference signal to form said input signal; and correction means responsive to the output signal of said discriminator means when said gate means is actuated for retuning said automatic frequency control system to cause said mixer means to produce said intermediate frequency signal.

Claim 10

The automatic frequency control system of claim 7 wherein said discriminator means comprises a LC tuned network having a wide frequency bandpass, and said reference oscillator means comprises a crystal controlled oscillator using a piezoelectric crystal having a resonant frequency curve substantially narrower than the bandpass of said LC tuned network.

Claim 11

The automatic frequency control system of claim 7 wherein said correction means includes an analog memory for storing said correction signal while said gate means is unactuated.

Claim 12

The automatic frequency control system of claim 9 including program means for periodically generating a gating signal, means coupling said gating signal to said gate means to cause actuation thereof, and said analog memory comprises a RC network having a time constant longer than the repetition rate of the gating signal from said program means.

Claim 13

The automatic frequency control system of claim 7 wherein said discriminator means includes a frequency variable element tunable by said correction signal to change the level of said output signal for a given frequency of said input signal, and said correction means includes means responsive when said gate means is actuated for coupling said output signal from said discriminator means to said variable element to retune said discriminator means.