Automatic fine tuning system and method for use in super-heterodyne receivers

Claim 2

In a superheterodyne receiver having a tunable input stage with a local oscillator and mixer for purposes of converting a selected RF carrier to an IF carrier having a predetermined fixed frequency, a frequency determining system providing a DC voltage indicative of the direction and magnitude of the difference in frequency between said IF carrier and said reference signal, comprising: means for generating first and second reference signals of like predetermined frequency and related phase; a first phase discriminator means receiving as inputs said IF carrier and said first reference signal for developing a first error signal related in frequency to the frequency difference between said IF carrier and said first reference signal and dependent upon the instantaneous phase difference between said first reference signal and said IF carrier; second phase discriminator means receiving as inputs said IF carrier and said second reference signal for developing a second error signal related in frequency to the frequency difference between said IF carrier and said second reference signal and dependent upon the instantaneous phase difference between said second reference signal and said IF carrier input, the relative phase of said first and second reference signals and the relative phase of said IF carrier at the inputs to said first and second phase discriminator means being chosen to cause the instantaneous phase difference between the inputs to said first phase discriminator means and the instantaneous phase difference between the inputs to said second phase discriminator means to be unequal by an amount other than a multiple of 180*, whereby a comparison of the phases of said first and second error signals indicates the polarity of the frequency difference between said IF carrier and said reference signals; combining means receiving said first and second error signals for generating a DC output voltage having a magnitude dependent on the frequency of said error signals and a polarity dependent on said phase relationship between said error signals, said combining means including a multiplier, means for coupling one of said first and second error signals to said multiplier, and a frequency dependent phase shift network which is coupled between said multiplier and the other of said error signals, said phase shift network introducing a frequency dependent phase shift in said other error signal to provide said multiplier with two error signals of like frequency whose phase difference varies according to the frequency of said error signals.

Claim 3

A system as defined in claim 1 wherein said fixed phase difference between said first and second reference signals is such that the dot product of the vectors of said error signals applied to said multiplier changes sign as the frequency of said IF carrier becomes greater or less than the frequency of said reference signals.

Claim 4

A system as defined in claim 1 including a summing network wherein said DC output voltage of said multiplier and the output of one of said first and second phase discriminator means are both applied to said summing network to generate a control voltage having AC and DC components.

Claim 5

In a superheterodyne receiver having a tunable input stage with a local oscillator and mixer for purposes of converting a selected RF carrier to an IF carrier having a predetermined frequency, a frequency determining system providing a DC voltage indicative of the direction and magnitude of the difference in frequency between said IF carrier and said reference signal comprising: means for generating first and second reference signals of like predetermined frequency and related phase; a first phase discriminator means receiving as inputs said IF carrier and said first reference signal for developing a first error signal related in frequency to the frequency difference between said IF carrier and said first reference signal and dependent upon the instantaneous phase difference between said first reference signal and said IF carrier; second phase discriminator means receiving as inputs said IF carrier and said second reference signal for developing a second error signal related in frequency to the frequency difference between said IF carrier and said second reference signal and dependent upon the instantaneous phase difference between said second reference signal and said IF carrier input, the relative phase of said first and second reference signals and the relative phase of said IF carrier at the inputs to said first and second phase discriminator means being chosen to cause the instantaneous phase difference between the inputs to said first phase discriminator means and the instantaneous phase difference between the inputs to said second phase discriminator means to be unequal by an amount other than a multiple of 180*, whereby a comparison of the phases of said first and second error signals indicates the polarity of the frequency difference between said IF carrier and said reference signals; combining means receiving said first and second error signals for generating a DC output voltage having a magnitude dependent on the frequency of said error signals and a polarity dependent on said phase relationship between said error signals, said combining means including a multiplier, a first phase shift network coupling one of said first and second error signals to said multiplier, and a second phase shift network coupling the other of said error signals to said multiplier, said first and second phase shift networks having predetermined time constants associated therewith for introducing in said error signals predetermined frequency dependent phase shifts, thereby causing said multiplier to generate a DC output voltage having one or more zeros at preselected frequencies of said error signals.

Claim 6

In a superheterodyne receiver having a tunable input stage with a local oscillator and mixer for purposes of converting a selected RF carrier to an IF carrier having a predetermined frequency, a frequency determining system providing a DC voltage indicative of the direction and magnitude of the difference in frequency between said IF carrier and said reference signal comprising: means for generating first and second reference signals of like predetermined frequency and related phase; a first phase discriminator means receiving as inputs said IF carrier and said first reference signal for developing a first error signal related in frequency to the frequency difference between said IF carrier and said first reference signal and dependent upon the instantaneous phase difference between said first reference signal and said IF carrier; second phase discriminator means receiving as inputs said IF carrier and said second reference signal for developing a second error signal related in frequency to the frequency difference between said IF carrier and said second reference signal and dependent upon the instantaneous phase difference between said second reference signal and said IF carrier input, the relative phase of said first and second reference signals and the relative phase of said IF carrier at the inputs to said first and second phase discriminator means being chosen to cause the instantaneous phase difference between the inputs to said first phase discriminator means and the instantaneous phase difference between the inputs to said second phase discriminator means to be unequal by an amount other than a multiple of 180*, whereby a comparison of the phases of said first and second error signals indicates the polarity of the frequency difference between said IF carrier and said reference signals; combining means receiving said first and second error signals for generating a DC output voltage having a magnitude dependent on the frequency of said error signals and a polarity dependent on said phase relationship between said error signals, said combining means including a multiplier, a first phase shift network coupling one of said first and second error signals to said multiplier, and a second phase shift network coupling the other of said error signals to said multiplier, said first and second phase shift networks having predetermined time constants associated therewith for introducing in said error signals predetermined frequency dependent phase shifts, thereby causing said multiplier to generate a DC output voltage having one or more zeros at preselected frequencies of said error signals.

Claim 7

In a superheterodyne receiver, an automatic frequency control system comprising: a tuner having a local oscillator and mixer for purposes of converting a selected RF carrier to an IF carrier having a predetermined nominal intermediate frequency; means for generating first and second reference signals having a like predetermined frequency f0 and a predetermined fixed phase difference; a first phase discriminator means receiving as inputs said IF carrier and said first reference signal for developing a first error signal related in frequency to the frequency difference between said IF carrier and said first reference signal, dependent upon the instantaneOus phase difference between said first reference signal and said IF carrier, and having a phase related to the phase of said first reference signal; a second phase discriminator means receiving as inputs said IF carrier and said second reference signal for developing a second error signal related in frequency to the frequency difference between said IF carrier and said second reference signal, dependent upon the instantaneous phase difference between said second reference signal and said IF carrier input, and having a phase related to the phase of said second reference signal, the relative phase of said first and second reference signals and the relative phase of said IF carrier at the inputs to said first and second phase discriminator means being chosen to cause the instantaneous phase difference between the inputs to said first phase discriminator means and the instantaneous phase difference between the inputs to said second phase discriminator means to be unequal by an amount other than a multiple of 180*, whereby a comparison of the phases of said first and second error signals indicates the polarity of the frequency difference between said IF carrier and said reference signals; combining means receiving said first and second error signals for generating a DC output voltage having a magnitude dependent on the frequency of said error signals and a polarity dependent on said phase relationship between said error signals, said combining means includes a multiplier and means for coupling one of said first and second error signals to said multiplier, and a frequency dependent phase shift network which is coupled between said multiplier and the other of said error signals, said phase shift network introducing a frequency dependent phase shift in said other error signal, thus providing said multiplier with two error signals of like frequency whose phase difference varies according to the frequency of said error signals; and means responsive to the DC voltage generated by said combining means for adjusting the frequency of said local oscillator toward a condition whereby the frequency of said IF signal is equal to that of said reference signal.

Claim 8

In a superheterodyne receiver, an automatic frequency control system comprising: a tuner having a local oscillator and mixer for purposes of converting a selected RF carrier to an IF carrier having a predetermined nominal intermediate frequency; means for generating first and second reference signals having a like predetermined frequency f0 and a predetermined fixed phase difference; a first phase discriminator means receiving as inputs said IF carrier and said first reference signal for developing a first error signal related in frequency to the frequency difference between said IF carrier and said first reference signal, dependent upon the instantaneOus phase difference between said first reference signal and said IF carrier, and having a phase related to the phase of said first reference signal; a second phase discriminator means receiving as inputs said IF carrier and said second reference signal for developing a second error signal related in frequency to the frequency difference between said IF carrier and said second reference signal, dependent upon the instantaneous phase difference between said second reference signal and said IF carrier input, and having a phase related to the phase of said second reference signal, the relative phase of said first and second reference signals and the relative phase of said IF carrier at the inputs to said first and second phase discriminator means being chosen to cause the instantaneous phase difference between the inputs to said first phase discriminator means and the instantaneous phase difference between the inputs to said second phase discriminator means to be unequal by an amount other than a multiple of 180*, whereby a comparison of the phases of said first and second error signals indicates the polarity of the frequency difference between said IF carrier and said reference signals; combining means receiving said first and second error signals for generating a DC output voltage having a magnitude dependent on the frequency of said error signals and a polarity dependent on said phase relationship between said error signals, said combining means includes a multiplier and means for coupling one of said first and second error signals to said multiplier, and a frequency dependent phase shift network which is coupled between said multiplier and the other of said error signals, said phase shift network introducing a frequency dependent phase shift in said other error signal, thus providing said multiplier with two error signals of like frequency whose phase difference varies according to the frequency of said error signals; and means responsive to the DC voltage generated by said combining means for adjusting the frequency of said local oscillator toward a condition whereby the frequency of said IF signal is equal to that of said reference signal.

Claim 9

A system as defined in claim 5 wherein said fixed phase difference between said first and second reference signals is such that the dot product of the vectors of said error signals applied to said multiplier changes sign as the frequency of said IF carrier becomes greater or less than the frequency fo of said reference signals.

Claim 10

A system as defined in claim 6 for purposes of locking a desired IF carrier to said predetermined frequency fo when said desired carrier is accompanied by an associated carrier having a frequency lower than fo by a fixed frequency difference f, f, wherein the frequency of said first and second reference signals, said fixed phase difference between said first and second reference signals, and said frequency dependent phase shift produced by said phase shift network are such that a phase relationship is created between said error signal received by said multiplier which causes the dot product of the vectors associated with said error signals to exhibit the following characteristics, namely: said dot product has a first predetermined polarity in response to a carrier having an IF frequency greater than fo, a second predetermined polarity in response to a carrier having an IF frequency between fo and predetermined frequency f1, which is lower than fo by less than Delta f, and said first predetermined polarity in response to a carrier having an IF frequency below f1 whereby, when the receiver is mistuned such that the IF frequency of said desired carrier is higher than fo while the frequency of said associated carrier remains lower than f1, the DC output voltage generated in response to said associated carrier is of the same polarity as that generateD by said multiplier in response to said desired carrier.

Claim 11

In a superheterodyne receiver, an automatic frequency control system comprising: a tuner having a local oscillator and mixer for purposes of converting a selected RF carrier to an IF carrier having a predetermined nominal intermediate frequency; means for generating first and second reference signals having a like predetermined frequency f0 and a predetermined fixed phase difference; a first phase discriminator means receiving as inputs said IF carrier and said first reference signal for developing a first error signal related in frequency to the frequency difference between said IF carrier and said first reference signal, dependent upon the instantaneous phase difference between said first reference signal and said IF carrier and having a phase related to the phase of said first reference signal; a second phase discriminator means receiving as inputs said IF carrier and said second reference signal for developing a second error signal related in frequency to the frequency difference between said IF carrier and said second reference signal, dependent upon the instantaneous phase difference between said second reference signal and said IF carrier input, and having a phase related to the phase of said second reference signal, the relative phase of said first and second reference signals and the relative phase of said IF carrier at the inputs to said first and second phase discriminator means being chosen to cause the instantaneous phase difference between the inputs to said first phase discriminator means and the instantaneous phase difference between the inputs to said second phase discriminator means to be unequal by an amount other than a multiple of 180*, whereby a comparison of the phases of said first and second error signals indicates the polarity of the frequency difference between said IF carrier and said reference signals; combining means receiving said first and second error signals for generating a DC output voltage having a magnitude dependent on the frequency of said error signals and a polarity dependent on said phase relationship between said error signals, said combining means including a multiplier, a first phase shift network coupling one of said first and second error signals to said multiplier, and a second phase shift network coupling the other of said error signals to said multiplier, said first and second phase shift networks having predetermined time constants associated therewith for introducing in said error signals predetermined frequency dependent phase shifts, thereby causing said multiplier to generate a DC output voltage having zeros at preselected frequencies of said error signals; and means responsive to the DC voltage generated by said combining means for adjusting the frequency of said local oscillator toward a condition whereby the frequency of said IF signal is equal to that of said reference signals.

Claim 12

In a superheterodyne receiver, an automatic frequency control system comprising: a tuner having a local oscillator and mixer for purposes of converting a selected RF carrier to an IF carrier having a predetermined nominal intermediate frequency; means for generating first and second reference signals having a like predetermined frequency f0 and a predetermined fixed phase difference; a first phase discriminator means receiving as inputs said IF carrier and said first reference signal for developing a first error signal related in frequency to the frequency difference between said IF carrier and said first reference signal, dependent upon the instantaneous phase difference between said first reference signal and said IF carrier and having a phase related to the phase of said first reference signal; a second phase discriminator means receiving as inputs said IF carrier and said second reference signal for developing a second error signal related in frequency to the frequency difference between said IF carrier and said second reference signal, dependent upon the instantaneous phase difference between said second reference signal and said IF carrier input, and having a phase related to the phase of said second reference signal, the relative phase of said first and second reference signals and the relative phase of said IF carrier at the inputs to said first and second phase discriminator means being chosen to cause the instantaneous phase difference between the inputs to said first phase discriminator means and the instantaneous phase difference between the inputs to said second phase discriminator means to be unequal by an amount other than a multiple of 180*, whereby a comparison of the phases of said first and second error signals indicates the polarity of the frequency difference between said IF carrier and said reference signals; combining means receiving said first and second error signals for generating a DC output voltage having a magnitude dependent on the frequency of said error signals and a polarity dependent on said phase relationship between said error signals, said combining means including a multiplier, a first phase shift network coupling one of said first and second error signals to said multiplier, and a second phase shift network coupling the other of said error signals to said multiplier, said first and second phase shift networks having predetermined time constants associated therewith for introducing in said error signals predetermined frequency dependent phase shifts, thereby causing said multiplier to generate a DC output voltage having zeros at preselected frequencies of said error signals; and means responsive to the DC voltage generated by said combining means for adjusting the frequency of said local oscillator toward a condition whereby the frequency of said IF signal is equal to that of said reference signals.

Claim 13

In a superheterodyne receiver, a dual mode automatic fine tuning system capable of both automatic frequency and phase control, comprising: a tuner having a local oscillator and mixer for purposes of converting a selected RF carrier to an IF carrier having a predetermined frequency; means for generating first and second reference signals of like predetermined frequency and predetermined fixed phase difference; a first phase discriminator means receiving as inputs said IF carrier and said first reference signal for developing a first error signal related in frequency to the frequency difference between said IF carrier and said first reference signal and dependent upon the instantaneous phase difference between said first reference signal and said IF carrier; second phase discriminator means receiving as inputs said IF carrier and said second reference signal for developing a second error signal related in frequency to the frequency difference between said IF carrier And said second reference signal and dependent upon the instantaneous phase difference between said second reference signal and said IF carrier input, the relative phase of said first and second reference signals and the relative phase of said IF carrier at the inputs to said first and second phase discriminator means being chosen to cause the instantaneous phase difference between the inputs of said first phase discriminator means and the instantaneous phase difference between the inputs to said second phase discriminator means to be unequal by an amount other than a multiple of 180*, whereby a comparison of the phases of said first and second error signals indicates the polarity of the frequency difference between said IF carrier and said reference signals; combining means receiving said first and second error signals for generating a DC output voltage having a magnitude dependent on the frequency of said error signals and a polarity dependent on said phase relationship between said error signals; summing means receiving one of said first and second error signals plus said DC output voltage of said combining means to generate a composite control signal having AC and DC components; means responsive to said composite control signal generated by said summing means for adjusting the frequency and phase of said local oscillator toward a condition of synchronization wherein the frequency and phase of said IF signal are locked to the frequency and related to the phase of said reference signals.

Claim 14

In a superheterodyne receiver, a dual mode automatic fine tuning system capable of both automatic frequency and phase control, comprising: a tuner having a local oscillator and mixer for purposes of converting a selected RF carrier to an IF carrier having a predetermined frequency; means for generating first and second reference signals of like predetermined frequency and predetermined fixed phase difference; a first phase discriminator means receiving as inputs said IF carrier and said first reference signal for developing a first error signal related in frequency to the frequency difference between said IF carrier and said first reference signal and dependent upon the instantaneous phase difference between said first reference signal and said IF carrier; second phase discriminator means receiving as inputs said IF carrier and said second reference signal for developing a second error signal related in frequency to the frequency difference between said IF carrier And said second reference signal and dependent upon the instantaneous phase difference between said second reference signal and said IF carrier input, the relative phase of said first and second reference signals and the relative phase of said IF carrier at the inputs to said first and second phase discriminator means being chosen to cause the instantaneous phase difference between the inputs of said first phase discriminator means and the instantaneous phase difference between the inputs to said second phase discriminator means to be unequal by an amount other than a multiple of 180*, whereby a comparison of the phases of said first and second error signals indicates the polarity of the frequency difference between said IF carrier and said reference signals; combining means receiving said first and second error signals for generating a DC output voltage having a magnitude dependent on the frequency of said error signals and a polarity dependent on said phase relationship between said error signals; summing means receiving one of said first and second error signals plus said DC output voltage of said combining means to generate a composite control signal having AC and DC components; means responsive to said composite control signal generated by said summing means for adjusting the frequency and phase of said local oscillator toward a condition of synchronization wherein the frequency and phase of said IF signal are locked to the frequency and related to the phase of said reference signals.

Claim 15

A system as defined in claim 9 wherein said combining means includes a multiplier, means for coupling one of said first and second error signals to said multiplier and a frequency dependent phase shift network which is coupled between said multiplier and the other of said error signals, said phase shift network introducing a frequency dependent phase shift in said other error signal to provide said multiplier with two error signals of like frequency whose phase difference varies according to the frequency of said error signals.

Claim 16

A system as defined in claim 10 wherein said fixed phase difference between said first and second reference signals is such that the dot product of the vectors of said error signals applied to said multiplier changes sign as the frequency of said IF carrier becomes greater or less than the frequency of said reference signals.

Claim 17

A system as defined in claim 10 for purposes of locking a desired IF carrier to said predetermined frequency fo when said desired carrier is accompanied by an associated carrier having a frequency lower than fo by a fixed frequency difference Delta f, wherein the frequency of said first and second reference signals, said fixed phase difference between said first and second reference signals, and said frequency dependent phase shift produced by said phase shift network are such that a phase relationship is created between said error signals received by said multiplier which causes the dot product of the vectors associated with said error signals to exhibit the following characteristics, namely: said dot product has a first predetermined polarity in response to a carrier having an IF frequency greater than fo, a second predetermined polarity in response to a carrier having an IF frequency between fo and a predetermined frequency f1 which is lower than fo by less than Delta f, and said first predetermined polarity in response to a carrier having an IF frequency below f1 whereby, when the receiver is mistuned such that the IF frequency of said desired carrier is higher than fo while the frequency of said associated carrier remains less than f1, the DC output voltage generated in response to said associated carrier is of the same polarity as that generated by said multiplier in response to said desired carrier.

Claim 18

A system as defined in claim 9 wherein the gain of the Automatic frequency control loop including said first and second phase discriminators, said multiplier and said summing network is such that, when said automatic fine tuning system attempts to lock onto an undesired carrier whose frequency is different from that of the desired carrier, the AC component of said composite control signal generated in response to said desired carrier is of a magnitude sufficient to cause the DC component of said composite control signal to lose control of said local oscillator, thereby forcing the local oscillator to adjust its frequency in response to said AC component so as to lock onto said desired carrier.

Claim 19

A system as defined in claim 9 wherein said combining means includes a multiplier, a first phase shift network coupling one of said first and second error signals to said multiplier, and a second phase shift network coupling the other of said error signals to said multiplier, said first and second phase shift networks having predetermined time constants associated therewith for introducing in said error signals predetermined frequency dependent phase shifts, thereby causing said multiplier to generate a DC output voltage having one or more zeros at preselected frequencies of said error signals.

Claim 20

In an RF receiver, a dual mode automatic fine tuning system capable of both automatic frequency and phase control with a synchronous type modulation detection system comprising: a tuner having a local oscillator and mixer for purposes of converting a selected modulated RF carrier to an IF carrier having a predetermined frequency; means for generating first, second, and third reference signals of like predetermined frequency and having predetermined fixed phase differences; first phase discriminator means receiving as inputs said IF carrier and said first reference signal for developing a first error signal related in frequency to the frequency difference between said IF carrier and said first reference signal and dependent upon the instantaneous phase difference between said first reference signal and said IF carrier; second phase discriminator means receiving as inputs said IF carrier and said second reference signal for developing a second error signal related in frequency to the frequency difference between said IF carrier and said second reference signal and dependent upon the instantaneous phase difference between said second reference signal and said IF carrier input, the relative phase of said first and second reference signals and the relative phase of said IF carrier at the inputs to said first and second phase discriminator means being chosen to cause the instantaneous phase difference between the inputs of said first phase discriminator means and the instantaneous phase difference between the inputs to said second phase discriminator mans to be unequal by an amount other than a multiple of 180*, whereby a comparison of the phases of said first and second error signals indicates the polarity of the frequency difference between said IF carrier and said reference signals; combining means receiving said first and second error signals for generating a DC output voltage having a magnitude dependent on the frequency of said error signals and a polarity dependent upon said phase relationship between said error signals; summing means receiving one of said first and second error signals plus said DC output voltage of said combining means to generate a control voltage having AC and DC components; means responsive to the control voltage generated by said summing means for adjusting the frequency and phase of said local oscillator toward a condition of synchronization wherein the frequency and phase of said IF signal are locked to the frequency and phase of said third reference signal; a synchronous-type detector receiving said IF carrier and said third reference signal for developing an output consisting of the product of said IF carrier and said third reference signal, said product including the information contained in the modulation of said carrier.

Claim 21

In an RF receiver, a dual mode automatic fine tuning system capable of both automatic frequency and phase control with a synchronous type modulation detection system comprising: a tuner having a local oscillator and mixer for purposes of converting a selected modulated RF carrier to an IF carrier having a predetermined frequency; means for generating first, second, and third reference signals of like predetermined frequency and having predetermined fixed phase differences; first phase discriminator means receiving as inputs said IF carrier and said first reference signal for developing a first error signal related in frequency to the frequency difference between said IF carrier and said first reference signal and dependent upon the instantaneous phase difference between said first reference signal and said IF carrier; second phase discriminator means receiving as inputs said IF carrier and said second reference signal for developing a second error signal related in frequency to the frequency difference between said IF carrier and said second reference signal and dependent upon the instantaneous phase difference between said second reference signal and said IF carrier input, the relative phase of said first and second reference signals and the relative phase of said IF carrier at the inputs to said first and second phase discriminator means being chosen to cause the instantaneous phase difference between the inputs of said first phase discriminator means and the instantaneous phase difference between the inputs to said second phase discriminator mans to be unequal by an amount other than a multiple of 180*, whereby a comparison of the phases of said first and second error signals indicates the polarity of the frequency difference between said IF carrier and said reference signals; combining means receiving said first and second error signals for generating a DC output voltage having a magnitude dependent on the frequency of said error signals and a polarity dependent upon said phase relationship between said error signals; summing means receiving one of said first and second error signals plus said DC output voltage of said combining means to generate a control voltage having AC and DC components; means responsive to the control voltage generated by said summing means for adjusting the frequency and phase of said local oscillator toward a condition of synchronization wherein the frequency and phase of said IF signal are locked to the frequency and phase of said third reference signal; a synchronous-type detector receiving said IF carrier and said third reference signal for developing an output consisting of the product of said IF carrier and said third reference signal, said product including the information contained in the modulation of said carrier.

Claim 22

A system as defined in claim 15 wherein said combining means includes a multiplier, means for coupling one of said first and second error signals to said multiplier, and a frequency dependent phase shift network which is coupled between said multiplier and the other of said error signals, said phase shift network introducing a frequency dependent phase shift in said other error signal to provide said multiplier with two error signals of like frequency whose phase difference varies according to the frequency of said error signals.

Claim 23

A system as defined in claim 16 wherein said fixed phase difference between said first and second reference signals is such that the dot product of the vectors of said error signal applied to said multiplier changes sign as the frequency of said IF carrier becomes greater or less than the frequency of said reference signals.

Claim 24

A system as defined in claim 16 for purposes of locking a desired IF carrier to said predetermined frequency fo when said desired carrier is accomplished by an associated carrier having a frequency lower than fo by a fixed frequency difference Delta f, wherein the frequency of said first and second reference signals, said fixed phase difference between said first and second reference signals, and said frequency dependent phase shift produced by said phase shift network are such that a phase relationship is created between said error signals received by said multiplier which causes the dot product of the vectors associated with said error signals to exhibit the following characteristics, namely: said dot product has a first predetermined polarity in response to a carrier having an IF frequency greater than fo, a second predetermined polarity in response to a carrier having an IF frequency between fo and predetermined frequency f1 which is lower than fo by less than Delta f, and said first predetermined polarity in response to a carrier having an IF frequency below f1 whereby, when the receiver is mistuned such that the IF frequency of said desired carrier is higher than fo while the frequency of said associated carrier remains less than f1, the DC output voltage generated in response to said associated carrier is of the same polarity as that generated by said multiplier in response to said desired carrier.

Claim 25

A system as defined in claim 15 wherein the gain of the automatic frequency control loop including said first and second phase discriminators, said multiplier and said summing network is such that, when said automatic fine tuning system attempts to lock onto an undesired carrier whose frequency is different from that of the desired carrier, the AC component of said composite control signal generated in response to said desired carrier is of a magnitude sufficient to cause the DC component of said composite control signal to lose control of said local oscillator, thereby forcing the local oscillator to adjust its frequency in response to said AC component so as to lock onto said desired carrier.