Automatic frequency control using split-band signal strength measurements

Claim 2

In a communications receiver having a tuner containing a frequency tunable local oscillator, and a demodulator, wherein the tuner mixes an output frequency of the local oscillator with a received signal to generate an intermediate frequency (IF) signal for application to the demodulator, the received and IF signals have an equal modulation bandwidth and first and second center frequencies, respectively, and wherein the first and second center frequencies are different, apparatus for generating a frequency control signal applied to the tuner for tuning the output frequency provided by the local oscillator in order to bring the second center frequency into coincidence with a pre-defined frequency for the IF signal, the apparatus comprising: a first filter, connected to said tuner, for filtering the IF signal so as to produce a first filtered signal, said first filter having a first pre-defined frequency characteristic with a first bandwidth equivalent to a pre-defined upper portion of the modulation bandwidth and a third center frequency set to a frequency higher than the second center frequency; a second filter, connected to said tuner, for filtering said IF signal so as to produce a second filtered signal, said second filter having a second pre-defined frequency characteristic with a second bandwidth equivalent to a pre-defined lower portion of the modulation bandwidth and a fourth center frequency set to a frequency below the second center frequency, wherein the upper and lower portions represent separate substantially non-overlapping portions of the modulation bandwidth; means, connected to both said first and second filters, for producing a first tuning signal responsive to a difference, in terms of signal strength, between said first and second filtered signals; a carrier recovery circuit, operative in conjunction with the demodulator, for producing a second tuning signal for adjusting, within a pre-defined capture range of the demodulator, the output frequency of the local oscillator so that the second center frequency coincides with the pre-defined frequency for the IF signal; and means, connected to both the first and second tuning signals and responsive to the demodulator, for selecting, as the frequency control signal, either the first or second tuning signals whenever the second center frequency lies respectively outside of or within the capture range of the demodulator whereby the first and second tuning signals function to provide coarse and fine tuning, respectively, of the output frequency of the local oscillator.

Claim 3

The apparatus of claim 1 wherein a passband response of either the first filter, the second filter or both is shaped to compensate for any asymmetries in a spectral response of said intermediate frequency signal.

Claim 4

The apparatus of claim 1 wherein said received signal is associated with a pre-defined channel within a plurality of channels wherein said first and second bandwidths and the third and fourth center frequencies are collectively defined by an expected frequency drift of signals contained in channels adjacent to said pre-defined channel.

Claim 5

The apparatus of claim 1 wherein said difference signal producing means comprises first means for squaring said first filtered signal so as to produce a first squared output signal; second means for squaring said second filtered signal so as to produce a second squared output signal; means, connected to said first and second squaring means, for subtracting said first squared output signal from said second squared output signal so as to form a difference output signal; and a low pass filter, connected to said subtracting means, for filtering the difference output signal to produce a difference signal, said difference signal representing a power difference between signals passing through said first and second filters.

Claim 6

The apparatus of claim 1 further comprising means, connected to said difference signal producing means, for scaling said difference signal to produce said first tuning signal.

Claim 7

The apparatus of claim 5 wherein said scaling means is a linear amplifier.

Claim 8

The apparatus of claim 5 wherein said scaling means is a look-up table containing pre-defined stored values for providing, in response to a value of said difference signal applied as an address thereto, one of said stored values as said first tuning signal.

Claim 9

The apparatus of claim 1 wherein said first and second filters are digital finite impulse response (FIR) filters.

Claim 10

The apparatus of claim 1 wherein said first and second filters are digital infinite impulse response (IIR) filters.

Claim 11

The apparatus of claim 1 wherein said first and second filters are complex filters.

Claim 12

In a communications receiver having a tuner containing a frequency tunable local oscillator, and a demodulator, wherein the tuner mixes an output frequency of the local oscillator with a received signal to generate an intermediate frequency (IF) signal for application to the demodulator, the received and IF signals have an equal modulation bandwidth and first and second center frequencies, respectively, and wherein the first and second center frequencies are different, apparatus for generating a frequency control signal applied to the tuner for tuning the output frequency provided by the local oscillator in order to bring the second center frequency of the intermediate frequency signal into coincidence with a pre-defined frequency for the IF signal, the apparatus comprising: a first filter, connected to said tuner, for filtering the IF signal so as to produce a first filtered signal, said first filter having a first pre-defined frequency characteristic with a first bandwidth equivalent to approximately an upper half portion of the modulation bandwidth and a third center frequency set to a frequency higher than the second center frequency; a second filter, connected to said tuner, for filtering said IF signal so as to produce a second filtered signal, said second filter having a second pre-defined frequency characteristic with a second bandwidth equivalent to approximately a lower half portion of the modulation bandwidth and a fourth center frequency set to a frequency below the second center frequency, wherein the upper and lower portions represent separate substantially non-overlapping portions of the modulation bandwidth; means, connected to both said first and second filters, for producing a first tuning signal responsive to a difference, in terms of signal strength, between said first and second filtered signals; a carrier recovery circuit, operative in conjunction with the demodulator, for producing a second tuning signal for adjusting, within a pre-defined capture range of the demodulator, the output frequency of the local oscillator so that the second center frequency coincides with the pre-defined frequency for the IF signal; and means, connected to both the first and second tuning signals and responsive to the demodulator, for selecting, as the frequency control signal, either the first or second tuning signals whenever the second center frequency lies respectively outside of or within the capture range of the demodulator whereby the first and second tuning signals function to provide coarse and fine tuning, respectively, of the output frequency of the local oscillator.

Claim 13

In a communications receiver having a tuner containing a frequency tunable local oscillator, and a demodulator, wherein the tuner mixes an output frequency of the local oscillator with a received signal to generate an intermediate frequency (IF) signal for application to the demodulator, the received and IF signals have an equal modulation bandwidth and first and second center frequencies, respectively, and wherein the first and second center frequencies are different, apparatus for generating a frequency control signal applied to the tuner for tuning the output frequency provided by the local oscillator in order to bring the second center frequency of the intermediate frequency signal into coincidence with a pre-defined frequency for the IF signal, the apparatus comprising: a first filter, connected to said tuner, for filtering the IF signal so as to produce a first filtered signal, said first filter having a first pre-defined frequency characteristic with a first bandwidth equivalent to approximately an upper half portion of the modulation bandwidth and a third center frequency set to a frequency higher than the second center frequency; a second filter, connected to said tuner, for filtering said IF signal so as to produce a second filtered signal, said second filter having a second pre-defined frequency characteristic with a second bandwidth equivalent to approximately a lower half portion of the modulation bandwidth and a fourth center frequency set to a frequency below the second center frequency, wherein the upper and lower portions represent separate substantially non-overlapping portions of the modulation bandwidth; means, connected to both said first and second filters, for producing a first tuning signal responsive to a difference, in terms of signal strength, between said first and second filtered signals; a carrier recovery circuit, operative in conjunction with the demodulator, for producing a second tuning signal for adjusting, within a pre-defined capture range of the demodulator, the output frequency of the local oscillator so that the second center frequency coincides with the pre-defined frequency for the IF signal; and means, connected to both the first and second tuning signals and responsive to the demodulator, for selecting, as the frequency control signal, either the first or second tuning signals whenever the second center frequency lies respectively outside of or within the capture range of the demodulator whereby the first and second tuning signals function to provide coarse and fine tuning, respectively, of the output frequency of the local oscillator.

Claim 14

The apparatus of claim 11 wherein said difference signal producing means comprises first means for squaring said first filtered signal so as to produce a first squared output signal; second means for squaring said second filtered signal so as to produce a second squared output signal; means, connected to said first and second squaring means, for subtracting said first squared output signal from said second squared output signal so as to form a difference output signal; and a low pass filter, connected to said subtracting means, for filtering the difference output signal to produce a difference signal, said difference signal representing a power difference between signals passing through said first and second filters.

Claim 15

The apparatus of claim 12 further comprising means, connected to said difference signal producing means, for scaling said difference signal to produce said first tuning signal, wherein said scaling means is a linear amplifier.

Claim 16

The apparatus of claim 13 wherein said scaling means is a look-up table containing pre-defined stored values for providing, in response to a value of said difference signal applied as an address thereto, one of said stored values as said first tuning signal.

Claim 17

The apparatus of claim 11 wherein said first and second filters are digital finite impulse response (FIR) filters.

Claim 18

The apparatus of claim 11 wherein said first and second filters are digital infinite impulse response (IIR) filters.

Claim 19

The apparatus of claim 11 wherein said first and second filters are complex filters.

Claim 20

In a communications receiver having a tuner containing a frequency tunable local oscillator, and a demodulator, wherein the tuner mixes an output frequency of the local oscillator with a received signal to generate an intermediate frequency (IF) signal for application to the demodulator, the received and IF signals have an equal modulation bandwidth and first and second center frequencies, respectively, and wherein the first and second center frequencies are different, apparatus for generating a frequency control signal applied to the tuner for tuning the output frequency provided by the local oscillator in order to bring the second center frequency into coincidence with a pre-defined frequency for the IF signal, the apparatus comprising: means, connected to the tuner, for digitizing said IF signal so as to produce a digitized IF signal; means, connected to said digitizing means, for producing an in-phase (I) signal and a quadrature-phase (Q) signal from said digitized IF signal; a first complex filter, connected to said producing means, for filtering said I and Q signals so as to produce a first filtered complex signal, said first complex filter having a first pre-defined frequency characteristic with a first bandwidth equivalent to a pre-defined upper portion of the modulation bandwidth and a third center frequency set to a frequency higher than the second center frequency; a second complex filter, connected to said producing means, for filtering said I and Q signals so as to produce a second filtered complex signal, said second complex filter having a second pre-defined frequency characteristic with a second bandwidth equivalent to a pre-defined lower portion of the modulation bandwidth and a fourth center frequency set to a frequency below the second center frequency, wherein the upper and lower portions represent separate substantially non-overlapping portions of the modulation bandwidth; means, connected to both said first and second complex filters, for producing a first tuning signal responsive to a difference, in terms of signal strength, between said first and second filtered complex signals; a carrier recovery circuit, operative in conjunction with the demodulator, for producing a second tuning signal for adjusting, within a pre-defined capture range of the demodulator, the output frequency of the local oscillator so that the second center frequency coincides with the pre-defined frequency for the IF signal; and means, connected to both the first and second tuning signals and responsive to the demodulator, for selecting, as the frequency control signal, either the first or second tuning signals whenever the second center frequency lies respectively outside of or within the capture range of the demodulator whereby the first and second tuning signals function to provide coarse and fine tuning, respectively, of the output frequency of the local oscillator.

Claim 21

In a communications receiver having a tuner containing a frequency tunable local oscillator, and a demodulator, wherein the tuner mixes an output frequency of the local oscillator with a received signal to generate an intermediate frequency (IF) signal for application to the demodulator, the received and IF signals have an equal modulation bandwidth and first and second center frequencies, respectively, and wherein the first and second center frequencies are different, apparatus for generating a frequency control signal applied to the tuner for tuning the output frequency provided by the local oscillator in order to bring the second center frequency into coincidence with a pre-defined frequency for the IF signal, the apparatus comprising: means, connected to the tuner, for digitizing said IF signal so as to produce a digitized IF signal; means, connected to said digitizing means, for producing an in-phase (I) signal and a quadrature-phase (Q) signal from said digitized IF signal; a first complex filter, connected to said producing means, for filtering said I and Q signals so as to produce a first filtered complex signal, said first complex filter having a first pre-defined frequency characteristic with a first bandwidth equivalent to a pre-defined upper portion of the modulation bandwidth and a third center frequency set to a frequency higher than the second center frequency; a second complex filter, connected to said producing means, for filtering said I and Q signals so as to produce a second filtered complex signal, said second complex filter having a second pre-defined frequency characteristic with a second bandwidth equivalent to a pre-defined lower portion of the modulation bandwidth and a fourth center frequency set to a frequency below the second center frequency, wherein the upper and lower portions represent separate substantially non-overlapping portions of the modulation bandwidth; means, connected to both said first and second complex filters, for producing a first tuning signal responsive to a difference, in terms of signal strength, between said first and second filtered complex signals; a carrier recovery circuit, operative in conjunction with the demodulator, for producing a second tuning signal for adjusting, within a pre-defined capture range of the demodulator, the output frequency of the local oscillator so that the second center frequency coincides with the pre-defined frequency for the IF signal; and means, connected to both the first and second tuning signals and responsive to the demodulator, for selecting, as the frequency control signal, either the first or second tuning signals whenever the second center frequency lies respectively outside of or within the capture range of the demodulator whereby the first and second tuning signals function to provide coarse and fine tuning, respectively, of the output frequency of the local oscillator.

Claim 22

The apparatus of claim 18 wherein the bandwidth of each of said first and second complex filters is equivalent to approximately half of the modulation bandwidth.

Claim 23

The apparatus of claim 18 wherein said difference signal producing means comprises first means for squaring said first complex filtered signal so as to produce a first squared I output signal and a first squared Q output signal; second means for squaring said second complex filtered signal so as to produce a second squared I output signal and a second squared Q output; first means, connected to said first squaring means, for combining said first squared I and Q signals so as to produce a first sum signal; second means, connected to said second squaring means, for combining said second squared I and said second squared Q output signals so as to produce a second sum signal; means, connected to said first and second combining means, for subtracting said first sum signal from said second sum signal so as to produce a difference output signal; and a low pass filter, connected to said subtracting means, for filtering said difference output signal to produce the difference signal, said difference signal representing a power difference between signals passing through said first and second complex filters.

Claim 24

The apparatus of claim 20 further comprising means, connected to said difference signal producing means, for scaling said difference signal to produce said first tuning signal.

Claim 25

The apparatus of claim 21 wherein said scaling means is a linear amplifier.