Automatic frequency control using split-band signal strength measurements

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.

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.