First adjacent canceller (FAC) with improved blending using a parametric filter
A method for processing a radio signal includes: receiving an FM in-band on-channel radio signal including a plurality of digitally modulated subcarriers in upper and lower sidebands; sampling the FM in-band on-channel radio signal to produce an input signal including complex digital samples of a combination of a desired one the upper and lower sidebands and an FM interferer; removing FM interferer components from the first signal by notch filtering to produce a notch-filtered signal; weighting the notch-filtered signal to produce a weighted notch-filtered signal; using a parametric filter to filter the input signal to produce a parametric-filtered input signal; and combining the weighted notch-filtered signal and the parametric-filtered input signal to produce an output signal. A radio receiver that implements the method is also included.
1. A method for processing a radio signal comprising:
receiving an FM in-band on-channel radio signal including a plurality of digitally modulated subcarriers in upper and lower sidebands;
sampling the FM in-band on-channel radio signal to produce an input signal including complex digital samples of a combination a desired one the upper and lower sidebands and an FM interferer;
removing FM interferer components from the first signal by notch filtering to produce a notch-filtered signal;
weighting the notch-filtered signal to produce a weighted notch-filtered signal;
using a parametric filter to filter the input signal to produce a parametric-filtered input signal; and
combining the weighted notch-filtered signal and the parametric-filtered input signal to produce an output signal.
2. The method of claim 1 , wherein the relative proportion of the weighted notch-filtered signal and the parametric-filtered input signal is determined by measuring the relative amount of interferer removed by the notch filter.
3. The method of claim 1 , wherein removing FM interferer components of the input signal by notch filtering to produce a notch-filtered signal comprises:
producing a first signal representative of the magnitude of the input signal;
producing a second signal representative of the mean magnitude of the first signal;
subtracting the second signal from the first signal to produce a notch-magnitude signal; and
multiplying the notch-magnitude signal with a normalized version of the input signal to produce the notch-filtered signal.
4. The method of claim 1 , wherein prior to using a parametric filter to filter the input signal to produce a parametric-filtered input signal, zero-value signal samples are appended to ends of an input signal vector.
5. The method of claim 1 , further comprising:
separating the complex digital samples from the upper and lower sidebands;
frequency shifting the complex digital samples from the upper and lower sidebands to produce complex baseband digital samples from the upper and lower sidebands; and
using the isolation filters to separately process the complex baseband digital samples from the upper and lower sidebands.
6. The method of claim 3 , further comprising:
determining a mean of the notch-magnitude signal;
determining a ratio of the mean of the notch-magnitude signal and the second signal;
using the ratio to compute first and second blend parameters; and
using the first blend parameters to produce the weighting for the notch-filtered signal.
7. The method of claim 5 , wherein:
complex baseband digital samples from the upper and lower sidebands upper and lower sideband isolation filters each have a passband located between 100 kHz and 270 kHz from a center frequency of the FM in-band on-channel radio signal.
8. The method of claim 6 , further comprising:
using the second blend parameters as coefficients of the parametric filter.
9. The method of claim 6 , wherein the ratio represents the power of the FM interferer relative to the power of the desired digital sideband.
10. The method of claim 8 , wherein the parametric filter shapes the spectrum of the input signal to apply more attenuation to a spectral portion of the input signal that is most affected by the FM interferer.
11. A radio receiver comprising:
an input receiving an original FM in-band on-channel radio signal including a plurality of digitally modulated subcarriers in upper and lower sidebands; and
processing circuitry for sampling the FM in-band on-channel radio signal to produce an input signal including complex digital samples of a combination of a desired one the upper and lower sidebands and an FM interferer, removing FM interferer components from the first signal by notch filtering to produce a notch-filtered signal, weighting the notch-filtered signal to produce a weighted notch-filtered signal, using a parametric filter to filter the input signal to produce a parametric-filtered input signal, and combining the weighted notch-filtered signal and the parametric-filtered input signal to produce an output signal.
12. The radio receiver of claim 11 , wherein the relative proportion of the weighted notch-filtered signal and the parametric-filtered input signal is determined by measuring the relative amount of interferer removed by the notch filter.
13. The radio receiver of claim 11 , wherein the processing circuitry removes FM interferer components of the input signal by notch filtering to produce a notch-filtered signal by: producing a first signal representative of the magnitude of the input signal, producing a second signal representative of the mean magnitude of the first signal, subtracting the second signal from the first signal to produce a notch-magnitude signal, and multiplying the notch-magnitude signal with a normalized version of the input signal to produce the notch-filtered signal.
14. The radio receiver of claim 11 , wherein the processing circuitry appends zero-value signal samples to ends of the input signal prior to using a parametric filter to filter the input signal to produce a parametric-filtered input signal vector.
15. The radio receiver of claim 11 , wherein the processing circuitry separates the complex digital samples from the upper and lower sidebands, frequency shifts the complex digital samples from the upper and lower sidebands to produce complex baseband digital samples from the upper and lower sidebands, and uses the isolation filters to separately process the complex baseband digital samples from the upper and lower sidebands.
16. The radio receiver of claim 13 , wherein the processing circuitry determines a mean of the notch-magnitude signal, determines a ratio of the mean of the notch-magnitude signal and the second signal, uses the ratio to compute first and second blend parameters, and uses the first blend parameters to produce the weighting the notch-filtered signal.
17. The radio receiver of claim 15 , wherein:
complex baseband digital samples from the upper and lower sidebands upper and lower sideband isolation filters each have a passband located between 100 kHz and 270 kHz from a center frequency of the FM in-band on-channel radio signal.
18. The radio receiver of claim 16 , wherein the processing circuitry uses the second blend parameters as coefficients of the parametric filter.
19. The radio receiver of claim 16 , wherein the ratio represents the power of the FM interferer relative to the power of the desired digital sideband.
20. The radio receiver of claim 18 , wherein the parametric filter shapes the spectrum of the input signal to apply more attenuation to a spectral portion of the input signal that is most affected by the FM interferer.