Radio frequency digital filter group delay mismatch reduction

1. A radio frequency, RF, filter, the RF filter having:

a first feed forward path having a delay line;

a second feed forward path, the first and second feed forward path arranged to be electrically in parallel, the second feed forward path having:

a splitter configured to provide a first signal path and a second signal path, the first and second signal paths arranged to be electrically in parallel;

a digital filter in the first signal path of the second feed forward path;

a tunable analog filter in the second signal path of the second feed forward path;

a delay device electrically in series with the tunable analog filter, a delay of the delay device being adjustable, the tunable analog filter and the delay device combining to reduce a difference between a delay of the first signal path and a delay of the second signal path; and

a combiner arranged to combine outputs of the first signal path and the second signal path.

2. The RF filter of claim 1 , wherein the tunable analog filter provides filtering at baseband.

3. The RF filter of claim 1 , wherein the delay of the delay device is adjusted to offset a delay between an impulse response of the tunable analog filter and an impulse response of the digital filter.

4. A radio frequency, RF, filter, the RF filter having:

a first feed forward path having a delay line;

a second feed forward path, the first and second feed forward path arranged to be electrically in parallel, the second feed forward path having:

a splitter configured to provide a first signal path and a second signal path, the first and second signal paths arranged to be electrically in parallel; and

a digital filter in the first signal path of the second feed forward path;

a tunable analog filter in the second signal path of the second feed forward path; and

a delay device in the second signal path of the second feed forward path, the delay device being coupled in an electrically series arrangement with the tunable analog filter, a delay of the delay device being adjusted to offset a delay between an impulse response of the tunable analog filter and an impulse response of the digital filter, the delay being adjusted so that a last non-zero value of the impulse response of the tunable analog filter occurs one digital clock period before the first non-zero value of the impulse response of the digital filter.

5. The RF filter of claim 1 , wherein the digital filter is adaptive and has coefficients based on feed back from a signal resulting from a combination of signals from the first feed forward path and the second feed forward path.

6. The RF filter of claim 1 , further comprising a band stop filter in the second feed forward path, the band stop filter suppressing signals in proximity to a center frequency of a signal of the second feed forward path.

7. A radio frequency, RF, receiver, comprising:

a first RF coupling device, the first RF coupling device coupling a RF signal received from an antenna into a first feed forward path, the first feed forward path having a first delay, and a second feed forward path, the second feed forward path having:

a splitter, the splitter splitting a signal on an input path of the second feed forward path to:

a first signal path, the first signal path having an adaptive digital filter; and

a second signal path, the second signal path being electrically in parallel with the first signal path, the second signal path having a tunable analog filter and a delay device electrically in series with the tunable analog filter, the delay device having an adjustable delay, the tunable analog filter and the delay device combining to reduce a difference between a delay of the first signal path and a delay of the second signal path; and

a combiner, the combiner combining signals from the first signal path and the second signal path; and

a second RF coupling device, the second RF coupling device coupling a signal of the second feed forward path with a signal of the first feed forward path to produce an RF output signal.

8. The RF receiver of claim 7 , further comprising:

a digital signal processor, the digital signal processor providing filter coefficients for the adaptive digital filter, the filter coefficients based on samples of the RF output signal.

9. The RF receiver of claim 8 , the digital signal processor further providing coefficients for the tunable analog filter.

10. The RF receiver of claim 7 , further comprising a band stop filter in the second feed forward path, the band stop filter arranged to suppress signals in proximity to a center frequency of the signal of the second feed forward path.

11. A radio frequency, RF, transmitter, comprising:

a first RF coupler, the first RF coupler coupling RF energy from a power amplifier to a first feed forward path and a second feed forward path, the first feed forward path including a delay, the second feed forward path including:

a splitter, the splitter splitting a signal of the second feed forward path to:

a first signal path, the first signal path having a digital filter; and

a second signal path, the second signal path having a tunable analog filter and a delay device electrically in series with the tunable analog filter, the delay device having an adjustable delay, the tunable analog filter and the delay device combining to reduce a difference between a delay of the first signal path and a delay of the second signal path; and

a combiner, the combiner combining signals from the first signal path and the second signal path; and

a second RF coupler, the second RF coupler device coupling a signal of the second feed forward path with a signal of the first feed forward path to produce an RF output signal.

12. The RF transmitter of claim 11 , further comprising:

a third RF coupling device, the third RF coupling device feeding back a portion of the RF output signal;

an analog to digital converter, the analog to digital converter converting the feedback to a digital signal;

a digital signal processor, the digital signal processor producing a plurality of coefficients of the digital filter based on samples of the digital signal.

13. The RF transmitter of claim 11 , wherein the tunable analog filter operates at baseband.

14. The RF filter of claim 1 , wherein the tunable filter is dynamically tunable during operation of the RF filter.

15. The RF filter of claim 14 , wherein the tunable filter is dynamically tunable during operation by dynamically adjusting tap values of the tunable filter based on feedback from a digital signal processor.

16. The RF receiver of claim 7 , wherein the tunable filter is dynamically tunable during operation of the RF receiver.

17. The RF transmitter of claim 11 , wherein the tunable filter is dynamically tunable during operation of the RF transmitter.