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Digital RF bandpass filter for FM radio translators


There was no response to this problem in DSP, so I thought maybe someone involved with RF or software-defined radio might be able to help.

Our public radio station uses an FM translator to receive a 91.5 MHz signal and rebroadcast it at 106.7 MHz.  The problem is that there is a nearby station on 91.1 MHz causing interference, and our RF analog bandpass filter skirts are only -10 dB down at that frequency.  Using multiple tuned RF filters in series helps, but the filter response of our cavity filters vary too much with temperature.

Is it possible to implement a digital RF bandpass filter for this?  It wouldn't need to be tuned, and I was wondering if it's possible to apply a (FIR?) 500 kHz bandpass filter at 91.5 MHz.  I think we'd need to sample at 275 MHz, and our 91.5 MHz analog bandpass filter could function as the anti-alias filter.  Since our translator/amplifier input is 91.5 MHz, is it possible to do this at RF, without any downconversion and upconversion to/from baseband?

Does Analog Devices have a demo board that can do this?  I started looking at software-defined radio systems, but don't know if any can receive and transmit at 91.5 MHz simultaneously.



  • Benjamin,

    Thanks for your reply.  The desired 91.5 MHz signal is about 40 dB SNR, and the interfering 91.1 MHz signal is a few dB higher than that, say 43 dB SNR.  Our tuned-cavity bandpass filters only attenuate the interference by about 10 dB, but if we could attenuate it by 30 dB, I think that would be enough.  Our output bandwidth is 400 kHz, which puts us too close to the 91.1 MHz station's upper sideband.

    I haven't been able to find commercial off-the-shelf digital filters like this, but with improvements in ADC performance and software-defined radio technology, I was wondering if it was feasible.


  • Hi, Douglas,

        It all comes down to dynamic range: in general the passive components gives you better dynamic range, subject to available Q of the components that define the filter skirts (selectivity).  You can get better selectivity with digital filters only if the analog path and ADC/DAC can support the dynamic range that you need.

       Before a blanket dismissal of the possibility, can you tell us the desired and interfering signal levels, the output BW, in-band SNR & output interferer level you are trying to achieve?  I have to admit that this is probably more of an academic curiosity than a practical solution.


  • Hi, Doug,

        The requirements may not be as stringent as I first envisioned!  A 12-bit RF ADC-DAC system should be able to provide 70+ dB of dynamic range at Nyquist, and much better once the BW is decimated down to 400 kHz.  So a line up as below may work:

        Cavity Filter - LNA - VGA - ADC - Digital Filter FPGA - DAC - Filter

    where the Digital Filter block includes Decimation, FIR filtering, and up-sampling.  Please double check my assumptions below:

        Input power (within pass band of the filter) ~= -75 dBm (40 uV)

        Gain required ~= 1/(12*40u) = 2096 => 66 dB,

                to convert 40 uVrms of random signal to 1 Vpp for ADC

        Fsamp(input) = 250 MHz

        Decimation ratio = 32 => Fsamp(output) = 7.8 MHz, i.e. > 8x oversampling

    Suitable ADI components would include:

      LNA - ADL5545 (among others) x 2

      VGA/Driver - ADL5101

      ADC - ADL9634-250 (12-bit 250 MS/s)

      FPGA - ADI does not make FPGA's, but some evb's have them

      DAC - AD9753 (12-bit 300 MS/s)

      Output filter - may need to be a cavity filter, or a dielectric resonator could work

    The parts cost would be much higher than an analog implementation.  But since you are looking at small quantities (1 or 2 ?), development cost would outweigh parts cost.  The filter you need is probably a bandpass with 250kHz 0.5 dB passband and 33 dB suppression at 400 kHz.  You may want to repost this on the high-speed ADC forum, and see if they may have an evb that already has an appropriate FPGA on, or even a bandpass sigma-delta ADC that would do the job.   Digital filtering is outside the scope of this forum and my expertise.


  • Benjamin,

    Thanks very much for your reply.  I tried responding the day before yesterday, but my message apparently didn't get through.

    I'm not familiar with bandpass sigma-delta ADCs.  If a sigma-delta ADC has an integral bandpass filter, could one be connected directly to a DAC and bypass the digital FPGA filter altogether?

    I'm also not familiar with dielectric resonator filters.  Can you recommend a source for a custom filter like this?


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