Digital RF bandpass filter for FM radio translators

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

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