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Paul Keinanen · June 23rd 07, 11:17 AM posted to rec.radio.amateur.homebrew,comp.dsp

On Thu, 21 Jun 2007 17:44:13 -0700, wrote:

I'm working on a SDR design using the AD9874 to
digitize the IF producing 280 Ksamples/sec and
am trying to size the DSP. I'm leaning towards
the ADSP-BF532 which is a fixed point DSP rated
at 400 MIPS / 800 MMACS and is available in a LQFP
package. I'd like to be able to handle everything
from decoding standard shortwave SSB signals to
broadcast FM stereo signals (including RDS).

I am not familiar with that specific component, but does it really
contain a decent S&H circuit for decimation ?

Even with a proper S&H (ten nanosecond sampling and several
microsecond hold times) for decimation, the 280 Ksamples/s sounds a
bit low for FM broadcast detection. The higher order (Bessel function)
sidebands are still quite strong with the modulation indexes used, so
truncating the bandwidth to the 100-140 kHz range might not be a good
idea.

If you after all want to include broadcast FM reception, I would
suggest that you do the reception in analog way and only go to digital
after the ratio detector, when 150 kHz sampling rate might be
sufficient for stereo and RDS separation.

For the other modulation modes, I would suggest using a high 1st IF
(40-80 MHz) with a proper roofing filter combined with a few IF stages
and AGC. This signal might then be decimated directly (or after
converting to 455 kHz for steeper filters) and processed at sampling
rates well below 100 Ksamples/s.

With the roofing filter and the AGC in the first IF, the ADC should be
able to handle large signal variations, as long as the offending
signal is not within the roofing filter bandwidth.

Paul OH3LWR

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