writes:

On Jun 23, 6:17 am, Paul Keinanen wrote:
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 IF is 1.7 MHz which is sampled using a 13.6 MHz clock by the
AD9874
and decimated by a factor of 48 to produce the 280 Ksamples/s output.

Looking at:

http://en.wikipedia.org/wiki/Fm_broadcasting

I see that FM audio goes from 0 to 53 KHz and RDS is at 57 KHz.
The AD9874 decimation filter should prevent any signal from aliasing
into the bandwidth of interest and the resulting sample rate is
above Nyquist.

I'm missing something ... assuming no aliasing problems why is
a higher sampling rate necessary for recovering the audio and RDS?

You're looking at the audio baseband bandwidth, i.e., the signal after
being FM demodulated. From your description the signal you're getting
has not yet been FM-demodulated. Thus you need to account for the +/-
75 kHz peak deviation of the FM signal, at a minimum. If you can
swallow up the whole 200 kHz, you gain slightly better performance due
to the admission of more the bessel function sidebands, as others have
noted.
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