On Mon, 30 Aug 2004 17:50:10 +1200, MikeN
wrote:


Has anyone given thought to producing NBFM using the AD 9854 or
similar DDS chips, for multiplying or heterodyning up to 2m or 70cm.

Doesn't the smaller DDS chips have serial loading of the phase
accumulator addend ?

To get communication quality audio, at least 8 kHz sampling rate is
required. If the phase accumulator addend is 32 bits, the serial clock
would have to be more than 2 MHz.

On 9854, the 14 bit phase offset word could also be used, thus
reducing the serial data clock to 1 MHz, well below the maximum 10 MHz
data clock frequency.

Maybe the FSK ramp rate could be used to reduce the clock rate needed.

This could be an interesting project. I see Analog Devices has an
application note AN-543 which describes a stereo WBFM generator with
output on 90.3 MHz.

A stereo WBFM generator would require at least 120 kHz sampling rate,
which would be quite trivial with parallel tuning words, but would be
quite demanding if 32 or 48 bit tuning words needs to be loaded each
audio sample time.

Paul OH3LWR