On 16 Mar 2005 12:58:44 -0800, wrote:

Direct conversion (DC) won't be effective on this
application for reasons of the civil aviation band
being AM with no pilot carrier or other reference.

If you are running an I/Q DDS feeding I and Q demodulators (mixers)
and low-pass filter the quadrature demodulator, producing a DC voltage
proportional to the phase error. This could be used through an ADC to
control the phase modulator input (or the phase accumulator addend) of
the DDS to track the signal. However, DDS chips with only serial input
might be too slow to track the signal. The I demodulator output would
provide the amplitude modulated information.

Paul OH3LWR