Sorry - I misunderstood, thought you were trying to digitally
downconvert directly !
Yes, that would work quite well. In fact G4GUO did just that on 73kHz a
few years ago using the 56002EVM with left and right channels for I/Q
and a downconverter fed with quadrature square waves in a mixer made
from 4066 analogue gates. There is no need for a DDS with quadrature
outputs, switching mixers works best if driven from a clean square wave.
The LF input has to be filtered to remove frequencies mixing with the
third harmonic of the LO, which of course would have to be done with any
mixer type - again the 100dB attenuation at 1MHz freqs, you can't get
away from that so easily !
One problem is the 3kHz (or whatever) lowpass filters. These have to be
analogue and very very precisely matched to preserve the 90 degree phase
relationship. Switched capacitor designs, such as used on the Codec
input, will work to an extent, but even these require some analogue
filtering to remove frequencies interfering THEIR alias products.
I'm not really a proponent of wide band digitisation then subsequently
filtering, unless that is really what you want and need for simultaneous
band monitoring purposes. Better to optimise gain / dynamic range,
analogue filter to a few 100 Hz bandwidth then digitise at a lower rate.
Andy G4JNT
perhaps I didn't make myself clear. What I
was suggesting was not
to digitize directly at 136 kHz.
My idea is along the following line :
Using a DDS with quadrature outputs (AD9854, if memory
serves), feed the
two signals to two mixers, each of them gets also the wanted
RF (in phase).>
--
The Information contained in this E-Mail and any subsequent correspondence
is private and is intended solely for the intended recipient(s).
For those other than the recipient any disclosure, copying, distribution,
or any action taken or omitted to be taken in reliance on such information is
prohibited and may be unlawful.
|
