Ken Scharf wrote:
Leon wrote:
JJ wrote:

"Leon" wrote in news:1142720911.911739.64900@

Bus switches like the FST3125 make very good mixers and quadrature
sampling detectors, as in the SoftRock SDRs.

Leon



And how about the IF amp? And VFO?
DDS like AD9850 look interesting.


ADI makes some very nice VCAs, like the AD603. They are probably the
best IF devices available, now. Racal was using FETs for IF amplifiers
in their military radios when I worked for them a few years ago.

PLLs are better than DDSs, but it can be diificult to make a good one.
A DDS may be used to provide a reference for a PLL, of course, for fine
tuning.

Leon

Actually there is nothing really wrong with a DDS all by itself.
They are capable of very low phase noise. The new AD995x series
have 14 bit D/A's and are therefore VERY clean. They are rated at
up to 400mhz clock, but if you don't use the built in clock multiplier
and provide a 1:1 external clock they have been over clocked to
as high as 600mhz! Running with a clock rate of 10x the desired
output frequency this chip should have VERY low spurs!

Instead of using the DDS as a reference for a PLL, consider using it
AS the divider in a pll. This will give fractional N divide ability
allowing you to tune to the nearest hertz without using dual loops.
The math to calculate the DDS tuning word, might be a bit heavy on
a pic though....

I was thinking of the spurs on the older devices. The newer one are
better.

Leon