The DDS is actually cleaner than the PTO for "close-in" noise. If I
inject an S9+60dB signal into my CorsairII and position it just outside
the receiver pass-band, then swap between the DDS in the external VFO
and the PTO in the rig, the _improvement_ in noise with the DDS is
noticeable.
The DDS' Achilles' heal is its wideband spurs. They can be reduced by
keeping the output frequency low compared to the clock frequency, and by
increasing the number of bits in the DAC. The AD9851 is an improvement
over the AD9850 in this respect; but both are surpassed by more recent
14-bit parts. I've never seen a level of spurs on my spectrum analyser
that has caused me concern, but you do have to search carefully because
those spurs appear and disappear with just tiny shifts of frequency.
One test is to inject a high level signal into the receiver and then
sweep the DDS over the complete 5.0-5.5MHz - listen for any audio
responses away from the main frequency. (Don't forget to then repeat the
exercise with the PTO rather than the DDS - you may be surprised!).
Steve G3TXQ
On 27/10/2017 17:10, lstoskopf@cox.net lstoskopf@cox.net wrote:
Just remember that TT went to great lengths to have pure LOs. The Orion was a
beast in making sure that the LO was as good as the time allowed. Wonder what
engineers designed that. Never have heard the story. Don't expect some cheap
DDS to do as well. It would be fun to compare a well built analog VFO to
cheaper DDS as far as clean signals are important. Of course, your
application may not require extreme close in performance and you will be
perfectly happy. That's what this hobby is all about. N0UU
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|