[TenTec] DDS PTO replacement for old Ten Tec rigs

[Steve Hunt]

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 at cox.net lstoskopf at 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
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