Perhaps I could comment:
I can't claim to have made exhaustive measurements. What I _have_ done
is fit the DDS to one of my remote VFOs so that I can A/B switch
instantly between the original internal PTO and the DDS. What I found
was that the DDS has noticeably less close-in noise than the PTO. If I
injected a large signal - say -33dBm - and tuned it just outside the Rx
passband, I heard more noise when switched to the PTO than when switched
to the DDS.
On the other hand the DDS _does_ have in-band spurs - typically -80dBc -
which the output filter can do nothing about. But I've been surprised
how seldom those spurs actually seem to cause a problem in practice -
maybe because most of them shift frequency rapidly with just slight
shifts of tuning, or maybe because you're very unlucky if you get a
station 70dB stronger than the one you're trying to listen to that also
happens to be right at a spur frequency.
Having the DDS unit in the remote VFO, and the original PTO in the
Corsair, gives me the flexibility to switch back to the PTO should I
ever need to for improved performance; but so far I have never needed to.
One other thing worth mentioning is that the DDS output filter is not
particularly "tight", and I deliberately didn't limit the upper
frequency of the DDS. In fact you can pretty much continue tuning until
the front-end Corsair filter cuts off signals; that over-run is very
useful on 10m where you can run up well into the next band segment
without needing to operate the band switch.
73,
Steve G3TXQ
On 09/07/2012 16:49, Jerry Haigwood wrote:
Perry,
Steve hunt- G3TXQ, who is the original designer of the DDS VFO that Dave
refers to, has made spectral purity measurements. His design uses a five
element low pass filter in the DDS output which helps with harmonics, etc.
All DDS have spurs at different frequencies. Most of these are way below
the desired output. However, the filters in the Corsair II take care of any
spurs that might cause a problem. Steve has documented his design at
<http://www.karinya.net/g3txq/corsair_pto/>.
BTW, I am running the same DDS on my Corsair II. I also have the 263G
remote VFO which still has the original PTO and I can use it as a reference.
Jerry W5JH
-----Original Message-----
From: tentec-bounces@contesting.com [mailto:tentec-bounces@contesting.com]
On Behalf Of w8au@sssnet.com
Sent: Monday, July 09, 2012 2:03 AM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] FS--N4YG DDS VFO Conversion Kit for Corsair II and
Others--Brand New & Unused
At 07:43 PM 7/8/2012, you wrote:
This past April I finished converting the PTO in my Corsair II to a DDS
VFO from the design of Steve-G3TXQ with great success. (I discussed my
project in a May 24th posting on this reflector.)
Have you made TX spectral purity measurements before and after?
Perry
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