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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