[Collins] DDS-2A?

[Gerald]

On Thu, 2005-05-19 at 06:04 -0500, cory wrote:
> The topic of DDS generators for VFO's came up on the QRP list.
> >From there I looked up the AOR DDS-2A, which is a high-end
> implementation. 
> 
> Has anyone built a lower-end solution, just to replace
> the crystal bank?   
> 
> I see the crystal packs on eBay from time to time and am still
> thinking about Glen's internal switch arrangement.  
> 
> de ah6gi/4  Collins 75S-1 serial number 55. 

In a dds, it takes the same complexity to clean up spurs whether a few
frequencies are needed or continuous coverage is needed. That especially
applies to the number of bits used in the adder and the D/A. I tried
that once going to the extreme of using the most significant bit out of
the counter as my synthesized signal. It had spurs all over the desired
spectrum. Maybe if the frequencies needed are round numbers, you can
simplify the controls but not the synthesis. Thing is these days, you
can buy a single chip to do all the synthesis and so save on real estate
on the PC board and probably on project cost.

In my 75S-3B, I mounted a little aluminum plate on the top end of the
tuning rack with a SPDT miniature toggle switch and a crystal socket. I
wired them to the oscillator circuit with miniature coax (RG-174 or
RG-196) removing fixed C to account for the capacitance of the coaxes.
Its been working that way for decades. I can change crystals with only
lifting the lid a couple inches. Somewhere in a box, I have a 30
position rotary switch in a box with 30 ceramic crystal sockets epoxied
together figuring hanging that outside the receiver case. Its not been
wired yet. The single socket has worked well enough.

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer