[time-nuts] HP 5065A A1 replacement with DDS

[cdelect at juno.com]

Hi,

I've repaired a few 5065A A1 synthesizer modules recently and lets just
say that they are not my favorite repair!
I decided to go back to a project I started a while back to try and
replace the A1 module with a DDS implementation.
I built up two different styles to evaluate. 

One I call the DDS-FE uses a synthesizer board removed from a dead
FE5650A 
Rubidium module. It applies the 5Mhz input to a 74HC14 schmitt trigger
which
drives a 570A multiplier giving a 50Mhz output. This is applies to the 
synthesizer board (AD9830A) which is configured for the 5.31XXXXMhz
output. This signal is 
sent through a one transistor buffer amp with a tuned transformer output.
Frequency is adjusted via RS-232 (pretty much a one time adjustment) and
after
saving is nonvolatile.
 
The second I call the DDS-BJ. It is a small board that Bert and Juerg
designed
that has schmitt trigger gate and 570 multiplier as well as an 8 pin PIC.
It multiplies the 5Mhz to 100Mhz. The 100Mhz is input to one of the cheap
Chinese
DDS (AD9850) that are available pretty much everywhere. The DDS is
configured
for a 21.24XXXX Mhz square wave output and divided by 4 on the board to 
get the 5.31XXXX Mhz output. This is applied via series resonant LC
to the input to the buffer amp. Frequency is adjusted by an up and a down

pushbutton and is nonvolatile. It is cleverly designed to allow the DDS
board to
plug right into the PIC board.

First I installed one into a 5065A that had the super mod installed and
the 
performance stayed the same. So that proved  DDS was not degrading the
performance.

Then I tried both styles in a standard 5065A that was performing well.
and then plotted the performance. I was a bit surprised that the DDS
units 
gave better performance than the original A1!

Either style fits easily inside the original module once the original
circuitry is removed.
Also you could install onto an L shaped aluminum bracket made to fit in
the A1 position.

Attached is a combined plot showing the performance of each and also PIX 
of the two DDS styles.

Cheers,

Corby
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