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

[Richard (Rick) Karlquist]

On 6/24/2018 10:21 AM, cdelect at juno.com wrote:
> 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.

The original 5065 used an M/N synthesizer.  It seems to be stated
(as if it is obvious) that a DDS either has (1) better performance
or (2) is easier/cheaper, etc than an M/N.  It is very simple and cheap
to make an M/N loop using synthesizer-on-a-chip IC's from ADI,
TI, etc.  You should be able to clone the M and N numbers which
represent a proven design.

DDS's are much riskier as others have pointed out.  For the
5071A we used a DDS to make an offset frequency.  But there
are some qualifications:

1.  It was only for offset, not in the main multiplier chain.

2.  It locked a VCXO, so only close in spurs of the DDS mattered.

3.  It didn't use COTS DDC chips but was custom built using
74ACXXX logic (SOTA 30 years ago) by Robin Giffard (an
extremely talented engineer) and had a special "blanking
circuit", as Robin called it, that cleaned up the DAC.

Did I miss the reason as to what was wrong with M/N?

Rick N6RK