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

[Bob kb8tq]

Hi

One of the things I’ve run into doing Rb’s this way is that the spurs out of the DDS
are not always the same device to device. They also change a lot with small tuning
changes. The result can be a very close in spur ( like << 1 Hz) that really rips up 
your ADEV since it passes through all the cleanup PLL’s …. It’s a rare occurrence, 
but it does actually happen.

Bob

> On Jun 24, 2018, at 1:21 PM, <cdelect at juno.com> <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. 
> 
> 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<ORIGINALvsDDSA1.pngi.jpg><ddsbjpix.jpg><feddsrs.jpg>_______________________________________________
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