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

Bob kb8tq kb8tq at n1k.org
Sun Jun 24 14:22:14 EDT 2018


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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