[time-nuts] HP 3586A/B/C entirely referenced to 10MHz: Asolution

[WB6BNQ]

Bert,

I am curious to know what caused you and Paul to select the AD5932 device ?

Admittedly, I haven't verified the Analog Devices simulator with real components,
but I suspect their simulator is spot on or damn close.  Using the Adsim page I
looked at a few different DDS’s to see what could be done.  With little additional
cost better choices are available allowing better on-frequency accuracy relative to
the offset values of the AD5932.

The problem with AD5932 is the frequency tuning word [FTW] is too small.  So,
clearly, increasing the FTW would give an immediate improvement as to accuracy.  A
simple low pass filter would clean up the spurs as they are all associated with the
clock frequency and well removed from the fundamental signal.  Some DDS selections
included an uncommitted internal comparator stage (notably the 9834 and the 9851)
that would serve well for squaring the signal after filtering.

I ran simulations for two different DDS devices.  I picked ones that operated off of
5 volts of which there is damn few good ones.  The first one is the AD9834 with a 28
bit tuning word with a 10 MHz clock.  Here are the results:

13775 = 13775.0059366226 Hz = error of +0.0059366226
14125 = 14124.9969601631 Hz = error of -0.0030398369
14275 = 14275.0144004822 Hz = error of +0.0144004822
16425 = 1642500.01311302 Hz = error of -0.0088095665
16625 = 1662500.01639128 Hz = error of +0.0020265579
16975 = 1697500.01281500 Hz = error of -0.0069499016
17125 = 1712500.00596046 Hz = error of +0.0104904175
17475 = 1747500.00238419 Hz = error of +0.0015139580

As you can see, with the additional 4 bit tuning word, the error improves for all
except 17125 where it is equal.  The second run was upping the frequency by 100
times to reduce the size of the filter components.  For the AD9834, this did not
turn out well at all.  The wave form had a hard staircase appearance due to the low
clock rate relationship (5:1) to the higher output frequency.  The same problem
exists for the AD9851.  So, I scrapped that whole idea.

The second run was using the AD9851 with a 32 bit tuning word with a 10 MHz clock.
Here are the results:

13775 = 13774.9989517033 Hz = error of -0.0010482967
14125 = 14124.9992884696 Hz = error of -0.0007115304
14275 = 14275.0004306436 Hz = error of +0.0004306436
16425 = 16425.0005036592 Hz = error of +0.0005036592
16625 = 16624.9996982515 Hz = error of -0.0003017485
16975 = 16975.0000350177 Hz = error of +0.0000350177
17125 = 17124.9988488853 Hz = error of -0.0011511147
17475 = 17474.9991856515 Hz = error of -0.0008143485

As you can plainly see, increasing the tuning word by, yet, another 4 bits allowed
for shifting the error further to the right.  Maybe enough to put it beyond the
resolution of the total measurement system and thus, perhaps, removing it from the
systemic error list (i.e., less to worry about in the calculation).

Unfortunately, Analog Devices has stopped producing some of their easier to use 48
bit DDS devices.  The current run of 48 bit DDS’s are way more complicated and
specialized, have issues with the clocking methods (time nut unfriendly), besides
using lower voltages, they are harder for the hobbyist to mount to a board and they
are more expensive.  Truly a sad circumstance for the occasional hobbyist.

Bill....WB6BNQ




"Bert, VE2ZAZ" wrote:

> Hello Everyone,
>
> I have spent the last few weeks modifying the HP 3586 Selective Level Meter. The
> intent was to modify the unit and reference the SSB product detector stage to
> the internal reference. This would make the entire SLM locked to the 10MHz
> reference, whether internal or externally fed.
>
> I have completed the modification with satisfactory results. I use a DDS-Based
> Function Generator 16-pin IC and a small 8-pin PIC micro.  For more detail,
> visit my website at http://ve2zaz.net/HP3586_AD5962/HP3586_AD5962.htm .
>
> This has been a very good exercise from a DDS perspective and I have learned a
> lot on the topic. I would be interested in hearing from those who proceed with
> the mod.
>
> Thanks,
>
> Bert, VE2ZAZ
>
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