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

paul swed paulswedb at gmail.com
Tue Mar 22 09:07:19 EDT 2011


It would take little to go to another DDS like the ad 9831 $8 at digikey. It
does not have the msb though so Bert its back to bypassing the one resistor
feeding the modulator.
Changing the program for more bits in the tuning word would be quite easy.
But the AD5932 is doing a great job so not sure when anything would actually
change.
Now if someone was going to build a project or kit that might be a different
consideration.
Regards
Paul
WB8TSL

On Tue, Mar 22, 2011 at 7:48 AM, Bert, VE2ZAZ <ve2zaz at yahoo.ca> wrote:

> Hi Bill,
>
> It is a great analysis you did. The truth is I found the AD5932 more or
> less
> randomly while searching for DDS chips. I computed the accuracy with a
> 100KHz
> MCLK and found that I would get better than 0.001 Hz. I knew that I would
> never
> be bang on, so I did not try to pinch more bits. This of course later
> became 0.3
> Hz as I increased to a 10MHz MCLK. But the unit was already on the bench,
> installed on the A22. If I had to do it again, I would consider other DDS
> chips,
> as long as the price is similar.
>
> In the end, since I know what the offset is, I just take it into account in
> my
> frequency calculations. Even with a better DDS, I would have still taken it
> into
> account. Spectrum Lab can go quite far in its averaging calculations.
> Besides,
> my main objective was to stabilize the HP 3586 more than try to be exactly
> on
> frequency. So in that sense, I have met my objectives.
>
> I am glad to see that my SLM mod will be used as a starting basis for
> future
> improvements!
>
> Thanks for your input.
>
> Bert, VE2ZAZ
>
> ------------------------------
>
> Message: 6
> Date: Mon, 21 Mar 2011 18:32:07 -0700
> From: WB6BNQ <wb6bnq at cox.net>
> To: Discussion of precise time and frequency measurement
>    <time-nuts at febo.com>
> Subject: Re: [time-nuts] HP 3586A/B/C entirely referenced to 10MHz:
>    Asolution
> Message-ID: <4D87FC17.892DE535 at cox.net>
> Content-Type: text/plain; charset=iso-8859-1
>
> 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
>
>
>
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