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

Tue Mar 22 16:02:52 UTC 2011

I'm watching this thread with great interest.  I have never worked 
with PIC's before and I'm not really knowledgeable with the other 
devices you're discussing.  For me, the 3596B is a tool that I use in 
my over the air broadcast frequency measurement service.  I'm also 
very active in the FMT's run by the ARRL and K5CM.  The FMT's put on 
by Connie, K5CM, as well as the MVUS FMT's have raised us to a much 
higher level of accuracy than those of the ARRL.  After all, we as 
time and frequency nuts are after absurdly accurate results!  As a 
result of all this I needed a way to get around the errors that 
resulted when using the 3586B's 1850 Hz tone that relied on the 
accuracy  and stability issues associated with the Product Detector's 
L.O.  That's when I came up with the idea of using the 15625 Hz I.F. 
rather than the audio output.  This totally removed the issues 
associated with the Product Detectors L.O. and it's source - the two 
crystals.  What I have found is that now my measuring accuracy is 
limited only by my external reference's accuracy and more so by propagation.

I would really like to see a method for getting around the Product 
Detector's crystal issues, but with the same accuracy and stability 
I'm achieving by measuring the I.F. directly.  Spectrum Lab does work 
on my laptop at 15625 Hz and I have been successful in calibrating my 
laptop to give me short term results at sub-millihertz accuracy.  My 
one complaint is that at 15625 Hz, the (Spectrum Lab) flywheel takes 
a long time to get up to speed.  Once it's up to speed, I can resolve 
reasonably fast changes that are under one-tenth Hz.  At least I've 
convinced myself, right or wrong, that this is true based on tests 
I've run driving the system with my GPS referenced HP-3336A.

Within about 5 miles of my home I have two AM broadcast stations I 
work with that are GPS referenced.  In spite of being so close to 
these stations it's amazing to watch the phase rock slightly due to 
propagation.  Marv Collins, W6OQI who is one of the FMT transmit 
stations for both ARRL and the K5CM group is 8 miles airline from 
me.  Since Marv and I are both GPS referenced, it's fascinating to 
watch the phase changes as he tunes his Johnson Viking II driver transmitter.

Burt, K6OQK

>From: WB6BNQ <wb6bnq at cox.net>
>
>     <time-nuts at febo.com>
>Subject: Re: [time-nuts] HP 3586A/B/C entirely referenced to 10MHz:
>
>
>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
>
>
>
>

Burt I. Weiner Associates
Broadcast Technical Services
Glendale, California  U.S.A.
biwa at att.net
www.biwa.cc
K6OQK 