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 <[email protected]>
<[email protected]>
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.
[email protected]
www.biwa.cc
K6OQK
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