[time-nuts] Problem with HP 83623A 20 GHz sweep generator stepping up/down 100 Hz when not wanted.
brooke at pacific.net
Wed Dec 21 20:35:13 EST 2016
Do you have a comb generator?
The lesser of evils is still evil.
-------- Original Message --------
> I run a test over the weekend with a fellow radio ham. I transmitted 100 mW
> or so at 10368.115 MHz from an HP 83623A 10 MHz to 20 GHz sweep generator
> into a small horn antenna inside my lab.
> He was able to receive me about 7 km away, although the signal was quite
> week - it was 20 dB above the noise in a 2.9 Hz bandwidth.
> What is odd, is that his transceiver + transverter combination indicates
> the signal generator is shifting frequency up/down 100 Hz. This is not slow
> drift, but a step change - see waterfall picture, where time is on the
> vertical axis, and frequency is on the horizontal. Unfortunately I don't
> know what the scale is on the vertical axis - I am trying to find out. The
> frequency on the x-axis is not the true frequency, but that shown on an 2 m
> amateur transceiver, so the true frequency is more than 10 GHz higher.
> The step size on this HP sweeper is 1 kHz, so the 100 Hz up/down shift is
> not due to a rotary encoder that might be just on the limit of two
> frequencies. Both the internal oscillator and a GPS locked frequency
> standard were used during this test. Going from internal to external
> reference caused a 450 Hz step in frequency, but did not change this
> up/down 100 Hz behavior. So the problem is certainly not the crystal in a
> 10 MHz reference oscillator, as two have been tried, one of which was
> locked to GPS.
> Does anyone have an idea what may cause this behavior?
> Does anyone have any ideas on the best way to investigate this, given the
> signal is at more than 10 GHz?
> What I do *not* have is any other signal generator capable of operation at
> 10 GHz. The only other sig gen I have is a 30 MHz Stanford Research DS345
> function generator.
> I don't have a TI counter at the minute, but had an offer of $300 accepted
> on eBay for a 5370B a couple of days back, so should have a TI counter
> soon. (Yes, I have had an 5370B and SR620 in the past, but for various
> reasons no longer have them). The 5370B at $300 was a lot cheaper than an
> Other equipment I have include
> * 22 GHz spectrum analyzer
> * 30 MHz signal generator
> * 20 GHz VNA
> but no other signal generator capable of anywhere near 10 GHz.
> I do have a couple of double balanced mixers which have RF and IF inputs
> that will take 10 GHz, and an IF output that will go from DC to 4 GHz.
> Introducing a REALLY long delay might allow the steps to be seen, as the
> frequency at the LO and RF inputs of the mixers will be different. But
> that's not really practical, as I'd need an awfully long bit of coax.
> The 20 GHz vector network analyzer, which could be pressed into service as
> a poor (rich) mans microwave signal source, but I suspect the output of
> that is quite dirty, as the output is generated from a step recovery diode.
> I have not yet tested it on a spectrum analyzer, but the SA has quite a few
> spurious signals, so I'm never exactly confident of the SA. But one thing
> to possibly is
> * Set sweep generator to 10.368 GHz
> * Set sweep generator to 10.378 GHz, so there's a 10 MHz difference.
> * Mix the VNA + sweeper down to 10 MHz using a double balanced mixer
> * Compare the 10 MHz at the output of a mixer to that of a 10 MHz crystal.
> Steps of 100 Hz should then be seen I guess.
> Any better suggestions?
> Someone had kindly given me a key to change the step size of the signal
> generator from 1 kHz to 1 Hz. I've not applied that yet, as it is quite a
> complex procedure. But the fact the step size of this is 1 kHz, but it is
> shifting up/down 100 Hz, does not make sense. Especially given the unit is
> capable of 1 Hz resolution, but HP decided to charge extra for 1 Hz steps.
> Luckily this is just a software option.
> I did wonder if the signal generator was incapable of output the exact
> frequency needed, so it was stepped up/down periodically so it gave the
> right number of cycles over a long duration. But again, the fact it can
> step 1 Hz with just a software upgrade suggest that's not the case.
> The radio ham that noticed this step change in frequency is well used to
> listening on 10 GHz, and hearing beacons. So I think its reasonable to
> assume that the problem is not his end, but my end.
> Any thoughts, which do NOT include purchasing a second expensive 20 GHz
> signal generator. They are around $10,000 each, so a bit out of my price
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