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Old 12th Jul 2015, 12:14 pm   #161
Chris Wilson
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Default Re: HP8568B calibrator noise / spurious signals

Hi again Jeremy. I do still have the EIP counter, not sure if its sensitivity is a bit suspect, but it worked for aligning the A22 board which required reading frequencies of around 3.7GHz, and it worked OK. I can give it a go. I can't recall where the LO2 test point is, but I'll unbutton it again and have a poke about, does cable length matter much? I have my counter in a 19 inch rack mount, and it's about 6 feet from where I am able to work on the 8568B?

The only other "tool" I may have of use is a dongle SDR that goes quite high, and SDR software that gives a spectrum display (HDSDR at www.hdsdr.de ) 24 TO 1766 Mhz native frequency span. Not sure if it could be pressed into any kind of useful service here?

As an aside, will the fact that the noise is present both with SHIFT UP ARROW pressed and SHIFT DOWN ARROW pressed, which i think shifts the second LO high and low show you anything of significance?

I am attempting to see if the scope issue is easily addressed, i am told it may just be a fairly simple adjustment. if not the Philips can be used, or I might just bite the bullet and get a Rigol scope and have something modern that shouldn't need fiddling with

Thanks for your understanding re the scope shots, I had never considered i was posting garbage
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Old 12th Jul 2015, 1:25 pm   #162
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Default Re: HP8568B calibrator noise / spurious signals

What did I *really* mutter?

Nothing special, none f those words saved up for when nothing else will really do. I just thought "Ah, that explains it"

Back at the beginning, when you were looking at the ramp generator, you had the appearance of spikes coming out of it. This looked almost impossible. The ramp voltage was spiking off a long way AND THEN COMING BACK. To change a capacitor voltage quickly takes a high current, but it can be done. However, this one seemed to come back just as quickly right to where it should have been which seems like a coincidence. What remembered where it should have been?

I wondered if the subsequent amplifier was adding spikes to a correct capacitor voltage.

I was a bit disappointed that you went off looking elsewhere when you'd already seen something that shouldn't be possible. There was something were were all missing and it needed hunting down. I never suspected it was the scope.

Anyway you're now down to a YTO unlock fault. The popular cause of these is dried out wet slug tantalum capacitors in the coil drive circuitry. Go round them with an ESR meter and see what they're like.

David
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Old 12th Jul 2015, 2:13 pm   #163
Chris Wilson
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Default Re: HP8568B calibrator noise / spurious signals

Hi David, why do you say I am now down to a YTO unlock fault, sorry, I am not sure what shows this to be the cause? Wouldn't the YTO UNLOCK text be shown if that was happening when the noise occurred?

Bearing in mind the driver board seems totally different to the one you photographed in another thread, as mine is an 8568B and seems different in this regard, I did lift a leg on the tantalums and they all measured zero ohms ESR.

Thanks.
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Last edited by Chris Wilson; 12th Jul 2015 at 2:27 pm.
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Old 12th Jul 2015, 5:03 pm   #164
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Default Re: HP8568B calibrator noise / spurious signals

Maybe I got confused, I thought you were getting occasional unlock messages, and I've got an 8566 I keep doing a bit of work on, so context switching to 8568 may be throwing me as well. It has a completely different LO system.

David
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Old 13th Jul 2015, 10:26 am   #165
Chris Wilson
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Default Re: HP8568B calibrator noise / spurious signals

No worries David, I have caused a lot of confusion here myself, apologies.

More observations (I am unable to find bench space to open it up safely right now, as I am building car stuff on the bench I sell commercially).

If I let the machine self calibrate, which it always completes without drama, quite often if I do a Recall 9 some time later the horizontal line is at the bottom of the display or needs significant re-tweaking to bring it to the maximum level again.

A Recall 8 at any time shows the -10 dBm peak wavering a bit, both vertically and horizontally.

If I connect my EIP counter to the BNC LO out socket on the back of the SA, or the IF Out on the back of the display section, the frequency is far from rock solid when I feed the SA with its own cal signal of 20MHz, with either a below or just above 1MHz span. Should it remain solidly on a fixed frequency?

Thanks!
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