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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 6th Jul 2020, 11:03 am   #121
Alan_G3XAQ
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Default Re: Replacement for HP8640B signal generator

The guy on the HP forum who has successfully modified an 8568B+8444A to use all 3 LOs and keep the TG transparent down to the SA noise floor has gone to some lengths to keep the reverse isolation high. He has a chain of isolators on the LO1 and LO2 signal feeds and a lot of bandpass filtering on LO3.

I was being tempted by all this because I need to replace the broken LSI output stage on the 8884A that is coming as part of the Cyprus package. One route might have been to buy a second TG just for its output stage and then I'd have most of that one left doing nothing. In the event the local chap with a TG for sale wants more than I can thoil so I'm going cold on the idea anyway. You two telling me it it's a bad idea is another nail in the coffin.

It's a shame that I'll be ending up with a system that is in some ways less functional than my old TF2370. I don't have space to keep both. Maybe I need to become more used to using my NanoVNA and the PC screen grabber although cluttering up the bench with a laptop whenever I want to use it feels clunky to me. Of course, if David has one of those HP VNAs he mentioned for sale for a lot less than the prices on eBay...

73, Alan G3XAQ
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Old 6th Jul 2020, 11:09 pm   #122
G0HZU_JMR
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Default Re: Replacement for HP8640B signal generator

OK but the main mechanism for generating internal birdies is when harmonics of each LO can meet somewhere (non-linear) like in one of the mixers in the signal path. If the beat frequency they generate falls in the IF passband then there will be an internal birdie. The order of the LO1 and LO2 harmonics that can cause this issue can be quite high so the system needs to be able to maintain really good isolation between the mixers up to many GHz. Any cabling you add could spoil the isolation. However, with general use you would be unlucky to spot a spurious term as they are likely to be down at the -110dBm level.

The HP8568B has an inbuilt feature to try and dodge this internal spurious issue where it can fudge the LO2 cavity sideways a few MHz. I think it does this with a diode based RF switch. When the shift happens then it forces the PLL to shift LO1 to counter the shift in LO2.

This happens really quickly and it should be transparent to the user. The change in LO1 and LO2 will then cause the spurious term to go off the CRT display.

I have a lot of experience designing RF downconverters that have to achieve low internal spurious so I have been down this path many times in the past. In the early 1990s I wrote various software programs (in Quickbasic!) to try and design a good frequency plan for various downconverters that worked up to many GHz with a digital IF of up to about 20MHz bandwidth. The software then predicted where the spurious terms will be.

I had a go at using this old SW to go hunting for internal spurious in my HP8568B and was able to find one fairly easily by turning off the LO2 shift feature. This prevents the HP8568B from dodging its own internal spurious.

It should be noted that this LO2 sidestep will affect the use of a tracking generator with the HP8568B unless you disable it. Otherwise you will see 'gaps' in the tracking generator response where LO2 does the shift sideways. It could be that LO2 is more stable in one shift position than the other. My guess is that the most stable will be when the diode is biased off.
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Old 7th Jul 2020, 1:47 pm   #123
Alan_G3XAQ
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Default Re: Replacement for HP8640B signal generator

OK, you've convinced me not to pursue the idea of extracting LO2 and LO3. Yes, I believe the operating instructions for using the 8444A with the 8568B tell you to disable the IF shift.

I had a play with my NanoVNA looking at a 455KHz IF tail-end filter idea I had using a ceramic resonator and the "phasing" trick used in HROs and similar receivers to drop a notch into the unwanted sideband to reduce the effects of broadband noise from the IF amplifiers. This needs the VNA to be run in repetitive sweep mode while the peak and null trimmers are adjusted. It works OK with the NanoVNA stand alone but the screen is tiny. When I tried the third party NanoVNA-Saver software on my PC in continuous sweep mode it crashed!

While the NanoVNA is great for things like aerial measurements down the garden I think I'm going to look for a proper benchtop instrument.

73, Alan G3XAQ
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Old 7th Jul 2020, 2:48 pm   #124
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Default Re: Replacement for HP8640B signal generator

First, get a strong bench...


David
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Old 7th Jul 2020, 3:46 pm   #125
Alan_G3XAQ
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Default Re: Replacement for HP8640B signal generator

I just looked... 28Kg... Does everything made by HP have to weigh so much? Did they have a disposal contract with Sellafield?

My shack/workshop has 2" fire doors on 2-drawer filing cabinets for benching. It wont collapse. But I might.

The DG8SAQ (N2PK-type) VNA looks more attractive, despite it still needing a PC for interfacing. It is 400+ and Xmas is a long way off.
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Old 8th Jul 2020, 12:25 pm   #126
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Default Re: Replacement for HP8640B signal generator

2" fire door will probably do, otherwise I was going to suggest taking up welding as a hobby. Nice, relaxing so long as you don't get suckered into doing it underneath things with burning underseal dripping past your ears.

HP didn't use scrap depleted uranium. But if anyone studied their artificial gravity generators, they could be reverse engineered into anti-gravity or be used to make space stations more comfortable and less dizzying.

David
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Old 8th Jul 2020, 1:55 pm   #127
Alan_G3XAQ
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Default Re: Replacement for HP8640B signal generator

[drifting further and further off topic...]

I can splash a bit of weld around, both stick and MIG but not TIG, but I've never been able to see what I'm doing once the peril sensitive sunglasses go dark. I've tried various auto-darkening helmets but I have yet to actually see the weld pool. So for me, welding is a case of getting the electrode near where I want the weld and then hoping for the best once the arc strikes. Maybe my eyes are a bit "special".

Alan
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