6th Jul 2020, 11:09 pm | #121 |
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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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7th Jul 2020, 1:47 pm | #122 |
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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 |
7th Jul 2020, 2:48 pm | #123 |
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Re: Replacement for HP8640B signal generator
First, get a strong bench...
David
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7th Jul 2020, 3:46 pm | #124 |
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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. |
8th Jul 2020, 12:25 pm | #125 |
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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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4th Sep 2020, 5:53 pm | #126 |
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Re: Replacement for HP8640B signal generator
My HP 8568B spectrum analyser and 8444A tracking generator have finally arrived from Cyprus. All seems well.
The fans in the analyser are REALLY loud so I've asked on the HP forum if anyone has successfully reduced the noise by fitting modern computer fans. By successfully I mean making it quieter, not cooking it, and not introducing excess electrical noise. I'll now go advertise my old Marconi TF 2370 in Sets Offered. Cheers, Alan G3XAQ |
4th Sep 2020, 11:39 pm | #127 |
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Re: Replacement for HP8640B signal generator
Wow, that took a while to arrive!
Sadly, the fan noise really is loud on these analysers. One (possible?) option would be to add a fan speed vs temperature controller board because I think this analyser was designed to operate up to +55degC ambient temperature. This is because the analyser was designed for use in ATE racks. I think that is why it sounds like it is about to take off each time it is powered up. I think the existing fans run from the AC mains supply. However, this is one area I've not looked at with this analyser. If it was possible to add a cheap and reliable fan controller then this would transform this analyser. I still have my HP8568B on my bench if you want to do any checks or comparisons.
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5th Sep 2020, 11:52 am | #128 |
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Re: Replacement for HP8640B signal generator
Hello Jeremy. Yes, the 8568B was a long time coming but seems none the worse for its boat ride.
Interesting idea about the fans. I'll look into it once the decorators (ordered by SWMBO) have been and gone. And yes, that will mean moving the hundredweight 8568B twice more. I have end user operating and performance check manuals but no documentation on servicing the internals. I'm thinking of something like the two inch thick 8640B-style of manual with full circuit diagrams. My google-fu is weak today. Does such a document exist? Is it available online? Thanks, Alan |
5th Sep 2020, 12:59 pm | #129 |
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Re: Replacement for HP8640B signal generator
I suppose if they are mains fans then speed control done "properly" needs a variable frequency AC drive, as is fairly common with workshop machine tools. I would have thought those things would have switching waveforms that were really hard to suppress.
Alan |
5th Sep 2020, 1:03 pm | #130 |
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Re: Replacement for HP8640B signal generator
I think part 2 of the manual for the RF section is available on the Keysight website here:
https://literature.cdn.keysight.com/...pdf?id=2208481 The other parts are probably there somewhere too but I couldn't find them easily. If you get desperate I have the other parts here for the RF and display sections but they are 16Mb and 25Mb in size. I think the fans are enabled via a relay when the analyser is taken out of standby so this would be a reasonable place to tap in to provide some sort of (triac?) fan controller. There is also a thermal trip and this switches off all the power supplies but this doesn't happen until a scary 85degC. Obviously, any fan controller module would have to be a safe and reliable design but it would be quite a popular mod for this analyser if it were possible!
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5th Sep 2020, 9:20 pm | #131 | |
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Re: Replacement for HP8640B signal generator
Quote:
David |
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6th Sep 2020, 10:36 pm | #132 |
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Re: Replacement for HP8640B signal generator
I've asked on some other groups about reducing the fan noise on these old HP units. A common response is to fit more modern/quieter fans but I can't find anything near the original 115V AC 95x38mm 75 CFM spec. Most modern fans shift a lot less air. So I think I will stick with the original fans.
The spec for the HP8568B is up to 55 deg C ambient so there should be lots of scope for slowing down the fans. I just took a measurement with a probe at the warmest point on the outlet grilles and it is 32.5 deg C, which is 10 deg C above ambient (ambient is a warm 22.5 because I have my amateur radio amplifier running). So I reckon if I slow the fans to an exhaust temperature below 40 or 45 deg C I will still be well within HP's limits. There are two fans in an HP8568B, one in each of the two cases. The cleanest and least intrusive suggestion I've had is to run the two fans in series and thus at about half speed until/unless the exhaust reaches, say, 40 degrees and then revert back to the standard fan configuration. The fan wiring will need extending (out to the rear aprons?), a thermal switch such as RS 339-291 added in the warmest part of the airflow (or even to the outside of the case), a 115V AC relay to switch between normal and series modes and the job is done. Well, on the face of it at least. Alan |
6th Sep 2020, 10:43 pm | #133 | |
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Re: Replacement for HP8640B signal generator
Quote:
David
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7th Sep 2020, 12:46 am | #134 | |
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Re: Replacement for HP8640B signal generator
Quote:
Of course, if you're really a dedicated aficionado of HP T & M kit, there are always two fans: one built in at the back - and one in front - you. Al. Last edited by Skywave; 7th Sep 2020 at 12:54 am. |
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11th Sep 2020, 9:45 pm | #135 |
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Re: Replacement for HP8640B signal generator
This has gone way off topic, sorry. I keep thinking I should start a new thread but I always have "just one last posting". Anyway...
The 20MHz CALibrator output on my HP8568B spectrum analyser is duff: -43dBm out instead of -10dBm. I fed the SA a -10dBm signal from my signal generator and led myself a merry dance wondering why the SA calibration seemed to be out by several dBm. It was caused by my twiddling of the front panel CAL adjust screw, which doesn't change the calibrator output level but instead adjusts the gain of the whole instrument. So I need to fix the calibrator, which is in module A16 of the RF section. I only have the manual for the 8566 RF and can find no trace of the circuit diagram for A16 therein. So, please Jeremy, can I scrounge the relevant manual PDF from you? Thanks, Alan |
12th Sep 2020, 4:11 am | #136 |
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Re: Replacement for HP8640B signal generator
8566 RF section is completely different to the 8568 RF section. You really need the right manual. The top boxes are the same, though.
David
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12th Sep 2020, 9:53 am | #137 |
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Re: Replacement for HP8640B signal generator
I asked on the HP group and was directed to Artek Manuals. I've ordered a full set of what I hope are the 8568B-specific manuals from there.
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12th Sep 2020, 11:27 am | #138 |
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Re: Replacement for HP8640B signal generator
It was in production for a good while, so even with B specific manuals, you may need to find out about manual update sheets to get up to your serial number.
Update sheets are ****** messy. David edit * adjective pertaining to human circulatory fluid. The forum software doesn't seem to like it. Can I say damnably? that'd be an alternative..
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12th Sep 2020, 12:19 pm | #139 |
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Re: Replacement for HP8640B signal generator
Just noticed the scans from Artek for the 8568A manuals were not all done the same year, Vol 1 & Vol 4 are four years apart, I hope they are from the same set, otherwise it maybe confusing.
How much apart from the processor boards will be different between the A & B models? The 8658A still has an A16 20MHz ref board; David |
12th Sep 2020, 12:52 pm | #140 | |
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Re: Replacement for HP8640B signal generator
Edit:
I just looked at the first page of the scans of the manuals I have here and there is a big red copyright notice put there by the people who scanned it. Therefore, I can't really provide a link to my manuals in case it breaks the forum rules. Quote:
I've never needed to repair my HP8568B and I think it has been fault free since it arrived at the company about 30 years ago. I do recall there was some issue with it one year during calibration at HP. This was enough to cause our MD a lot of stress. He did not like the idea of HP service techs digging about inside it to fix a minor cal issue. I do think it needs a service though. I tested the log accuracy recently and it was right on the calibration limit down at about -70dB.
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Regards, Jeremy G0HZU Last edited by G0HZU_JMR; 12th Sep 2020 at 1:02 pm. |
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