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Intermittent noise on HP 8568B
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While I was working on my power sensor thing, I wanted to check the spectrum of some signal generator by using my 8568B spectrum analyzer.
About one year ago, I had the problem that the analyzer had some intermittent noise, i.e. it was mostly working fine, but for short periods, its noise rose quite a bit. I was not able to determine the source of the problem, but after a while, it somehow fixed itself and didn't appear anymore. UNTIL NOW! ARGH! I need to fix this. I made a video (it is quite loud, so lower the volume a bit! :-)): The signal shown on that screen is an ultra low phase noise OCXO which is powered up since a week or so, so it has definitely settled in frequency. One can clearly see that most of the time, the signal is displayed nicely, but for a short time, the noise rises quite a bit and there are some artifacts visible. I have already checked the power supplies, and they seem to be fine and within spec. The spectrum analyzer is also powered since about two weeks or so, so its internal OCXO is definitely also settled. What else could cause this type of problem? has anyone ever seen something like that? I was also thinking whether it is some sort of ground loop or so, so I removed my OCXO signal and connected the calibrator signal. Same behaviour, so I assume it's something generated within the analyzer. Any ideas? Best Tobias HB9FSX |
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I had something very similar on my 8568B.? It turned out that one of the coax connectors on the A23 assembly was loose.? The noise would come and go as I moved the coax cables inside the RF unit.? The connector in question is completely invisible with the bottom cover off.? I think you can get to this connector by following the A23 removal instructions until you can see it.? It was difficult to find this problem as the cable in question meanders quite a way around the instrument and moving other cables tended to move it too. On Mon, Nov 19, 2018 at 10:53 AM Tobias Pluess <tobias.pluess@...> wrote: While I was working on my power sensor thing, I wanted to check the spectrum of some signal generator by using my 8568B spectrum analyzer. |
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On Mon, Nov 19, 2018 at 1:53 PM Tobias Pluess <tobias.pluess@...> wrote: I made a video (it is quite loud, so lower the volume a bit! :-)): I had a bad diode in the log amp in IF section of my HP 8566B that manifested in a similar manner. See </g/HP-Agilent-Keysight-equipment/message/73067> for the thread. I have a couple of YouTube videos of the behavior mine exhibited. Here's one <>. Key points that would have helped me isolate it at the time are: 1. Occurs in zero span at low levels for all frequencies - so not frequency dependent. 2. Does not occur in linear mode. Good luck, Siggi |
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Hello Tobias,
Monday, November 19, 2018 I had something remarkably similar (I can link to a video if you wish) and after i gave up home diagnosis a professional UK firm found the YIG faulty. They replaced it at some cost and it's been perfect since then. Best regards, Chris mailto:chris@... TP> While I was working on my power sensor thing, I wanted to check TP> the spectrum of some signal generator by using my 8568B spectrum analyzer. TP> About one year ago, I had the problem that the analyzer had some TP> intermittent noise, i.e. it was mostly working fine, but for short TP> periods, its noise rose quite a bit. I was not able to determine TP> the source of the problem, but after a while, it somehow fixed TP> itself and didn't appear anymore. TP> UNTIL NOW! ARGH! I need to fix this. TP> I made a video (it is quite loud, so lower the volume a bit! :-)): TP> TP> The signal shown on that screen is an ultra low phase noise OCXO TP> which is powered up since a week or so, so it has definitely TP> settled in frequency. One can clearly see that most of the time, TP> the signal is displayed nicely, but for a short time, the noise TP> rises quite a bit and there are some artifacts visible. TP> I have already checked the power supplies, and they seem to be TP> fine and within spec. The spectrum analyzer is also powered since TP> about two weeks or so, so its internal OCXO is definitely also settled. TP> What else could cause this type of problem? has anyone ever seen TP> something like that? I was also thinking whether it is some sort TP> of ground loop or so, so I removed my OCXO signal and connected TP> the calibrator signal. Same behaviour, so I assume it's something generated within the analyzer. TP> Any ideas? TP> Best TP> Tobias HB9FSX |
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Try setting the SA sweep to trigger from line frequency. If the noise pattern becomes stationary, then you've got line ripple problems, either on a supply, or via ground loops. Did you check the supplies for ripple too, or just the DC? If not, I'd recommend scoping them out - this includes the supplies in both the RF and IF/display unit. Likewise, set the scope for line trigger, so you'll be able to easily spot anything bad.
Ed |
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