HP141T + HP8555A + HP8552B problem - phantom signal detection - Page 1
Forum section: electronics / repair

author: neklid
author_rank: Newbie
timestamp: March 21, 2020, 05:23:29 pm
content: Hello!I would like to ask you about the HP8555A problem as follows: when applying the calibrator signal (30 MHz, - 30 dBm) of the HP 8552B IF plugin to the HP8555A plugin main input, I can see the signal spectrum not only in corresponding frequency range (0-2 GHz), but also in some upper ranges. Similarly, when using external signal with the frequency around 9 GHz, I can see the signal not only in correponding range (6.2-14 GHz), but also in some lower ranges. When switching through ranges, the signal varies slightly as in amplitude as in screen position. My best guess is that for ranges outside signal spectrum there should be no signal detection... Could you, please, help?Thank You!

author: wr4t
author_rank: Newbie
timestamp: March 26, 2020, 10:35:00 pm
content: The 8555a plug-in uses harmonic mixing, so without a preselector you will see the sub-harmonics of the selected frequency band. This behavior is explained well in the user manual, there is a switch on the 8555a that allows you to identify if you are seeing sub-harmonic or a fundamental, its use is also explained in the manual.Ted WR4T

author: neklid
author_rank: Newbie
timestamp: March 28, 2020, 07:59:15 pm
content: Dear Ted!Many thanks for your response! I have also gone through the application note 136 from Hewlett-Packard which confirmes your words. Anyway, for users familiarized with up to date equipment this HP8555A behaviour is quite confusing. But after exploring wiring diagrams understandable... Thx! Karel

author: KE5FX
author_rank: Super Contributor
timestamp: March 28, 2020, 08:15:05 pm
content: This is basically what the 8445A preselector was for. They're useful to have around the house.

author: neklid
author_rank: Newbie
timestamp: March 31, 2020, 02:35:18 pm
content: Anyway, probably I have some problem with HP8555A. I have tested it for the harmonic signal of 9.4 GHz. It was a klystron source. According the harmonic mixing there were bars at the scale of LO at 2.4 GHz, 2.9 GHz, 3.6 GHz and 3.85 GHz. But what is surprise, I saw also bar at 2.25 GHz - but this one was about 20 dB in amplitude smaller. Is this correct? What a frequency product is it? Many thanks for your opinion!Karel

author: wr4t
author_rank: Newbie
timestamp: April 03, 2020, 04:35:31 pm
content: On what bands were you seeing the 2.25 GHz spur, you also have to consider mixing products for the harmonics of the fundamental as well as the fundamental and possibly IF images (I can't recall the image rejection for the analyzer's harmonic bands). For example is the 2.25 GHz spur 2x the IF from the 2.4 GHz signal, with a 70 MHz IF that might be what you are seeing? Calculating all these spurious responses can be tedious.Rather than going through all the math, to test the analyzer I would use the fundamental band with a LPF (1.5 GHz). If that band shows no spurious responses, then the 8555A is likely okay, and I would look at the test setup. Also do you have the required 50 Ohm terminators on all the LO and IF connectors? These terminators are needed to prevent some spurious responses.Ted WR4T

author: neklid
author_rank: Newbie
timestamp: April 15, 2020, 08:47:13 am
content: Dear Ted!Thank You for Your contribution! It looks like the fundamental band is OK. Some time ago I tested it with 1 GHz signal and I am not conscious of any spurious responses. But I can try again. Anyway, before doing this I have to add the termination on my rear AUX-A connector. I have terminated only front panel connectors what is half job ...Please, stay in touch! Thank You!Karel