How to test a Spectrum Analyzer? Rohde & Schwarz - Page 1
Forum section: electronics / rf-microwave

author: Dan44
author_rank: Newbie
timestamp: December 21, 2020, 12:18:33 pm
content: I know very little about RF equipment. I have a Rohde & Schwarz FSH3. I would like to know how to actually test this and similar RF equipment. It has the tracking generator, preamp, and a calibration T. Is there a cheap RF source or something I can use to test it? I'd also wonder how that works since I believe it's like a multimeter where it's auto ranging and a certain range can be broken (eg 2.7ghz-3ghz). I put an antenna on it and saw traffic around 2.4ghz (Wi-Fi). Any pointers would be appreciated?

author: Grandchuck
author_rank: Super Contributor
timestamp: December 21, 2020, 12:31:27 pm
content: Another place to look is the 88 to 108 MHz FM broadcast band. The signals there are often strong enough for a good display.

author: CJay
author_rank: Super Contributor
timestamp: December 21, 2020, 12:43:43 pm
content: First, be careful with it, it's a very nice piece of test equipment and is likely to be worth a good chunk of cash.To fully test it and get the best out of it you need some expensive kit but you can buy cheap signal generators based around PLL chips like the ADF4351, they're readily available but really aren't good enough to do much more than very basic function tests as their performance is nowhere near as good as the FSH6.Something like this (you can get much cheaper ones)https://www.ebay.co.uk/itm/NEW-23-5-6000M-RF-Signal-Generator-Frequency-Sweep-Touch-Screen-PC-Control-/383756982815They're not really 'auto-ranging' in the way a multimeter is, I think maybe you mean the way they adjust signal path as they 'sweep' across a range of frequencies?To do that it will switch in different multipliers, between oscillators, mixers etc. to get the necessary span and performance so for 10MHz to 6GHz you might have a few PLL circuits, several mixers and multipliers 'in line', on top of that each 'band' will have a range of calibration parameters to try and make the measurements as consistent as possible, I.E. 0dBm energy input will always measure 0dBm at every frequency.Handy guide here:https://www.electronics-notes.com/articles/test-methods/spectrum-analyzer/how-to-use-spectrum-analyzer-operation.phpWhat other RF gear do you have, it's possible you may have more than enough to get started with?

author: Grandchuck
author_rank: Super Contributor
timestamp: December 21, 2020, 02:51:13 pm
content: Yes, looks good. Decrease the resolution BW and post another screen shot if you care to ... I am curious

author: Grandchuck
author_rank: Super Contributor
timestamp: December 21, 2020, 03:00:45 pm
content: Here is what I see.

author: geggi1
author_rank: Frequent Contributor
timestamp: December 21, 2020, 09:39:26 pm
content: Quote from: Dan44 on December 21, 2020, 12:18:33 pmI would start with some on the air testing with an antenna whip and some known signals. Test on some airband, tetra, PMR FM or similar narrow band modulation. If you have a HF or MF time signal available you also have a very accurate test frequency. Another good test source is a 10MHz GPSDO with a square-wave output.Combine this with a comb generator with a design similar to one in the links and you have markers every 10 MHz form 10 and up to maybe the 15 order or higher. Remember to put a DC block on the output of the comb generator to protect your Spectrum analyzer.https://www.rudius.net/oz2m/milli_beacon.htmhttp://lea.hamradio.si/~s57uuu/astro/sidi_debug/harmgen_w_rtx.htmhttps://interferencetechnology.com/harmonic-comb-generators-are-useful-tools/http://www.vk2zay.net/article/41But you have to be careful about the voltage levels so you should frist check the level of the output on an oscilloscope or at least have an 10dB attenuator in between to ensure that you dont damage the front-end.The output of the tracking generator is simplest to test on another spectrum analyzer but you can to some level also check it by using attenuators and start on for instance 0dBm and loop from tracing to input with a quality coax. Buy or borrow a few good attenuators and check the attenuation in steps for instance 3dB, 10dB, 20dB, 30dB, 40dB and even further attenuation if you can get other values. Several anttenuators can be connecter in series to get higher attenuation. The test will depend of the quality and linearity of the attenuators.Do also check what licences that comes with your analyzer, there can be several nice licenses like 1port SWR measurement and FM/AM demodulation.I have used this analyzer once or twice in the past and is a very nice instrument to use. I used it to find RFI in a process control room. The RFI interfered with the portable UHF radios used.

author: radiolistener
author_rank: Super Contributor
timestamp: December 21, 2020, 11:30:58 pm
content: cool device, where did you get it?

author: Bicurico
author_rank: Super Contributor
timestamp: December 22, 2020, 11:05:34 am
content: Dan,Let me give the following advise:1) Your R&S spectrum analyzer is a premium test equipment that is worth a lot of money! Treat it with great care!2) A spectrum analyser can be easily broken! Do not just hook up any cable carrying some random signal! On the RF INPUT port you will see the specification of the maximum power permitted. If you feed more than that, you will break your device! Use attenuators to lower the signal power. If you do not know how much signal power there is on the cable, use a switchable attenuator and start with maximum attenuation. Watch this video:3) If you want to "measure anything" for the fun of it, do use a passive UHF/VHF antenna and take a look at the frequency range from 60MHz up to 1800MHz. You will see radio stations, mobile phone communications, etc. If you use attenuators and know what you are doing, you can take a look at the CATV signal or even SAT signal, using the LNB LOOP THROUGH of a satellite receiver. *** MAKE SURE TO USE A DC BLOCKER FOR CATV AND ESPECIALLY SAT SIGNALS *** The spectrum analyzer MAY NOT BE FED WITH ANY DC VOLTAGE!!! A satellite receiver uses 13V/18V to operate and switch the LNB. Without a DC blocker you will break your spectrum analyzer!4) You can use ADF4350/ADF4351 based devices (NWT4000, D6, LTDZ) as signal generators. You can buy a cheap noise source, too, and use it to calibrate/measure antennas and filters.Take a look at my blog:https://vma-satellite.blogspot.com/I discuss all of this stuff plus I have some videos, too.Regards,Vitor

author: Dan44
author_rank: Newbie
timestamp: December 22, 2020, 12:11:58 pm
content: Thanks for the reply. I watched the video. When a spectrum analyzer is damaged from receiving to high of a signal how does that usually manifest itself? Lets say the mixer is damaged, does it usually read just noise and no signal or will it usually break on a certain path (eg low freq). I'd imagine it varies but still...

author: Bicurico
author_rank: Super Contributor
timestamp: December 22, 2020, 12:14:35 pm
content: In my case, on a cheap SMA4400 the mixer broke (M810 IC, which I then successfully replaced). As a result I got only random noise, but no measurable signal.Regards,Vitor

author: Bicurico
author_rank: Super Contributor
timestamp: December 22, 2020, 01:12:53 pm
content: I am not qualified to comment on the measurement you are doing.But note that no spectrum analyzer will show a flat TG signal over a 6 GHz span.Also, your cable needs to be of high grade.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: December 28, 2020, 11:32:42 pm
content: Dan:If you get reception of various radio signals like wifi and FM radio stations, the thing works. At least kinda.The next thing I would do is get some really good test cables as has been suggested. You will need some to use the unit anyway.Make sure that they are what you need like 6 GHz cables if you are only going to use the lower end of the range. For measuring 6 GHz I would get cables and connectors rated at over 10 GHzAlso get some good attenuators rated for the appropriate GHz range also. You will need these also in the future.You can use the unit to test itself by using tracking generator thru the good attenuator to see if the reading on the screen is attenuated by the correct amount. If you have a cheap lower freq attenuator you can see where it stops working so well....Then you can pick up some passive filters (maybe there are some cheap wifi or other filters available) and check them to see if the freq range is kinda close...At the low MHz frequency end maybe you could put some crystals between the tracking gen and the SA to see how that looks.As has been pointed out there will always be some blips or irregularities in GHz measurements.I imagine you are not going to use this thing for things that do not require exquisite calibration so if the above tests show a fairly understandable flat reading, I suppose I would conclude that the thing works well.Look up coax and see what kind meets your needs. Some people make their own test cables. It can be a lot less expensive.You need good connectors and good coax at GHz frequencies.Crappy cables can show you their limits on the SA however, so if you have some low freq cables you can see at what point they start to fail in transmission.As has been said, do not put any power into the unit (with the exception of the tracking generator) till you have familiarized yourself with its function.A way to safely see the spectrum of transmitters is to put a couple wraps of wire around the outside of the transmission coax to use as a pickup and look at the SA to see if there are spurious frequencies. Or just an antenna.Looks like a really nice piece of equipment!!!