10MHz signal from internal in spectrum analyzer, is this normal ? - Page 1
Forum section: electronics / rf-microwave

author: BravoV
author_rank: Super Contributor
timestamp: May 09, 2019, 02:21:01 am
content: My SA is equivalent to Aeroflex IFR-2399B 9kHz-3GHz, while trying to check the noise floor with input shorted with a 50 Ohm terminator, there is little bump at 10MHz, although its near to noise floor, is this normal ? Wonder where it came from ? Leaked from it's own internal 10MHz reference ?

author: radiolistener
author_rank: Super Contributor
timestamp: May 09, 2019, 08:17:42 am
content: -110.6 dBm is very weak signal. It is close to noise level of environment on such frequency (about -130 dBm). It may leak from some electronic equipment near spectrum analyzer (for example notebook, cell phone or something like that). TFT display in the spectrum analyzer also may be source of such interference.

author: worsthorse
author_rank: Super Contributor
timestamp: May 09, 2019, 01:45:45 pm
content: Quick test. Carry the SA to another room and see if the signal is still there. Second test, put a shorting cap on the input connector.

author: Ice-Tea
author_rank: Super Contributor
timestamp: May 09, 2019, 01:48:54 pm
content: Quote from: radiolistener on May 09, 2019, 08:17:42 amIt would be an incredible coincidence that any of those would radiate at *exactly* 10MHz. One way or another, that's the reference of some piece of test equipment.

author: DaJMasta
author_rank: Super Contributor
timestamp: May 09, 2019, 03:30:17 pm
content: Yes and no? On your lowest sensitivity ranges it is fairly likely to find tones like this, especially around key frequencies to the band planning of the downconversion of the instrument, but -110dB, while low, is higher than I'd want in such an instrument, so I'd probably call that one an error. Since it's exactly 10MHz, it's probably some leakage from the 10MHz reference. You're on the internal timebase, so you'd expect it to be at 10MHz, but I suspect that if you hooked up an external reference it would appear in the same place, since it's the tone being used to reference the frequency of everything displayed.If you have lower RBW settings, I'd try it and see how it looks, but I think either the filters removing the 10MHz component have deteriorated or there's some shielding that's out of place or something allowing it to bleed through. You could probably buy an inline high pass filter that would remove 10MHz tones and stick it on the LOs leaving the synthesizer board (provided it is a separate board with some RF plumbing connecting it) if you really wanted to troubleshoot.

author: worsthorse
author_rank: Super Contributor
timestamp: May 09, 2019, 04:04:37 pm
content: Quote from: DaJMasta on May 09, 2019, 03:30:17 pmYup. If the signal is still there with other test gear turned off or if the SA is tested in a different location, or there is a shorting cap on the input connector, then the signal you are seeing is probably internal.And -110dBm is indeed high for an instrument like this and certainly within the working range for everyday RF signal measurement, so you want to do something about it.When you look at a wider span, are you seeing harmonics? What do you see when you lower the RBW?edit: I took a quick look at the manual and the IF is 10.7MHz and there's a 10MHz reference, too. There are BNC connectors for both on the back. Is there a way to insure that both are turned off at these connectors? If so, does that make a difference?

author: radiolistener
author_rank: Super Contributor
timestamp: May 09, 2019, 05:58:41 pm
content: Quote from: Ice-Tea on May 09, 2019, 01:48:54 pmWhy? Exactly 10 MHz may be reference for PLL which produces any required frequency.

author: Ice-Tea
author_rank: Super Contributor
timestamp: May 09, 2019, 06:04:49 pm
content: Quote from: radiolistener on May 09, 2019, 05:58:41 pmCould a PLL have a 10MHz reference? Sure. Do they exist? Probably. Have I seen one? No. Have I seen 10MHz T&M equipment? Roughly all of them.Occam's razor.

author: BravoV
author_rank: Super Contributor
timestamp: May 10, 2019, 08:35:31 am
content: Ok, moved to other room, same result, and shorted the input with thru hole 10nF cap, still the same.Btw, that RBW and VBW are the lowest, can not go any further.Next, I will test with external 10MHz reference when I bring it home, disabling the internal one, as currently this SA is at friend's place.Btw @DaJMasta, open this thing up and tinker & tweaking is out of option, as thats beyond my skills & experience.

author: worsthorse
author_rank: Super Contributor
timestamp: May 10, 2019, 02:53:20 pm
content: Quote from: BravoV on May 10, 2019, 08:35:31 amThat means most likely source is internal. You you have an external 10MHz source, so try switching to that and see if the signal goes away. If it does, you can use the external source as a workaround.One other thing to try. If you have a tracking generator option on this scope, and it has a normalize function: connect the tracking generator to the input and run the normalize function. If the signal goes away, at least you know that you can adjust for it when using the tracking generator.Since opening the box is not an option, you may have to otherwise live with it.

author: BravoV
author_rank: Super Contributor
timestamp: May 10, 2019, 03:01:15 pm
content: This SA has no TG.

author: worsthorse
author_rank: Super Contributor
timestamp: May 10, 2019, 03:04:20 pm
content: Quote from: BravoV on May 10, 2019, 03:01:15 pmOkay... so the external reference is your best option, otherwise you will have to account for the birdie whenever you are using the SA. You may want to feed signals around 10MHz into the SA to see what it looks like... are there mixing products, for example? All of that will give you a way to understand how the signal modifies what you are seeing in operation. Good luck!