Spectrum Analyzer Newbie - What's Going on Here? - Page 1
Forum section: electronics / testgear

author: edpalmer42
author_rank: Super Contributor
timestamp: August 04, 2013, 01:59:56 am
content: I recently picked up my first Spectrum Analyzer. It's an Avcom PSA-65C. Nothing fancy - 100KHz to 1.25 GHz, Dual conversion Superhet design with a digital overlay to give you some frequency info. I've attached a block drawing of the system. I have the owner's manual, but the service manual isn't available.Everything looked okay, but then I happened to sweep a signal generator through 750 MHz and found that at that frequency, strange things happen. If the input signal level is within about 20 db of the reference line, whatever level the reference line really is, the baseline goes crazy as shown in the second picture. Move 2 MHz in either direction and the problem is gone. The problem also occurs at 500 and 250 MHz, but not as strongly. The horizontal scale in the picture is DC - 1 GHz, reference line is 0 dbm, 10 db per division. Here are the things I've done to try and figure out why this is happening:1. Checked the levels on my signal generator. The generator agrees with my RF level meter within 1 db from 100 MHz to 1 GHz and from +10 dbm to -70 dbm so I'm not overloading the input of the spectrum analyzer.2. Checked the analyzer's frequency and level response. At 100 MHz, level matches the generator from +10 dbm down to -80 dbm. At 1 GHz the response is low by a few db. Doesn't seem like a damaged mixer on the front end.3. Added a 10db pad to the input to isolate the generator from the analyzer. No change except for needing 10 db more signal to show the problem.4. Added a 1 GHz low pass filter to the input to block any 2nd harmonic from the signal generator from entering the spectrum analyzer. Measured attenuation of > 30 db at 1.5 GHz. No change.5. Tried another signal generator. Same situation.Are there any other tests I can or should do? I suspect that the explanation is related to the 1.5 GHz bandpass filter as compared to the 750 MHz input frequency, but I expected the lowpass filter to fix it. If the frontend mixer was damaged, I wouldn't expect the 100 MHz and 1 GHz levels to be as close as they are.Am I dealing with a design deficiency, component failure, calibration issue, or "that's just the way it is"?Thanks,Ed

author: kg4arn
author_rank: Unknown
timestamp: August 04, 2013, 03:29:01 am
content: I had the same issue with my homebrew SA when there was signal energy at the IF.All the gain is at the IF(s) so if there is insufficient shielding you will see the "bumpy" baseline.To test, connect a piece of coax to the signal gen and dangle a few inches of wire from the end of the center conductor. Tune to 750 Mhz. Wave it near the SA and see if you get the baseline articact. My guess is that your first IF is at 1.5 GHz and insufficient shielding is letting the second harmonic into the IF stage.

author: kg4arn
author_rank: Unknown
timestamp: August 04, 2013, 03:38:42 am
content: Oh, the LPF won't be enough. The analyzer will be very sensitive to 1.5 GHz. That's why many superhets place the first IF > 2x the highest input frequency. It helps avoid this issue and image responses.

author: KJDS
author_rank: Super Contributor
timestamp: August 04, 2013, 06:36:32 am
content: The effect you are seeing is an unwanted signal at 1500MHz coming out of the first mixer. It could be poor LPF design, though there's not a LPF shown on the block diagram I suspect it's really there, filters are cheap and easy to design. More likely is a mixing product issue. Mixers, especially high performance ones are very expensive.The mixer not only creates the sum and difference of the VCO and input signal, but also harmonics of both. So if your input signal is at 250, 500 or 750MHz then harmonics will arrive at the first IF of 1500MHz. In a high end spectrum analyzer, the RF signal at the first mixer is too low to cause this issue.

author: G0HZU
author_rank: Super Contributor
timestamp: August 04, 2013, 10:12:54 am
content: It looks like the classic half IF response in a spectrum analyser but at a very high (poor) level. If you inject a big signal at the half IF frequency (half IF = 750MHz) the mixer will generate a second harmonic of this and it will therefore be at 1500MHz and will jam the receiver slightly across the whole sweep just like your image because the harmonic will constantly appear at the IF port of the mixer and will jam the analyser slightly across the whole sweep although the jamming effect will tend to vary across the sweep for various reasons.However, your analyser shows a poor performance here because most analysers are designed to keep the half IF response very low. On a typical lab grade analyser you can kind of predict the half IF response if you know the typical 2HI spec for the mixer (2HI = 2nd Harmonic Intercept Point). A good analyser would achieve a mixer 2HI figure of about +50 to +60dBm but this will vary with frequency and LO frequency etc.Eg for a typical lab grade analyser with a 2HI of +50dBm you can crudely predict the effect as below if we assume it has the same block diagram as yours.If you set the input attenuator to 10dB and inject a clean 0dBm at half the IF frequency (in your case 750MHz) the signal arriving at the mixer is -10dBm.You can then crudely predict the 1500MHz harmonic level from the mixers typical +50dBm 2HI figure. eg the -dBc response of the 1500MHz term will be (2HI - mixer level) so I would expect to see 50 - (-10) = -60dBc.So I'd expect to see a response a bit like yours but with the 'strange' lifting of the noise to only appear at a level 60dB below the level of the 750MHz term on the display.Your analyser appears to be quite prone to this. What do you have the input attenuator set to? Because this is a second order effect the 'strange' response you see in your plot should appear less if you drop the drive level to the mixer by increasing the input attenuator by 10dB. eg if you drop the mixer drive level 10dB with the internal attenuator then the dBc figure should improve by about 10dB.

author: edpalmer42
author_rank: Super Contributor
timestamp: August 04, 2013, 01:26:45 pm
content: Quote from: kg4arn on August 04, 2013, 03:38:42 amI was expecting that the external filter would work with the internal filter that I hope is in there somewhere and solve the problem by knocking the level down some more.Quote from: kg4arn on August 04, 2013, 03:29:01 amI'll try that later today, but I don't think I'll see anything. The RF section is all machined aluminum boxes with SMA hardline connections between them. Come to think of it, I'll have to try and check all of those connections to see if any of them are loose. That would be a weird cause for the problem, but it could happen.Thanks,Ed

author: edpalmer42
author_rank: Super Contributor
timestamp: August 04, 2013, 01:37:03 pm
content: Quote from: KJDS on August 04, 2013, 06:36:32 amI wondered about the lack of a LPF on the block diagram. I can't tell if there's one or not - it could be buried inside one of the RF boxes. It's really hard to get at anything in there. It's like a 3D jigsaw puzzle. But I just did a test that I think proves that the problem isn't coming from outside. I moved my input signal to 500 MHz and added a 550MHz LPF in series with the 1 GHz LPF that I had used in my earlier tests. That gave me over 100 db of attenuation at 1.5 GHz. There was no change in the baseline disturbance with or without the filters. That tells me that the problem is being generated internally rather than some harmonic coming in with the signal.I know the basics of mixer operation, but I think I'm going to spend some time with an external mixer and this spectrum analyzer to explore them further. A while ago I breadboarded a mixer out of discrete components. It worked surprisingly well considering how crude my construction was. I'm going to put it together again and see what happens to the output signals when one or more of the diodes is degraded by adding resistors in series or parallel. Maybe the front end of this analyzerisdamaged.Thanks,Ed

author: edpalmer42
author_rank: Super Contributor
timestamp: August 04, 2013, 01:53:30 pm
content: Quote from: G0HZU on August 04, 2013, 10:12:54 amI can't really tell you what the input attenuator is set to. This analyzer only has a control to set the reference level. It changes both the input attenuator and the IF amp gain. Notice on the block drawing that a line connects the Reference Level control to both the input RF attenuator and an attenuator in the IF path. However, if I add an external 10 db pad, the highest parts of the baseline drop by about 20 db for an improvement of about 10 dbc. This is only 5 db above the normal baseline so the problem is almost gone.EditI just figured out how the Reference Level Control works. I realized that I might be able to see some VCO signal coming out of the analyzer so I put an RF level meter on the input. Here are the levels that I measured and my interpretation of how the RF Attenuator and IF Gain works.Ref Level VCO level RF Attenuator IF Gain+20 dBm -53 dBm 30 dB -10 db0 -43 dBm 20 0-20 -23 dBm 0 0-40 -23 dBm 0 +20I showed the IF Gain as +20 dB for the -40 dBm Reference Level because when you select this level, the noise floor rises by ~15dB which suggests that amplification is being added.FYI, my RF Level Meter is a Boonton 4210 e/w a 4200-4E sensor (+10 to -70 dBm, 200 KHz to 18 GHz).The bad news is that when I look at VCO leakage vs. frequency, for frequencies from 1.5 to 2.7 GHz, the level wanders around and finally ends up within a dB of where it started. There doesn't seem to be a low pass filter on the input.Thanks,Ed

author: KJDS
author_rank: Super Contributor
timestamp: August 04, 2013, 02:05:48 pm
content: It's possible that a diode has blown in the mixer. The drive level into the mixer could also be a little low and cause the same problem.I'd drop and email to the manufacturers asking if it is supposed to behave like that.