Spectrum Analyzer Rigol TG-815 - Page 1
Forum section: electronics / testgear

author: JoeAtl30319
author_rank: Contributor
timestamp: March 03, 2015, 02:39:32 am
content: I have not had any luck figuring this out. I have a Rigol TG-815. When I have a direct connection from the output of the tracking generator to the input of the analyzer, everything works fine and I have a relatively smooth line. When I take this same setup and add attenuation via coaxial attenuators, say 70db, to the point when the signal is close to the noise floor, ripples begin to appear (roughly 10db). The frequency spread of the ripple changes as I change the coaxial connector length. Also, it does not matter where I place the attenuators (e.g. at the output of the tracking generator or input to the spectrum analyzer. Any ideas as to what's causing the ripple?

author: ion54
author_rank: Contributor
timestamp: March 03, 2015, 02:52:33 am
content: Looks to me like a lack of impedance matching. Are you sure your coaxial attenuator is pure resistive?

author: JoeAtl30319
author_rank: Contributor
timestamp: March 03, 2015, 02:56:58 am
content: I thought of an impedance mismatch at first as well. But I'm not sure where it would be coming from. The attenuators are from mini circuits and rated at 50ohms. Also, the ripples I'm describing only occur when the attenuation pushed the signal near the noise floor. Maybe it's a minor mismatch that I'm only seeing when the attenuation is that high. Not sure though if anyone else has encountered this and has a simple explanation?

author: JoeAtl30319
author_rank: Contributor
timestamp: March 04, 2015, 12:23:19 am
content: Maybe an impedance mismatch this is amplified, somehow, as the signal approaches the noise floor?

author: ion54
author_rank: Contributor
timestamp: March 04, 2015, 12:48:14 pm
content: I don't have the capability to test your set-up and the picture is not very good. Could you explain all the steps from full signal to the level you start seeing that amplitude variation? How many attenuators are cascaded. Does the amplitude start oscillating with the first attenuator installed? The noise floor of the Rigol TG815 is about -120 to -130. Where is your signal starting to become affected? If you connect more attenuators are they attached back to back or are you using cables between attenuators? Some more pictures of the set-ups, with better resolution, might help too.

author: G0HZU
author_rank: Super Contributor
timestamp: March 04, 2015, 09:22:10 pm
content: I've not used a Rigol 815 but I'd expect that this effect will be due to poor leakage/bypass performance inside the analyser when the TG is enabled. i.e. the TG signal can bypass your attenuators by another path within the analyser itself.So as you add more and more attenuation the bypass/leakage signal will begin to become significant wrt your wanted (attenuated) signal. So, depending on the relative phase of the two signals where they eventually meet inside the analyser, there will be a summing or cancelling effect that will appear as ripple on the display. The ripple will get worse the more attenuation you add up until the point the two signals are at the same amplitude where they meet. Also, the longer the cable you use the more peaks and troughs you will see as there will be a steeper phase shift wrt frequency in a longer cable resulting in more cases where the relative phase of the two signals sums or cancels.

author: JoeAtl30319
author_rank: Contributor
timestamp: March 06, 2015, 12:02:30 am
content: Thanks! I can see this occurring. When a low level of attenuation is used, the TG primary signal path dominates and you don't see much of a ripple. When the signal is attenuated by, let's say 70db, the primary signal path is weak, and the weak primary is adding with the weak leakage and you see a substantial ripple. Makes sense.

author: Howardlong
author_rank: Super Contributor
timestamp: March 06, 2015, 12:39:27 am
content: I just tried it on mine. This is the result. Attenuation was from four SMA pads and one N pad. I normalised with the same setup minus the pads. I tried to duplicate the OP's setup, but it showed up a little blurred so I'm not sure I have exactly the same setup.

author: JoeAtl30319
author_rank: Contributor
timestamp: March 06, 2015, 12:45:10 am
content: I will say that the ripple I noticed was significantly more prominent in the 500+ MHZ range. For me, it did not matter where the attenuators were located so I think your setup is probably fine. What does it look like from 500Mhz to 1.5Ghz?

author: markone
author_rank: Frequent Contributor
timestamp: March 06, 2015, 12:46:18 am
content: Quote from: JoeAtl30319 on March 06, 2015, 12:02:30 amI also do not have TG-815 but i'm considering to buy it and i have a couple of questions about this situation :1) what happen if you disconnect the attenuator leaving the cable on place during the sweep ?2) what happen if you disconnect everything during the sweep ?If the leakage is inside the instrument (i hope is not the case...) you should see a responce curve quite far from the noise floor (let's say -120dBm ) even at "open circuit" condition.Am i wrong ?

author: Howardlong
author_rank: Super Contributor
timestamp: March 06, 2015, 12:59:03 am
content: Quote from: JoeAtl30319 on March 06, 2015, 12:45:10 amI'll take a look in the morning, it's 1am here and I Just powered down the shop and put everything away. I chose the span 1MHz-500MHz as that looked like what you had, but it was a bit blurry.What cables are you using? Are you sure they're not 75 ohm? Some BNCs are 75 ohm, as indeed are some N types but that's much rarer.

author: JoeAtl30319
author_rank: Contributor
timestamp: March 06, 2015, 01:08:26 am
content: FYI, here are some better pictures. Everything in my system is 50 ohm. Top line in each photo is the TG signal with a direct connection from output to analyzer input (saved line). The bottom line is the noise floor at a band width of 300 Khz over the full scale of the instrument (again, a saved line). The middle line in the first picture is the the signal with 20db of attenuation. As you can see, there is not much ripple. The second picture is with 50db of attenuation. As you can see, there is significant ripple. The ripple is close to the noise floor.markone, I like the machine and it is great performance for the price.

author: JoeAtl30319
author_rank: Contributor
timestamp: March 06, 2015, 01:10:19 am
content: Second picture

author: markone
author_rank: Frequent Contributor
timestamp: March 06, 2015, 01:34:33 am
content: >The bottom line is the noise floor at a band width of 300 Khz over the full scale of the instrument (again, a saved line)So the noise floor level is around -70 dBm ?

author: Howardlong
author_rank: Super Contributor
timestamp: March 06, 2015, 01:34:38 am
content: Right, couldn't sleep!Full span, 30kHz RBW, 70dB attenuator:Full span, 30kHz RBW, TG on, no cable:Full span, 30kHz RBW, TG off, no cable:So above about 800MHz, the TG leakage is about the same level as your attenuated signal, leading to the in phase and out of phase peaks and troughs you're seeing. What it doesn't explain is why in your OP, which looked like it was a span from 1MHz to 500MHz, why you were seeing the same peaks and troughs.

author: georges80
author_rank: Frequent Contributor
timestamp: March 06, 2015, 02:04:04 am
content: Instead of taking camera pictures, a screen capture provides better 'detail'....http://peter.dreisiebner.at/rigol-bildschirmkopie-lan/download.htmcheers,george.

author: JoeAtl30319
author_rank: Contributor
timestamp: March 06, 2015, 02:06:16 am
content: Right, the first picture was something I actually noticed on a friend's machine. Honestly, I don't know why his is acting up at that lower frequency. He could have been using cheap coax, but I'm not certain. On my setup, it behaved much better. Your setup even looks better than mine. Maybe a better coax or the use of sma attenuators?markone - the noise floor can get as low as -135 db, but to get there, like with any spectrum analyzer, you have to limit the input bandwidth. To get to -135db, I have to 1) turn on the pre-amp 2) reduce the bandwidth to about 100 or so hertz, and possibly reduce input attenuation. I did not do this for my demonstration as it would have taken too long to sweep the entire 1.5 Ghz and was not necessary. Again, I think it's a good buy. It won't perform the same as a $10k+ unit, but then again, unless you want to spend 10k+, your options are limited.

author: markone
author_rank: Frequent Contributor
timestamp: March 06, 2015, 03:02:26 am
content: >markone - the noise floor can get as low as -135 dbMy initial assessment was made thinking that the test condition was different, with a much lower noise floor.I'm trying to understand which is the actual dynamic range of this instrument, it would be great to see what happen if you could manage RBW and preamp status to obtain a noise floor at least around -90 dBm and repeat the 50dB attenuator analysis.Thanks for your attention.

author: G0HZU
author_rank: Super Contributor
timestamp: March 06, 2015, 02:11:23 pm
content: QuoteI've not used the 815 but can you alter the TG power level in the menus? The OP's plot looked to be a normalised measurement and I'd expect you could see this effect get worse if it had been normalised with a lower TG power level.Also, depending on where the signals 'meet' inside the analyser you could also see changes wrt the internal attenuator setting.There could also be some common mode effects here as well. However, The results for leakage don't look very good on any of the analyser plots above 1GHz.

author: Howardlong
author_rank: Super Contributor
timestamp: March 06, 2015, 02:21:25 pm
content: Quote from: G0HZU on March 06, 2015, 02:11:23 pmIn the test I wound it up to full smoke, 0dBm. Interestingly, the difference in an unconnected (leaked) sweep at 0dBm and -20dBm isn't much, I would imagine a lot of the leakage is pre-attenuator.

author: G0HZU
author_rank: Super Contributor
timestamp: March 06, 2015, 02:30:57 pm
content: QuoteMy first impression when looking at your plot and Joe's second set of images was that you had (perhaps) each done your tests at different TG drive levels prior to normalisation. Was Joe's test normalised at -10dBm TG drive level? That might explain why yours looks to have lower leakage relative to the 0dB normalised reference (when compared to Joe with the -10dBm TG power level).

author: Howardlong
author_rank: Super Contributor
timestamp: March 06, 2015, 02:59:10 pm
content: It's a very valid question. Indeed we should be very clear on the parameters used beyond defaults when making these tests.

author: JoeAtl30319
author_rank: Contributor
timestamp: March 07, 2015, 12:10:21 am
content: FYI, here is a nice clear shot, for those who are interest, of an analysis I just performed. On this analysis, I set the bandwidth to 3khz. This had the effect of lowering the noise to around -95db. I re-preformed the -50db test (top line). The middle line is with 69db of attenuation. The bottom line represents the noise at the 3khz bandwidth. Here are some other setting I used with this test:TG level set to -20dbmNormalizing not usedCoax = 2ft RG58/U (50Ohm)Internal attenuator set to 20dbm (30dbm was default)Pre Amp = OnAttenuator = 50 OhmLet me know if I missed anything.

author: Electro Fan
author_rank: Super Contributor
timestamp: March 07, 2015, 12:42:01 am
content: Slightly tangential to the discussion but maybe helpful for anyone new to SA operations:http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CB4QFjAA&url=http%3A%2F%2Fwww.birdrf.com%2F~%2Fmedia%2FBird%2FFiles%2FPDF%2FResources%2Fapp-notes%2FConfiguring-Spectrum-Analyzer-to-Measure-Low-Level-Signals.ashx&ei=uEj6VOjRBcnUgwTkh4SYAQ&usg=AFQjCNGJgYaK0WugcKwy8CY3zl0oOL7VVg

author: fqahmad66
author_rank: Regular Contributor
timestamp: March 07, 2015, 05:30:25 am
content: Quote from: JoeAtl30319 on March 07, 2015, 12:10:21 amI think if you move the cable and those bumps change then it is cable...

author: N8AUM
author_rank: Regular Contributor
timestamp: March 07, 2015, 09:54:12 pm
content: Electro, tnx for info, good read !So I was not amused seeing the TG bleed into the front end. Here`s what it looks like on my brand new unit.Yellow trace TG offBlue trace TG on and both input & output terminatedPurple trace TG on input & output left openTried same on my HP 8591 ZIP !

author: markone
author_rank: Frequent Contributor
timestamp: March 08, 2015, 01:55:54 am
content: Quote from: N8AUM on March 07, 2015, 09:54:12 pmThe terminated I/O result does not seem promising ...

author: N8AUM
author_rank: Regular Contributor
timestamp: March 08, 2015, 02:50:58 am
content: Quote from: markone on March 08, 2015, 01:55:54 amI agree if trying to look for something at those low levels.Still, cant beat the price !

author: Andy2
author_rank: Contributor
timestamp: March 08, 2015, 10:38:20 am
content: That's an interesting screenful. I have a friend with an 815 and I'll ask him to run the same test. Just for the record, here is my Rigol 1030a-TG running the same test. Same settings (I think) and same trace colours. The 1030a is a 3GHz instrument, so I limited the sweep to 1500 MHz to make the comparison easier. Interesting that your 815 seems to have a few dB lower noise especially at the lower end.If I get a spare few minutes, I'll run the same procedure on my Signal Hound and see what it says. As the TG and SA are physically seperate units, the leakage should be lower.

author: N8AUM
author_rank: Regular Contributor
timestamp: March 08, 2015, 02:43:02 pm
content: Quote from: Andy2 on March 08, 2015, 10:38:20 amPretty sure you will nothing unless both are close in proximity to each other. Cant wait to see your results.

author: Andy2
author_rank: Contributor
timestamp: March 08, 2015, 03:01:49 pm
content: OK, I've just attempted to run the procedure on the SigHound and it's rather difficult. For a start, I cannot make a normal sweep and a TG sweep share the screen, but there is a further problem - selecting a TG sweep seems to invoke a whole new setup of the instrument and the noise levels are simply not comparable. Maybe I'm missing something, but it was more trouble than it was worth! I later set the SH to do a normal (non TG) sweep and used the TG in CW mode at spot frequencies, like a signal generator. The leakage level was so low that a 30KHz RBW would not see it. I had to get down to 10KHz or less to lower the noise floor enough to see any leakage at all (using 2dB per div). At this level (around -110 - 120dBm), the analyser's own internal spurii are showing up anyway, so I stopped there.This test was done with the units in their normal configuration, side-by-side in an instrument case but with the SMA extension leads disconnected. All interesting stuff for a Sunday afternoon!

author: G0HZU
author_rank: Super Contributor
timestamp: March 08, 2015, 04:06:34 pm
content: My old Advantest TR4172 analyser has an internal Tgen and the analyser shows a noise trace just below -100dBm with a 30kHz RBW.If I turn on the Tgen and set it to 0dBm and look at the leakage at 1500MHz then it doesn't show anything. i.e. the noise floor stays the same. So I can't see any leakage effects at this RBW setting. If I search around up to 1800MHz I can see places where the noise trace lifts up to about -100dBm.I also have a HP8566 analyser with an external HP85644A TGen and I'd expect this to perform even better. But it's too much hassle to connect it all up to test it...

author: N8AUM
author_rank: Regular Contributor
timestamp: March 08, 2015, 04:36:46 pm
content: It only makes sense that it would be difficult to keep a strong signal source from "leaking" in due to the 815 being physically small.Now I have to go back and re read this entire thread to see if Joe ever did come up with an answer or solution.

author: markone
author_rank: Frequent Contributor
timestamp: March 09, 2015, 07:39:51 pm
content: Quote from: N8AUM on March 08, 2015, 02:50:58 amI'm trying to understand if this instrument is capable to trace a passive filter response curve with a decent dynamic range ( 80dB ?) ....

author: G0HZU
author_rank: Super Contributor
timestamp: March 09, 2015, 07:47:36 pm
content: In my experience, pretty much all spectrum analyser TGens aren't really up to making critical measurements of high performance RF filters. (as in filters with low insertion loss and decent stopband performance)I think the recent images of the 3GHz Rigol look OK for tracking gen leakage and the 815 leakage plots look reasonable up to about 1GHz.I think the problem will get worse if the analyser ports are padded with external attenuators to improve the port VSWR of the analyser in an attempt to reduce measurement uncertainty of low insertion loss filters.This will compromise the ability to measure stopband performance accurately and one way around this is to measure the passband loss using the external attenuators but then remove the attenuators (and renormalise/recal) for the stopband measurements.A bit tedious but probably worth it in a lot of cases...

author: Mr Simpleton
author_rank: Unknown
timestamp: March 10, 2015, 10:14:01 pm
content: Beeing late to the party... I had a simple tubular filter at hand, and ran it through Signal Hound and TG... As Andy said, one is not able to adjust RBW, so a bit hard to dig deep down, still I do have close to 100 dB range here. Not too shabby.

author: G0HZU
author_rank: Super Contributor
timestamp: March 10, 2015, 10:39:40 pm
content: The Signalhound performance looks really good. However, I suspect that this is because it is housed in a separate box and the 'tracking' will presumably be software based. So this gives it an edge.Note:I looked up the specs for my old HP85644A external tracking gen when used with my HP8566B analyser and they claim a dynamic range (in terms of max TGen level to the analyser noise floor) of 141dB up to 1800MHz.However, I seriously doubt that this is based on a leakage test. For the leakage test I would hope/expect it to be better than 100dB across this range but it could be a lot better (or worse!)