SSA 3021x  Siglent Spectrum analyzer bug.  Spurious when doing RMS Power average - Page 1
Forum section: electronics / testgear

author: Andy Granis
author_rank: Contributor
timestamp: March 10, 2017, 10:08:02 am
content: When changing Average type from "Log Power" to "Power" , in menu "BW" Spurious signals in spectrum is generated, if you are measuring in a CW signal.Please see my video recording.I guess this must be a bug inside the FPGA Spartan 6?

author: tautech
author_rank: Super Contributor
timestamp: March 10, 2017, 10:23:02 am
content: Welcome to the forum.Please upgrade the firmware in your unit with the latest version, link below:http://siglentamerica.com/gjjrj-xq.aspx?id=5128&tid=15Please report back with any changes to your observations.

author: rf-loop
author_rank: Super Contributor
timestamp: March 10, 2017, 11:10:14 am
content: Quote from: Andy Granis on March 10, 2017, 10:08:02 amI have tested my test unit now using same signal - pure sine wave from HP8644B.Same settings. Center freg same, span same, RBW, VBW sameTrying all detectors. (Pos Peak, Neg Peak, Sample, Normal, Average RMS*/Average Video*/Average Voltage*)*depends selected average type in BW menu.Trying with all (in BW menu) Average Type: LogPower, Power, Voltage(all combinations with detectors and average types)I can not see anything what you show except just pure carrier.One (not important, just wonder) question but not related to this miracle what you see. Why you use Average detector for sinusoidal type of signals? In many cases it leads to signal level measurement error. It takes average from whole bucket. This works as fast freq step sweep full IF SA working principle. In many cases signal is only part of bucket.Average detector is more suitable for noise.But first. Please use latest FW.

author: Andy Granis
author_rank: Contributor
timestamp: March 10, 2017, 11:36:07 am
content: Quote from: tautech on March 10, 2017, 10:23:02 amThanks for tip!Upgraded to FW 1.2.8.2 and that resolved the problem with spurious signals.However there seems to be an artifact, with this version of FW, when using 1 MHz span (only) and "power" detector, see my enclosed spectrum plot, where the marker is placed. I have averaged the spectrum 100 times.

author: Andy Granis
author_rank: Contributor
timestamp: March 10, 2017, 11:58:05 am
content: I have tested my test unit now using same signal - pure sine wave from HP8644B.Same settings. Center freg same, span same, RBW, VBW sameTrying all detectors. (Pos Peak, Neg Peak, Sample, Normal, Average RMS*/Average Video*/Average Voltage*)*depends selected average type in BW menu.Trying with all (in BW menu) Average Type: LogPower, Power, Voltage(all combinations with detectors and average types)I can not see anything what you show except just pure carrier.One (not important, just wonder) question but not related to this miracle what you see. Why you use Average detector for sinusoidal type of signals? In many cases it leads to signal level measurement error. It takes average from whole bucket. This works as fast freq step sweep full IF SA working principle. In many cases signal is only part of bucket.Average detector is more suitable for noise.But first. Please use latest FW.Hi,Firmware upgrade solved the problem with the spurious signals, see above.This is only something I stumbled on when testing my new spectrum analyzer.I am trying to measure "Man-made noise " and therefore I am interested in measuring True RMS power.See my enclosed "noise report".I have also measured a noise source (diode) and using an 23 dB amplifier to increase the noise level, I have more than 20 dB margin to the noise floor of the spectrum analyzer.What I find is that when measuring noise level using the "noise marker" function I get about -125.0 dBm/Hz when using "power" and -124.0 dBm/Hz when using "logPower" averaging. What I understand from the manual, this should be compensated for by a correction factor (guessing white Gaussian noise is assumed).From the manual from "Noise Marker": "During this process, certain compensation is always made on the basis of the detection and trace types."

author: rf-loop
author_rank: Super Contributor
timestamp: March 10, 2017, 12:09:10 pm
content: Quote from: Andy Granis on March 10, 2017, 11:36:07 amYes. Lets hope next FW correct this.Btw, it is in FFT mode. If change mode to sweep, this artefact disappear.Also it do not affect signal peak level.Also if look with some different settings there can see also automatic ADC level control steps in noise trace near carrier...(ref Agilent appnote 150, there explained)

author: Andy Granis
author_rank: Contributor
timestamp: March 10, 2017, 02:25:09 pm
content: According to the manual, "Power" averaging should be used to measure the True RMS power of complex signals, see enclosed cutout from the manual.I have tried to measure the True Power of a sum of two sinewaves, see youtube link.However the SSA3021x is not measuring the correct power of this "complex" signal. The True power is equal to -7.1 dBm, and the 10 kHz beat from the signals are not averaged out until 100 traces are averaged.https://www.youtube.com/watch?v=3ayb0sox_V4&feature=youtu.be

author: Andy Granis
author_rank: Contributor
timestamp: March 10, 2017, 02:28:41 pm
content: Sorry! Here is the cutout, from the manual.

author: videobruce
author_rank: Frequent Contributor
timestamp: March 10, 2017, 04:12:24 pm
content: Why isn't this in the dedicated SSA thread??

author: Andy Granis
author_rank: Contributor
timestamp: March 11, 2017, 09:14:51 am
content: Quote from: videobruce on March 10, 2017, 04:12:24 pmBeacause I am a Newbie.

author: rf-loop
author_rank: Super Contributor
timestamp: March 11, 2017, 04:41:50 pm
content: Different peoples do different.Here is two CW carrier. Both around -10dBm level. Other is 8000000Hz and other 8010000HzFirst three images show settings how they are in menu system..Sweep mode Sweep.RBW=VBW=1kHzBW menu average type PowerDetector Average RMSSpan 100kHzCenter 8005000HzAs can see SSA markers show around -10dBm for both peaks. (absolute accuracy is not this and I have not measured true perfect accurate power for these carriers. But sure is that error to real -10dBm is less than 0.5dBm.Then change RBW so that both are inside center of filter. Span still same 100kHzResult: OK around -7dBm - which was of course expectedThen change Span to 1MHz keep same RBWResult: OK around -7dBm - which was of course expectedThis kind of measurement need do like this and it need also care that sweep time is enough long even if SSA try select faster sweep time.SA can not do thinking, it is just stupid simple machine - the user must perform brainwork. For help thinking there is lot of free literature available around.This is one what can say - it is mandatory to read:http://cp.literature.agilent.com/litweb/pdf/5952-0292.pdfSpecially things what are there about full digital IF SA's.The user must note that these fast hopping modern full digital IF SA's work very very different if compare to old analog sweep analog IF SA's. If the user is accustomed to using an analog spectrum analyzer. One of the most important task is to learn new things, but at the same time need tounlearnsome things. Otherwise, may fall in to some trap. Who is then wrong, equipment or user?One of most important thing is: Know your equipment!These first 3 images same but show different menu settings. Peaks both -10dBmSSA is running and just sweeping without trace average. Snap shot on the fly.These first 3 images same but show different menu settings. Peaks both -10dBmSSA is running and just sweeping without trace average. Snap shot on the fly.These first 3 images same but show different menu settings. Peaks both -10dBmSSA is running and just sweeping without trace average. Snap shot on the fly.Signal is exactly same what in first 3 images. --> RBW 100kHz Level now -10dBm + -10dBm = -7dBm = OKSSA is running and just sweeping without trace average. Snap shot on the fly.Signal is exactly same what in first 3 images. --> Span 1MHz Level now -10dBm + -10dBm = -7dBm = OKSSA is running and just sweeping without trace average. Snap shot on the fly.For thinking: there is one "secred" word...and it is.............. phase(signals phase difference)....oh well there is also other things but...

author: Andy Granis
author_rank: Contributor
timestamp: March 12, 2017, 02:31:56 pm
content: Quote from: rf-loop on March 11, 2017, 04:41:50 pmYes I agree you must know your measurement equipment. That is why I am testing these different measurements,and this forum gives me excellent help, to understand my equipment! Thank you Mr rf-loop!The Agilent appliction note is really good.I have tried your setup and lowering the sweep time and increasing the RBW and then I get the correct power level, as you have shown in your spectrum plots.In my recorded video, using 10 MHz span, I agree that it is not possible to get a stable power measurement value with, by the SA choosen sweep time (28.5 ms), since the measurement time on the frequency of intrest (assuming 200 frequency points measured across the frequency sweep) is only 28.5 ms / 200 = 142 us, and therefore only a little more than one cycle of the beat period (10 kHz => T= 100us) is measured. However, I still think that when averaging 100 Sweeps using "Power average" this should have given the correct powerlevel(using default SA setting), in my case -7.1 dBm, since RBW used is 100 kHz and the spacing between the CW-signals are 10 kHz.However, this discussion has given me new insights how my equipment should be used to produce the correct measurement results.