SOLVED-- SVA1032X -- Why is VNA noisier than SA-TG same measurement?  - Page 1
Forum section: electronics / testgear

author: NY2KW
author_rank: Regular Contributor
timestamp: February 22, 2020, 04:12:23 pm
content: I just got a nice deal on a Eagle RLB150x5 rated beyond 3GHz for my SVA1032X and wanted to check it out. First test is always to make sure previous owner didn't zap it, so using the SA TG I wanted to check the specs on the internal loss for a maximally reflected source. The specs say if not zapped, with a good 50 ohm load (mine is a Narda to 12GHZ) , the reflected signal should be -35 to -40 dB or better from normalized. So TG (0 dbm) to Source port, SA to Reflected port normalized with 50ohm load shown in first two pictures.Then I did the same in VNA mode with S21 Log Mag format, which should be the same signal paths, keeping everything the same:[ Specified attachment is not available ]Why is the trace for the same load so much noisier in VNA mode, this is averaged - it as even noisier than SA-TG mode given all conditions are otherwise equal? There seems to be better sensitivity but more noise with VNA mode. Aren't the internal paths for this measurement identical?

author: bson
author_rank: Unknown
timestamp: February 22, 2020, 09:23:05 pm
content: Is it actually noise, and not ripple?

author: tautech
author_rank: Super Contributor
timestamp: February 22, 2020, 09:34:35 pm
content: I think it's an interpolation issue with just 201 data points (default). Change it to the max701751points and the interpolation result becomes more accurate.Post another screenshot with 751 points please.Edit, corrected max # of data points.

author: NY2KW
author_rank: Regular Contributor
timestamp: February 22, 2020, 09:38:43 pm
content: Makes sense, will change points and post new image. Any way to change the default # points?Jerry

author: tautech
author_rank: Super Contributor
timestamp: February 22, 2020, 09:41:50 pm
content: Quote from: NY2KW on February 22, 2020, 09:38:43 pmNot near a SVA but IIRC it's in one of the top row of buttons or in the Meas menu. If you do a full Preset to factory default it returns to 201 points.

author: hendorog
author_rank: Super Contributor
timestamp: February 23, 2020, 12:32:12 am
content: To compare the paths while using the same amount of averaging in both cases, try this:SA Mode:Set attenuator to zero (Edit)Leave RL Bridge test port openSet averaging to 100Normalise once averaging reaches 100Attach 50 ohm load and restart averagingSet trace to View once averaging reaches 100Switch to VNA mode:Remove load from test portSet averaging to 100Save trace to Mem once averaging reaches 100Select Math, Data/MemAttach 50 ohm load and restart averagingOnce averaging reaches 100 compare the two traces.Edited to add attenuator setting.

author: NY2KW
author_rank: Regular Contributor
timestamp: February 23, 2020, 03:09:18 am
content: This wont work with Attenuator set to zero. I need to set the SA TG to 0 dBm because I believe the VNA sets the TG output is fixed at zero dBm. I get a over power beep with TG=0 dBM and Att=0. I will try it with Att=5.

author: hendorog
author_rank: Super Contributor
timestamp: February 23, 2020, 03:27:46 am
content: Quote from: NY2KW on February 23, 2020, 03:09:18 amNo problem - that was just a fine tuning.

author: NY2KW
author_rank: Regular Contributor
timestamp: February 23, 2020, 04:50:28 am
content: OK here is the SA, TG=0 dBm, with DUT open (full reflection), averaged 100x then used this to normalize, then added precision 50 ohm load to DUTthen did same with VNA, and looks differentAny ideas what might be going on? Can you try this on your SSA or SVA with an RF bridge?

author: NY2KW
author_rank: Regular Contributor
timestamp: February 23, 2020, 04:51:28 am
content: for some reason, the images are displayed in wrong sequence but are correctly labeled

author: NY2KW
author_rank: Regular Contributor
timestamp: February 23, 2020, 04:54:18 am
content: I have an older RF/SWR bridge from Amtronics up to 1.5GHz. I will do the same comparison in morning and post. Need to know if this is a quirk of the SVA or the Rerun Loss Bridge

author: NY2KW
author_rank: Regular Contributor
timestamp: February 23, 2020, 09:15:58 pm
content: I am no expert, but by process of elimination it seems most of the S21 noise in VNA mode comes from just having an open port near the bottom of the noise floor.Here are my comparisons:First, in SA mode, TG= 0 dBm, RG405 4” cable directly from Port 1 to Port2 and the SA displays a very low noise trace showing signal at 0 dBM for 1 MHz RBW, full span. Doing same in VNA mode gives near identical trace.Next I placed two Mini-circuits 18GHz 20 dB attenuators onto each Port and connected Port 1 to Port 2. I did this in SA mode and VNA mode and the Thru traces look pretty clean and almost the same in both modesVNA 40dB THRUI wanted to see the noise floor under very low signal, so I added an additional 10 dB attenuator to each Port (total 60 dB)The SA Thru trace looks good though for the BW setting, pretty flat, low noise across the full span not quite getting down to -60dBM.On the other hand, doing the same in VNA yields that very noisy trace (even averaging 100x with 700 points).This looks like the noisey trace I saw when checking out the reflection on my Eagle RLB:VNA 60dB THRU 700ptsNext I just removed the cable from Port 1 and left the 60dB of attenuators connected to Port 2 and you can see most of the noise is still there even without a input signal:[ Specified attachment is not available ]VNA 60dB NO TGThe SVA1000x datasheet says there should be a minimum dynamic range for the SVA 60 dB between 10MHz – 1.5 GHz and 55 dB above 1.5GHz. The Trace noise should be < 0.1 dB rms with 50 averages.So I am puzzled if this is considered out of spec. Can anyone else with a SVA1000x try to duplicate my observations?

author: NY2KW
author_rank: Regular Contributor
timestamp: February 23, 2020, 09:18:23 pm
content: I don't know why my uploaded images are getting scrambled in sequence and some missing Here is the VNA with 40dB attenuation Thru.As you can see, the noise only begins at the lowest limits of TG input

author: eb4fbz
author_rank: Regular Contributor
timestamp: February 24, 2020, 10:19:38 pm
content: S21 ripple due to reflections, because analyzer ports doesn't present 50ohm.Check placing a good 10dB attenuator right at the nanoVNA port 2 and calibrating with it in place. If my theory is right, the measurement ripple will be lower. However the S21 dynamic range will be 10dB lower too.

author: NY2KW
author_rank: Regular Contributor
timestamp: February 25, 2020, 02:56:50 am
content: Port 2 (input) in VNA and SA mode is specified as being 50 ohm input. The easiest way to replicate my observations is:1) In SA mode (no TG connected !) - connect a precision 50ohm load rated > 6 GHz load directly to the Port 2 connector. Set SA full span, 1 MHz RBW, TG OFF and average about 10 sweeps. You will find a relatively smooth trace that goes from -60dBm at low end up to about -55dBm on the 3 GHz end.2) Now switch to VNA mode, S21, with same 50 ohm load on Port 2 and average the traces. You will see a trace spiking up and down ranging from -50 to -80 dBM. Calibration doesnt help and when using a bridge or coupler and connect the TG if the reflected or Thru signal you are trying to measure is less than -40 or -50 dBm then all you see is this noise. Doing the same identical type of measurement in SA mode, even with TG attenuated down to -60 or more show a nice clean trace.I dont know if its a problem with my SVA or something intrinsic to the design but I ahve sent it all to Siglent Tech support and waiting to hear back.

author: TheSteve
author_rank: Unknown
timestamp: February 25, 2020, 03:27:59 am
content: I've never used an SVA1032X but in VNA mode what are you using for IF bandwidth or equivalent? I don't think you can expect the same trace in both SA and VNA modes. You should also be terminating port 1 for any testing.

author: tefe
author_rank: Contributor
timestamp: February 25, 2020, 03:50:00 am
content: The TG OPEN floor is actually the SA noise floor, even if it is normalized.But VNA S21 OPEN floor is a ratio of SA noise floor/TG output full reflection, so it will be the shape of TG output full reflection.If this box has an access of Port1 reflection, I think you will get the truth.

author: NY2KW
author_rank: Regular Contributor
timestamp: February 25, 2020, 04:18:25 am
content: QuoteCan you elaborate that a little more? Why does the SA+TG mode Thru to Port 2 traces compared to VNA mode S21 traces look virtually identical in terms of noise up to attenuating the TG (Port 1 ) signal to just about below -50dBM - at that point in VNA mode the noise increases dramatically with flux +/- 30 dBM ( from -50 to -80). In SA+TG mode the Thru trace looks same low noise well below -60dB attenuation.

author: NY2KW
author_rank: Regular Contributor
timestamp: February 25, 2020, 04:22:27 am
content: QuoteOne of the limitations of the SVA1032X is you cannot adjust the TG output level in VNA mode, its fixed at 0 dBm and as far as I can see the VNA bandwidth seems the same as the default SA (1 MHz) when full span, at least by sweep speed and trace noise -- up to about -50to -55 dBm input when the trace falls apart with noise flux ranging from -50 to -80 dBm.

author: TheSteve
author_rank: Unknown
timestamp: February 25, 2020, 05:25:15 am
content: Quote from: NY2KW on February 25, 2020, 04:22:27 amThe datasheet mentions an IF bandwidth of 10 kHz for the VNA. It also lists a dynamic range of 10 kHz - 10 MHz @ 75 dB, 10 MHz - 1.5 GHz @ 60 dB and 1.5 GHz - 3.2 GHz @ 55 dB. So what you're seeing could be normal. The dynamic range really isn't too great.

author: tautech
author_rank: Super Contributor
timestamp: February 25, 2020, 05:28:06 am
content: Quote from: TheSteve on February 25, 2020, 03:27:59 amNope, SVA models have just a single active port in VNA mode; Port 1 (also TG for SA mode)DUT reflections are then passed back through port 1 to an internal coupler that's not in the signal path in SA and TG ON mode.In VNA mode just port 1 is used for basic antenna and DTF measurements.Examples of just the single VNA port 1 usage are here:https://www.eevblog.com/forum/rf-microwave/what-really-is-this-antenna/

author: TheSteve
author_rank: Unknown
timestamp: February 25, 2020, 05:31:14 am
content: Quote from: tautech on February 25, 2020, 05:28:06 amHow do you measure S21 with only a single port?

author: tautech
author_rank: Super Contributor
timestamp: February 25, 2020, 05:48:32 am
content: Quote from: TheSteve on February 25, 2020, 05:31:14 amYou don't.Watch TSP from 33 mins in:https://youtu.be/ToVJTKCyIU8?t=1979Take note of the changes from VNA to SA mode for some measurements.

author: TheSteve
author_rank: Unknown
timestamp: February 25, 2020, 06:03:04 am
content: Quote from: tautech on February 25, 2020, 05:48:32 amI know, lolI thought he was looking at the S21 noise floor with port 2 terminated. If that is the case port 1 should also be terminated as RF is being generated there when looking at S21.

author: tautech
author_rank: Super Contributor
timestamp: February 25, 2020, 06:09:33 am
content: Quote from: TheSteve on February 25, 2020, 06:03:04 amThis is where I reach the limits of my VNA understanding with my somewhat limited single port experience however as I understand it a S21 measurement is obtained from the stimulus generated from port 1.Am I wrong ?

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: February 25, 2020, 10:37:32 am
content: I don't see any real noise in your pictures unless I'm missing something (you talking about all the notches?). Those are classic standing wave effects you get from mismatches. At certain frequencies (and their harmonics) your structure will happen to just form a 1/2 wavelength or something and reflect, giving you ripple because at those frequencies you happen to have some resonance. Can also be caused by poor calibration or the addition of pieces of cable/shift in reference plane after calibration.

author: tautech
author_rank: Super Contributor
timestamp: February 25, 2020, 10:57:59 am
content: So I had some time tonight to have a play for the benefit of NY2KW.Equipment used.SVA1032X, running with FW 2.5 and full factory reset therefore VNA running on factory Cal. Small # of averages just to remove any fuzz.N-SMA adapters and cheap 6" SMA leads.Narda 4-8 GHz -20dB SMA bridge....all measurements are outside the rated frequency for this -20dB coupler.50 ohm SMA cap not cal load quality as demonstrated by this full sweep Smith chart.SA mode, full 3.2 GHz sweep, TG ON, -20dB coupler between TG and RF IN ports and coupling port capped with 50 ohm SMA cap.Now things get ugly for a S11 measurement as the coupler is not intended to be used in this way. Port 1 in VNA mode is applying a stimulus and reading the reflection which means nothing in the big picture for us. That we are also operating it below its design parameters and with a substandard 50 ohm load could both also be adding to the mess.A S21 measurement makes much more sense as it's examining the through pass of the coupler and the full sweep performance of the 50 ohm cap although its response is attenuated 20 dB by the coupler.When we change the scale to 0.5dB/div the nonlinearities of the measurement system become much more apparent yet they are barely 1 dB.So how bad is this 50 ohm load, well let's look at a single port 1 connection of it on just the N-SMA adapter but without any port extension corrections.Some further examinations of it using Smith charts and min and max scaling.

author: tautech
author_rank: Super Contributor
timestamp: February 25, 2020, 11:25:29 am
content: Quote from: NY2KW on February 23, 2020, 04:51:28 amYeah it's a forum bug, the first image you add is always last so I now add thumbnails and link them into IMG flags.Completely lost one image from the above post so had to add it again in an edit.

author: NY2KW
author_rank: Regular Contributor
timestamp: February 25, 2020, 09:24:05 pm
content: Thanks Tautech… I want to focus on the S21, your 4th picture. I get this as well but please try this -- in VNA mode without changing the Y-scale, insert a total of 60 dB of attenuation between Port 1 and Port 2 and you will see the trace go wild with oscillations varying from (-)50dBm to (-)80dBm. It becomes apparent when the signal from Port 1 is reduced below -50dBm.

author: tautech
author_rank: Super Contributor
timestamp: February 25, 2020, 09:40:30 pm
content: Quote from: NY2KW on February 25, 2020, 09:24:05 pmSorry but I haven't got anywhere near that amount of attenuation available.

author: NY2KW
author_rank: Regular Contributor
timestamp: February 25, 2020, 11:31:19 pm
content: I was trying to replicate the condition of a coupler input attached to Port 1 going to a near-perfect 50 ohm load at the coupler's DUT and I wanted to know the lower limits of sensitivity for measuring a reflected signal from DUT port back to Port 2. In SA+TG mode this is a power measurement and I was able to measure a reflected signal down to almost (-) 100 dBm. I understand in VNA mode, the S21 is a ratio of the incident/reflected ratio measured at Port2. But I was uncertain why there was such an abrupt transition in S21 measured noise when the incident power dropped below -50 dBM in contrast to my reflected measurement in SA+TG mode. In VNA mode, if the impedance at Port 2 remains purely real 50 ohms across the span, my non-mathematical intuition was telling me that any signal reflected from DUT to Port 2 should be fairly constant as increasing attenuation to the Port 1 signal.... until such a point when the reflected signal approaches "measurable" zero and then the ratio becomes infinite. I was surprised that this happened just below a incident signal of (-) 50-55 dBM as determined by my adding sequentially more and more attenuation to Port 1. I don't think this has any practical implication, but I was more curious if this represented a situation of design limits or a problem with my SVA1032X. I think I spent too much time describing the conditions rather than focusing on the fundamental question. My apologies.Jerry

author: tautech
author_rank: Super Contributor
timestamp: February 25, 2020, 11:54:27 pm
content: I think the datasheet offers clues to the lower limits of dynamic range.Outside those who know how it behaves.Calibration P12https://siglentna.com/wp-content/uploads/dlm_uploads/2018/06/SVA1000X_DataSheet_DS0701X_E02A_JCEdit.pdf

author: NY2KW
author_rank: Regular Contributor
timestamp: February 26, 2020, 12:26:09 am
content: OK, so here is the datasheet:Dynamic Range S21, IFBW=10 kHz, Port1 level=-5 dBm, Log Mag, Average=50 100 kHz ~ 10 MHz 75 dB 10 MHz ~ 1.5 GHz 60 dB 1.5 GHz ~ 3.2 GHz 55 dBSo an S21 measurement between 10 MHz - 1 GHz should be able to detect 60 dbM down from TG of -5 dBM and that is about the limit of my experiment.Thanks !! How can I change the title of this thread to "SOLVED" ??