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Siglent SDS1x04X-E BodePlot II (SFRA) features and testings
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rf-loop:

--- Quote from: Performa01 on June 12, 2019, 11:39:46 am ---
Signal level measurements in the stopband usually need not be that accurate and a couple of dB plus or minus doesn’t really matter – all the more so as component tolerances and test setup can have a much higher influence on the results. At levels that low, double shielded cables and high quality connectors should be used throughout in order to get meaningful results.


--- End quote ---

Yes - but then also no. It is true that usually stop band level accuracy is not so important but stop band itself can be  very important or even more important - often overlooked or underestimated. Why we do band pass filters, example IF filters for receivers. Because we want block all but pass band signals and there this is simple - more is better. Filter stop band performance is very important when we talk example good analog HF radio. High end and state of art class analog radio can do only if IF filters stop band is excellent with good filter shape. In some most bottom level radios like example just home radios they can be nearly what ever. But in state of art receivers IF filters stop band performance is critical. How ever nice and excellent and perfect and expensive things there is but if IF filters stop bands are crap it is end of game.

Now we have under 1000 euro entry level oscilloscope SFRA system (BodePlot II) what can do lot of for measure also these things - but still not down to excellent IF filters stop band, not even close if we use "normal" test signal levels (without extra tools and even then - bit difficult). Not at all up to state of art class, not even high end class, not even real professional class (shift right price tag decimal point some steps) but still it leave competitors far behind.

Using more low test signal levels still we knock our nose to noise wall very fast.  We are still working with ~<7bit (oh yes digital data bus width is 8bit) ADC oscilloscope in 500 price  group.  And when thinking this, its  BodePlot II performance is amazing in this and also in bit upper price group.



Here is one image what give some tip how this frequency selective sweep works and how it adjust selectivity depending frequency. It also may give partial explanation about this noise floor levels (and dynamic range changes)

During this BP sweep there is fixed frequency generator what generate lot of harmonics (square wave) to DUTout channel (in this case I have made Ch1 as DUTout channel and Ch4 is ref/DUTin.
Most left narrow peak is 230kHz fundamental.

As can see BPII frequency sweeping selective level detector ("sweeping selective level meter") change its RBW filter width depending frequency when it sweep. (if want more deep inspection it need sweep using many separate frequency bands for get enough freq resolution)
As can see there with fundamental freq, ~230kHz BW is narrow (scale is linear), then it step bit more wide, again bit more, then more and finally after ~3.5MHz*) it have ~300kHz width. If go higher frequencies then width again change. (stepping filter width somehow proportionally with sweep freq).

*) Note reply #3 picture 2  change after 3MHz.

Performa01:
Inter-Channel Crosstalk up to 120MHz.

I’ve stated before that I don’t think that crosstalk would be significantly different for the various channels. Now I’ve actually tested this and the screenshot below shows the crosstalk between channel 1 and all other channels:


SDS1104X-E_Bode_Crosstalk_+18dBm_Ch1

The reference signal is +18dBm, swept from 1kHz to 120MHz. Channel 1 was fed through a high quality double shielded low loss cable with >105dB screening efficiency up to 2GHz and a HP10100C through terminator (50 ohms). All other channels were terminated by the usual 50 ohms BNC end terminators.

As we can see, there is not much difference indeed, except for the frequency range 60kHz – 900kHz, where channels 3 and 4 are actually significantly better.

We can conclude that for passive structures, the dynamic range is limited by the crosstalk effects as listed below:

-   9.5MHz: 110dB
-   55MHz:  100dB
-   100MHz:  90dB
-   120MHz:  87dB

We can see that there is actually a significant benefit in using an external amplifier for boosting the signal into a passive DUT. In a previous posting I have demonstrated how the combination of +27dB amplifier and step attenuator could provide some 100dB usable dynamic range up to 120MHz. This hints on an overall improvement of at least 10dB.

EDIT: Corrected the reference level from +13dBm to +18dBm.
rf-loop:
Ch-Ch cross talk depends also what channels are in use. 
Also base noise level depends (some amount) frequency due to selective level measurement bandwidth what is stepping more wide when frequency rise. But still effect is marginal. One BW change is roughly around 3.5MHz and this can see if carefully look. We can perhaps see many steps if there is trace average function but there is not.


Here is some individual examples pick-up from previously made quite big set of tests.

There is 4 equal test pairs  so that CH1 is DUTin and CH2, 3 and 4  DUT out.  There is no DUT. These channels are terminated and sensitivity 500uV/div.
Top image DUTin get quite constant level  from start to stop freq. Level is 17.9dBm
Bottom image all same but DUTin signal attenuated 20dB using external accurate HP step attenuator.
Test generator is SDG1062X.  All cables M17/84 with Suhner's of course. Signal from generator to DUTin channel terminated using Tektronix 011-0049-01 feed thru.

Next image CH2 is DUTin, then CH3 and last CH4 and other channels as DUT out.

I have used Amplitude measurement/display mode Vout (instead of default Vout/Vin) so display scale is also dBm  not dB vs DUTin.
Channel Gain is in Hold mode (instead of Auto)
(this setup is ok for this purpose)

In images can clearly see that different channels have different cross talk.
Around 0dBm DUTin cross talk start mostly fall in to noise.

As can see CH1 is quite good for DUTin channel and it is also BodePlot II default DUTin.
(note, channel cross talk may be different in individual scopes)

If think example typical IF filters around 100kHz or around 450kHz or 1.4MHz  it is good to take into account. Depending of course test setups. Typical filter input level is not so high so  there is not so much dynamic range available due to level of noise floor. But still example if max input level is 0dBm there is still over 80dB range with quite good level accuracy and over 90dB with reduced accuracy.  When DUTin signal level is 0dBm or less it do not rise other channels floor or effect is insignificant. If input level need be more low it is all away from dynamic range, noise floor do not change.

Not bad at all.


Note display scale. It is NOT dB vs DUTin signal. It is abs level in dBm!












KungFuJosh:
I setup a bode plot using an SDG2042X and the SDS1104X-E. The DUT is a supercardioid microphone. I'm assuming I set this up wrong...just because. ;) The test has been running for over 12 hours and is still going.

The settings are in the screenshots. Control is via network. I'm using a 50ohm terminated coax between the SDG and SDS on ch1. DUT is connected with a 10x Pico TA131 probe unterminated on ch3.

Side note: the frequency response for the mic is supposed to range between 40Hz to 16kHz.
Rerouter:
Sound correct to me, It never stops, it just sweeps across the range endlessly until you stop it. that way you can use it to adjust circuit parameters and see the result
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