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Two Tone Test with Scope and SA
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mawyatt:

--- Quote from: rf-loop on June 11, 2022, 04:59:52 pm ---Generator SDG1032X. It need need tightly understand that this with splitter is not enough for serious dual tone test. Who knows what it itself generate...

2*f2 - f1 = 10.3 Peak Marker 4 (-85dBc)
2*f1 - f2 =   9.7 Peak Marker 1 (-83dBc)


--- End quote ---

Well we may not know just how good the signal from the SDG1032X is (could be verified with a quality SA tho), but the result you've shown is quite good indeed!! If we "assume" the IMD result is root sum square of the AWG and the DSO, then the DSO worst case IMD is as shown (assuming AWG IMD is zero)!!!

Siglent evidently got this right just like they did with the 8 bit SDS2000X+ :)

Thanks for taking the time to show this :-+

Best,
David Hess:
Run the same test with a low distortion summing amplifier and see if the results agree.
G0HZU:
For comparison, a fairly modern lab spectrum analyser should achieve about 95dB SFDR in a two tone test with a 100Hz RBW selected. Greater SFDR than this is possible with a narrower RBW but the measurement time will obviously be longer.
Performa01:
Since we have seen some LF results so far, I thought I’ll add some measurement at the upper end of the spectrum, specifically at 90% of the specified bandwidth of 500 MHz.

So, this IMD test is about 450 MHz and 200 kHz frequency offset.

It has already been brought up several times before: for comparable results, we need a bunch of conventions and preconditions. I cannot meet all of them, for instance I cannot guarantee that the dual tone test signal has any lower than -74 dBc IMD.

As has been already discovered, the results also depend on the scope settings. Like a “real” spectrum analyzer, we won’t get impressive results if we feed the scope input with full scale signals (in SA terms: right below the compression level). In general, the lower the input signal(s), the lower the distortion. For testing the third order dynamic range of an SA, we usually have signals in the realm of at least 30 dB below the 1 dB compression point. Consequently, I think it is only fair to use signal levels at least about 10 dB below full scale for the scope as well.

Now let’s calculate the settings for 0 dBm test signals (sine waves assumed throughout these calculations).

0 dBm into 50 ohms means 223,61 mVrms or 632,46 mVpp.

With two 0 dBm signals combined, the peak output voltage doubles to 1.265 Vpp.

The SDS2000X HD ADC input range is roughly 9.6 divisions, so we need at least 132 mV/div to have the dual tone signal within full scale. If we want to keep the signal at or below -10 dBFS, this makes for 4 Vpp full scale.

In the end I chose 500 mV/div, which is equivalent to 4.8 Vpp full scale.

We can get ~74 dBc IMD3 in this scenario, this is equivalent to a TOI (third order input intercept point) of +36.7 dBm.

See attached screenshot:

SDS2504X HD_IMD_500mV_C450MHz_O200kHz_0dBm_L

Please note that this is not a bad result at all. There were (and still are) plenty of low(er) end spectrum analyzers that might not even reach 70 dB third order dynamic range, whereas top instruments can provide up to 110 dB.

For example, the ancient HP8590A has a third order dynamic range of 70 dB at -30 dBm input.

 
mawyatt:

--- Quote from: Performa01 on June 13, 2022, 05:46:23 pm ---Since we have seen some LF results so far, I thought I’ll add some measurement at the upper end of the spectrum, specifically at 90% of the specified bandwidth of 500 MHz.

So, this IMD test is about 450 MHz and 200 kHz frequency offset.

It has already been brought up several times before: for comparable results, we need a bunch of conventions and preconditions. I cannot meet all of them, for instance I cannot guarantee that the dual tone test signal has any lower than -74 dBc IMD.


--- End quote ---


However you can guarantee that the DSO (or the AWG) is better than this....which is impressive for the DSO, and not so bad for the AWG either IMO ;)


--- Quote ---As has been already discovered, the results also depend on the scope settings. Like a “real” spectrum analyzer, we won’t get impressive results if we feed the scope input with full scale signals (in SA terms: right below the compression level). In general, the lower the input signal(s), the lower the distortion. For testing the third order dynamic range of an SA, we usually have signals in the realm of at least 30 dB below the 1 dB compression point. Consequently, I think it is only fair to use signal levels at least about 10 dB below full scale for the scope as well.

--- End quote ---

For the DSO case the lower signal level may not be the "Sweet Spot" of IMD, this is because you need to partially fill up the ADC range to capture as many "bits" as possible and this conflicts with the smaller the signal the better the amplifiers IMD performance. So the overall DSO "Sweet Spot" is more likely somewhere in the middle DSO range where you've positioned your input signal.


--- Quote ---
We can get ~74 dBc IMD3 in this scenario, this is equivalent to a TOI (third order input intercept point) of +36.7 dBm.

See attached screenshot:

SDS2504X HD_IMD_500mV_C450MHz_O200kHz_0dBm_L

Please note that this is not a bad result at all. There were (and still are) plenty of low(er) end spectrum analyzers that might not even reach 70 dB third order dynamic range, whereas top instruments can provide up to 110 dB.


--- End quote ---

Agree, this is very good indeed and indicative that the Siglent engineers got this front end design right just like the X+ version, of course they had the advantage of that excellent design to start with!!

This speaks very well for the new HD version, and shows that Siglent took the time and energy to not just replace the 8 bit ADC with a 12 bit version, but to reengineer the front end to support the new 12 bits ADC and not degrade it's performance!!

Makes one wonder just how good this DSO is, we know it's better than -74dBc IMD at 450MHz from your tests. Maybe someone with a really good SA and a couple quality signal sources with much better than -74dBc IMD when resistive attenuator summed can duplicate your tests. Also especially interested if the lower frequency Two Tone Test can be done with the ERES function (if possible within FFT) which I understand is hardware based in the HD version.

Anyway, thanks so much for the testing and reports!!

Edit: Maybe this front end design is from the 6000 series that they did for LeCroy with the 12 Bit ADC??

Best,
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