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| Two Tone Test with Scope and SA |
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| Performa01:
I love challenges. Quite obviously, we need a trustworthy low distortion dual tone signal in order to perform meaningful IMD tests. To generate this seems to be a challenge in some cases already – all the more so to confirm it without a high-dynamic range SA like the R&S FSEA30. In my case it was a matter of finding the sweet spot on the existing average SA. By this I mean a combination of input signal level, input attenuation and IF-gain that adds as little distortion as possible. My tests didn’t reveal any hints where the distortion actually comes from. Changing the input level, by internal and/or external attenuation didn’t show the expected behavior. If 1 dB lower drive level doesn’t make the 3rd order IMD products drop by more than 1 dB (theoretically 3 dB), then the analyzer frontend shouldn’t be the source of the distortion. If on the other hand, additional attenuation at the generator outputs, i.e. additional isolation, doesn’t change anything, then the generator shouldn’t be the culprit either. That leaves the power combiner/splitter, which might introduce non-linearities even when it’s purely resistive – at least when we’re approaching distortion levels in the realm of -100 dBc. But my earlier tests got -92 dBc IMD, with a generator output of only +6 dBm and the isolation pads to the power combiner were 10 dB each, with a 20 dB attenuator at its output for a total of 36 dB of attenuation. The actual test setup is the dual channel AWG with +10 dBm output levels for each tone, i.e. 4 dB higher than before. These signals are connected to the power combiner via 20 dB inline attenuators, but the output of the resistive wideband combiner is now terminated by only a 10 dB inline attenuator. Consequently, there is 36 dB total attenuation again. The resultant -26 dBm signal is then fed into the SA. The attached screenshot shows what I have achieved when taking advantage on the sweet spot of the SA. Ref_10MHz_O1kHz_-26dBm_Iso40dB Finally I see distortion figures better than -100 dBc 😉 |
| Performa01:
The question about the internal combiner in the Siglent AWGs has been answered in reply #238 here: https://www.eevblog.com/forum/testgear/how-much-noise-floor-and-other-things-matter-in-oscilloscope-usability/msg3906974/#msg3906974 --- Quote ---The integrated digital combiner does indeed generate some IMD products on its own, so it is not suitable for reliable 3rd order dynamic tests. IMD can be as bad as 45 dBc at high output levels. --- End quote --- Of course, the -45 dBc are a worst-case figure and at lower frequencies and/or levels it performs a lot better than this, yet it cannot even come close to a proper external power combiner. |
| rf-messkopf:
Okay, I've re-run my measurement with the ZFSC-2-6-n+ splitter, but now with 20 dB pads at each input, plus a 6 dB pad at the sum port. That gives me 70 dB of isolation between the two signal generators. Using the same generators (R&S SML and SMU200A at 10 dBm each) I get exactly the same results as before, i.e., 3rd order products at about -75 dBc. I've also increased the reference level so that the fundamentals are now approx. 20 dB below in case there is intermodulation at an IF stage in the analyzer. Next I tested the same setup with two function generators (Keysight 33521B and an old HP 3325A). Again, exactly the same result. Except that the function generators spit out some spurs (I think it's mainly the 3325A) -- see attachment. I'm stating to believe that it is the darned splitter itself that is causing the IMD. |
| mawyatt:
--- Quote from: Performa01 on June 20, 2022, 04:46:34 pm ---The question about the internal combiner in the Siglent AWGs has been answered in reply #238 here: https://www.eevblog.com/forum/testgear/how-much-noise-floor-and-other-things-matter-in-oscilloscope-usability/msg3906974/#msg3906974 --- Quote ---The integrated digital combiner does indeed generate some IMD products on its own, so it is not suitable for reliable 3rd order dynamic tests. IMD can be as bad as 45 dBc at high output levels. --- End quote --- Of course, the -45 dBc are a worst-case figure and at lower frequencies and/or levels it performs a lot better than this, yet it cannot even come close to a proper external power combiner. --- End quote --- Yeah, forgot about that thread since it's 1/2 year old...thanks for pointing there!! My short-mid term memory is getting worse with age, but long term seems to holding up well. Need to get a much of those long term experiences documented before they disappear!! Not surprised the analog combine outperforms the digital combine in some cases. Siglent likely chose the easier digital path since the source of all these non-linearities are quite involved as has been shown. Anyway, very interesting discussions. Best, |
| G0HZU:
To explore the limits of a decent modern swept spectrum analyser (swept LO with a wideband digital IF) it's best to use wide tone spacing and then select a very narrow span to zoom in and just look at the IMD tone itself. This assumes the analyser has a suite of analogue pre-filters ahead of the ADC and this is generally the case for a high end spectrum analyser. These pre-BPFs will typically be 2 or 3 times wider than the digital RBW filter and the analyser should select the optimum pre-BPF automatically. The BPF protects the wideband ADC from having to cope with the main test tones if you set the span much narrower than the tone spacing. It should be possible to achieve the SFDR on the datasheet for the analyser using this method assuming the signal generators and the combiner are not limiting the performance. With a RBW of 1Hz some analysers should manage 115dB IMD3 SFDR. Older classic analysers like the HP 8568B use analogue RBW filters and this analyser can typically achieve just over 100dB IMD3 SFDR using the smallest RBW of 10Hz. This classic old analyser typically has a mixer IP3 of +13dBm and a DANL of -140dBm in a 10Hz RBW. So the IMD3 SFDR will typically be about 102dB. |
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