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| Two Tone Test with Scope and SA |
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| G0HZU:
No, what matters is the relative phase of the distortion term from stage 1 delivered to stage 2 with respect to the phase of the distortion term generated in stage 2. The theory and equations are at their best when the distortion terms are very low in amplitude compared to the amplitude of the test tones. Note that mawyatt could maybe take a look at the graph in my earlier post and see that his calculation of <1dB uncertainty when there is a 10dB difference in IMD is wrong and my calculation of 2.4dB was correct. The blue curve shows about 2.4dB error at a 10dB difference. |
| mawyatt:
--- Quote from: G0HZU on June 14, 2022, 05:56:13 pm ---Wow, just wow... this is taking much longer than it should... Have a look at page 13 of this white paper from Rohde and Schwarz https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_application/application_notes/1ma219/1MA219_2e_IM_Distortion.pdf If you think they are as clueless as me you could also look at page 21 of this article from Keysight. https://www.testunlimited.com/pdf/an/5980-3079EN.pdf They use the same equation as me. The same equation I've been using for decades. --- Quote ---Nothing overlooked, the two signal generators are NOT coherent, as shown in just about every app note anywhere these source are completely independent sources --- End quote --- You simply aren't listening to what I'm saying. The generated distortion terms are coherent in both stages. That's the point I'm trying to make. --- End quote --- Agree it's taking too long, and I'm listening very well thank you!! These are good references for a swept SA and assumes a separate DUT is being evaluated and measured by the SA. Whereas the DSO under discussion here are FFT based and the actual DUT is themself being measured, so effectively DSO measuring themselves. These references also state the signal sources are coherent, whereas we've stated all along they are not coherent, and as such can use the RSS approach which assumes incoherent signals. With coherent vs. incoherent signals this is likely the source of the 3dB difference we are discussing, and maybe even the swept SA vs FFT based analysis. Also suspect if the 2 Tone IMD beat waveform were analyzed exactly over a integer number of complete cycles which might be difficult with a conventional SA, then the results might converge to a similar value. Here's a couple papers on IMD for ADCs, and this is actually what's being discussed regarding the Two Tone Test, that and the scope input. https://www.analog.com/media/en/training-seminars/tutorials/mt-012.pdf https://www.ti.com/lit/an/slyt090/slyt090.pdf?ts=1655216827186 This one discusses placement of the two tones so the IMD products fall within the FFT frequency bins. https://pdfserv.maximintegrated.com/en/an/AN728.pdf Anyway, interesting discussions. Best, |
| mawyatt:
--- Quote from: G0HZU on June 14, 2022, 06:53:17 pm ---A good practical example would be to take two identical 12dB gain amplifiers and put them in series with an attenuator in between them that has 12dB attenuation. For a narrowband system the IMD of the first and and the second amplifier usually sums in phase so the IMD levels for the two amplifiers in series will usually be about 6dB worse (higher in level) compared to the case where you just measure a single amplifier on its own with no attenuation. This is because the two IMD terms sum together in phase so you get twice the voltage (four times the power) hence a 6dB increase in IMD level seen with the dual amplifier setup. --- End quote --- A) With the attenuator the total IMD should be just slightly more than the IMD of the second amp since this distortion products of the 1st are attenuated by 12 dB. B) But what would the IMD be if the single amp had the same gain (read twice) as both amps in series wo the attenuator, 6dB worse maybe?? --- Quote ---If you won't take my word for it then read the app notes I linked to in post #109 written by R&S and Keysight. There is a graph there if it helps. The graph shows that the IMD level rises 6dB when the IMD level in stage 1 is the same as the IMD in stage 2. The two identical amplifiers in series (with an attenuator between them) is a neat way to demonstrate this. It can be demonstrated on a decent RF simulator or you can do the tests on a pair of real amplifiers and use a decent spectrum analyser. I've demonstrated both methods in the past. The result is usually very close to a 6dB increase. This assumes the IMD terms generated in each stage are in phase so they sum and this gives a 6dB boost. See the blue curve below. It shows a 6dB boost just as I told you it would. --- End quote --- Yes, saw the graph and it clearly states coherent input sources, not incoherent sources as we've stated all along as this is the basis for utilizing RSS as stated. Edit: Added here Figure 2–17 Amplitude Uncertainty due to Two Coherent 8.00 CW Tone Adding 6.00 Together Best |
| nctnico:
--- Quote from: G0HZU on June 14, 2022, 04:48:39 pm ---What matters here is the relative phase of a small IMD term generated in stage 1 (and this IMD tone is then amplified by stage 2) vs the phase of the IMD term actually generated in stage 2 from the two main test tones. If the phase of these is not the same then there will be some cancellation in stage 2. Do you understand it now? --- End quote --- After reading your other posts: what you are saying is that the measurement uncertainty depends on whether the intermodulation products of various devices in the chain are in phase or not? For two identical amplifiers the assumption is that the intermodulation products are in phase and thus -worst case- add up (no matter whether the input signal is a single tone or multiple tones). Edit: better wording |
| G0HZU:
In the case of a DSO I guess a lot depends on the signal handling performance of every stage of the analogue front end before it gets into the digital domain. All these stages can generate IMD ahead of the ADC. The same applies to a spectrum analyser. Often it's the front end that defines the IMD performance. --- Quote ---A) With the attenuator the total IMD should be just slightly more than the IMD of the second amp since this distortion products of the 1st are attenuated by 12 dB. --- End quote --- I'm confused why you come to this conclusion. This is for a classic two tone IMD test. The 12dB attenuation is offset by the 12dB gain of the first stage. So the second stage is being driven by a signal that now has (say) -60dBc IMD. The second stage will amplify these IMD terms by about 12dB but it will also generate coherent -60dBc IMD terms of its own at a similar level at the output. These two mechanisms are usually in phase so the IMD terms will appear 6dB worse for the dual amplifier compared to the single amplifier case. The single amplifier is being fed the same amplitude drive level but there are no IMD terms in the drive signal. So it just generates IMD3 terms close to that predicted by the datasheet for that drive level. The dual amplifier will have IMD terms 6dB higher. |
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