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Two Tone Test with Scope and SA
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mawyatt:

--- Quote from: G0HZU on June 14, 2022, 03:12:11 pm ---
--- Quote --- If the source has -77dBc IMD (3dB difference), then the Scope has -77dBc IMD
--- End quote ---

It isn't that simple, you have to think in terms of voltage level and phase rather than power.

--- End quote ---

These were all actually computed based upon the RSS method as outlined above. Here one must convert to actual levels (voltage in the case of the DSO) in linear terms not dB, do the proper calculations, then revert back to dB scales. Since you can't add or subtract linear terms in dB (actually a exponential/logarithmic scale, so adding in dB is multiplying, and subtracting in dB is dividing), you can't do the RSS computations without converting to linear terms, do the squares, then summation, then square root, then convert back to dB scales ....elementary signal processing!!

BTW phase has nothing to do with 3rd order IMD as outlined in this thread about Two Tone IMD, it's all about the mathematical relationship between odd order system non-linearity and a method to represent the DUT in a simple way to convey such. This 2 Tone IMD is utilized in all sorts of various fields and why we wanted to see how well these new DSOs behave. It's also useful in extrapolating the well known RF metric of 3rd Order Intercept as was shown earlier (without sweeping the input), however this extrapolation does have a baseline assumption that the DUT has a well behaved non-linearity and exhibits gain compression as signal levels increase. There is a little know amplifier where this metric is not applicable, where the gain actually expands over a range before going into compression, but this is out of the scope for discussion here (will discuss if folks want, but better in another separate thread).

Anyway, hope this helps clear up any confusion regarding the Two Tone IMD test and how they infer the performance of the instruments and DUT involved.

Best
G0HZU:

--- Quote ---BTW phase has nothing to do with 3rd order IMD as outlined in this thread about Two Tone IMD, it's all about the mathematical relationship between odd order system non-linearity and a method to represent the DUT in a simple way to convey such.
--- End quote ---

When you combine two (same amplitude) IMD tones in phase the voltages sum in phase so you get twice the voltage. This means the level seen on a spectrum analyser goes up 6dB relative to each tone. If the tones are out of phase they cancel so the IMD level will tend to cancel. So I think phase can be important.

Normally with a narrowband test setup the IMD terms tend to be in phase. However, in theory at least, the phase shift can be sufficient to cause significant cancellation of the IMD terms when two non linear stages are connected in series. So you have to consider phase in any analysis.
G0HZU:

--- Quote ---In your example of a -74dBc IMD measurement, if the source has no IMD then the Scope IMD is obviously -74dBc. If the source has -77dBc IMD (3dB difference), then the Scope has -77dBc IMD, if the source has -80dBc (6dB difference) then the Scope IMD is -75.3dBc, if the source has -84dBc (10dB difference) then the Scope IMD is -74.5dBc, and if the source has -94dBc (20dB difference), then the Scope has 74.04dBc. Of course you can do this analysis with a true DUT IMD and include the Source and Scope/Analyzer IMD and find the 10dB separation is sufficient for the 1dB uncertainty margin. This tends to confirm the usual 10dB minimum separation in measurements for a reliable resultant, however because the measurement instrument IS the DUT the 10dB margin actually confirms a 1/2dB uncertainty.
--- End quote ---

If the source and the scope both truly had -77dBc IMD, the resultant IMD would typically appear at -71dBc on the analyser (a 6dB difference and not the 3dB you imply). This assumes the IMD terms are in phase. In reality there might be a small phase shift. If the test was done with wide tone spacings the phase cancellation effects could be more pronounced (in theory at least).

Your other calculations above are wrong as well.

mawyatt:

--- Quote from: G0HZU on June 14, 2022, 04:15:47 pm ---
--- Quote ---BTW phase has nothing to do with 3rd order IMD as outlined in this thread about Two Tone IMD, it's all about the mathematical relationship between odd order system non-linearity and a method to represent the DUT in a simple way to convey such.
--- End quote ---

When you combine two (same amplitude) IMD tones in phase the voltages sum in phase so you get twice the voltage. This means the level seen on a spectrum analyser goes up 6dB relative to each tone. If the tones are out of phase they cancel so the IMD level will tend to cancel. So I think phase can be important.

Normally with a narrowband test setup the IMD terms tend to be in phase. However, in theory at least, the phase shift can be sufficient to cause significant cancellation of the IMD terms when two non linear stages are connected in series. So you have to consider phase in any analysis.

--- End quote ---

Don't think you understand the Two Tone IMD, what you are missing here is that the Two Tones are not the same frequency, but separated by a small fraction of the absolute frequency of either. In the last IMD test graph shown the Two Tones were at ~450MHz, with a 200KHz frequency difference, and both Tones at equal amplitude. So the Two Tones wander in and out of phase and the amplitude varies between twice the individual Tone amplitude (in phase) to zero (out of phase) at the difference frequency rate. Note how the composite amplitude varies between the maximum of twice the single tone level to identically zero, this exercises the total amplitude range of the DUT between these extremes and why this simple 2 tone test is so reveling of the DUT non-linearities.

Best,
G0HZU:
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?

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