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Two Tone Test with Scope and SA
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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 ---

Like I said, your calculations above are totally incorrect. I think you should have used this equation for the case where the terms are in phase.

Amplitude Error = 20*log( 1+ 10^(d/20))   where d is a negative number that represents the difference between the two IMD levels.

In your analysis a 10dB separation delivers <1dB uncertainty. This is obviously wrong and the correct answer is just under 2.4dB.

mawyatt:

--- Quote from: G0HZU on June 14, 2022, 04:40:16 pm ---
--- 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.

--- End quote ---

Well let's just go over those very calculations that you infer are wrong.

Scope IMD is -77dBc and Source IMD is -77dBc,

then -77dBc = 10^ (-77/20) or 10^-3.85 or 141.25 E-6 or 19.953 E-9 squared

So 19.953 E-9 + 19.953 E-9 = 39.905 E-9

Square root of 39.905 E-9 = 199.76 E-6

10 base LOG of 199.76 E-6 = -3.6995

20*(-3.6995) = -73.99 or ~ -74dBc

BTW you can perform the above using dBc as power ratio, i.e. 10*Log rather than voltage ratio which is 20*Log and the result is the same as it should be, since dBc is a ratio of like terms!!

Care to point out where this is wrong!!! How about you post your calculations here and shown how you come to the incorrect conclusion that this is all wrong!!

Best,



G0HZU:
What you are overlooking is that the IMD terms are coherent. If you combine two coherent terms in phase the level on the spectrum analyser goes up 6dB with respect to the power of each tone and not 3dB.

I'm not sure I need equations to prove this as this should be common knowledge.

You have made the common mistake of just doubling the power. The power goes up by 6dB in the in-phase/coherent case which is a multiplication of four not two.

Put the numbers in to my earlier equation for the case where the IMD terms are at the same level. This is when d = 0.  You should get 6.02dB as the answer (not 3.01dB).
G0HZU:
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. So you end up with the same 12dB gain of a single amplifier. Then compare the narrowband IMD level of this arrangement to the IMD seen on a single 12dB gain amplifier with no attenuator. Note that this analysis assumes a perfect measuring tool. Or you could consider the second amplifier stage to be the limiting factor of the measurement tool if you like.

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.

mawyatt:

--- Quote from: G0HZU on June 14, 2022, 05:08:29 pm ---
--- 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 ---

Like I said, your calculations above are totally incorrect. I think you should have used this equation for the case where the terms are in phase.

Amplitude Error = 20*log( 1+ 10^(d/20))   where d is a negative number that represents the difference between the two IMD levels.

In your analysis a 10dB separation delivers <1dB uncertainty. This is obviously wrong and the correct answer is just under 2.4dB.

--- End quote ---

Think you are talking about your very own calculations being "totally incorrect"!! And your thinking is totally incorrect!!

The simple straightforward analysis we have shown above shows that the result we arrived at is correct as stated, best review how and where to use the Amplitude Error equation you proposed.

Best,
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