Products > Test Equipment
Two Tone Test with Scope and SA
<< < (18/33) > >>
G0HZU:

--- 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 ;)
--- End quote ---

Note that the displayed result on the scope or analyser screen can appear artificially higher or lower than the real result due to the linearity limitations of the scope/analyser.

Normally, the distortion performance of the analyser will be fairly well known and understood beforehand and the usual aim is to ensure that the IMD level in the analyser is >20dB lower than the IMD caused by the device under test. This keeps the overall uncertainty of the displayed result below about +/- 1dB.

So to measure -74dBc within +/- 1dB  the scope or analyser needs to be capable of -94dBc IMD. To measure -74dBc within about +/- 3dB  the scope or analyser needs to be capable of -85dBc IMD.

gf:

--- Quote from: G0HZU on June 14, 2022, 09:44:12 am ---
--- 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 ;)
--- End quote ---

Note that the displayed result on the scope or analyser screen can appear artificially higher or lower than the real result due to the linearity limitations of the scope/analyser.

Normally, the distortion performance of the analyser will be fairly well known and understood beforehand and the usual aim is to ensure that the IMD level in the analyser is >20dB lower than the IMD caused by the device under test. This keeps the overall uncertainty of the displayed result below about +/- 1dB.

So to measure -74dBc within +/- 1dB  the scope or analyser needs to be capable of -94dBc IMD. To measure -74dBc within about +/- 3dB  the scope or analyser needs to be capable of -85dBc IMD.

--- End quote ---

But if A+B = -74dBc, can't we still conclude that A <= -74dBc and B <= -74dBc?
[ Btw, this leads to the question: How do IMD products from two sources actually add up? ]
mawyatt:

--- Quote from: G0HZU on June 14, 2022, 09:44:12 am ---
--- 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 ;)
--- End quote ---

Note that the displayed result on the scope or analyser screen can appear artificially higher or lower than the real result due to the linearity limitations of the scope/analyser.


--- End quote ---

The scope or any instrument will always show a result for the Two Tone IMD that is the Root Sum Square (RSS) of the signal source and measuring instrument. Thus, the shown result is the worst case of such, either source or measuring instrument. If the instrument were an older analog display types then there could be a display uncertainty in reading, however these DSOs are actually digital data acquisition systems which utilize ADCs and digital techniques so this isn't an issue.

Can't think of a case were the instrument linearity issues would cause an "artificially lower" IMD result (read better), if you have such a case please provide details.


--- Quote ---Normally, the distortion performance of the analyser will be fairly well known and understood beforehand and the usual aim is to ensure that the IMD level in the analyser is >20dB lower than the IMD caused by the device under test. This keeps the overall uncertainty of the displayed result below about +/- 1dB.

--- End quote ---

Here the device under test IS the analyzer, so the source needs to be better than the analyzer. Again the result is always the RSS of both the source and the analyzer.

--- Quote ---So to measure -74dBc within +/- 1dB  the scope or analyser needs to be capable of -94dBc IMD. To measure -74dBc within about +/- 3dB  the scope or analyser needs to be capable of -85dBc IMD.

--- End quote ---

See above, the scope IS the DUT.

Normally when measuring a device the actual measured IMD result is square root of {(Source IMD)^2 + (DUT IMD)^2 + (Measurement Instrument IMD)^2}, so if the Source and Instrument IMD is << than DUT IMD, then the result becomes the DUT IMD.

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.

Best,

mawyatt:

--- Quote from: gf on June 14, 2022, 02:13:21 pm ---
But if A+B = -74dBc, can't we still conclude that A <= -74dBc and B <= -74dBc?
[ Btw, this leads to the question: How do IMD products from two sources actually add up? ]

--- End quote ---

Yes, the result is the Worst Case of -74dBc IMD for all things involved. See directly above for more details.

Best,
G0HZU:

--- 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.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod