Products > Test Equipment
CMRR measurement, am I doing it right?
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Randy222:

--- Quote from: ballsystemlord on September 12, 2024, 02:24:23 am ---Wow! Thanks so much! I never expected you to go the extra mile to test my setup.

--- End quote ---
I did not test your setup. I only put that feedthrough on my VNA.
1/2 the battle in testing is knowing how all the pieces in the test setup work as individual components, and when all connected together. If the signal entering the scope channel is the thing that determines results, you need to make sure you understand how that input signal changed from actual source, then add in your adjusting factors accordingly. Some test gear might allow you to add those adjusting factors (like an offset) into the gear, others will not and you will need to do it manually.

Golden rule - when a number is way off from expectation, it's either the DUT, or the test setup/config.
ballsystemlord:

--- Quote from: Randy222 on September 12, 2024, 03:09:29 pm ---
--- Quote from: ballsystemlord on September 12, 2024, 02:24:23 am ---Wow! Thanks so much! I never expected you to go the extra mile to test my setup.

--- End quote ---
I did not test your setup. I only put that feedthrough on my VNA.

--- End quote ---

When I re-read what I had wrote before posting I thought, "You should say, 'part of my setup.'" But I decided otherwise because a small exaggeration would better convey my feelings on the matter (or so I thought). I'm saddened to see that it just led to misunderstanding. I'll be more careful from now on.


--- Quote from: Randy222 on September 12, 2024, 03:09:29 pm ---1/2 the battle in testing is knowing how all the pieces in the test setup work as individual components, and when all connected together. If the signal entering the scope channel is the thing that determines results, you need to make sure you understand how that input signal changed from actual source, then add in your adjusting factors accordingly. Some test gear might allow you to add those adjusting factors (like an offset) into the gear, others will not and you will need to do it manually.

Golden rule - when a number is way off from expectation, it's either the DUT, or the test setup/config.

--- End quote ---

In my own case, the setup is so simple I don't really see how anything other than the terminator would be affecting my results. That is to say, my BNC tee and BNC cable. Mind, I'm not disagreeing with you here nor trying to be difficult. I just lack the experience and know how to understand and isolate what I'd need to discover any flaw in the setup. Granted, if I had different equipment I could do A/B testing, but I don't.
Randy222:

--- Quote from: ballsystemlord on September 12, 2024, 11:52:14 pm ---
--- Quote from: Randy222 on September 12, 2024, 03:09:29 pm ---
--- Quote from: ballsystemlord on September 12, 2024, 02:24:23 am ---Wow! Thanks so much! I never expected you to go the extra mile to test my setup.

--- End quote ---
I did not test your setup. I only put that feedthrough on my VNA.

--- End quote ---

When I re-read what I had wrote before posting I thought, "You should say, 'part of my setup.'" But I decided otherwise because a small exaggeration would better convey my feelings on the matter (or so I thought). I'm saddened to see that it just led to misunderstanding. I'll be more careful from now on.


--- Quote from: Randy222 on September 12, 2024, 03:09:29 pm ---1/2 the battle in testing is knowing how all the pieces in the test setup work as individual components, and when all connected together. If the signal entering the scope channel is the thing that determines results, you need to make sure you understand how that input signal changed from actual source, then add in your adjusting factors accordingly. Some test gear might allow you to add those adjusting factors (like an offset) into the gear, others will not and you will need to do it manually.

Golden rule - when a number is way off from expectation, it's either the DUT, or the test setup/config.

--- End quote ---

In my own case, the setup is so simple I don't really see how anything other than the terminator would be affecting my results. That is to say, my BNC tee and BNC cable. Mind, I'm not disagreeing with you here nor trying to be difficult. I just lack the experience and know how to understand and isolate what I'd need to discover any flaw in the setup. Granted, if I had different equipment I could do A/B testing, but I don't.

--- End quote ---

This is why having something like a VNA and SA would be beneficial, you could then characterizes your BNC T and the feedthrough, then adjust your readings accordingly within reason. If the adjusting makes no sense then perhaps CMRR is just horrible and your tests validate that.

Keep studying it.
ballsystemlord:

--- Quote from: ballsystemlord on September 09, 2024, 04:22:35 am ---Hello,
I followed Dave's video here: https://youtu.be/vDe_BHvRpks
And arrived at the following formula:
(20 * log10( RMS_diff_probe / input )) + dB
dB == 20 for 10x probe and dB == 40 for 100x probe.

...

My calculation is as follows:
(20*log(0.018349/2.4508))+30 == -67.8919dB
The CMRR on the datasheet says >= 50dB at 1MHz.
...

--- End quote ---

Serendipity!
I was testing my setup again, and used a TI-85 calculator instead of my PC's. Now I get about 12dB of CMRR. I then plugged the same numbers into my PC's calc but wrote "log10(X/Y)" and got the same result as the TI. I'm surprised no one else noticed!

Thanks for your help guys!

PS: Now I need to see if I can return this very under performing product.
PPS: My PC's calc is doing log1, in case anyone's curious.
EDIT: PPPS: Because I was used to working with the TI, which actually doesn't accept the syntax "log10(X/Y)", I didn't think to try that with my PC's calculator. It's not that I was lazy or something.
Update: I returned it and got a refund.
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