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how to choose an active oscilloscope probe for 0-2v antenna measurements?

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David Hess:

--- Quote from: edavid on January 22, 2019, 06:23:32 pm ---NFC operates at 13.56MHz.
--- End quote ---

This would be a good candidate for a home built active probe like that shown below; replace Q1 and Q2 with your favorite RF JFETs.  In the past when faced with this problem, I just included a JFET input based probe as part of the circuit which went unpopulated in production.

k8943:

--- Quote from: David Hess on January 23, 2019, 04:30:03 pm ---In the past when faced with this problem, I just included a JFET input based probe as part of the circuit which went unpopulated in production.

--- End quote ---

If you were making large numbers of boards, might guess you had access to commercial active probes, so are you suggesting a built-in circuit gives a better result?

Initially thought "yippee, maybe I can find a dual SMT package RF JFET.... and maybe the gain stage is not really necessary....". Then started looking at datasheets and became more concerned. Then followed the reasoning behind a related circuit in HH and now have the impression this is a project in itself!

k8943:
@davidh A day or two later and find the appeal of your in circuit approach is only growing. Found modern SOT-23 equivalents (‎MMBF5486,  ‎MMBT3904,  ‎MMBT3906 - which assuage some of the earlier concern about unbalancing a finely equilibrated circuit with 47 and 51 ohm resistors) and a nice 3mm*3mm trimpot. Plenty of room left for two holes to wire in the bench PSU!

Since my objective will specifically be to get accurate peak voltage readings (at 13.5Mhz) any recommendations on how to calibrate? Is it reasonable to use DC? Oh, and what about the optional 30M resistance at probe tip?

k8943:
I should be able to get my hands on one.

Is the idea to measure the sig gen output at DC and then crank it up to 13.5Mhz and assume the peaks are the same as voltage measured earlier? Then to calibrate the scope reading accordingly? Presumably one would use a sine wave since that's comparable to the kind of signal to be measured?

David Hess:

--- Quote from: k8943 on January 24, 2019, 11:39:21 am ---
--- Quote from: David Hess on January 23, 2019, 04:30:03 pm ---In the past when faced with this problem, I just included a JFET input based probe as part of the circuit which went unpopulated in production.
--- End quote ---

If you were making large numbers of boards, might guess you had access to commercial active probes, so are you suggesting a built-in circuit gives a better result?
--- End quote ---

Using a probe would have required some type of probe tip adapter built into the circuit anyway and the built in circuit works better or at least good enough unless the bandwidth requirements are too high which is not the case for your application.


--- Quote ---Initially thought "yippee, maybe I can find a dual SMT package RF JFET.... and maybe the gain stage is not really necessary....". Then started looking at datasheets and became more concerned. Then followed the reasoning behind a related circuit in HH and now have the impression this is a project in itself!
--- End quote ---

It is actually better to use separate JFETs in most cases to prevent coupling between them.


--- Quote from: k8943 on January 26, 2019, 01:17:24 pm ---Since my objective will specifically be to get accurate peak voltage readings (at 13.5Mhz) any recommendations on how to calibrate? Is it reasonable to use DC? Oh, and what about the optional 30M resistance at probe tip?
--- End quote ---

For calibration short the input and trim for zero output.

The input resistor is needed if the source is AC coupled to absorb the JFET's gate leakage.

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