Electronics > Projects, Designs, and Technical Stuff
how to choose an active oscilloscope probe for 0-2v antenna measurements?
k8943:
Following NXP guidelines for selecting the right passive components between antenna and Rx pins of PN7462 NFC controller, they recommend using an Active probe with a low capacitance (<2-3pF) for taking voltage measurements at the pins during communication.
I looked on a few sites and had the impression that there are two kinds of active probes:
1) those that are designed for lower voltages (say up to 20v or 50v) and cost around 1000USD upwards;
2) those that seemed to be designed with measuring higher voltages in mind - and whose pricing starts lower down;
The least expensive active probe on Digikey looks to be: https://www.digikey.be/product-detail/en/cal-test-electronics/CT4066/CT4066-ND/9342238
Since this one seems to offer the required capacitance am wondering if it could do the job. (Which amounts to checking peak voltage levels at Rx inputs to the controller chip and making adjustments so they're somewhere around 1-1.5v.)
Probably the best would be one of the "low voltage" probes but so far they all look a bit expensive (given this is all I really think I need it for)?
One of the things that concerns me though is that it comes with two long wires which look like they're going to make a huge pickup loop for noise?
Any recommendations?
macboy:
If the signal is single-ended and ground referenced, then you don't need a differential probe and you can probably use any ordinary FET probe. These typically have an input impedance of 100 kOhm to 1 MOhm in parallel with 1 to 2 pF capacitance, and have a typical bandwidth around 1 GHz. Of course a few very old or very cheap ones have less bandwidth, and the most expensive modern ones can go much higher. You should be able to get a used working one for $100-$250 USD. Note that all FET probes need a power supply, and the Tek ones usually require either a Tek scope with TekProbe connections or an external power supply such as Tek 1103. I use one of those with my Lecroy scope and Tek P6243 probe. Likwise, Lecroy active probes need a matching scope connection, and I haven't ever seen an external supply for those. Used Lecroy probes usually cost more than comparable Tek ones, but used Lecroy scopes themselves are usually a bargain.
If you can't measure the signal w.r.t. ground then you will need a differential active probe, which will cost much more. You still need to think about the power supply.
cdev:
You didn't say what frequencies you want to measure.
You can build your own FET RF probe for under $5 in parts or find kits on auction sites for under $15.
A switch to a broadband high input impedance MMIC right at the input (assuming stability - likely only possible to assure with your own probes) might improve the flatness substantially at a tiny increase in cost. Also you could make small (loop and e-field, or a balun to give you a balanced input- but not a DC coupled one of course. Baluns are designed to cover a certain range - a balun that works great for HF likely wont work so well for VHF or UHF or LF, so you might want the ability to interchangeably use several of them) Making additional probes could extend the functionality a great deal. (That is how commercial antenna measurement equipment do things, some of that gear is basically just what I described, quite simple, but nonetheless, typically insanely expensive).
edavid:
Since the voltage you are measuring is reasonably high, and you don't need DC coupling, you can use a capacitive divider instead of a FET probe. Just put a 2pF capacitor between the measurement point and your lowest capacitance passive scope probe.
You are correct in thinking that short leads are required.
Ice-Tea:
Do you need differential measurements? If not Testec may have something for you...
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