Simple E-field probe

Simple probe for sniffing electric field on PCBs.

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Contactless high impedance E-field probe for sniffing noise sources on PCBs.

This probe was made for probing (what else) of electric field. Its input impedance should be about 1 MOhm in parallel with the gate of BF998 (about 1.5 pF). Its output impedance is 50 Ohms (tuned using both simulation and TDR).

Just connect one SMA cable with 9V low-noise power supply. The other one goes into your 50 Ohm terminated oscilloscope / spectrum analyzer / detector.

Use it to find sources of noise or interference on PCB - this includes cheap power supplies, oscillators designed by idiots, unstable amplifiers and so on.

For better directivity, make the "sniffing end" shorter and solder female SMA connector to it. Then use some copper foil or some sheet metal to shield the probe.

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  • 1 × BF998 RF, IF, RFID, ZigBee Semiconductors and ICs / Transistors and FETs
  • 2 × 10k 0805 resistor
  • 1 × 2k 0805 ferrite bead Use some ferrite bead you feel appropriate
  • 1 × 1M 0805 resistor
  • 2 × 100 Ohm 0805 resistor

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Qbort wrote 12 hours ago point

I don't know why you think this project is boring. I think this is fascinating!

Would you be able to write a little bit as to how it works? Some questions below on what you could include:

What makes it an E-field probe (as opposed to "H-field"). I imagine you still pick up some H-field?

Also, what frequency range does it work at? How is the frequency response /directivity?

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nike9307 wrote 03/27/2020 at 15:05 point

Hey man really cool project. How much is the amplification? Standard 12 or 20 dB?

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MS-BOSS wrote 03/28/2020 at 20:46 point

The amplifier is just a buffer, therefore its voltage amplification should be near 0 dB. Its purpose is to have high-Z input  (several Megaohms in parallel with several picoFarads) and 50Ohm output.

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nike9307 wrote 03/30/2020 at 09:39 point

I was under the impression that you needed some application for e filed probing.
Examples - or the Rohde & Schwarz HZ-15 (Probe set) + HZ-16 (amp). 

To be fair, i've never had to measure e or h field. i'm currently looking to get a set of probes + spectrum analyzer for the office, so i'm really curios about the subject. 

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