Radio transmissions causing high current readings

Technical Discussion
1

I have a JS110 and I am using it to test a small battery powered powered sensor that transmits in the sub-GHz range ( <= 1W at ~400MHz). The current when idle is correct, but when transmitting I measure several amps, when it should be in the 450-600mA range. Other measurement methods give the expected measurement (sense resistor, PPK).

I saw the discussion in a different thread about how RF transmissions could cause the higher current readings. I followed the advice there to place the joulescope as far away as I can (about 4 feet) and twist my power wires. This does seem to solve the problem.

I have two questions:

  1. is anything in my description surprise? or does it more-or-less track with what others have seen.
  2. Is this mitigated at all in the JS220?

Thanks!

2

Hi @eslattery and welcome to the Joulescope forum!

Yes, the JS110 is affected by high-power transmitters. The JS220 was designed to improve RF rejection significantly.

While the JS110 has some RF filtering, 1W is a lot of power compared to the JS110’s 20 mV burden voltage. The JS110 measurement range for the shunt resistor voltage is asymmetric and saturates at approximately +22 mV and -5 mV. When overloaded and saturating, this rectifies large RF signals to introduce a positive offset. I am not surprised that 1W at 400 MHz causes problems. 1W is 20V p2p in a 50 Ohm transmission line. 400 MHz => 0.73 meters. So 4’ => 1.2 meters is 1.6 wavelengths, enough to avoid near-field effects.

Yes, the JS220 is designed to have significantly more RF rejection filtering. In addition to improved filtering, it also has a fully symmetric response. This symmetric response means that even if the JS220’s frontend does become overloaded with RF, it will not rectify the signal. However, in all cases, it is best to configure your setup to avoid huge RF signals coupling into your sensitive current measurement signals.

3

Thank you for the quick reply, and the technical details. You are certainly right about about the setup, and I have some plans now to improve the layout on my workbench. But I was also looking for an excuse to upgrade to the JS220, so I am glad to hear that it could also help a bit :slight_smile: .

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