New coax cable assembly causing increase in current consumption

[setebos_rc]

Hi all,

UPDATE: I feel pretty silly, but I think I've figured out that the signal is getting picked up in the leads going to the Joulescope causing a false value to be read. The difference between when I tested various cables is that I had the cable running outside the enclosure and away from the wires. If I stick the antenna right up next to the wires the current spike gets even higher but the voltage drop stays the same. 

I'm working on a project that involves a battery-powered wireless device that periodically collects some sensor data and transmits it to a gateway. I am using an XBee-900HP Pro RF module for the wireless communication which specifies that the transmit current is ~215mA on the highest power level. When I first started designing this I was using an XBee module with a U.FL connector and a U.FL->SMA cable (DigiKey PN: ACX1893-ND, they don't seem to stock this anymore) to connect the antenna. I analyzed the power consumption using a Joulescope using this setup and confirmed it was drawing about 200mA or so during a transmit pulse.

At some point I had issues finding stock of the U.FL model XBees and had to purchase some RP-SMA models, so I switched to a different cable assembly (DigiKey PN: 2072-CABLE234RF-0200-A-1-ND). After making some changes I tested the power consumption again to make sure it was still as expected, and I noticed that the total power consumption was quite a bit higher than before. I confirmed that the current was the same as before during sleep/data collection, but during the transmit pulse it was now measuring ~600mA for the same duration. I plugged the U.FL model XBee into the same board with the original connector I was using and it was back to 200mA pulses. I had another RP-SMA to RP-SMA cable assembly laying around from a different manufacturer (DigiKey PN: 2868-CBG-SARF3OGTX9-SARMGT-8IN-ND) so I tried that as well and with that cable I was measuring 300mA pulses.

Does anyone know what would be causing these big differences? The two RP-SMA cables I tried are both the same length and both use RG-178 cable, just from different manufacturers. Could it just be related to the quality of the cable/connectors?

Thanks.

[radiolistener]

such behavior may happens if cable has a bad assemly which leads to a high impedance change for RF path. As result it may lead to a bad VSWR and as result to a higher power consumption for output amplifier. Such issue may damage your output RF amplifier, because it is not intended to work in such mode.

[buta]

As mentioned above, the cable assembly is the first component to be checked for open, short and VSWR.

Assume the SMA is connected to a 50R termination or an antenna.  Have you applied the same way to mount the SMA connector in both measurements?

 

[mliberty]

Hi @setebos_rc,

[disclaimer: I am the Joulescope creator and owner of Jetperch LLC]

Which Joulescope are you using?  The JS110 is susceptible to rectifying powerful RF signals, so you do need to be careful with the test setup.  You can find more info on the Joulescope forum:

https://forum.joulescope.com/t/radio-transmissions-causing-high-current-readings/578

The JS220 is now fully symmetric and includes improved RF filtering so it performs much, much better in the presence of powerful RF transmitters.

However, for both instruments, you want to avoid near-field coupling effects.  A rule of thumb is to separate the transmitter/antenna and the Joulescope by at least one wavelength.  Also, twist the cables to minimize the loop area.