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A Dilemma when evaluating a RF Power Sensor

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I bought a Agilent 8481D diode power sensor at a great price and wanted to verify it’s linearity and sensitivity down to its rated -70dBm or better.  I have a R&S SM300 signal generator and a Siglent SVA1032X and I know these correlate well if I set the SM300 for any signal greater from -100 dBm signal and up.  I also have a recent NIST traceable calibrated HP438A (I am the original owner) that has worked fine for many years.  After calibrating the 8481D sensor at 50MHz to the HP438A, I found the lowest power I could read was with the SM300 set down to -55 dBM.  Any lower on the SM300 and the power meter/sensor just stayed at -55dBM.   OK maybe the sensor had seen better days,  but then I noticed that when I turned off the SM300 RF, the power level on my HP438A was still at -55dBm.    I disconnected the sensor from the SM300 output and the HP438A+sensor reading dropped to -90 dBm.     Now I wondered if my SM300 was leaking RF even with RF switched off,  so I hooked the SMA300 (with RF turned off) it up to my SA with same coax and looked at many frequency segments from 1 Mhz up to 3GHz and could not find any leaking signal and the floor remained below -90 dBm.   All instruments are grounded together and I did an quick check around the workbench using the SVA1032x and some EMI probes and saw nothing.   I even wrapped the sensor with copper foil but no difference. As a final check that maybe the sensor was somehow screwed up – I used a Rigol signal generator and some attenuators and found the 8481D sensor and HP438A read RF power accurately down to -80 dBM.   

Any suggestions as to what’s going on?

Did you check the whole range of frequencies? These sensors will measure average power across the entire range so if you have harmonics you'll measure those.

You might also want to set it to trace max hold in case you're getting spikes rather than constant values.

Finally, are you getting any DC out on the SM300? The 8481D will have a DC blocking cap but maybe that cap leaks.


Is it possible that the SM300 leaks 50/100Hz from the PSU ? It wouldn't be visible on the SA as it starts from 9kHz.

I know it doesn't apply as they're not diode sensors, but even if they're specified from 100kHz onwards my 8482A and 8483A seem to be able to pick up signals that low.

Edit: that would be 60Hz/120Hz on your side of the pond :).

Thanks for the suggestions.  DC block didn't help.  I did check different freq ranges and no spikes.  Since the sensor is a diode, I think it would be relatively impervious to spikes/transients, these would need to be pretty big to give an average power reading that is 20dB over the background noise power floor (-80 to -90dbM)

Weird huh?

George Edmonds:

When calibrating the 8481D sensor to the HP438A power meter 0dBm output did you have the 11708A 30dB attenuator in place?

George G6HIG


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