Author Topic: Pira175 as a power meter??  (Read 2927 times)

radiohead

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Pira175 as a power meter??
« on: September 20, 2013, 07:31:31 am »
Well I did my first acceptance test with our Regulator using the P175 the other week.  Deviation, pilot, RDS etc was aligned perfectly - I think the guy from the Regulator was quite impressed!   10/10 for the P175.

The one thing that was not correct was power output level - unsurprising as I don't have a power meter.  What did surprise me was how inaccurate the power meter on the TX was (a BW TX50).  It was 2.8dB out.

I was therefore wondering if there is any potential to use the P175 as a rudimentary power meter to get better accuracy than 3dB?   Obviously it needs adequate attenuation to get below the maximum input level of the P175 (+7dBm, but personally I would not go above 0dBm).  I know it clearly says in the manual the thing cannot be used as a calibrated receiver - but I wonder if it is "good enough"...

If I've done the maths right, then dbuV = dBm+77.   ie: 0dBm = 77dBuV  in 50 ohms.

If anyone has access to a calibrated power meter I'd be interested to see results for a few frequencies across the band and a few different power levels (say 0, -10, -20dBm).   If a few people have access then it will also be interesting to see if the measurement accuracy across a few units is consistent enough to generate an average calibration curve.

Disclaimer - if you want to do this, don't exceed the maximum input of your P175!!!

Jan

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Re: Pira175 as a power meter??
« Reply #1 on: September 20, 2013, 02:46:11 pm »
The P175 is a radio receiver. It cannot provide <3 dB accuracy on absolute RF power metering. Receivers are not suitable for RF power metering at all due to additional functional blocks preceding the signal level detector. Such blocks include tunable input bandpass filter, front-end amplifier, 1st mixer, IF filters, IF amplifier. These blocks are necessary for receiving the signal via antenna, however each block adds some portion of inaccuracy for RF power metering.

Another problem is the attenuation. It is not only an attenuator what you need. In fact, you need a shielded accurate inductance-free 50 ohms dummy load with ability to sustain full RF power of the transmitter. Once having such device, you can use a simple method of RF voltage measurement. The result will correspond to the real operational output RF power if your antenna has a SWR of 1.

radiohead

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Re: Pira175 as a power meter??
« Reply #2 on: September 20, 2013, 03:56:38 pm »
OK thanks Jan - very clear.   I guess my hopes were too high - encouraged by the 1dB resolution of the dBuV display in the software and LCD display ;-)

Luckily I do have a good, resistive 20dB attenuator/dummy load, so I'll try that RF voltage technique.  I've never done that before (always had access to proper power meters before now!).  Looks like I need to knock-up a simple diode/capacior circuit, do some maths and then check if it measures a known power level correctly....  If you have any tips please say!