Author Topic: Can't seem to actually get +20dBm out of RFM69HCW  (Read 304 times)

mah115

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Can't seem to actually get +20dBm out of RFM69HCW
« on: February 13, 2020, 06:39:12 PM »
Hi, has anyone actually measured +20dBm out of the RFM69HCW radio?  I can only seem to get between 17 and 18 dBm measured with a spectrum analyzer.  I'm using the M0 Moteino.  Might there be some bit we have to set somewhere to get the full power?  Or is that power level only achievable with specific settings?

Felix

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Re: Can't seem to actually get +20dBm out of RFM69HCW
« Reply #1 on: February 14, 2020, 01:38:44 PM »
Can you please explain how you make that measurement too, it is greatly important.

mah115

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Re: Can't seem to actually get +20dBm out of RFM69HCW
« Reply #2 on: February 16, 2020, 05:49:30 AM »
I'm using one of the methods described by the FCC for testing: I'm setting the RBW and span of the spectrum analyzer to 1MHz, and setting the trace to max hold mode, detector to peak, and the measuring the recorded peak after the trace has stabilized.

Felix

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Re: Can't seem to actually get +20dBm out of RFM69HCW
« Reply #3 on: February 18, 2020, 09:29:14 AM »
Ok and how do you receive the signal?

mah115

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Re: Can't seem to actually get +20dBm out of RFM69HCW
« Reply #4 on: February 21, 2020, 04:44:35 PM »
I'm just transmitting packets with a pseudo random payload, the antenna port is connected directly to the spectrum analyzer with coax cable.  I've characterized the cable and lose about 1dB at 900MHz, and I'm measuring 16-17dBm on the spectrum analyzer.

Felix

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Re: Can't seem to actually get +20dBm out of RFM69HCW
« Reply #5 on: February 21, 2020, 06:38:29 PM »
Given the cost of these modules I would not expect miracles, and would say just 1dB loss is great.
So 1db lost in cable - you know about that, hence you expect ~19dB.

Assuming analyzer and procedure are perfect, I fully expect another 1-1.5dB is lost in translation (ie. RF switch, impedance matching imperfections, PCB signal routing discontinuities). Think about how many twists and turns that signal has to go through from exiting the TX pin of the transceiver chip, and into the port of the analyzer. So you loose 1-2dB which lands you at 17-18dB you first mentioned you get.

But since you're chasing that last dB - do you actually need to reach full 20dB?

As soon as you add the elephant variable - ANTENNA! - that starts attenuating in ways you cannot fully control (mostly based on antenna type/parameters).

mah115

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Re: Can't seem to actually get +20dBm out of RFM69HCW
« Reply #6 on: February 23, 2020, 12:47:07 AM »
It's not so much about chasing that last dB, it's more about verifying that my register settings are correct, especially in light of the setPowerLevel() bug which was robbing me of another dB before I fixed it..  +18dB is 64% of +20dB, I didn't expect to lose 30% of my power in the traces, but maybe this is normal?

Felix

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Re: Can't seem to actually get +20dBm out of RFM69HCW
« Reply #7 on: February 24, 2020, 03:55:34 PM »
It's not so much about chasing that last dB, it's more about verifying that my register settings are correct, especially in light of the setPowerLevel() bug which was robbing me of another dB before I fixed it..  +18dB is 64% of +20dB, I didn't expect to lose 30% of my power in the traces, but maybe this is normal?
What was your fix for the setPowerLevel() bug?