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Old 28th May 2022, 4:46 pm   #1
6SN7WGTB
Hexode
 
Join Date: Apr 2021
Location: Crawley, West Sussex, UK.
Posts: 442
Default Bit confused with RF power readings

I have a RF voltmeter that has a dB scale marked "1mW 600Ω".

The actual meter impedance is >1MΩ.

I have connected my pantry Tx to this via a 50Ω load.

I get -15dBm, which I believe equates to about 32µW.

But the meter has this 600Ω reference which is confusing me.

If I use a spectrum analyser with same 50Ω termination (plus a 30dB attenuator) I get -45dBm, so I'm back at -15dBm.

So, is my output into 50Ω the 32µW I seem to be reading?
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Old 28th May 2022, 8:24 pm   #2
Mr 1936
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Join Date: Mar 2017
Location: Romsey, Hampshire, UK.
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Default Re: Bit confused with RF power readings

Hi

Your RF voltmeter is just that, it reads voltage. For convenience it also has a dB scale but this is only calibrated for (Mainly Audio) measurements in a 600 ohm system. If it is presented with 0.775 volts RMS, it will also display 0 dBm i.e. 1 mW

If you drop the level to 0.138 volts RMS, it will display -15 dBm i.e. 31.6 uW

In your case, your system impedance is 50 ohms so the meter's dB scale will not tell you the correct power in 50 ohms. It will under-read by 10*log(600/50) which is 10.8 dB i.e. a factor of 12 in power.

If you add 10.8 dB to the displayed level of -15 dBm you get -4.2 dBm i.e. 0.38 mW which is the power in 50 ohms.

Add a 30 dB pad and the level should drop to -34.2 dBm. I don't know why you aren't seeing this on your spectrum analyser.
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Old 29th May 2022, 11:52 am   #3
6SN7WGTB
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Join Date: Apr 2021
Location: Crawley, West Sussex, UK.
Posts: 442
Default Re: Bit confused with RF power readings

Thanks for clarifying the ratio issue due to impedance mismatch.

The RF attenuator has a 50Ω impedance so that would surely explain the SA result being the same with the 30dB pad?
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