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Author Topic: True RMS meters and RF signals  (Read 49 times)

m0rjc

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True RMS meters and RF signals
« on: January 12, 2018, 05:07:02 AM »
I was looking at the Brymen 869 meter after reviews here, and thought that its ability to show dBm relative to 50ohm load looked useful. I guess it won't work for RF signals though, as these will be well above the bandwidth of the meter? Could I even use it to find RF signals through the circuit or will these always show up as 0V? Would a non-true-RMS meter perform better if it's using a simple rectify approach to measuring?

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m0rjc

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Re: True RMS meters and RF signals
« Reply #1 on: January 12, 2018, 08:05:02 AM »
Answering myself based on various searches:

Stack Exchange has a comment that any DMM is likely poor at RF, not only due to its measuring technique but also because the impedance of its input at RF is likely to be lower and complex. To date I've used a PC based oscilloscope which is designed for use up to 20MHz. I'd need specialised kit to go higher.

Brymen give accuracy over frequency ranges in their spec sheet http://brymen.eu/wp-content/uploads/biall/102092/102092.KARTA_EN..2015-07-09.1.pdf
Accuracy is spec'd up to 100kHz in the low voltage and current ranges that may be used on an audio pre-amplifier or processing circuit. It drops to about 5% at the higher frequencies, which to be fair is not too bad when thought of in dB.  (20*log(1.05) ≅ 0.5dB)

It could be interesting to compare against other meters including an non-true-RMS. Fluke in their "Extended Specs" for the 170 series (similar price range) only specify an accuracy up to 1kHz. The 280 series specifies 100kHz and impressive accuracy, but costs well over twice as much in the UK.


SeanB

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Re: True RMS meters and RF signals
« Reply #2 on: January 12, 2018, 10:18:48 AM »
The voltage measurement side will probably only run to around twice the spec amount, but only for largish signals for the range. Yes the input impedance will take a nasty dive as frequency rises, but the thing to remember is the DMM does not normally provide the 50R load, it just shows the voltage relative to an external 50R load with the meter in parallel.

The Brymen frequency counter will go pretty high in measurement mode, JoeQSmith on YT did review it, and the input was quite good at showing frequency up in the MHz region.

https://www.youtube.com/watch?v=K-4L2JarVxA

However for best RF measurement it probably would be best to use a RF probe with a diode detector in it, and use the meter as a display only, least loading on the circuit and the least disturbance due to stray capacitance.

 

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