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  • Author Author: michaelkellett
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A Really Simple HF True RMS detector

michaelkellett

In a recent thread Shabaz was wondering how easy it might be to make a wideband true RMS detector  using a surface mount thermistor and resistor.

This is a first very crude and simple experiment to find out.

I soldered tiny bare wires to a 10K 0603 thermistor and a 100R 0805 resistor and glued the pair together. Its supported by its wires on a piece of strip board.

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It needed a little bit of wind shielding since its a warm day and I had a fan on.

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I drove the resistor with a range of pulses and settled on 0.66V at 0.01Hz 50% duty for plotting. This is 4.35mW. An 0805 resistor can take 1t least 100mW so we are operating at 13.6dB below full scale.

image

The response time is a bit slow but I think 0603 or 0402 parts would sort that !

The signal was clearly detectable but rather swamped by ambient thermal noise at 70mv (-43dB ref 100mW).

A differential sensor with an ambient sensing thermistor and better thermal design might well fix that.

Time for tea, more later.

MK

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  • Hi Michael,

    Very interesting results! That's good to know that 43 dB could be possible. Nice to see the clean output at just 4 mW. The long settling time doesn't seem so bad, since perhaps it might reduce to 10 sec or so if the design is scaled down, and then the delay for a measurement to me at least seems fine considering the cost. Even a multimeter can take a few seconds to auto-range and settle down.

  • in reply to shabaz

    I did a pretty rough and ready attempt at measuring frequency response.

    No change in sensitivity (discernable with this rig) between 1kHz and 20MHz (didn't try any higher frequencies yet, need to trim the tracks on the stripboard first !). I'll need to set up something better for measuring and do a proper frequency sweep.

    I'm thinking that the best way to operate this sensor would be at constant temperature - keeping the sensor at 50C +/- 0.01C (or better) shouldn't be that hard or power hungry. Trying to temperature compensate with thermistor bridges requires very closely matched thermistors, and that it not easy to do with 0402 (or even 0603) parts.

    A constant temperature instrument  should be able to self calibrate with DC.

    MK

  • in reply to michaelkellett

    Couldn't resist doing another quick measurement without any additional changes, approx -2dB at 100MHz.

    Not bad !

    MK

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