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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.

image

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

image

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

    Hi Michael,

    That's really impressive! Regarding your comment to keep it at a constant 50 deg C, I searched Google for info on small ovens, looking at OCXO teardowns etc, and have not come up with anything conclusive, except that some DIY OCXOs use a BJT driving a resistor, and an op amp and temperature sensor to control it.

    For a suitable PCB for the thermistor and resistor, I don't know if this sort of thing could work in the diagram below. It's intricate work though, since that hole would be just 1.1mm diameter.

    Alternatively, are you thinking it could just be a normal SMD footprint for the resistor, and then some slots cut around it, to keep it low thermal mass, and then glue the thermistor on top? It may be a lot easier, and still fairly low thermal mass.

    image

  • in reply to shabaz

    I was thinking of making  a board with two footprints, both variations on interleaving resistors and thermistors on standard footprints.

    (One with two resistors with a thermistor in between and another with three resistors and two thermistors. I suspect that with temperature control of the board one thermistor will work better.

    But now I've seen your suggestions I think the board might need some extra patterns to try.

    I was hoping to get it to work with standard pcb assembly, but I think it will almost certainly work better with a more custom mechanical arrangement.

    MK

  • in reply to michaelkellett

    Neat idea to have a testboard, the interleaving sounds interesting! Also, two resistors allows for more standard 100 ohm resistors to be used rather than 49.9 ohm. 

    Regarding the hole method, I just realized that diagram above had an issue, in that the right side had a lot of thermal conduction. 

    The lengths are so small at this scale, that some additional inductance will have no significant impact as your 100 MHz result shows:

    image

  • in reply to michaelkellett

    Another idea for the PCB layout:

    In this layout, the hole is 0.8mm. Since a typical resistor is a bit thinner than a typical thermistor, the resistor is soldered to the PCB first, it is the orange part in the diagram. It is likely about 0.3mm tall.

    Them, the blue thermistor is glued on top, and then soldered (the vertical gap is merely 0.3mm since that's the thickness of the resistor underneath, so it's easy for solder wire or paste to accommodate it.

    image

  • in reply to shabaz

    I quite like this one - I may get time to do a pcb today.

    I've been thinking about thermal isolation from the rest of the board.

    MK

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