A Really Simple HF True RMS detector

RE: A Really Simple HF True RMS detector

michaelkellett — Thu, 22 Jun 2023 13:27:36 GMT

I've complete the initial pcb design.

After a lot of reflection and thinking about how other devices are put together I decided that it's important that there should be a clear thermal path from the resistor to the "cold sink" at controlled and measured temperature and that heat loss via this path should be the dominant source of cooling for the resistor. I could have done a lot of sums but decided to just make it instead.

The design uses one of [mention:b0bc65b9ecdc4307bd967592f00e340a:e9ed411860ed4f2ba0265705b8793d05] ideas for layout with the resistors soldered to the pcb and the thermistor glues crosswise on top. The twist is that I shall machine most of the board away from the back rather than just have a hole under the resistor. The machining will be 4mm diameter and I'll attempt to get to within 0.2mm of the top surface of the board.

It'll be a couple of weeks before i see any boards.

The additional 4 thermistors are to measure the temperature of the board. It has 4 layers and will be 0.8mm thick so a lot of it is copper.

RE: A Really Simple HF True RMS detector

ggabe — Tue, 20 Jun 2023 15:36:38 GMT

I suggest making a pair of detectors - one to receive the signal, the other to balance to the same thermistor output level with a DC stimulus, just how HP did in their thermal RMS converters. It will bring another challenge - the symmetry of the two devices. Would be easy to characterize though, under DC conditions.

RE: A Really Simple HF True RMS detector

shabaz — Mon, 19 Jun 2023 18:19:28 GMT

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.

RE: A Really Simple HF True RMS detector

michaelkellett — Mon, 19 Jun 2023 17:06:52 GMT

1t least = at least