Rms converter to match HP3400A?

www.analog.com/.../1968f.pdf

LTC1968’s bandwidth is not more than 1MHz. 

Frequency response is up to 15MHz.

-3dB is 29% error. The 1968 samples at 2MHz, there will be a response for sure, just accuracy and frequency response will be off of a measurement grade.

My current thinking is an AGC looped AD8361, like on fig38 of https://www.analog.com/media/en/technical-documentation/data-sheets/AD8367.pdf, with the exception I’m about to use a linear VCA, not a decibel scaled, so that the control voltage is the rms output.

I'm also thinking creating two signal paths, for low frequencies up to 1KHz with traditional log-antilog converter or LTC1968. Not sure though how to manage an accurate transition between the two.

Those are interesting parts. I have three  HP435B with no sensors and no prospect of getting any.

It would be nice to get them doing something useful.

The AD eval board for the 8367 is a bit pricey (>£100) but cheapo boards for both designs on the data sheet are available from Aliexpress. The pair cost me £32 which is a bit more reasonable.

Won't arrive to July. May work, may not but worth a punt. At the worst I can replace the chips with AD ones

MK

I've always wondered if it could be possible to buy a 50 ohm (or 600 ohm) 0402 resistor, and super-glue a 0402 thermistor onto it, rotated 90 degrees, and it could be extremely sensitive even without much amplification, since the thermistor would be able to sense even micro-watt level changes and be fast-responding maybe, since it's so tiny. However I know nothing about how such a thing could be insulated, and if it would even be accurate, and how to calibrate! I've never through to experiment since I didn't have a real use-case, just curiosity.

I've always wondered if it could be possible to buy a 50 ohm (or 600 ohm) 0402 resistor, and super-glue a 0402 thermistor onto it, rotated 90 degrees, and it could be extremely sensitive even without much amplification, since the thermistor would be able to sense even micro-watt level changes and be fast-responding maybe, since it's so tiny. However I know nothing about how such a thing could be insulated, and if it would even be accurate, and how to calibrate! I've never through to experiment since I didn't have a real use-case, just curiosity.

I've got 0402 resistors and thermistors (2k and 10k) - no time today but I'll give it a go.

Also got 0603 Pt sensors but I think they do 0402 as well.

MK

Would you actually put two thermistors into some sort of bridge, such that changes in the ambient temp would not influence the reading?  At a place I worked in the mid-1960s, they made their own matched transistor pairs, thermally-coupled and potted to get around that sort of thing.

Hard to know without some experimentation, but maybe even some slow chopping, with a reed relay, could help, for instance measure for 3 seconds, and then disconnect the source for 3 seconds and so on. Wouldn't be a very fast way of getting a measurement though : ( I don't know how the existing sensors do it.

Many 'true r.m.s.' detectors in legacy audio signal-processing gear paired incandescent light bulbs with either CdS sensors or, later, silicon solar cells, either of which might (or might not!) exhibit flatter temperature response than a thermistor, which, after all, is a temperature-sensitive device by design.  But whatever is used for conversion, the desired r.m.s. waveform response may be a trusted issue, but the linearity of conversion over a 20dB or better range might require some additional nonlinear network or a lookup table to get the numbers to line up.  A daunting task for sure. 

So far the plan is a dual RMS converter setup: an AD637 from 10Hz to 50KHz, an AD8361+VCA824 from 20KHz to15MHz. In the crossover region a "track the highest" analog stage will combine the high and low frequency path. 

I've seen separate DC and AC path is several professional designs, never one that combined two frequency ranges. Have you seen anything like that?

Sounds like a solid plan, actually.  And, no, I've never encountered anything like that in a measurement application.  Be sure to post your progress!

Did it, see blog.

community.element14.com/.../a-really-simple-hf-true-rms-detector

MK

The exit criteria is different in my case: I'm to verify the rebuilt voltmeter can met or exceeded the original 3400A specs.