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My analog adventure begins

A project log for All-hardware frequency meter and LCR

Look mom, no MCU!

hummusprinceHummusPrince 06/27/2020 at 21:420 Comments

So now that I have a proper frequency meter, I want to use it to measure electrical resistance, capacitance and inductance. The natural path to take is to make some oscillating circuit whose frequency is proportional to the values we want to measure.

Alas, this seems like a problem - Traditional RC circuits has their frequency inversely proportional to both capacitance and resistance:

With the less popular RL circuits, this is somewhat better, with frequency proportional to the resistance and inverse to the inductance: 

Hence naively using my frequency meter, I can get, maybe, a circuit that measures resistance only, and requires using one of these bulky inductors, which probably would be a pain to work with.


It seems, if so, that physics and human conventions for electrical quantities put me in a difficult position for this project - how could I measure these values using my frequency meter?

I was able to come up with 4 possible solutions:

  1. Leave it as is, measuring the inverse of these quantities - that is, measure admittance rather than impedance.
  2. Add some digital circuit which can invert the admittance into impedance, displaying these wanted values on the display.
  3. Switch the meter's input and latch signals, effectively measuring the period rather than the frequency generated, which should be proportional to the relevant electrical quantities.
  4. Use some analog wizardry to effectively invert the impedance measured.

The first one is a no-go: Nobody uses such quantities, we all use the old LCR values - not their inverse. To use conventional units, I'd have to use a calculator whenever I measure these values, making the device plain stupid.

The second solution won't do either - It's almost like saying that it can't be achieved without some computing. Was it so, I'd rather put there an MCU to begin with and call it a day, and that would be lame.

The third option might seem appealing, yet it has some drawbacks:

I'm less then keen to walk into this sort of electrical swamp, more so when the result is so non-elegant.

Thus, it seems, the first route to pick is the route of analog wizardry. Wizards back in the analog days had plenty of cool tricks up their sleeves, and I'm sure that there's some trick for, say, effectively inverting the impedance of some component, thus solving all of my problems in an elegant manner.

I am, however, not an analog wizard. Despite being sent to a wizard school, my knowledge is rather rudimentary and my experience dull. To accomplish, I'll have to learn some new analog spells and trick, and maybe capture and train some analog dragons.

Will analog wizardry save the day? Time will tell.

Let the analog adventure begin :D

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