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LC Meter -- colpitts oscillator

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Nov 9, 2014
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I have tried to build a meter to measure capacitor like the above.

Finally, I build it but it does not work.

Both sides are 0V.
It does not have any oscillation.
What wrong am I?

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In the kerrywong drawing the L and C settings of the switch are mixed up.
You should have a parallel LC circuit there to the ground for the oscillator to run. When measuring C, Cx is parallel with the 1nF cap, and when measuring L. Lx is in series with the the 221uH coil. But you still need that modified (with Lx or Cx) parallel LC circuit to the ground.

As a side note, I have a similar circuit in a "secohmmeter", a flea bay purchase of a cheap LC meter kit, except it uses a 100uH coil and a 2nF cap for the reference oscillator.
With the pull-up resistor for the first comparator (the oscillator) being 1KOhm, I only got about 100mVpp signal at the output of that comparator. With 3.3K I got the full 0 - 5V signal.
Interestingly, a simulation of the circuit with 1K pull-up resistor would not start, but would run fine with 3.3K.
This probably has to do with the loop gain being higher with the 3.3k resistor.
According to the spec sheet of that comparator it could sink 5mA current, so the 1K pull-up resistor should be fine with 5V supply voltage, but in all their sample application circuits they used 3.3K!

Hope this helps,

The LCR-meter circuit is usually implemented with LM311, see e.g.

Using slower LM339 reduces the usable frequency range, but should basically work. 220 µH/1nF may be out of range for LM339. As orbanp explained, the circuit is intended to operate in LC parallel resonance.

I have redrawn the schematic.

I put C = 22pF. I am wonder that I should choose 10K or 100K.
When I pick a 100K, the frequency is 0.
When I have a 10K, the frequency is 353310.
But when I calculate the frequency, it seems it gets wrong answer.

Have you tried to build that using Arduino Uno?
I have many problems with that.
And I am going to build LC meter to measure capacitance only.
Btw, how is a circuit designed to have very small resolution?

If I have a specific range and resolution, how should I design it?
It is very difficult!

I didn't pay attention to the second stage, but I believe that LM339 should have a lower pull-up resistor than 10k. I don't see the purspose of mixing up the pull-up resistor selection with a positive feedback network, I won't do that.

I tried this circuit and it doesn't work, also in theory it is not supposed to work, once you put capacitor to the positive feedback the hysteresis stops and also the oscillation.
I use now this meter:

But I can get the frequency.
Do you get the frequency at that time?
I still have not tried the code.

The circuit I tried is using the comparators of pic16f628. There is an extra reed operated by the pic and the contact connects 1000pF in parallel to the other 1000pf. I'm sure that the square wave oscillator of the comparator doesn't oscillate. What that happens is that when the reed opens and closes there are a few decaying oscillations in the resonant frequency of the coil and 1000pf, the 2 comparators have enough gain to give 5V square wave for the pic to measure the frequency.
I wasn't successful in getting it to work in a stable way.
To my opinion, the circuit as you have it wont give any oscillations relating to the coil inductance.

The comparator oscillator is known to work. You can see it in a simulation or with real hardware.

"known to work" I've heard this one before.

Here is a more detailed explanation:
**broken link removed**


Hi Peter.
Thank you for the doc.
The circuit in the doc has an extra SW1 which is missing from the OP drawing. I gave my theory to that in post #9.
I spent more than 10 hours trying to get it to work and I failed. I've never seen the code, only the firmware. It is possible that it works only with some conditions.
I don't accept the theory of operation in the doc you sent. I hope that you also don't think that "if it's on the net it must be true".

I am still trying to build this one but I think it cannot measure down to 0.0001pF. How should I do to have this resolution?
Using other circuit?
Change the values of components to smaller?

- - - Updated - - -

I am still trying to build this one but I think it cannot measure down to 0.0001pF. It may be 0.01pF. How should I do to have this resolution 0.0001pF?
Using other circuit?
Change the values of components to smaller?

Hey Peter,
I have watched your doc.
I see that the circuit has a switch to measure 2 frequencies.
But for Kerry Wong one, I do not see any switch to measure 2 frequencies.

I am still trying to build this one but I think it cannot measure down to 0.0001pF.
Apart from your general problems to operate the circuit which can be overcome as I believe, it surely can't measure 0.0001 pF.

Referring to the original LM311 circuit which I rated as "known working": It has a parallel capacitance of 1000 pF. Consider a frequency resolution resolution of 10 sec/10 MHz (measurement time/clock frequency), you get a relative frequency resolution of 10^-8, corresponding to a capacitance resolution of 10^-4 or 0.1 pF. Take this as a ballpark number. To extend the capacitance range to lower values, you could re-dimension the base L and C. A factor of 10 to 50 may be achievable. 0.0001 pF is far beyond anyway.

These previous threads have a list of many occurences of the classical LCR meter circuit:

To discuss possible ways to measure 0.0001 pF, we have to talk about total capacitance and measurement frequencies first.

To have 0.0001pF, we should have higher resolution of frequency.
Is there any method to get higher resolution of frequency?
Can I prolong the time for measure the occurences and later on, just divide the measurement by the time?

It's not possible with the hardware under discussion (PIC frequency meter). Another constraint that hasn't been considered yet is reference oscillator frequency stability and phase noise respectively jitter of both oscillators. These constraints are making it difficult to extend the relative frequency resolution far beyond 10^-8.

You didn't mention yet an application of your intended sub fF measurement. I guess it's related to this previous topic

The result should be in range of 0m - 10000m with 10cm resolution.
If the range of capacitance is 0 - 4pF, it should be 0.00004pF resolution.
Am I right?
I am trying to use this method to have smaller resolution as I can.

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result is the altitude
sensor is changing according to pressure
Does the PIC need to program?
Because I cannot see any code in there.

ADI capacitance-to-digital converters like AD7745 achieve a similar resolution.

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