Extech LCR meter schematics

[Joe Gwinn]

A number of people noticed my comment on having the electrical schematics for the Extech LCR meter, and expressed interest. Accordingly, I have added the schematics to my website Joe Gwinn's website homepage.

No Principles-of-Operation text has been found, but I did work out how it works. I described the basic approach in a posting on the effect of eddy currents on inductance. Specifically message #44 of http://music-electronics-forum.com/t14110-2/#post113076.


Here is the relevant text from posting #44:

I found the circuit diagram of the Extech (under a different brand name) on the web. For the record, it measures in the following way:

The 1000 Hz (or 120 Hz) oscillator has two outputs, one at 0 degrees (I) and one at 90 degrees (Q) phase shift. The 0 degrees output is amplified and imposed on the unit under test (UUT), a pickup in this case. Both 0 and 90 degree outputs are fed to a pair of synchronous demodulators, as will be discussed later.

The voltage across the UUT is sensed by one opamp circuit, and the current through the UUT is sensed by another opamp circuit, yielding two voltage signals, one proportional to UUT voltage and the other to UUT current.

These two voltages are fed one at a time to to the pair of synchronous demodulators, yielding a pair of complex numbers.

The first complex number consists of the I (real) and Q (imaginary) values of the UUT voltage, and the second complex number consists of the I and Q values of the UUT current.

The complex impedance (I and Q) is the UUT voltage (I and Q) divided by the UUT current (I and Q).


Not mentioned above is one practical complication, a circuit that compensates for the non-zero input impedance of the current-to-voltage converter.

[-Elepro-]

i hope to start my LCR meter project in september....
now i'm waiting for the documentation that i need... thanks for schematics...

i would like minimize component around mcu.... but is not simple and cheap...
.....this was very useful but too expensive...

bye

[RedHouse]

i hope to start my LCR meter project in september....
now i'm waiting for the documentation that i need... thanks for schematics...

i would like minimize component around mcu.... but is not simple and cheap...
.....this was very useful but too expensive...

bye

Simple and (fairly) cheap are XR2206 and IC8038, and Maxim has one too buy I can't recall the part number off the top of my head.

[-Elepro-]

... max038... but these are not programable with 16 bit serial input....

with ML2035 or similar we could have sine from dc to 25khz without any other component...and it is very easy to control by PIC....

[RedHouse]

...with ML2035 or similar we could have sine from dc to 25khz without any other component...and it is very easy to control by PIC....

Oh yes, which reminds me here's an app note on that chip: http://www.fairchildsemi.com/an/AN/AN-42025.pdf

[Joe Gwinn]

.... but is not simple and cheap...
.....this was very useful but too expensive...

A bigger problem is the lack of the necessary two test signals, in strict quadrature even as the frequency is varied. One signal is applied to the UUT, but there are two synchronous detectors (mixers), one signal to a mixer, to yield the desired I and Q signals.

I would look at DDS (Direct Digital Synthesis) chips say from Analog Devices. Many provide quadrature outputs, are easily controlled from a mcu, and will have far cleaner signals.

Also note that the signal applied to the UUT must be a clean sinewave, or the fact that the inductance and loss of practical cored inductors varies with frequency will cause significant measurement errors.

[-Elepro-]

.....from Analog Devices...

nothing in dil package

[Joe Gwinn]

nothing in dil package

True enough. Less and less modern stuff comes in DIL (Dual In Line) packages these days.

Part of the problem is that system-on-a-chip things like a complete DDS would require far too many pins for any of the older package styles to be economically practical.

While we can check other manufacturers, my guess is that quadrature dual output DDS chips will all be surface mount. Perhaps there is a single-output chip that is designed to be used in coordinated pairs to generate the needed quadrature outputs.

Actually, we need only one sinewave output, to drive the UUT, and two squarewave logic outputs in quadrature, to drive the I and Q synchronous detectors, so the quadrature clocks could be generated digitally, with the I phase being cleaned up by a PLL chip to yield the sinewave drive.

For instance, use a simple DDS to generate a quad-frequency digital clock, followed by a divider circuit that generates the I and Q clocks, with the I clock being turned into a sinewave by a PLL chip.

[RedHouse]

True enough. Less and less modern stuff comes in DIL (Dual In Line) packages these days.

Part of the problem is that system-on-a-chip things like a complete DDS would require far too many pins for any of the older package styles to be economically practical.

While we can check other manufacturers, my guess is that quadrature dual output DDS chips will all be surface mount. Perhaps there is a single-output chip that is designed to be used in coordinated pairs to generate the needed quadrature outputs.

Actually, we need only one sinewave output, to drive the UUT, and two squarewave logic outputs in quadrature, to drive the I and Q synchronous detectors, so the quadrature clocks could be generated digitally, with the I phase being cleaned up by a PLL chip to yield the sinewave drive.

For instance, use a simple DDS to generate a quad-frequency digital clock, followed by a divider circuit that generates the I and Q clocks, with the I clock being turned into a sinewave by a PLL chip.

What happened to the 1 sine circuit in your MaxwellBridge.pdf that won't work now? seemed a simpler solution, load a pickup, apply Vs, have the PIC sample and tweak R1 & R2, then take the readings at Vdet, move on.

Now we need 3 signals, one sine and two digital clocks in quadrature. Are we really needing to clone an Extech -or- can we just build a meter than can characterize and document a pickups parameters better than what's currently available?.

[Joe Gwinn]

What happened to the 1 sine circuit in your MaxwellBridge.pdf that won't work now? seemed a simpler solution, load a pickup, apply Vs, have the PIC sample and tweak R1 & R2, then take the readings at Vdet, move on.

With the Maxwell bridge, one adjusts two pots, to bring both I and Q into balance. The Extech measures the I and Q values directly, rather than deducing them from the bridge equations. This is far faster to accomplish than manual balancing of a bridge.

Now we need 3 signals, one sine and two digital clocks in quadrature. Are we really needing to clone an Extech -or- can we just build a meter than can characterize and document a pickups parameters better than what's currently available?.

The reason Elpro wants to clone the Extech is to allow more test frequencies than just 120 Hz and 1000 Hz, without spending thousands of dollars (the cost of comparable commercial instruments).

[RedHouse]

...The reason Elpro wants to clone the Extech is to allow more test frequencies than just 120 Hz and 1000 Hz, without spending thousands of dollars (the cost of comparable commercial instruments)....

I think he already had something in mind when this came up post #89 it's really good you offered Extech schems.

Speaking of those schems, now don't take this wrong but being the kind of guy that questions absolutely everything sourced from the web, when I looked at those schematics I was immediately wondering what makes us think these are the Extech meter? I understand you posted "I found the circuit diagram of the Extech (under a different brand name) on the web" is there something that lead you to believe this schem is the Extech Joe?

I tried doing some googling for RC-00041-S1 and 140-00041-F and LIC 940330 which are the only things I can read on those pages but didn't get any hits on meters of any kind. Again I'm not throwing water on ya Joe, I've just got into the habit for some years now of finding and verify sources for myself when their origin is the iNet if ya know what I mean.

[Joe Gwinn]

Speaking of those schems, now don't take this wrong but being the kind of guy that questions absolutely everything sourced from the web, when I looked at those schematics I was immediately wondering what makes us think these are the Extech meter? I understand you posted "I found the circuit diagram of the Extech (under a different brand name) on the web" is there something that lead you to believe this schem is the Extech Joe?

In July 2008, someone asked if the Pont LCR meter would work, so I looked into it. From the Pont datasheet it was clearly the same instrument as the Extech, but with a different logo. And the seller (not Pont) provided schematics on their website, and these schematics were clearly of an LCR meter. (Don't laugh - lots of times I have been given a schematic that on inspection cannot be for the claimed instrument, and in some cases could not be for anything at all, being effectively an artist's impression of a real schematic.)

I just looked where I got the schematics, Conrad Electronic - Europas führendes Versandhandelsunternehmen für Elektronik und Technik, and the schematics are there but the product has changed for the better: The MT-4080 is now an expensive LCR meter that has test frequencies from 100 Hz to 100 KHz. The circuit wouldn't change as the frequency changes. Conrad Electronic UK

[RedHouse]

...(Don't laugh - lots of times I have been given a schematic that on inspection cannot be for the claimed instrument, and in some cases could not be for anything at all, being effectively an artist's impression of a real schematic.)...

The very reason of my inquiry, thanks. I wouldn't laugh because that very same thing has happened to me a number of times, now I always question internet "data".

[Steve Conner]

Just thought I would chime in my $.02 worth, as I've designed gear using the *038, AD9850 and so on.

Do they really need to be sine waves at all? You could maybe assume that the UUT acts as a low-pass filter and neglect the harmonics of square waves.

Have you seen the Tayloe detector circuit?

You can generate a sine wave using a shift register and a weighted summing circuit, see Horowitz and Hill.

Or use a long-tailed pair of transistors to distort a triangle wave.

There's also the state-variable oscillator, close cousin to the state-variable filter used in parametric EQs. It produces I and Q sine waves (out of what would be the bandpass and lowpass outputs if it were a filter) when it's set up right. A cookie for anyone who can figure out how to tune it digitally....

Finally, how hard would it really be to make a full 1-port audio network analyzer that runs off your soundcard? I'm sure you could use FFT techniques and something like those hybrid circuits from phone patches, and get a plot of your UUT's complex impedance over the whole audio band.

[Joe Gwinn]

Just thought I would chime in my $.02 worth, as I've designed gear using the *038, AD9850 and so on.

Do they really need to be sine waves at all? You could maybe assume that the UUT acts as a low-pass filter and neglect the harmonics of square waves.

You would have to have different filters for each frequency in the range 100 Hz to 10 KHz, or variable filters. Easier to generate sinewaves from the start.

Have you seen the Tayloe detector circuit?

Yes. This would work as a way to get I+Q directly. One can use a single digitizer and implement Tayloe mathematically.

You can generate a sine wave using a shift register and a weighted summing circuit, see Horowitz and Hill.

Or use a long-tailed pair of transistors to distort a triangle wave.

Yep to both. They all have their advantages and disadvantages.

There's also the state-variable oscillator, close cousin to the state-variable filter used in parametric EQs. It produces I and Q sine waves (out of what would be the bandpass and lowpass outputs if it were a filter) when it's set up right. A cookie for anyone who can figure out how to tune it digitally....

I had forgotten this approach, but yes it could work quite well.

One can tune it with computer controller potentiometers, and by using logic-controlled analog transmission gates to switch capacitors in and out.

Or implement the whole thing in a DSP.

In any event, we would need to digitally measure the actual achieved frequency.

Finally, how hard would it really be to make a full 1-port audio network analyzer that runs off your soundcard? I'm sure you could use FFT techniques and something like those hybrid circuits from phone patches, and get a plot of your UUT's complex impedance over the whole audio band.

A while ago, we were discussing soundcards to generate the test signal and to receive the I and V signals, and doing the fancy math (like digital tayloe) in software. We would build the electronics to interface the soundcard to the drive amplifier and IV inputs. Mike Sulzer built some interface circuits. The effort seems to have petered out, but the idea still lives.

A lot of people would prefer a handheld or bench instrument with buttons.