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inductance meter using synchrounous demodulation

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Hi

I need to measure coil inductance. Searching the web I have found this schematic. The method is based on synchrounous demodulation.

https://www.qsl.net/va3iul/L_meter/L_meter.htm

With this measure strategy error contribuition (as for example from iron losses) are avoided.

Observing schematic, Lx is feed by the improved Howland voltage-current converter. Is the Lx also connected to the ground through the shield!?

The other question is: what is the goal of operational amplifier that receive the +90º reference signal and is connected with the differential amplifier that senses Vx.

Please can someone help me?!

thanks in advance

Best Regards
Pendrangon
 

Yes Lx is connected to GND through cable shield.
+90° reference through 2k pot and 470k resistor serve as zero compensation of instrument. By shorting test leads the inductance reading should be zero. You can set it by 2k pot.
 

Hi

Thanks by your answer...

But, I can not understand why we must use the +90 signal!? why not a DC value!?

Best Regards
Pendragon
 

Voltage Vx is -90° on top of inductor (noninverting input of differential amplifier) regarding current generator signal and is AC of cause. To zero parasitic inductance of measuring tips can be compensated by applying +90° signal to the other end of inductance (inverting input of diff amplifier).
 

DC would not help befere demodulator .
 

If I understand, +90º reference AC signal is used to obtain a null output at diferencial stage!?!... for that I must shunt Lx terminal and change 2K resistor...

is this correct!?

AES
 

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