inductance meter using synchrounous demodulation

[Guest]

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

 

[Borber]

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.

 

[Guest]

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

 

[Borber]

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).

 

[artem]

DC would not help befere demodulator .

 

[Guest]

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

 

[Borber]

Yes that is correct.