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XJW01 Auto LCR meter review ($120 bench top LCR meter)

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dannyf:
Here is how the phase detector works, in a nutshell.

By alternatingly shorting the input to ground, here through two switches and in the xjw lcr meter through two pins (PA0/PA1 in the STM32 version, K7 and P2.0 in the 8051 version), you are essentially controlling how the two rc filters are going to be charged up.

The output trace is the differential voltage between the upper rc filter and the lower rc filter. Traces are plotted with the input signal in 45 degrees apart, starting from 0 degrees at the top.

The traces are just as you would have expected.

So in his program, I assume he is using this approach to measure the phase differential between V and his reference signal (that drives the shorting of the input signal), and then measure the phase differential between I and his reference signal. Once you have the two, you get the phase difference between V and I.

Vgkid:

--- Quote from: nctnico on June 04, 2016, 10:24:57 am ---I also have some experience with building LCR-ish circuits and one of the parameters you forgot about is noise. Also even when using ratios you'll still need a known-good reference. In my case I used a 0.1% resistor to achieve better than 0.5% accuracy.

--- End quote ---
You do need relatively stable resistors for that. In my .1% gr1658, 0.02% resistors are used. When it comes to higher end/ more advanced lcr meters. The internal resistor reference is not as important, ,but the stability is. For those, you calibrate them using a known external standard.+|- 10ppm for the 1689/93. With a known dq value to be entered as well.

happydad:
Hi Danny,
Thanks for the sim on synchronous detector.  I speculate that he is switching between PA0-high/PA1-low and PA0-low/PA1-high at 1/4 the signal rate, thus producing 0  and 90deg sampling of both voltage and current for a total of 4 readings.  That will give you all you need to know about ESR and C.  Sounds about right?

Yansi:
Yes, you measure the voltage and currect accross the DUT, both as complex variables (in phase and quadrature), so you can use the standard formulas for complex impedances to calculate any impedance.

happydad:
OK Thanks.  So to recap, I am picturing sine waves of V and I  that are 90 degrees apart right up to the synchronous detector.  After the synchronous detection, there is an rc filter at 16Hz?  Isn't that a little low?

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