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

--- Quote from: timofonic on June 03, 2016, 04:37:43 pm ---I wonder why aren't there a DIY high end bench DMM. Are they so difficult beasts.

--- End quote ---

Our forum member Mickle T builds 6.5-8.5 digits multimeters :)

See here
https://www.eevblog.com/forum/metrology/lm399-based-10-v-reference/msg589601/#msg589601

and http://bbs.38hot.net/ forum.

I suppose going above 5.5 digits is not impossible with DIY, it is just very expensive. Aging the references, buying Vishay resistors in small quantities, implementing and testing multi-slope ADC and then in the end you'll need 3458A / SR104 / Fluke 732B / KVD to test the performance. So at this point it is much cheaper and easier to buy 6.5 digit meter, even a new one (e.g. 3446XA) or look for a second hand on eBay.
Vgkid:
Running off memory. Remember we are talking about lcr meters. Which work rather easily if you break it down to a dimpler model. Lets use components in series. The basic portions to a auto balancing bridge, what almost all (digital)bridges use. To calc the unknown impedance compare the voltage on the unknown to the voltage on the known resistor. Using dome math, we can ratiometrically determine the the impedsnce on the unknown resistor(this scored me bonus kudos in my physics class). Then to calc the parameter(phase shift) we measure the timeit tskes for the waveform to either lead/lag/stsy the same. When compared to the reference resistor. Now talking components: when making precise(relatively speaking) with good resolution. The main things that need to be reasonably stable is the amplitude of the sine wave.  If you tbink about this, since we are dealing with ratios, an lm399 is overkill. Heck the GR1689 used used a 3 terminal voltage(adjusted to 2.5v) feeding the 8-bit dac. Wich was filtered to produce the sine wave. This signal was thrn passed through a high speed aml(lh002) before going through the reference and thd dut. For the reference resistor you needed stability, and low tc. Some units used flatt mica eound pww resistors. The lower end genrads used those. While the higher end oned used s102's.  The esi's and Wayne Kerrs used a similiar system. Though I  know that it seems like every ESI unit used a rather complex csl procedure. With frewuency high end ocxo's are not needed. Most units used a plain xo. With either a msnual frequency adjust(if it even has one), or a frequency offset stored in memory 1689/93.
* I can go into more detail if you guys want it.
** Im tired, and on my phone, so ignore spelling/grammar issues.
nctnico:
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.
dannyf:

--- Quote ---Are we trying to flip the sign here?
--- End quote ---

The instruments essentially measure sthe voltage across the DUT, and then the current through the DUT (via a I/V conversion).

PA0/PA1 is a simple synchronous detector: I suspect that the mcu alternately shorts PA0/PA1 to ground and float to measure V's and then I's phase relative to the control signals on PA0/PA1. With that, you get the relative phase between V and I.

I haven't looked at his code but I have used an identical approach myself on a LCR build with an AVR (AT90USB1286 to be precise). It is not very efficient btw and given the number of analog switches he is already using, it puzzles me who he didn't go down the route of a real synchronous detector route.
dannyf:

--- Quote ---Are they so difficult beasts.
--- End quote ---

With a build-in 24bit adc, it can be done fairly easily with a mcu (C8051F350 for example).

Using discrete parts, external adcs, it is beyond most people's capabilities if you want to go over 20-22 bits.
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