1 kHz is a commonly used test frequency, and I resolved to see if I could make an inductance meter that uses a 1 kHz test signal at a low level. I also wanted to use a low level test signal so that the test current would have a minimal effect on the measurement. Magnetic cores, and soft ferrites in particular, change in permeability as a function of the current through the winding. I chose 1.0 milliamps peak-to-peak as a test current. Given that I wanted to be able to measure inductance down to 100 uH, my attention turned to the problem of how to accurately measure the amplitude and the phase of as little as 63 millivolts across a 100 uH inductor.
Looking around on the web a little more, found the user's manual for an M3 Electgronix Digital LCRZ Meter kit, which, aside from spelling out the meter's impressive capabilities, also explained a few short paragraphs explained how the meter worked. That was enough to convince me to give this technique a try. By the way, if case you are starting to get the idea that this project is a quick and dirty knock-off of the M3 design, its not. The M3 kit has much more range, measures lot more than just inductance, and seems to be quite a bit more accurate.
My project is just an exploration of circuit and firmware techniques, mostly to see what it would take to get such an instrument operating. I think that if you have the time to build the kit, the M3 kit is an excellent way to get a full function LCRZ measurement capability that rivals that of some of the most expensive factory built instruments, but at fraction of the cost. You might want to have a look at the M3 Electgronix webiste, and in particular at the web page for their meter kit. You can even download the manual for their kit on the page at this link: http://www.m3electronix.com/lcr.html.
The result of this investigation is some circuit concepts, some firmware techniques, and some learning. Notably, the internal reference impedances are switched on and off without the use of electromechanical relays or switches, and a comparatively low parts count version can be...