[time-nuts] Help improving impedance measurement by having a better clock
kb8tq at n1k.org
Sun Apr 8 17:12:14 EDT 2018
I believe we are talking about material impedances here rather than electrical impedance. One of the
weird things about this is that frequency (as in frequency accuracy) *can* be a contributor to the resultant
error. Thus my original question about just what the concern actually is…..
> On Apr 8, 2018, at 4:52 PM, Brooke Clarke <brooke at pacific.net> wrote:
> Hi Daniel:
> If you're concerned with accuracy of impedance measurements you might want to spent a lot of time studying "The Impedance Measurement Handbook", it's the bible for this. The way the measurement is made has a huge impact on the accuracy. For example using a vector network analyzer to measure an impedance that's not near 50 Ohms causes poor results.
> For frequencies low enough to use 4 Terminal Pair, that's the way to go.
> Have Fun,
> Brooke Clarke
> -------- Original Message --------
>> I´m a long time time-nuts lurker (I posted here just a dozen times). I make
>> a few impedance measurement systems for material analysis (i´m a single man
>> shop doing custom hardware for clients). Usually they´re based around a
>> STM32F4 / F7 microprocessor: DAC generates sine signal (1-400KHz), ADCs
>> measure them back, a few calculations later we have modulus / phase. I
>> always used internal ADCs and DACs (12 bits each).
>> I now want to use external ADCs and DACs with more bits to push the limits,
>> but i´m afraid that the poor performance of the STM32 PLL that drives the
>> clock will get in the way, so I plan to drive the "load" of both DAC and
>> ADCs from an external signal derived from a TCXO using a clock divider.
>> To get some sense of how much things are improving (or not) I need to
>> somehow measure these clocks and get a meaningfull measurement about how
>> good (or bad) they are.
>> The tools I have are a Hameg HM8123 with a 10MHz OCXO I shoehorned inside
>> and a Picotest U6200A with original OCXO. I can log period information from
>> both using serial/USB port. I can make a histogram of the data. I don´t
>> have any better idea about what to do and would like to hear from you :)
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