[time-nuts] Help improving impedance measurement by having a better clock

Brooke Clarke brooke at pacific.net
Sun Apr 8 16:52:00 EDT 2018


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.
http://prc68.com/I/Z.shtml#KeyDocs
For frequencies low enough to use 4 Terminal Pair, that's the way to go.
http://www.prc68.com/I/HP4274_4275_LCR.shtml#MeasMtd

-- 
Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html

-------- 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 :)
>
> Daniel
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