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

Bob kb8tq kb8tq at n1k.org
Sat Apr 7 23:01:04 EDT 2018


I’m not at all sure that the clocks will really get into the measurements. The PLL does 
have jitter, but it has zero frequency error. 

One way to get around the whole issue is to use an outboard device with its own clock. 
400 KHz is a bit high for the integrated audio ADC / DAC parts. Drop down a bit and 
they will do quite well. Interface with an I2S port and away you go.

That said, are you concerned about frequency error or jitter? If it’s frequency error, you 
can get that pretty well a number of ways. The gotcha is an accurate reference. A GPSDO
is by far the cheapest way to solve that problem.

If you are concerned about jitter, the next question is: over what range? It is reasonable 
to guess that the PLL follows the input to a pretty good degree for long time periods. At
some point the PLL loop filter allows it to move around a bit. The issue here is that the loop 
filter likely is in the audio range. You are after jitter at time periods << 1 second. That gets
a bit gear specific. You also are after numbers in the nanosecond (or sub nanosecond) range.
That increases the complexity. 

So - what are you after? 


> On Apr 7, 2018, at 9:54 PM, Daniel Mendes <dmendesf at gmail.com> wrote:
> 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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