[time-nuts] Tektronix FCA3103 ADEV measurement tau setting problem

[Richard (Rick) Karlquist]

I have a Tek FCA3103 300 MHz counter that measures ADEV
as a built in function.  When I bring up the settings
menu for the measurement, it has an entry window for
"tau" (the averaging time, IE "sigma sub y of tau").
It defaults to 200 ms.  I can enter larger values and
ADEV gives reasonably results.  However, if I enter
smaller values, even 199 ms, I don't get any error
on the display, but I get clearly erroneous
results for ADEV. I read the manual and cannot find
anything to the effect that the instrument doesn't work
for less than 200 ms.

BTW, I asked Tek "customer no support" about it and
they were clueless.

1.  Is this pilot error?  Can anyone tell me the trick?

2.  Can anyone recommend a 300 MHz counter that measures
ADEV, correctly :-)?  Bonus question:  a counter that
measures Hadamard variance?

3.  Can anyone recommend a 300 MHz counter that can
measure ADEV and Hadamard using off the shelf software
that runs on a PC?  I don't write software :-)
Bonus question:  software to make these measurements
that works with the FCA3103 that I already have?

I have an NI GPIB-USB-HS to interface the counter
to the PC running the software if that helps.



Thanks

Rick N6RK