[time-nuts] multi input counter

Jim Lux jimlux at earthlink.net
Thu Oct 6 23:02:14 UTC 2011

since I rudely tagged onto someone else's search for a suitable counter, 
I'll restate my need here..
I want to set up an experiment  to characterize a bunch (several dozen?) 
of cheap XOs (e.g. SiTime parts) over temperature  and time and power 
cycles. I'm not looking for 1e-15 adev at 100 seconds  kind
of performance: maybe more like 1E-6 or 1E-7 ADEV at 100-1000 seconds 
(i.e. does the frequency of a 10MHz oscillator vary more than 1 Hz over 
20 minutes?)

Something like a programmable MUX into one counter would work, but if 
you have 20 odd oscillators, making ADEV measurements for tau of 1-10 
seconds on all of them would be tough.

If I were doing it in an FPGA, I'd just setup a bunch of counters and 
latch them once a second, then shoot the counts out a serial port in 
some fashion (might still wind up doing that).  Or, one could latch a 
single common counter with each of the unknowns divided down by, say, 
10million. I think the two measurements are basically equivalent (one is 
measuring period, the other frequency, essentially).

Or, any of a variety of microcontrollers can do it.

Or, a combination of microcontroller + FPGA.

I think what I was hoping is that there's some already existing box that 
someone sells (or sold in the past) that does this. If not, I'll just 
build something. Probably the FPGA approach.. it seems simplest.. any 
suggestions from the assembled multitude for a inexpensive eval board 
that has an FPGA with suitable input pins for the output from those 
SiTime oscillators (and any other grungy oscillators I scrounge up)?

Something with, say, 32 inputs/pins brought out to a header on the eval 
board would be nice.  maybe the Spartan 3A or 3E for $200?  (I'll have 
to look at the data sheets)

or, given that I'm not looking for ultimate performance, are there any 
particular FPGAs to stay away from that are notoriously bad in this kind 
of timing application.

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