[time-nuts] Fast frequency counting question

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun May 4 20:32:29 EDT 2008

John Miles wrote:
>> Not quite so easy. First of all, this type of transient response
>> signal is not
>> well suited for FFT. Then, I am not sure the repeatability of the
>> signal is
>> valid to such a degree that an accurate curve is achieved.
> If the chirp is just the startup behavior of an ordinary crystal oscillator,
> I don't see how it could help but be highly repeatable, given constant
> temperature, orientation and Vcc edge characteristics.  A
> repetitive-sampling scope sounds like the right tool to use, triggered with
> a fast-rising Vcc edge.
> In fact, I'll bet that variations in Vcc slope are going to dominate any
> effects intrinsic to the 40 MHz oscillator itself, by at least a few orders
> of magnitude.  Study the inputs to this device, not the outputs.
> -- john, KE5FX
Probably impractical as the sampling noise is so high that it will 
require  tens of thousands of measurements to get the noise down to the 
required level and meanwhile the temperature etc will have changed 
significantly. Also the relatively low ADC resolution and nonlinearities 
will tend to limit the effective resolution in determining the zero 
crossing locations.


More information about the time-nuts mailing list