[time-nuts] Fast frequency counting question

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun May 4 19:03:25 EDT 2008

Magnus Danielson wrote:
> From: "Pete" <peterawson at earthlink.net>
> Subject: Re: [time-nuts] Fast frequency counting question
> Date: Sun, 4 May 2008 16:07:02 -0600
> Message-ID: <004401c8ae33$336ee590$0200a8c0 at BASE1>
> Pete,
>> I may be out in left field (again), but isn't this
>> a textbook measurement for a fast sampling
>> 'scope? I think most of the recent devices in
>> this category have 500MHz to 10+GHz
>> sampling rate, with plenty of memory depth.
>> They can output a time record via GPIB, 
>> USB or whatever.
>> The time record can be post-processed to
>> yield actual zero crossings. You also get all
>> the amplitude data, which should help with
>> analysis.
>> Some of the newer 'scopes have the post-
>> processing software built in & there is at
>> least one source of  PC based timing
>> analysis available.
> I still think it will be hard to acheive the required resolution that way.
> But yes, modern scopes have moved considerably in the right direction lately.
> I also somewhat question the importance of this initial time. It should not be
> the make or break of the application as being claimed.
> One interesting thing one can do is to hook a spectrum analyser to the
> oscillator output and see how the spectrum changes over the first initial time.
> It should be an interesting experience trying to figure out what is happening
> and why... :)
> Cheers,
> Magnus
The quickest and easiest way to achieve the required resolution is to 
buy a high speed (~100MHz) sampling ADC evaluation kit from Analog 
Devices, Linear Technology etc, use a low phase noise crystal oscillator 
(eg Wenzel ULN or equivalent performance device) or equivalent bandpass 
filter the output and use as the sampling clock source. Then either:

1) Sample the 40MHz signal directly (possibly after some bandpass 
filtering) then post process the raw data.

2) Use  a mixer to produce a ~1MHz output bandpass filter it and sample 
this signal  (amplifying if necessary) with the ADC then post process 
the raw data. This will achieve lower noise. However the mixer LO has to 
have very low phase noise.

High speed ADC evaluation kits are readily available (at least from 
Linear) and considerably cheaper than a timer counter with equivalent 
resolution and noise f such an instrument is available at all.
The LTC evaluation kits include a board with local sample storage and a 
USB interface to a PC.

The sampling clock frequency need not be exactly 100MHz as long as its 
frequency is known.


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