freq counter?

[Magnus Danielson]

From: jdec at mindspring.com
Subject: Re: freq counter?
Date: Wed, 21 Jun 2000 13:32:59 -0400

> This discussion reminds me of some advice a wise
> technician told me in college.  He stated that when
> you are trying to measure the frequency of a waveform
> with a meter it's MORE accurate to have it measure the
> period (which you would then have to manually divide
> into 1 to get frequency) than the direct frequency.
> 
> Was he telling truth????

Yes and no. There are many methods for measuring frequency and they
have their own merit. A gated counter has good precission on high
frequencies while a period length counter (thus, counting the number
of reference clocks cycles for a period) has good precission on low
frequencies. However, there are several methods that extends the
precission beyond the limits of these relatively simple methods.
The reciprocal counter runs popular for generic counters of good
precission. Then, you can dive into several diffrent methods for
measuring small frequency diffrencies around some expected
frequency. Frequency comparision between atomic clocks is done by
measuring the time interval between their 5 MHz or 10 MHz outputs. By
multiple measurements you see the phase difference and can by this
find the linear line that is the frequency difference between the
signals. Then there are several other techniques to use. Time interval
measurements can be done downto 20 ps resolution in a single-shot
measurement using commercially available devices, it migth be that
someone got even lower now.

The trouble with high precission counters is not only the technique by
wich a signal is measured and the errors which is introduced due to
the method. No, the trouble that quickly arises is that what you
really is measuring is relative frequency, so you are measuring the
frequency of the input signal relative to your local reference
clock. Some counters allows you to provide an external reference and
there are also counters which allows you to measure the ratio between
two arbitrary input frequencies (the external reference is usually 1,
5 or 10 MHz). High precission counters can even include Rubidium
clocks and if you want to go beyond that you better look into a Cesium
or GPS frequency receiver. There are also other frequency
transmissions which can be usefull (DCF77, Loran-C, GOES, Glonass,
Omega etc).

Cheers,
Magnus (count on me to take my time)