freq counter?

[Dave Halliday]

Another way to go for low ( audio range ) frequencies is to measure the
period between waveforms.  This can be a royal bear with an irregular
waveform but for simple sine/square waves, it can give really good accuracy.
This doesn't give a direct frequency readout, it is 1/f but this can be
calculated fairly easily.



Another way is to use an o-scope and a precision frequency source - use the
scope in an XY mode and plug the known freq into one axis and the unknown
but variable freq into the other.  When you get close to a straight line,
the two are equal.

-----Original Message-----
From: Magnus Danielson [mailto:cfmd at swipnet.se]
Sent: Wednesday, June 21, 2000 9:45 AM
To: n0nspaz at loa.com
Cc: synth-diy at node12b53.a2000.nl
Subject: Re: freq counter?


From: mark scetta <n0nspaz at loa.com>
Subject: freq counter?
Date: Wed, 21 Jun 2000 00:45:45 -0400

> suddenly have a use for one (calibrating the phasers an arp).  can i build

> one? what is it exactly??

Yes, you could build one, but I am not sure that it would be worth the
effort.

A generic frequency counter will measure the frequency, the naive form
is to have a counter which is being reset at some time t1 and stopped
at some time t2. The counter is being clocked by the signal of
interest. The time t2-t1 is called the gate time. Given the count of m
at the time of t2 you have the frequency

       m
f = -------
    t2 - t1

Now, while this is really ultrasimple to implement the downside is
that the gate time can be pretty long if you want a decent accuracy of
f for low frequencies. Having a 1 Hz resolution requires a 1 second
gate time.

There are zillions of other ways to construct a frequency counter.

If you want to learn more about Frequency Counters I recommend you to
dig up Agilents (new name for what used to be the real
Hewlett-Packard) Application note 200. In there is a pretty good
explanation of frequency counters.

I recall that there was a Intersil chip called ICM 7216 which is a
simple all-in-a-chip 8 digit frequency counter. It includes the
7-segment decoding and driver so it is a really simple thing to build.
While it is by no means as good as a good system counter, it could
very well prove sufficiently good for DIY work. If you only get the
chip it should be a pretty straigth forware thing to build the rest of
the curcuit up. It does DC-10 MHz but could be extended upward in
frequency with the help of prescalers.

Cheers,
Magnus