N3ZI
Digital Dial in Low Frequency Applications
(Under 1 MHz)
My digital dial was designed for Radio Frequency Applications, but I
have had quite a few persons interested in using it for low frequency
applications. And for the most part it has been used successfully
for audio and low frequencies.
Some minor tweaks are required. The input coupling network has a
22pF coupling capacitor, this network rolls off below 1Mhz and
shouldn't be used below that frequency. Certainly you can change
C3 to a 1000pF capacitor which will work down to 10KHz. But if
you want to go very low in frequency you really should use DC
coupling. Assuming that the voltage swing of your source is
between 500mV and 5V peak to peak, the following circuit should work
fine.
This circuit will also provide a level of protection for over voltage
and under voltage. But the ground for the signal source and the
frequency counter should be tied together. But the signal will
need to swing across 2.5 volts to be counted.
For low frequencies you do not need the prescaler, Set the prescale factor to 100, turn off the decimal
points, and set the IF to 0.0. This will give you 1 Hz resolution
but a rather slow update rate (once ever 2 seconds) For a faster
update rate use a prescale factor of 10, then your resolution will be
10Hz, and you will update 5 times per second.
If you want a wide range counter, then use a Div/8 prescaler, and set the prescale factor to 80.
Set the decimal point for 2 digits right of the decimal point.
This will give you operation from 10Hz to 50MHz with 10Hz precision.
