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.
DC Coupling

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.
DC Coupling

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