N3ZI Digital Dial
Using my Digital Dial as a General Purpose Frequency Counter.

The info on this page applies to the counter sold in 2008 and 2009

Technical Note: Improving accuracy and precision

My digital dial was designed as a low cost digital readout for single band homebrew radios.  Although not intended as a laboratory frequency counter, many are using it for that, and some considerations must be allowed for, when doing so.

All kits are shipped uncalibrated.  As a digital dial, the until can be calibrated by adjusting the IF offset to cancel any errors.  This works fine over the narrow frequency range of typical VFO's  But if you want to us it as a wide range counter this type of correction will  not work well.  The low cost crystals used in this device have an error of +/- 150ppm, which means at 1Mhz, you could be off by as much as 150Hz, at 30MHz you might be 5 KHz off.  So you will need to tweak the crystal oscillators frequency.  This is done by replacing C2, a 22pf fixed capacitor with a 50pF trimmer. The PCB design allows for this with larger holes for C2.  If this doesn't get you enough range, you may also need to change the value of C1, email me if you have any problems.

Apply a calibrated test signal input, then adjust C2 for the proper frequency readout.  Whenever the "timebase factor" is changed you will need to recalibrate slightly. And the crystal is not temperature compensated, so you will need to recalibrate anytime there is a significant change in temperature.

HIGH RESOLUTION:  The normal modes are 10Hz and 100hz resolution, but there is an undocumented mode which allows up to 1Hz resolution, which is only useful with a well calibrated counter, and at frequencies below 6 MHz.  To use this mode, set the "timebase factor" to 100 and move the decimal point 3 places from the left.  You cannot use a prescaler in this mode.

If you are very very concerned about accuracy, you might want to purchase the vintage radio version.  This version uses a larger microprocessor which allowed me to  include a set up step that allows you to set the S/W calibration factor.  I ship all the counters with a 13.44MHz crystal, and the S/W will accept calibration factors in 400Hz steps, so you will still need a trimmer cap to tweak it exactly on.

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