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.
Back to the Manuals page
Back to my Main Web Site