5.10 Higher sensitivity contrast potentiometer adjustment
The contrast voltage required by the LCD is typically under one volt, but the potentiometer R1 is connected
b5V and Gnd. Therefore most of the available travel of the potentiometer adjustment is
redundant, and the contrast adjustment is rather sensitive. If you wish to improve this, you may fit a 220K
resistor at R4 instead of the wire jumper.
6. Calibration
This section is REALLY IMPORTANT! It is very important to realise that by far the hardest part of any
successful QRSS beacon operation, is tuning the oscillator to the correct frequency. For example, most
30m QRSS stations monitor a narrow 100Hz-wide band from 10,140,000 to 10,140,100. If you are much
outside this, the chances are that nobody will see your signal. It is therefore essential either to adjust your
output frequency using an accurately calibrated frequency counter, or an accurately calibrated receiver.
If you are going to use a GPS receiver module to provide a 1pps signal to the kit, no calibration is required:
the kit will self-calibrate from the 1pps signal.
In the case that you will not be using a GPS signal, you need to calibrate the kit by entering the correct
value of the 125MHz reference oscillator frequency into the “Ref. Frq.” configuration setting.
It is possible that you have an accurate means to measure the 125MHz reference oscillator frequency
itself by probing the correct point on the DDS module. However most kit builders will not have the
necessary equipment, and such a measurement at VHF is not easy to make without itself risking disturbing
the frequency.
The easiest method is to set the output frequency to something convenient such as 12.500000 MHz, and
measure it. Measurement can either be by using an accurately calibrated frequency counter, or by setting
up an accurately calibrated receiver with Argo and monitoring the output signal frequency that way.
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