5.4 Optional connection of GPS module
A GPS module may be connected to the kit, to provide
frequency stability, accurate time, and latitude and longitude
which can be converted to Maidenhead locator format for
encoding in the WSPR message transmission.
Check that your module is powered from 5V. Many modules
specify a 3.3V supply – in this case you will need to provide
an external 3.3V voltage regulator. Where a 3.3V GPS
module is used, the serial data and 1 pulse-per-second (pps)
inputs are not a problem for the 5V microcontroller on the kit
PCB – no voltage level conversion is required.
In cases where the location information is not required
(modes other than WSPR) or you wish to enter it manually,
the serial data input can be left unconnected. The 1 pps input
is enough for the frequency lock function. Note that the
frequency locking function can only operate if you have selected a wait period between message
transmission (i.e. a non-zero “Frame” parameter).
See operation instructions for details on how to configure the GPS interface.
5.5 Higher power output
The kit provides around 150mW of output power using a 5V supply and a single BS170 transistor
(depending on the band). The transistor gets slightly warm to the touch. If higher output powers are
desired, this can be achieved by using a higher supply voltage for the Power Amplifier (PA). The
microcontroller must still be run at a voltage not exceeding its 5.5V rating (5V is recommended). To
facilitate running the PA at higher voltages to provide more power, the PA supply connection is a separate
pin.
At higher supply voltages and power outputs, the heat
dissipation in the BS170 will increase and its temperature
will be higher. Provision is made on the PCB for two more
transistors in parallel with the first. This will slightly increase
the power output even at 5V supply, but more importantly
at higher voltages it should share the heat dissipation
between the devices.
It may be necessary to fix a heat sink to the transistors if the temperature rise is excessive. The transistors
are sited near the edge to the board to facilitate this. Experiment is needed in this area, with PA voltages
up to perhaps 12V.
5.6 Audio frequency output
The kit can be used to generate audio tones for feeding to an SSB transceiver etc. In this case you can
simply set the output frequency to the desired audio frequency, and feed the “RF” output to the
microphone input. It is very important to note that this output is a just a 5V peak-to-peak square wave – to
connect it to a microphone input (for example) you will need to reduce the signal amplitude considerably.
In testing, I used a potential divider made of an 18K resistor and a 10-ohm resistor to ground. The mic
input was connected across the 10-ohm resistor and provided a suitable signal amplitude.
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