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The lower half of the diagram shows the modified block diagram once the 5W HF PA kit is installed.
The RF path from the output of the Ultimate3S BS170 PA originally routed directly to the Low Pass
filtering. Now we must intercept this RF and insert the 5W HF PA kit in the signal path as shown.
Additionally, control signals are required if the Ultimate3S raised-cosine envelope shaping feature is
to be used.
The interception of the RF output signal to insert the 5W HF PA kit in the path, and the connection of
the control signals, require additional wiring between the PCBs. Unfortunately It is not a matter of
plugging header connectors!
3. Physical Assembly
Physically, if you are going to bolt the 5W HF PA kit into the Ultimate3S "stack", then you will need to
put the 5W HF PA kit at the BACK of the stack, due to the 65mm tall heatsink. You will also need to
purchase a set of four 25mm nylon spacers from the QRP Labs shop, spare parts section. The
physical module assembly is shown in the diagrams below, for the single band system (below left)
and multi-band (below right).
Please refer to section 5.4 of the PA kit assembly manual "Heatsink temperature rise; PA kit
installation in enclosure" to decide how to enclose the system – it may require additional heatsinking
or fan-assisted cooling.
4. Connections
When using the Ultimate3S transmitter to drive the 5W HF PA kit, use a single BS170 transistor in the
U3S PA, and run it at 5V. Use a 25-turn inductor in the drain circuit of the BS170 PA in the U3S, do
not use a bifilar transformer. Over-driving the 5W HF PA does not result in higher power output, but it
can result in lower efficiency (more heat dissipation).
Rev 1.01
2
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