Rev 1.01
1
AN004
HF PA kit with Ultimate3S transmitter kit
1. Introduction
The QRP Labs HF PA kit has an 8-bit shift register (74HC595) whose outputs control an 8-bit Digital-
to-Analogue converter (DAC). This DAC controls a discrete-component power modulator with
foldback current limiting. A microcontroller can load a sequence of values into the shift register in
such a way that a raised cosine shape is applied to the RF envelope.
This application note explains how to interface the PA kit to the QRP Labs Ultimate3S transmitter kit.
The assembly document for the PA kit is ESSENTIAL reading, prior to this application note!
2. Block Diagram
The basic Ultimate3S transmitter is shown in the top half of this diagram. The Low Pass Filter (the
LPF) is drawn as a separate block to the Ultimate3S transmitter. In reality the LPF is a plug-in module
on the Ultimate3S PCB. If you are running a multi-band Ultimate3S then there is a LPF plugged into
your Ultimate3S PCB, and a 5-band Relay-switched extension PCB bolted behind that. As far as the
5W HF PA kit is concerned, the principles are the same in the single-band and multi-band cases.
