Rev 1.03
1
PA
5W HF PA kit SIMPLIFIED assembly instructions
1. Introduction
This is a Power Amplifier kit (PA) for CW or FSK modes. The output power is approximately 5W in
the middle of the HF frequency range, using a 13.8V supply. A single inexpensive IRF510
MOSFET is the amplification device. An unusual feature of this PA is the built-in facility for
envelope shaping, which can create a precise raised-cosine keying envelope shape. This greatly
reduces key-clicks when using on/off keyed modes such as CW or at the
commencement/completion of an FSK mode transmission.
I highly recommend building the full kit as described in the assembly instructions. The kit has a
discrete component power modulator (voltage regulator) which is controlled by an 8-bit R-2R
Digital Analogue Converter, and allows an external microcontroller to control the amplitude of the
output by loading the on-board 8-bit shift register using three I/O signals, and thereby create the
raised cosine shape.
These instructions are for a simplified assembly of the PA kit, which omits raised cosine
envelope shaping, or the power modulator with fold-back current limiting.
It is easier to
build this way, but the functionality is less.
The kit has a PCB of size 80 x 37mm, which is the same size as the other QRP Labs modules
such as the Ultimate3S QRSS/WSPR transmitter kit, the VFO kit and the Relay-switched LPF kit.
It is therefore physically (as well as electrically) compatible with the other QRP Labs kits and can
be bolted behind them in the familiar PCB sandwich. Alternatively this PA can be used with your
own homebrew projects. The PCB has space for SMA connectors (not supplied by default with the
kit), if you wish to use SMA cables.
The PA kit must always be followed by a low pass filter to attenuate unwanted transmitter
harmonics, as usual for RF power amplifiers.
Carry out an inventory of the components according to the parts list to make sure all the
components required are present. Remember that there will be many surplus components that are
required only for the full build and will not be used here. Read the assembly instructions first
before starting the assembly, understand everything, and then finally start the assembly, following
all the steps carefully!
