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QRP Labs PA Assembly Instructions Manual

5w hf pa kit
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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!
1
Rev 1.03

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Summary of Contents for QRP Labs PA

  • Page 1 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.
  • Page 2 The following diagram shows the simplified circuit diagram of this HF PA. This diagram includes a 10-ohm resistor modification which is highly recommended, and will be described later in this document. Rev 1.03...

  • Page 3: Parts List

    2. Parts list Unused parts are coloured grey – these are used only in the full build. Resistors 0.33-ohm, 2 watt resistor (none) 10-ohm resistor R2, 19 15K resistor (2 pieces) R4..10, 28 1.1K resistor (8 pieces) R3, 11..18, 24 2.2K resistor (10 pieces) R20, 22 3.3K resistor (1 pieces used, 1 piece not used)
  • Page 4 The incoming RF is coupled to the IRF510 gate via a 1uF capacitor. The largish 1uF value is intended to help the PA work well on LF and MF. A DC bias voltage must be applied to the IRF510 gate, this is supplied by 4.7K preset potentiometer R1 via the 3.3K resistor R20.
  • Page 5 PCB space is limited. It was important to design it to fit on an 80 x 37mm PCB for potential use with other QRP Labs kits such as the Ultimate3S QRSS/WSPR transmitter. You will need to bend the resistor wires as shown in the picture below.
  • Page 6 PCB track diagram. Tracks shown in BLUE are on the bottom layer. Tracks shown in RED are on the top layer. There are only two layers (nothing is hidden in the middle). Not shown in this diagram are the extensive ground-planes. Everything on the bottom layer that isn’t a RED track, is ground-plane! Large areas of the top side are also ground-plane, connected to the bottom ground- plane layer at frequent intervals by vias.
  • Page 7 Inventory parts This is the full list of parts. Many are not used in this simplified build. Please refer to the parts list. Rev 1.03...
  • Page 8 The jumper wires are necessary to bypass the power modulator circuit, and to bypass the leakage gate circuit. These parts of the full PA kit assembly are not undertaken in this simplified build. Four jumpers need to be installed as per the following diagram. This diagram shows the PCB holes/pads in green to make it clear where the jumper wires need to be installed.
  • Page 9 Install trifilar transformer The trifilar transformer is installed in the position coloured red in this diagram. The photo (right) shows the board after installing the trifilar transformer. But it is a tricky part of the assembly, so please read and follow these steps carefully. Firstly, the wire.
  • Page 10 When you have unwound the wire and straightened it, cut it into three approximately equal pieces. These three pieces now need to be tightly twisted together to make the trifilar wire. My method for this is to tie one end in a knot around a small screwdriver shaft. Similarly tie the other end around another small screwdriver.
  • Page 11 Hold the core between thumb and finger. Pass the wire first from above, to below. Then take the wire from below, and bring it around to pass through the toroid again to form the second turn. After each turn, ensure the wire is fitting snugly around the toroidal core. Wind 10 turns on the core. Each time through the toroid’s central hole counts as one turn.
  • Page 12 Now trim the wires underneath the board, and tin the ends again. The EASIEST way to do this is simply apply solder so that it sits in the hole and surrounds the 1mm of wire ending, and hold the soldering iron in that position for a few seconds until the enamel is burnt away.
  • Page 13 Note that this does NOT necessarily check that you correctly orientated the windings. The only way to do that, was with the DVM continuity check before inserting the transformer; and after identifying the windings, being careful not to mix the wires again while inserting the transformer in the PCB;...
  • Page 14 10-ohm resistor on the bottom side of the PCB. The resistor location is shown in the above circuit diagram fragment which shows the IRF510 PA. The 10-ohm resistor is supplied in the kit and has colour code brown-black-black-silver-brown.
  • Page 15 The modification is illustrated in the following diagram and picture. Rev 1.03...

  • Page 16: Basic Connections

    1 or 1.5A of current so remember to use a well-regulated good quality power supply capable of this. Use thick wires to the PA to avoid voltage drop in the wiring. You may wish to connect an electrolytic capacitor close to the PA kit PCB, particularly if the wires to your supply are long.

  • Page 17: Bias Adjustment

    The zero idle current point can be found quite easily if you connect an ammeter (typically a DVM in current measurement mode) in series with the PA kit supply. The adjustment is carried out in the key-up state, or in other words, no RF input (you could disconnect the RF input, for example).
  • Page 18 Heatsink temperature rise; PA kit installation in enclosure The desired output power of this PA kit in your application has a significant bearing on how you enclose the project. Remember that the heatsink will dissipate a considerable amount of heat. The heat dissipation will cause the temperature of the heatsink to rise.
  • Page 19 40 minutes (see chart). In summary: Enclosing the PA kit in an enclosure will increase the temperature rise; using fan- assisted cooling would reduce it. There are many factors that will influence heat dissipation. The above discussion should be taken as a guideline only –...

  • Page 20: Document Revision History

    7. Document Revision History • 02-Jan-2017 Initial revision • 09-Mar-2017 1.01 Corrected typos in toroid core type • 04-May-2017 1.02 Corrected section 3.3 to say four jumper wires (not three) • 29-Nov-2018 1.03 Corrected BS170 in parts list, it is TO92 not TO220 package Rev 1.03...