Getting Started on 630 metres

Part 2: Transmitter Driver
15/02/2018
Introduction     Transverter     TX Driver Stages     TX Power Stage     Firing Up

The Transmitter Driver

This transmitter design has been well tested, with many working transmitters based on this design. There are both high and low powered versions, although this series will describe only the 150 W version. The design comes from Graham ZL1QM, who has built several amplifiers to this design, although I have adapted it slightly.

Like the transverter, the amplifier runs off 12V, but I suggest that the power supply for the low power stages be the same supply as the transverter, and a separate high power (and preferably adjustable) supply be used for the power stage. More about that when we come to it.

Driver schematic
The driver schematic
(Click on picture for full size view)

Description
This schematic shows the driver stages, which are built on a small piece of copper-clad board, ‘dead bug’ style. Drive at about +10 dBm level is applied to the input, and the first transistor TR1 amplifies it up to about 12V p-p, sufficient to drive the CD4093B device. TR1 can be just about any NPN RF transistor, the MPS9602L shown, a 2N2222A, even a BC547B.

The CD4093B IC U1 is a standard CMOS device, four NAND gates, chosen because the device will operate off 12V. On this schematic, the points marked ‘+V’ are derived from the same 12V supply as the transverter, and appropriately filtered. The first stage (U1a) acts as an inverting squarer, and has bias set on the inputs to ensure that the output duty cycle is exactly 50%.

At the top left of the schematic is the keying circuit. You can manually key the amplifier at the ‘Key’ input provided drive is already applied. I found it much easier to use an RF-keyed drive circuit (the parts shown in red). The MPS9608 transistor TR2 (a BC547 would do) operates in Class C, and draws current only when drive exceeds about 2 V p-p. It simply pulls down the Key input on every positive cycle of the RF drive. The CD4093B inverter U1b has a CW keying shaping network on the input which conveniently holds the input pin 6 low between RF drive cycles. The output at pin 4 is high to enable the transmitter.

The next gate, U1c, combines the drive and the keying. When the key is up or there is no drive, pin 10 remains high and there is no drive to the following stages. The last CD4093B gate U1d acts as an inverter, driving the CMOS driver U2, a TC4428. This is a push-pull device capable of driving high current into the gates of the power stage. The outputs of the TC4428 are AC coupled to the power FETs, so that when drive is removed, the FETs turn off very quickly. The diodes and 1 k Ohm resistors act as DC restorers to correctly zero-bias the FETs. The 1N5819s are 1A Schottky diodes. The 10 Ohm series resistors provide stability to the FETs by preventing high frequency feedback.

Construction
The small board containing the driver stages has been laid out so that it fits into the gap between the fins of the heat sink (see below). Most of the wiring is point-to-point, with the chips wired in a ‘dead bug’ fashion.

Driver board
The driver board
(Click on picture for full size view)

In the photo, input is at the left, and there’s a small tag strip to terminate the leads, RF input and DC power. You can see the small trim-pot used to adjust duty cycle. About in the middle of the picture is the up-turned CD4093B, and to its right the little TC4428. There’s another tag strip on the far right where the FET connections are made, and the gate resistors are secured. The FETs are connected via the 10 Ohm resistors which pass through holes in the heat sink.

Introduction     Transverter     TX Driver Stages     TX Power Stage     Firing Up
Copyright © M. Greenman 1997-2018. All rights reserved. Contact the author before using any of this material.