The 'Multi-Four'  - a VXO Transmitter for 20/30/40/80m

Contents:

1.   Features Summary
2.   Introduction
3.   Variable Crystal Oscillator (VXO) and Buffer
4.   Keyed Switched-Gain Amplifier
5.   Driver Stage
6.   Power Amplifier (PA) and 14 MHz Low Pass Filter (LPF)
7.   Low Pass Filter
8.   SWR Bridge
9.   Receiver Mute Circuit
10. Pin-out Diagrams
11. Additional Notes
 

This picture shows the partially completed Multi-Four (before fitting the front panel labels etc.).

1 - Features Summary

This transmitter has been designed for use on 20; 30; 40; and 80 m, and has the following features:
- 12 v DC operation;
- variable crystal oscillator (VXO);
- 5 kHz tuning range (depending on the band);
- six switchable crystals
- key-click filter;
- 5 watt output power, preset on each band;
- multi-section low pass filters;
- sidetone generator keying facility;
- receiver muting circuit
- built-in dummy load; SWR indicator; and antenna changeover;
- low level VXO output.

2 - Introduction

 I've had an interest in building a simple VXO transmitter for many years, but was unsure that the limited tuning range usually associated with VXOs would be useful on the busy HF bands.  However, the ability to switch a number of crystals, and cover a number of different bands in one transmitter greatly increases the chance of finding a clear frequency on which to operate. 

Much of the design is based on the Super-Sixty 5 MHz QRP transmitter, described in the Winter 2002/3 issue of SPRAT (SPRAT Nr. 113).  

The transmitter uses a FET VXO; push-pull buffer; 2N2222 pre-driver + BC212 keying transistor; 2N3553 gain-controlled amplifier; 2SC2166 driver and a pair of 2SC2166 transistors in parallel in the power amplifier (PA).  With limited shack space in mind, the Multi-Four was built into a small diecast box measuring only  255 (W) x 55 (H) x 150 (D) mm.

Since preparing this web page, the Multi-Four has undergone some minor changes to add 5 MHz (60 m) and to improve the keying waveform.  Details of the 5-band version of this VXO transmitter can be found here.

3 - Variable Crystal Oscillator (VXO) and Buffer Amplifier

The 50 pF tuning capacitor provides a tuning range of approximately 400 Hz on 3.5 MHz, rising to nearly 6 kHz on 14 MHz. 

It is easier to tune a crystal to the high side of its intended frequency of operation, so the crystals for the '20' and '30a' slots on the crystal switch (SW1a) were custom made to resonate at 2 kHz below the international QRP frequency, thereby providing a good tuning range above and below the usual centre of QRP activity.  The VXO drives a very effective push-pull buffer, which presents a chirp-free signal to the driver stages. 

The TOKO coil KANK 3334 is used to set the VXO to 14.060 MHz when VC1 is 25% meshed (corresponding to 135 degress of rotation of the spindle.)

Note that the green LED associated with the selected frequency is only illuminated when the correct band has been selected in the Low Pass Filter circuit. 

The actual frequencies covered when using the specified crystals (see circuit diagram) were:
14059.90  +/- 2.80 kHz
10116.28  +/- 2.25 kHz
10105.70  +/- 1.30 kHz
7030.40    +/- 1.25 kHz
3579.74    +/- 0.35 kHz
3560.16    +/- 0.15 kHz

4 - Keyed Switched-Gain Amplifier

The first stage of the switched-gain amplifier uses an untuned common emitter amplifier (2N2222), keyed by a BC212 PNP transistor.  In my experience, many simple TX designs fail to provide adequate shaping of the keying waveform, resulting in very hard keying. To reduce the likelihood of transmitting key clicks, the keying circuit in this transmitter provides rise and fall times of  around 8 ms. The waveform shaping - together with the effective VXO buffer results in a very pleasant T9 note.

The keyed signal is coupled via the 330pF capacitor to the pre-driver stage (2N3553).  Gain control is provided via the shunt attenuator (2N2222).  The band information from SW3c is used to set the drive level required to obtain 5 watts RF output on each band.

The KEYING OUT socket is used to key an external sidetone generator.   Point ( H ) is used to key an internal Murata piezo-electric sounder.

L1: 17 turns, 0.5 mm enamelled copper wire on FT50-43 ring core.  (Alternatively, a 100 uH RFC would probably work just as well.)
T1: 12 bifilar turns, 0.5 mm enamelled copper wire on FT50-43 ring core.  Twist two wires together at about one twist every 15 mm.  Wind 12 turns on the ring core, and label each of the two wires at the start of the winding with the identification numbers 1 and 3.  Then label the other end of each wire with 2 and 4, respectively.

One pass through the centre of the ring core counts as one turn; two passes as two turns, etc.  In practice, the wire gauge is unlikely to be critical for any of the inductors - I have simply detailed the wire gauges that I felt to be appropriate for this project. 

5 - Driver Stage

The driver stage uses a single 2SC2166 transistor.

L2: 17 turns, 0.5 mm enamelled copper wire on FT50-43 ring core.  (Alternatively, a 100 uH RFC would probably work just as well.)
T2: 12 bifilar turns, 0.5 mm enamelled copper wire on FT50-43 ring core. 

The current through the driver transistor should be between 50 mA and 80 mA on key down, depending upon the band in use.  The current through the transistor can be checked by measuring the voltage across its emitter resistor: 500 mV corresponds to a current of 50 mA. 

6 - Power Amplifier (PA) and 14 MHz Low Pass Filter (LPF)

L3–L5: 22 SWG enamelled copper wire on T50-6 ring core.
T3: 14 bifilar turns, 22 SWG enamelled copper wire on T68-2 ring core.

The current through each PA transistor should be between 400 mA and 600 mA on key down.  (The efficiency of the PA is a compromise of on each band, but will probably be optimum on 30 metres.)  The current through each transistor can be checked by measuring the voltage across its emitter resistor: 400 mV corresponds to a current of 400 mA. 

The PA transistors will require a small heatsink to survive extended periods of key-down.  Note that the tab of the 2SC2166 is internally connected to the collector, so be sure to use a TO220 insulating kit!  Although the driver transistor does not need a heat sink, I found it convenient to mount all three 2SC2166s on the inside wall of the diecast box. 

7 - Low Pass Filter

The low pass filter for the 20 m band is part of the PA circuit and is therefore always in circuit.  The switched low pass filters for the 30; 40; and 80 m bands are shown below.  A third pole on the switch is used to generate a 'current band' signal for illuminating the selected frequency LED (see Variable Crystal Oscillator and Buffer circuit), and for setting the output power (see Keyed Switched-Gain Amplifier circuit).

L6–L8: 22 SWG enamelled copper wire on T50-6 ring core.
L9–L14: 0.5 mm enamelled copper wire on T50-2 ring core.

8 - SWR Bridge

T4: 10 bifilar turns, 0.5 mm enamelled copper wire on FT50-43 ring core. 

The one turn primary for T4 is formed by passing the wire through the core.  Surprisingly, this provides enough coupling - even with only 5 watts of RF!

9 - Receiver Mute Circuit

The following circuit was used to ground the 'Send' (receiver mute) terminal on the companion receiver.  If your receiver has very good AGC characteristics, you can probably omit this circuit without fear of deafening yourself each time you switch to transmit!

10 - Pin-out Diagrams

11 - Additional Notes

Here is the set up used for the first contact  . . . taken shortly after completing the first contact with Tim G4ARI in Leicester.  The QSO was made on 3.560 MHz.

The station comprised:
- a 'K2' kit-built transceiver (used as the station receiver, lower left);
- WW2 German morse key (to the right of the station log book);
- Multi-Four transmitter (to the right of the morse key);
- 'Carlton' antenna matching unit (not visible in this picture).
The antenna was a G5RV.

General Construction Notes

The VXO stage was screened from the other stages within the diecast box using 'walls' of copper-clad board soldered to a 'floor' of the same material.  Earth (or 'ground') connections were made directly to the floor, keeping all earth connections as short as possible – especially around the driver and PA stages.  If this is done consistently, Mr Murphy will be more forgiving if you end up with longer connecting wires elsewhere. 

This method of construction is affectionately known as 'ugly construction' and more information about this rapid method of construction can be found by clicking here.

Here are some more pictures of the completed unit.  Click an image to enlarge.

Component Sources
The transistors may be obtained from either of the following suppliers:
1) Grandata Limited, K.P. House, Wembley, London, HA9 0HB
Tel: 020 8900 2329    Fax: 020 8903 6126
http://www.grandata.co.uk

2) Sycom, PO Box 148, Leatherhead, Surrey, KT22 9YW
Tel: 01372 372587    Fax: 01372 361421
http://www.sycomcomp.co.uk
(Also a good source for RF chokes; ring cores; capacitors; TO220 insulating kits; switches; etc.)

3) Two custom-made HC25 crystals (for 10.114 MHz and 14.058 MHz) were obtained from:
QuartSlab Marketing Limited.
PO Box 19
Erith
Kent
DA8 1LH
Tel: 01322 330830
The crystals used were made to 'Specification E'.  

4) The crystals for 10.106; 7.030; 3.5795; and 3.560 MHz were obtained from the G-QRP Club.

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