GW4ALG's QRP Radio Pages

[ GW4ALG went QRT in February 2007 ]

Home ] Antennas ] Equipment ] Introduction ] Links ] Logbook ] Operating ] Station Summary ]

Up ] Audio Filter ] Components ] FT707 ] IC756PRO ] K2 ] 'Multi-Four' VXO TX ] Portable ATU ] SP220 Power Meter ] Ugly Construction Workbench ] [ 60 m 'Super-Sixty' ] 60m QRPP TX ] 80 m Transceiver ]


View/Hide Contents Window

The 'Super-Sixty'  - a 5 MHz Transmitter & Receive Converter

Contents:

1.   Features Summary
2.   Introduction
3.   Overview
4.   Variable Crystal Oscillator (VXO) and Buffer
5.   Keyed Pre-Driver Stage
6.   Power Amplifier (PA) and Low Pass Filter (LPF)
7.   Receive Converter
8.   Pin-out Diagrams
9.   Introducing 'Ugly' Construction
10. Component Sources
11. Feedback from other constructors

 

This picture shows the completed Super-Sixty on top of the companion Elecraft K2 transceiver.  Here, the K2 is being used to receive 5 MHz via the receive converter section of the Super-Sixty.  The keying line of the K2 is connected to the 'Keying Out' line from the Super-Sixty, thereby providing sidetone on transmit (see below).

Super-Sixty with K2

Features Summary

This transmitter and receive front-end combo has been designed for use on 5 MHz and has the following features:
- 12 v operation
- VXO transmitter, with 2 kHz tuning range;
- key-click filter;
- 5 watt output power;
- 7-pole low pass filter;
- sidetone generator keying facility;
- 5 MHz to 29 MHz receive converter;
- built-in antenna changeover.

Introduction

Between August and October 2002, several hundred UK radio amateurs applied for permits allowing experimentation on 5 MHz (60 m).  Most applicants were successful in gaining the special authorisation (or 'Notice of Variation') required to carry out experiments on the five 3kHz-wide channels made available under these arrangements.

 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, when the channelised frequencies became available on 5 MHz, I thought that this would be an ideal application for a VXO TX/Receive Converter project to be used alongside my Elecraft K2 transceiver. 

While much of the design is based on the 'Marathon' 136kHz QRP transmitter, described in the Autumn 2001 issue of SPRAT (SPRAT Nr. 108).   I am grateful to G-QRP Club member Dave Sergeant G3YMC (Bracknell, Berkshire) for his ideas and advice during the design of the transmitter.   Those not having access to 5 MHz will find that all the circuit elements can be adapted for similar projects on other HF bands.

As things turned out, my first QSO using this TX design was with G3YMC  - who was also using a very similar 5 watt VXO TX.  You don't need high power to work around the UK on 5 MHz!  For details of  Dave's VXO TX, click here.

Overview

The transmitter uses: a bipolar VXO; FET buffer; 2N2222 pre-driver + BC212 keying transistor; 2SC2166 driver; and a pair of 2SC2166 transistors in parallel in the 5 watt power amplifier (PA).  The receive converter uses a single-ended FET mixer, and a 24 MHz oscillator to up-convert the 5 MHz band to 29 MHz.  With portable operation in mind, the Super-Sixty was ‘shoe-horned’ into a small diecast box measuring only  85 x 155 x 50 mm.

 

Variable Crystal Oscillator (VXO) and Buffer

For the transmitter, channels ‘FA’ (centre frequency of 5.260 MHz) and ‘FB’ (5.280 MHz) were chosen.  The 50 pF tuning capacitor provides a tuning range of approximately 2 kHz above the crystal frequency. 

It is easier to tune a crystal to the high side of its intended frequency of operation, so a 5.25900 MHz crystal is used for channel 'FA' (tuning 5.259 to 5.261 MHz); and a 5.27900 MHz crystal is used for channel 'FB' (tuning 5.279 to 5.281 MHz).  The VXO drives a very effective FET buffer, which presents a chirp-free signal to the driver stages. 

VXO and Buffer Stage

Keyed Pre-Driver Stage

The pre-driver stage uses an untuned 2N2222 common emitter amplifier, 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  4 ms. The result is a very pleasant T9 note.

The keyed signal is coupled via a 0.01 uF capacitor to the 2SC2166 driver transistor. 

The KEYING OUT line is for use with an external sidetone generator. In my case, I wired the KEYING OUT line to a panel-mounted phono socket, and then made up a connecting lead to the key jack of the K2.  (When using the K2 in this way, it is wise to inhibit CW transmit on the K2 by selecting the 'test' CW mode!) 

Pre-driver and Keying Stage

Power Amplifier (PA) and Low Pass Filter (LPF)

PA and Low Pass Filter

L1 –  14 turns, 26 SWG enamelled copper wire on T50-2 ring core.
L2 –  19 turns, 26 SWG enamelled copper wire on T50-2 ring core.
L3 –  20 turns, 26 SWG enamelled copper wire on T50-2 ring core.
L4 –  19 turns, 26 SWG enamelled copper wire on T50-2 ring core.
T1 – 14 bifilar turns, 22 SWG enamelled copper wire on T68-2 ring core.  Twist two wires together at about one twist every 15 mm.  Wind 14 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.  (In all cases, one pass through the centre of the ring core counts as one turn; two passes as two turns, etc..)

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.  Each PA transistor draws 400 mA on key down.  This can be checked by measuring the voltage across one of the 1 ohm emitter resistors: 400 mV corresponds to a current of 400 mA.  Increasing the value of the 10 ohm emitter resistor in the driver stage will reduce the drive level to the PA.

The 'RX' antenna terminal should be connected to the 5 MHz input terminal of the receive converter.  Alternatively, the 'RX' terminal may be connected directly to a separate 5 MHz receiver.  The neat thing about using a switched receive converter is that you don’t have to keep winding back every gain control on the main receiver each time you switch to transmit.  It’s well worth the trouble of incorporating a receive converter for that reason alone!  When using a separate 5 MHz receiver with the Super-Sixty, the ‘+12 R’ line could be used to generate a ‘mute’ signal for the receiver.

Although I made over 30 contacts using the first prototype without fitting heatsinks to the PA transistors, these devices 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.  Initially, the VXO coil was tuned for maximum signal, and then off-tuned very slightly (to a point where adjustment of the core had less effect upon oscillator frequency).   The top of the adjustable core ended up being about 2 mm below the top of the coil former.

Receive Converter

This simple two transistor design is similar to one that I have used for many years in a homemade 80 m transceiver, and performs very well indeed.  I have included some modest protection at the output (the pair of 1N4004 diodes), in case the associated 29 MHz rig is keyed accidentally.  To align the converter, initially set the top of all adjustable cores about 2 mm below the top of the coil former.  Then, using a weak off-air signal, peak all coils.  As with the VXO, I suggest off-tuning the 24 MHz oscillator coil very slightly. 

Receive Converter

Pin-out Diagrams

Pin-Out Diagrams

 

 

 

Introducing 'Ugly' Construction

Those with lots of experience of building homemade rigs will have no trouble building the transmitter into a smart equipment case.  Those with less experience would, perhaps, benefit by starting with a simpler approach. A 'bird's nest' version can be constructed quickly on copper-clad board using 'ugly' construction techniques.  This is exactly what I did when I made the prototype transmitter section of the Super-Sixty (see picture below).  It didn't look very pretty, but that really didn't matter - the important thing was that (even before fitting heatsinks to the PA transistors) it was possible to use the prototype to test the design - and also work a number of stations around the UK!

Prototype TX

For such an approach, start by fixing some copper-clad board to a piece of wood or 'chipboard'. (Prior to fitting the copper-clad board, be sure to remove any oxidation using metal polish, followed by a rinse with soap and water.) Odd scraps of aluminium sheet can be used as the front and rear panels by screwing them to the front and back edges of the chipboard. Start by getting the tuning capacitor; antenna sockets; TX/RX switch; NET switch; 12 v connector; and PA transistors/heatsink mounted in convenient positions, and in relation to their position in the circuit diagram.

The rest is easy: solder those components needing a connection to earth directly to the copper-clad board; and solder the remaining components directly to one another. Don't worry too much about lead lengths; but be sure to provide enough spacing between components to prevent short circuits, and to allow voltage measurements to be made during testing. To support larger components, solder one lead of a high value resistor to earth, and the other lead to the component requiring support - a sort of poor man's 'insulated' terminal post.  The bird's nest in the picture above worked just fine!

For more information about ugly construction techniques, click here.


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.)

The HC25 crystals and sockets may be obtained from:
QuartSlab Marketing Limited
PO Box 19
Erith
Kent
DA8 1LH
Tel: 01322 330830
The crystals used were made to 'Specification E', 5.25900 and 5.27900 MHz.

 

Feedback from other constructors

Use with the K2 (added 15/03/2003)
Dave G3YMC (G-QRP Club member #11029) has written to say that the frequency readout on the Elecraft K2 can be 'shifted' using the built-in transverter software to take account of an IF off-set (which, in my design, is 24 MHz). 

Dave's configuration of the K2 subtracts the conversion frequency (in my case, 24 MHz) from the K2 receive frequency (say, 29261 kHz), and hence displays the actual received signal frequency of 5261 kHz.  Neat, eh?

All the details can be found on Dave's web site at:
http://www.dsergeant.btinternet.co.uk/fivemegs/k2config.htm

Note that on my K2, I set the 'rF' option to '4'; and the 'IF' option to '28' (thus shifting the readout by the required 24 MHz).