Andover Radio Club 4m Transverter Project
(c)1995 Andover Radio Amateurs club

This project was originally conceived in about 1995 by members of the Andover Radio Amateurs club. The design concept was to produce a well documented, simple project to enable members to get on the amateur 4m (70MHz) band. The Transverter was designed to be driven by a 28MHz (10m) amateur transceiver or by a CB radio. This latter approach offering a very low cost solution but one restricted to FM only. Whilst the preferred option for “prime mover” is a multi mode 10m rig it is also possible to drive the transverter from a 145MHz “prime mover” with some changes to the oscillator and mixer stages and with the addition of a trap to further reduce the 75Mhz local oscillator signal from the transverter output.

Contents

Contents.............................................................................................................................................1
Figures.............................................................................................................................................. 2
Introduction........................................................................................................................................ 2
Circuit Design.................................................................................................................................... 3
Construction...................................................................................................................................... 5
Before you start................................................................................................................................. 5
The tools you will need....................................................................................................................... 5
Assembling the PCB.......................................................................................................................... 5
Anti-static Precautions....................................................................................................................... 8
Test Equipment.................................................................................................................................. 8
RF Sniffer.......................................................................................................................................... 8
RF Probe........................................................................................................................................... 9
Test and set-up Procedure.................................................................................................................. 9
Some Useful signals for tuning your Receiver:-.................................................................................... 11
Drive Level....................................................................................................................................... 12
Cautions......................................................................................................................................... 12
A Simple Antenna............................................................................................................................ 13
Errata.............................................................................................................................................. 14
Component Sources......................................................................................................................... 14
Appendix 1...................................................................................................................................... 15
Resistor Colour Code........................................................................................................................ 15
Capacitor Codes............................................................................................................................... 15
Test Points...................................................................................................................................... 15
Semiconductor Pin Out data............................................................................................................. 16
Appendix 2 CB Channels- 70Mhz...................................................................................................... 17
Parts List........................................................................................................................................ 18

Figures

Figure 1 Mixer U1....................................................................................................................................
Figure 2     Soldering..............................................................................................................................
Figure 3  Coil Positioning.........................................................................................................................
Figure 4 Wire tails..................................................................................................................................
Figure 5  Diode Sniffer.............................................................................................................................
Figure 6  Diode Probe.............................................................................................................................. Figure 7  4m Dipole.................................................................................................................................
Figure 8  Semiconductor "Pin Out"..........................................................................................................

Introduction

This project was conceived for the Andover Radio Amateur Club with the following objectives:-

1. To provide an opportunity to members to build a piece of Amateur Radio equipment themselves and to experience the pleasure and satisfaction of operating equipment you have made yourself .

2. To increase activity on an under used band and perhaps create a “club” or even “inter-club” frequency for the surrounding areas of NW Hants, S Wilts and West Berkshire.

A transverter was chosen rather than a complete transceiver because the latter was considered perhaps too ambitious and costly for a club project.

Various schemes such as transverting from two meters were considered but after some discussion, it was decided to transvert from 27MHz or optionally 28MHz. Using 27MHz as the drive source was particularly attractive for a number of reasons. Many members own or could cheaply obtain a 27MHz CB rig which would mean that the completed transverter could be permanently connected to a dedicated drive source without “tying-up” and restricting the use of equipment used regularly on other bands. This would make it more practical to establish a “club” frequency which could be monitored whenever one is in the shack. It was felt this would help to build up band activity. Although CB radios have 10khz channel spacing whilst the 4m band uses 25khz, the two spacings coincide on a number of frequencies including all the calling frequencies (70.45 FM, 70.26 All Mode and 70.2 SSB) and two of the most commonly used simplex FM working frequencies 70.35 and 70.40.

A key advantage of a Club project is that help is on hand to ensure a successful outcome. Please call a member of the design committee if you need assistance (during social hours please!).

Also, the test and set-up procedure necessarily assumes that you have some basic test equipment available. If you do not, don’t give up but seek our help when you get to that stage.

I understand that 70% of all electronic kits are never completed. I suspect that the main reasons for this are loss of interest following some problem or other. With this project there should be no excuse. All you need to do is to follow the instructions closely and to adhere to good assembly and soldering practices which are described in the instructions. Please check that the kit is complete and read the instructions in full before you make a start. Some of the information you might need is contained in the appendices toward the end of the manual.  Good Luck and BCNU on 4 metres soon!

Circuit Design

The design of this Transverter was not intended to push the frontiers of technology, but to provide a simple, repeatable design based on readily available components many of which could be found in the “junk box” thus keeping down costs.

The receive converter uses Dual Gate MOSFET R.F. and mixer stages which provide good gain, noise performance and stability. L1 is resonated by the series combination of C1 & C2 to provide the first stage of input filtering, while the ratio C2:C1 provides matching from the 50 Ohm input to the higher impedance of Q1 Gate 1. The R.F. and mixer stages are band-pass coupled (L2 and L3) to improve rejection of unwanted, out of band signals.

The Local oscillator uses a third overtone crystal and is shared between the Receive and Transmit mixers. The choice of crystal frequency depends on the drive source to be used. For 28MHz the crystal frequency is 42MHz, for a CB 27/81 driver a frequency of 42.49875MHz is required while for the newer PR 27/94 CB Rigs a frequency of 43.0850MHz is required. The crystal option is selected at the time of ordering your kit but could easily be changed later if required. (Please note that we are also investigating a 144/145MHz  IF Option using a 74 or 75MHz Crystal, but that this option will also involve changes to L4, L5, L6, {Cirkit 35-13444}and C16 {10pF}, C11 {39pF} and C12 {10pF}).  Also, a trap (L17 / C43) will almost certainly be required to reduce the Local Oscillator output level. The Zener diode ZD1 is an option which may be required to improve stability if the transverter is to be used for SSB or CW operation (Initial tests indicate that it is not necessary so long as a regulated supply is used).

The transmit Mixer uses a proprietary Doubly Balanced Mixer to provide additional suppression of the Oscillator and Driver frequencies and their products. There are a number of different mixer units that will operate satisfactorily, but if substituting a different  device beware of its pin connections as two different pin layouts are in common use and only the right one will work !  Also, some types are not as shown but have pins 2,5,6 and 7 bonded directly to the case. This is not a problem. Note that pin 1 is identified by a different coloured bead which may be lighter or darker than its neighbours.

These Doubly Balanced Mixers require about +7dbm of Local Oscillator injection and 0dBm (1mW) of Drive (I.F.). The output of the oscillator is loosely coupled to L6 providing a band-pass arrangement to reduce unwanted harmonics. C19 and C20 in series resonate L6 while their ratios provide an impedance transformation to match the oscillator output to the mixer (U1) at 50 Ohms.

The input from the Driving “rig” is first coupled through C42 to a voltage doubling detector D3/D4 which acts as an RF sensor driving Q7 to operate the change over relays RL1 and RL2. Note that the choice of device for Q7 was based largely on the need for a high current gain (hfe), the specified BC548C having an hfe min. of 420. Once a sufficient level of RF is present at the input, Q7 turns ON and relays RL1 (and RL2) close routing the RF through an attenuator, R29,R30 and R31 to reduce its level to about 0dbm (1mW). Please note that RF switching is generally acceptable for FM but can be problematic for SSB operation so direct switching is recommended for this mode. There are two options for external switching:- a positive voltage on Transmit applied to the EXT PTT + input or a contact closure to ground applied to EXT PTT LO. Note that the latter should be from voltage free contacts or possibly through a series diode.

Assuming that a standard CB rig is being used with an output of 4W, the attenuation required is -36dB. The values shown for these resistors produce about 36dB of attenuation. If you wish to use a different input level, you will need to change these values. Bear in mind that R29 will dissipate the bulk of the power output from the driving rig and should be rated accordingly. This design uses a TO220 style  non inductive power resistor for R29, and this is bolted to the front panel and hence chassis of the transverter to dissipate the heat. Although rated at 20 Watts, a maximum drive level of 10 watts should not be exceeded and if you intend to have long overs using FM, you should keep to a maximum input level of 4 watts or the case will get mightily hot!

Input Level             Required Attenuation                 R30           Note. All values are approximate.

1W                       30dBs                                      820

4W                       36dBs                                      1.5K                                                                    

10W                      40dBs                                      2.7K

The mixer is band pass coupled (L7/L8) to a pre driver stage Q4 which is in turn band-pass coupled (L9/L10) to the driver Q5. Note that space has been provided for a trap (L17 / C43) from the base of Q4 to ground. This trap which is expected to be necessary only if a 144 or 145Mhz IF is used should be resonated at the LO frequency (74/75Mhz).  With a 28mhz If and no trap, the Local Oscillator output level was measured at about 47dBs below full output power which was felt to be acceptable. As in the receive part of the transverter, split capacitance is used (C21/C22, C24/C25 and C30/C31) to effect impedance matching.

The Driver and PA devices were chosen on the basis of price and availability. The PA is characterised for FM (Class C) operation (5W at 150MHz) but it was felt that there was a good chance that it could be made to operate satisfactorily in Class B which would allow the transverter to be used for SSB (and AM) if required.

The first prototype appeared to provide about 4W and this power level was considered entirely adequate for the local activity that  this project is designed to stimulate particularly when stations as far afield as Peel (GD), Warrington, Lincoln and Jersey were contacted on SSB during the cumulative contests.

Subsequent improvements to the layout yielded the full 5 watts. For those wanting higher power, the layout provides for connection of an external Power Amplifier within the RF switching provided. Please note however that relays with a higher contact current rating may be needed to perform the D.C. switching of such a PA as their absolute maximum limit is 2A DC and in practice this should not be closely approached.

Space on the PCB has been left free for additional output filtering which it was thought may be necessary to reduce the harmonic output from the -35dB level observed on the prototype. Although not absolutely necessary at the 5 watt level, it would nonetheless represent good practice and tests suggests that this filter reduces harmonics to about 60dBs down on the wanted signal at full power. Tests on a sample filter suggest a loss of about 0.85dB reducing the output from 5W to 4W.  In real terms this is insignificant. If a PA is to be fitted a filter is essential and should be fitted to its output .

Construction

Before you start

First check that the kit is complete by checking all components against the Parts List (see appendix 1) and read the instructions right through.

It’s far easier to correct errors before you solder so careful attention to the instructions and step by step inspection and checking are the order of the day. Each time you fit a component, check it off on the parts list, the circuit diagram and the layout.

The tools you will need

To complete this project you will need:- fine, long nosed pliers; small, sharp side cutters, a small Pozidrive screwdriver and a soldering iron suitable for PCB work (either a conventional iron from 10 to 25 Watts or best of all a thermostaticaly controlled Iron. If you don’t have suitable tools please ask for help. In my view, the old adage that a good workman doesn’t blame his tools is not quite right because a good workman would never use bad ones anyway!

Assembling the PCB

PCB assembly (sometimes known as board stuffing) commences with the smallest components first as placement of the larger components makes it difficult to reach and inspect the smaller ones. Start with the pins, whose positions are marked on the PCB Component Ident as circles. You should push them firmly into place and this is most easily done before any other components have been fitted. Vero pins are supplied with the kits,  push them through from the top side of the PCB. The ridges on the pins hold them in place until soldering which need not begin until almost all the components are fitted. Leave the fitting of the following components until much later:- MOSFETS Q1 and Q2. The power resistor R29 is not fitted until the unit is assembled in its box.

When fitting components, pre-form all leads so that the component will sit as close to the PCB as possible except for the self supporting coils which should sit 2mm above the board. Some capacitors such as Cs 3,7,8,9 must be pre-formed to a slightly wider pitch (than 0.1”) as conductors pass between their pins and more clearance is required. Don’t cut the leads until you have fitted the component and bent its leads over to about 45 degrees from the board. Leave 1.5-2 mm protruding. This will hold them in place until soldering which should not start until all the components except the MOSFETS, Inductors and R29 have been fitted. Note that the PCB layout shows VC5 as a large capacitor, in fact a small (green) one should be fitted in this position. The circuit diagram and parts list are correct.

Note that Q5, the driver transistor should be mounted on a TO5 transipad

After fitting the small components, carry out a careful inspection to confirm that everything is correctly placed before soldering.

When soldering, remember that the objective is to use the minimum of solder and not the maximum !

Keep the Soldering Iron Bit meticulously clean by wiping on a clean damp (non synthetic) sponge or if this is not available on a clean damp cloth. Re-Tin the bit after cleaning with a minimum of solder. Apply the heat to both the component and the PCB track then without removing the iron, apply the (minimum) of solder to the joint NOT the BIT and keep the heat applied until the solder flows as shown.

Carefully inspect your soldering to ensure that all the joints are good. It is helpful to remove all surplus flux using a PCB cleaning solvent and a stiff brush (such as a 1/2” paint brush with its bristles cut short) before inspection if possible as surplus flux has been known to mask a bad joint. When all the small components have been fitted it is time to fit the pre wound inductors. These should be fitted one at a time and one pin “tack” soldered merely to hold them in place. Note that L17 and C43 are only required if a 2m I.F. is used.

Next, the self supporting coils should be wound using a mandrel such as a drill bit with the correct diameter (6mm). All coils are specified for close (no) spacing between turns. While each coil is still on its mandrel, carefully scrape off the insulation “enamel” at the ends, then remove it from the mandrel and pre-tin the ends. Next, pre form the leads to fit the PCB in the appointed space observing the orientation as shown on the component layout. Fit the coils so that they lie 2mm above the PCB and bend the leads over at the rear of the PCB by about 45 degrees to hold them in place as you did with the other components.

Coil winding Data

L1,2,3,7,8,9,10                       Crkit     35-11934

L4                                         Cirkit    35-13415

L5,6                                       Cirkit    35-13492

L11                                        5T        22SWG            6mm dia close wound

L12                                        6T        22SWG            6mm dia   - ditto -

L13                                        1T        22SWG            6mm dia   - ditto -

L14                                        7T        22SWG            6mm dia  - ditto -

L15                                        7T        22SWG            6mm dia   - ditto -

L16                                        6T        22SWG            6mm dia   - ditto -

L17                                        See text 2m I.F. version only.

LF1,2,3                                  6T        22SWG            6mm dia close wound

Be sure to observe the correct orientation of the axis of the self supporting coils. The sense (clockwise/anticlockwise) in which they are wound doesn’t matter. When all inductors are fitted, inspect the PCB again for correct placement then solder them. With the pre-wound coils in particular solder the unsoldered pin first to avoid them dropping out of the PCB !

Next fit the wire link LK1 and, if you have opted to fit the Low Pass filter, place a link from the TX output to the filter input which is nearby. If you are not fitting the filter, run a miniature co-axial link from the PA stage to the Change over relay RL2 using additional pins where CF4 would have been fitted..

Next, fit and solder the MOSFETS Q1 and Q2 keeping the leads as short as possible and taking sensible anti-static precautions (see below). First pre-form the leads downwards 2mm from the body so that they will pass through the holes on the PCB.

Fit “tails” of 22SWG Tinned copper wire about 1” (25mm) long  to the pins adjacent to the I.F. Socket and 50mm long to the ANT and GND Pins adjacent to RL2 by making one turn tightly around the pin then soldering. Bend these tails up at right angles to the PCB. They will be used to connect to the I.F. and Antenna sockets when the PCB is fitted in it’s box.

Finally, make a trial assembly of the PCB into its case, using the spacers provided and fit the I.F. BNC socket. Leave the other BNC socket off at this stage as it will prevent removal of the PCB from the box once fitted. Next, fit R29 in place, bolt it firmly in position and “tack” solder its leads on the top of the PCB. Remember that it will need to be un-bolted from the box when the PCB is removed. Now remove the PCB from the box and trim and solder R29’s leads on the underside of the PCB. It is recommended that initial testing is carried out before final assembly into the box.

Anti-static Precautions

Make sure your soldering iron is earthed.  Establish yourself at the same potential as the MOSFET by touching its anti static packaging before the device. Once you are holding the device, establish yourself at the same potential as the PCB by touching and holding it with your other hand before the MOSFET comes into contact with it.

Test Equipment

One of the advantages of a Club project is that help is on hand at all stages. The test and set-up procedure described later assumes that you want to have a go yourself and that you have access to a multi-meter with an input impedance of 20K Ohms per volt or better and either an Oscilloscope with an input sensitivity of from 10mV per division (Bandwidth immaterial) or a millivolt meter. A frequency counter a general coverage receiver and a 70mhz signal source are also helpful but not essential. If you have access to more equipment than this I shall assume that you know what to do with it! If you do not have access to these basic tools please feel free to call upon your club for further assistance. In particular, the 70mhz signal source might be a problem. A very local (and patient) station already with 4m is one possibility, but if you can’t manage that, there is a simple crystal oscillator source which can be made available to members.

In addition to the basic equipment mentioned, you will need two very simple and useful pieces of test equipment, an RF “Sniffer” and a diode probe to turn your oscilloscope or milivolt meter into a wideband RF level indicator.

In case you don’t already posses these, they are easily constructed as follows and are likely to be useful in many RF projects.

RF Sniffer

An RF sniffer consists of a loop of wire, a diode,

a capacitor and a small meter of about 100uA FSD connected as shown. The loop can consist of two turns of a large gauge (18swg or greater) of enamelled copper wire. The diode should ideally be a schottky or fast geranium diode but a humble 1N4148 will do. The sniffer is used by simply holding its loop close to the RF source inductor eg. the Oscillator L5. Its sensitivity can be adjusted by simply moving the loop closer to or further away from the source. The diode sniffer cannot be used on coils with screening cans.

The sniffer’s sensitivity is limited to levels above a few mW. For lower levels a diode probe is useful.

RF Probe

The RF Probe can be constructed in an old plastic Biro body. The capacitors should be of the low inductance axial types (so they fit in the biro body). Ideally the diode should be a Schottky type but again, a 1N4148 will do. The lead to the scope or mV meter can be of any reasonable length (say 1 m) and fitted with the appropriate connector e.g. BNC.

The 100pF Capacitor blocks any DC voltage which may be present on the point under test allowing only RF to Pass. The resistor provides a D.C. return path for the current rectified by the diode while the 10nF Capacitor provides decoupling.

The probe is used by grounding the flying lead to a suitable earth point on the PCB and touching the probe onto the desired test point at which RF should be present. The sensitivity of the ‘scope (using a D.C. setting) is increased until a deflection is observed, or if none is observed, to its maximum setting. The circuit under test can then be tuned for maximum smoke.

Test and set-up Procedure

Before final assembly into the box, connect a red 14/0.2mm lead to one of the pins at either end of LK1 and a black lead to any convenient point on the ground plane. Carefully apply 13.8 Volts DC from a current limited supply (if possible limit to 100mA to avoid damage if there are any serious errors in the construction. If you do not have a current limit on your supply or if it cannot be set to 1 Amp or less, connect the supply through a resistor of 47 to 100 Ohms.

With a Voltmeter having an input impedance of 20K Ohms per volt or more, set to a range which extends to at least 15 volts, check the following points.:-

All voltages are approximate. Allow about 15%.

Local Oscillator supply.                           TP1             12V

Local Oscillator Emitter.                          TP2             1.8V

Q1 Source                                              TP3             0.55V

Q2 Source.                                             TP4             0.8V

Note that at this stage the transmit side of the system is not powered up.

With a diode sniffer, tune L5 for maximum deflection, then tune L6. There will be slightly less deflection for L6 but the meter should move significantly. These two circuits interact so repeat the. Set VC1 so that it is about half engaged and re-tune L5 for the correct frequency. If you have a frequency counter, couple it loosely to L6 using a single or two turn loop and re-tune L5 for the correct crystal frequency. If you do not have a counter don’t worry as the oscillator is unlikely to be far out and can be trimmed to frequency using an off air signal later. For fine trimming adjust VC1.

Now increase the current limit on your power supply to 500mA, or reduce the series resistor to about 22 Ohms and connect the EXT PTT input (R26) to the +ve supply. You should hear the relays click. Now check the following voltages:-

TX Supply                                               TP5             13V

Q4 Emitter                                              TP6             1.8V

Q5 Emitter                                              TP7             0.9V

Q6 Emitter                                              TP8             0.3V

Satisfactory results to these tests gives us some confidence that resistors and semiconductors are correctly placed and there are no disastrous short circuits between tracks !

As taking the PCB out of the box to correct errors is very tedious, we have found it best to run through the tune-up procedure with the PCB out of the box first and carry out a final re-tune later after fitting the fully tested PCB into the box.

First, however, fit the 10mm spacers in the box and screw the PCB in place temporarily, Next, fit R29 in place and bolt it firmly to the box before soldering it in place. Now un-bolt R29 and remove the PCB for initial set-up and test. Fit “tails” of 22SWG tinned copper wire to the I.F. and Antenna pins and adjacent ground pins then connect the loose BNC sockets to these tails with the ground “tails” soldered to the large solder tags supplied with the sockets.

Having already tuned the oscillator, the front end, mixer and I.F. stages (L1, L2 and L3 and L4) can be tuned by connecting a receiver (or transceiver) to the IF socket, and using a strong local signal. In an ideal world, a signal generator is used. If this is not available a number of signal sources are available on loan from the Andover Club, or you may be able to persuade a local amateur to provide you with a strong signal to tune-up on. You will of course need an antenna if you adopt this approach. You may also be able to use the intermittent transmissions from your local fire service who occupy the band just above 4m running vertically polarised AM. If you are using a CB radio I.F. you will need  a general coverage receiver to receive these transmissions. Remember they transmit intermittently so it may be necessary to wait some time before you hear anything. Also, if you peak the tuned circuits on these transmissions you will need to re-adjust them on in-band signals later for best performance.