VK5 50MHz TRANSVERTER

144 MHz IF Version
by David Minchin, VK5KK

Transverters for the 6 Metre band have evolved over the last 30 years, in both Thermionic and Solid State technology, as a means to use HF transceivers in the 50 - 54 MHz band. While dedicated transceivers have become more prevalent since the mid seventies, the alternative of using a transverter has always remained.
There is a proliferation of 2 Metre multimode transceivers, also suitable for use as an IF driver. It is common practise to use 144 MHz as an IF for the uWave bands. The still popular IC202, if carefully restored and modified, it is still a good driver for other VHF/UHF bands. If you are really keen you can overcome the one major drawback of the IC202 (Frequency readout) by constructing a frequency counter/readout in the rarely used battery compartment. A 23 year old IC202 has better phase noise characteristics than all but the very best DDS designs.
In searching for a useable design, no single design combined all the desired features, so it was decided to throw some old and new ideas together to create something a little different. The final design evolved with the following design points in mind. While a HF "IF" version of the following transverter is also available, this article describes the original 144 MHz IF version. Over 50 Kits for this version have been supplied to Australian Amateurs since 1992.
PLEASE NOTE, THIS KIT IS CURRENTLY BEING UPDATED. A NEW PCB IS BEING DEVELOPED TO MAKE CHANGES REQUIRED FROM VARIOUS PARTS BEING SUPERCEDED (THAT's LIFE!!!) KITS WILL BE AVAILABLE from the WIA Equipment Supplies Committee EARLY 2000.
DESIGN FEATURES

 

 

A transverter is simply a bi-directional "Heterodyne" converter. An ideal transverter is one that reliably presents the same receiver performance (both weak and strong signal) and as a clean a RF signal as the driver. On the first point, 6 Metres tends to be a bit like 20 Metres during the CQ WW contest if someone mentions the possibility of a new country being on! The integrity of the IF transceiver will determine how much IMD your neighbour gives you and how much splatter you give him. Be warned, if you are going to use a doubtful transceiver, the final results will be impaired. Unfortunately, some of the early digital multimode transceivers, with poor VCO phase noise, fall into the doubtful area.
The transverter is to be looked at in six sections as follows, by referring to the circuit some idea of just how it all fits together will soon be apparent.
  1. IF Signal Processing. At the 144 MHz IF port a 50 ohm 2 Watt load terminates the driver transceiver. On transmit the 144MHz driver level is attenuated approximately 35db by a PI type resistive attenuator. On receive a Grounded Gate Fet amplifier compensates for the loss presented by the 50-ohm termination. The mixer port is selectively switched between receive and transmit by two PIN diodes. On receive this stage has approximately 0 db gain.
  2. DC Switching. 144 MHz RF is sensed at the IF port by a Germanium diode to produce a DC switching voltage. A 4049 CMOS IC provides the required fast attack/slow decay for R/T switching. The decay time is set by the 4.7uF capacitor. Its value can be increased if preferred; the current value gives about 250 mS delay. The receive and transmit rails are enabled by PNP switching transistors.
  3. Balanced mixer/Bandpass filter section. A single SBL-1 Double Balanced mixer (**replaced by RMS11X**) is used for receive and transmit. The IF and RF ports are terminated in 3db attenuators. This mostly ensures a degree of load isolation and hence a better 50-ohm load to each port. The extra 6-db loss is easily compensated for. The filter is slightly over coupled to drop the sharpness of tuning, without destroying the skirt selectivity. Rejection of near band products is not a huge problem when using a high side IF, 94 MHz away. The filter is about 1 MHz wide when peaked for single frequency use. It can be misaligned to cover 2.5 MHz with some iteration on the trimmers. Care must be taken as out of band responses can increase as you might end up resonating more than one element outside of 50/54 MHz.
  4. Local Oscillator. A standard overtone oscillator & Emitter follower design is used with a 94 MHz overtone crystal. The oscillator has a low phase noise compared to most circuits being lossely a derivative of the Butler oscillator. The output is passed via a 2-section low pass filter to reduce the harmonics of the Oscillator below -60dbC.
  5. Receive Amplifier. A single BF981/8 provides around 24db gain at 50 MHz. The input is tuned with a capacitive tap to enable easy noise matching/stable device loading. Noise Figure is usually not a priority at 50 MHz however it is nice to here a solid increase in noise level when you connect the antenna! The output is a simple broad band transformer. The overall noise figure of the transverter is between 1 and 2 db, depending on the performance of the IF transceiver. The gain contribution of the transverter is deliberately low (6-8 db), as strong signal handling is a priority. If your 144 MHz transceiver has a particularly poor noise figure you may need to compensate for this by increasing the gain of the IF pre-amp.
  6. Transmit Amplifier. The 10uW 50Mhz signal from the filter on transmit is amplified to 50mW by a three Class A stages. The first two use 50-ohm MMIC's (Monolithic Microwave IC's). They provide stable gain, well within their linear operating points. The third stage is a discrete Class A stage using a BFQ34T transistor. The final stage, a Mitsubishi M57735 Hybrid module, is fed through a two-stage low pass filter, again reducing harmonics below -60dbC. The PA will produce 15 W PEP with typically -30db third order IMD products. The module is "enabled" during transmit by the application of 9V to the bias terminal of the module. A final 2-stage low pass filter reduces any harmonics created by PA module. Antenna switching is provided by a SPDT relay.
CONSTRUCTION

PCB OVERLAY & Coil Data

It is assumed that the constructor has built some amount of equipment before and won't need explanation of some of the more basic procedures/items. While experience with RF circuitry is not essential, if it is your first go, enlist the help of a fellow Amateur or Club. As you install a component it is a good idea to check that component off the circuit. This saves double-checking afterwards. If a component lead is installed through a hole that is not cleared, on the ground-plane side, then it is soldered to both sides of the PCB (earth).
1. Fabricate two pieces of Double sided PCB, one 124 x 30 mm, the other 35 x 30 mm. Solder both pieces in, where shown. Fabricate two smaller pieces for the oscillator, one 20 x 25 mm, the other 20 x 50 mm. Solder both pieces in, as shown. The shields can be sprayed with PCB lacquer to prevent tarnishing.
2. Install all resistors. Do not forget the ferrite beads where indicated (4 places) slipped over the RF end of the resistor.
3. Install all Ceramic capacitors.
4. Install all electrolytic capacitors, except for the 470uf cap.
5. Manufacture all coils, three sizes of wire are used. All air wound coils are spread out to the limits of the mounting holes except for L3 (close wound). Toroids have turns spread over 80% of the core.
6. The Bifilar transformers are wound using a twisted pair of the thinnest wire provided. Make sure that the windings are connected as shown. The RF chokes are constructed as shown, the 6 Hole ferrites must use 0.5mm wire (mid sized).
7. Install all coils, chokes, transformers and Variable capacitors. Note the orientation of the Blue 10pF Oscillator trimmer. The toroids can be glued flat to the PCB.
8. Install the three Dipped Mica capacitors in the Antenna Low Pass filter. The 120pF capacitor has its Earth side soldered to the smaller PCB shield due to area limitations.
9. Install all diodes including the SBL-1 mixer, note correct placement of pin 1. The case of the mixer is soldered to the PCB in one spot.
10. Install all Transistors, Fets and MMIC's. Note the BF981 is mounted trackside, the designation faces the component side. The BF981 is sensitive to static discharge so take necessary precautions.
11. Insert the 7808T regulator into place so only 1.5mm of uncut lead protrudes trackside. The Heatsink tab should be flush to the PCB shield. Mark the tab hole on the shield, remove the IC and drill a 1/8 hole. Also drill another 1/8" hole 0.5" to the left (looking at the same side), this hole is for the Teflon coax to join the Receiver section to the Antenna relay. Re insert the 7808T and bolt the IC to the Heatsink using an insulating washer/bush. Solder in the IC and check that it is insulated from the shield with a Multimeter.
12. Install 8 PCB pins as shown.
13. Install the remaining IC's again checking for correct orientation.
14. Install the 470uF Cap, Crystal and the Antenna relay.
15. Install 2 x 1K resistors as shown to drive the Receive and Transmit indicator LED's.
16. Install a 125mm piece of Teflon coax provided between the points indicated (passing it through the previously mentioned hole in the PCB shield).
17. Check the PCB for poor solder joints, shorts, etc. You may proceed with preliminary alignment at this stage.
18. Prepare all metal work and mount the M57735 module to a suitable panel/Heatsink. Ensure that the contact area is smooth and flat to allow proper heat transfer between the module and the Heatsink. The module must be positioned close to the PCB as shown in the overlay. The best way to achieve this is to solder on small tabs, made from PCB, to the main PCB to enable it to be bolted flush against the same panel. The PCB will only need supporting at the remaining two mounting holes at the other end. It is preferable that the 144MHz section is well shielded from the 50Mhz final section.
19. The Heatsink should be approx. 75 x 75 x 25mm.
20. With final assembly, mount the PCB etc. into place. Use Heatsink compound between the Module flange and Heatsink/panel. Connect the BNC Input and SO239 output sockets with short (i.e.: 20mm) pieces of 0.7mm TCW. Indicator LEDS, DC source and switch are connected with suitable sized wire (Transverter will draw 3 Amps). Good DC continuity must be established between the PCB and the PA Module flange.
21. Finally connect the M57735 module to the PCB pins; do not forget the three Ferrite beads, on the three inner wires. Do not tension the leads, slightly loop them and twist around the stake.
ALIGNMENT

 

1. Preliminary tests. Check each of the 3 voltage rails (Common, Transmit and Receive) with a Multimeter for shorts. Check the PCB physically for poor solder joints etc. Adjust all trimmers to positions as described on the overlay. Ensure that a 5 Amp fuse is used in the Positive DC lead.
2. Preparation. The Transverter can be either tested with or without the PA module connected. The latter is preferable, before final installation of the PCB into the enclosure. Connect a power meter and dummy load to the 50 MHz socket or if testing without the PA use a 51 ohm resistor and diode detector at the PCB pin normally connected to the input of the PA module. Connect a 0-5 Amp meter in series with the Positive DC lead. Connect a 2 Metre transceiver to the IF connector. A maximum of 3 Watts should be available. Both LED's should be connected to indicate Rx/Tx state.
3. DC Test. Switch on the Power supply. Several things will occur. Firstly you will notice that the Tx Led will light for 0.25 seconds then revert to the Receive Led. The Relay should click in and out. Also while the Tx Led is on the Transverter will draw about 0.5 Amps standing current (If the PA module is not connected about 150mA) dropping to less than 100mA in Receive. If the LEDS don't light check voltage rails, you may have the LED's in backwards! The switching circuit, in various forms, has been built many times and has always performed consistently.
4. Oscillator adjustment. The best instrument to use is a frequency counter, however a FM receiver on 94MHz can be used as an indicator. If the coil is correctly dimensioned the oscillator should start at about half capacity. You will notice that the oscillator will continue to oscillate as you reduce the trimmer capacity (higher frequency) until it drops out. Turn the DC supply off then back on. Increase the Cap until the oscillator starts again. The oscillator should be close to frequency and can now be fine-tuned. If you can not get the oscillator to work, you can check the resonant frequency of the oscillator Tank circuit by substituting a 47-ohm resistor for the crystal. The oscillator will free run on the resonant frequency of the coil.
5. Bandpass filter adjustment. Input approximately .5 Watts (Handheld on low power?) on 144.2 MHz to the IF input. The transmit Led should light and some output be evident in either the power meter or RF detector. Adjust the three trimmers associated with L7,8,9 for maximum output. The ratio of the trimmers should remain similar to the original settings; the middle trimmer is quite broad but should always be 50% higher than the other two. With the PA, an output of about 5-Watts should be available (without PA should be around 15-20 mW). With some careful tuning, you can broaden the filter to cover 50- 52.5Mhz. Increasing the input to 2.0 Watts should give the rated output on SSB, rising to 20 Watts RMS with 3 Watts Carrier.
6. Receiver Adjustment. A Signal generator or steady carrier is required to tune the receiver. Place the transceiver into the FM mode and peak the input trimmers on 50.2/144.2 MHz. With some practice, this is the easiest way to optimize a receiver front-end. As both trimmers interact, it will take several adjustments. The IF trimmer should resonate broadly around the indicated area. The input trimmer to the Preamp should end up being 1/8 meshed for best noise figure.
7. Final Performance. You may fine-tune the transmitter drive level by adjusting the 1K8 resistor in the 144 MHz attenuator (values 1K2 - 2K2). Do not overdrive the mixer, the gain of the transmit stages is very repeatable, if you find low drive, check the Bandpass filter. Correctly aligned it has a loss of approximately 3 db.
The receiver should have enough gain in front of a 2 Metre Transceiver provided the noise figure is below 6-8 db. A worthwhile modification for early transceivers is to substitute a BF981 for the front end RF stage, taking care to reset bias levels etc. (perhaps copy the VK5 2M Preamp?).
CONCLUSION

 

At this stage you should have a working transverter. Additional circuitry can be added for external PTT control from a transceiver or Relay control of a linear. Another idea worth looking into, is another Relay to select a straight through 2 Metre output, to bypass the transverter for 2 Metre operation.
The Transverter will fit into a standard Horwood 93 Series 150mm W x 75mm H x 225mm long box or a large diecast box. Extra time taken in planning will give you a final product that works we
(C) VK5KK 20/3/92. revised 23/04/98