goto homepage go to 23cm index
High performance 23cm transverter
Click photo for higher resolution image (172kB)
A new transverter has been developed. Features include a very low noise figure and very simple alignment. Power output is sufficient to drive the Mitsubishi 1W module with good linearity. Out of band spurious signals are better than -30dBc, while the in-band 144 x 9 spurious is better than -60dBc. Some external filtering may be required for the transmit signal depending on the design of the rest of the station and local requirements.
The transverter is built on a single duroid pcb and uses SMT construction techniques. The new SMT LO can be be fitted in the other side of the box (see below for details). An IC202 compatible tx/rx interface is included on the board. With simple modification, the tx/rx switching can be adapted to other systems.
For an independent review of this design, please click here.
For a more detailed technical description, please click here.
For the circuit diagram, please click here.
Receive stability
One or two examples of this transverter have shown signs of instability on the reveive side when the lid is put on the box. In my prototype, this was cured by glueing a 1 x 2" piece ofr lossy rubber to the inside of the lid, above the HEMT stage. There was no degradation of noise figure.
Prototype Results
Click here for Excel file of above data
Wideband spectrum, centre = 1296MHz, sweep 800MHz
Wideband spectrum with extra filter, centre = 1296MHz, sweep 800MHz
For details of the filter, see 1296 accessories page of this site
Narrowband Spectrum, centre=1296.5MHz, sweep 5MHz
Installing the SMT LO chain into the transverter box
(draft)
It is possible to install the SMT LO described elsewhere in this site into the transverter box, on the groundplane side.
The following procedure aims to describe how to do this.
1. Cut about 12mm off the SMT LO board (output end)
2. Drill a 1mm hole next to remaining length of output line (above the line, crystal end to left)
3. Fit a veropin thru hole from track side and solder to groundplane side. Trim off excess length.
4. Remove oscillator coil, remove core and saw off about 2mm of the former. Refit on board
5. Remove interconnection wiring on groundplane side of transverter and "feedthrough" veropins.
6. Using thin insulated wire, replace the wiring, passing the wires through the holes vacated by the veropins. It helps if the +12V on receive wiring is made on the track side (see photo above), leaving the +12 on transmit and other connections to be made on the groundplane side. It is not recessary to remove the veropins near the top and bottom edges of the board, as they will not interfere with the mounting of the SMT LO board.
7. Drill a 1mm hole through the transmission line feeding LO power from the socket to the mixer and cut the track about 1.5mm from the hole towards the LO input socket. Relieve the groundplane around the hole using a proprietary tool or drill. Insert a veropin from the track side through the hole and solder to the track. Cut off on groundplane side, leaving about 2mm protruding.
8. Mount SMT LO into the box, as shown on the photo below. It may be necessary to bend the crystal towards the board to allow the lid to fit. The prototype was secured by soldering wires from ground pads on the SMTLO to the transverter groundplane (pads near LH 10pF trimmer and near 10R chip feeding final multiplier were used).
9. Make a connection between the new LO input veropin and the output of the SMT LO using a short length of thin 50 ohm cable. 0.085" semirigid or RG316 are suitable. Any type with PTFE dielectric should be OK. If using braided cable, cut outer plasic sheath back about 3mm, taking care not to cut through the braid. Fold the braid back over the outer sheath and tin the braid all round. Cut off the inner insulation about 1mm, and tin the inner conductor. The coax link may then be installed, inner to SMTLO output and transverter LO input, and outer to veropin on SMT LO and transverter groundplane.
10. Test the transverter functions correctly with the LO installed. In the protoype, some evidence of spurious oscillations were seen with the lid on (!). This was fixed by gluing a 2 x 1" piece of lossy rubber to the inside of the lid.
11. Depending on the tuning of the oscillator coil, it may or may not be necessary to use a core in the coil. If there is a problem with the core sticking out, consider drilling a hole in the box lid or sawing the core to size (unlikely to be a problem with the smaller oscillator coil now used in the design).
Notes on stability
If problems with oscillator stability (ie spurious oscillations) are encountered when the lid is fitted to the box, and these cannot be cured with the lossy rubber (see 10 above), a solution which is being tested at the moment is to add a small screen to the LO board. A drawing of the screen will be posted here in due course. The screen is soldered to the groundplane of the LO board in six places (ends and centres) for a length of about 8mm at each point. The lips of the screen are soldered as close as possible at the four corners to the groundplane of the transverter pcb (obstructions allowing). The other end of the pcb is mounted by soldering old component leads to the groundplane of the oscillator pcb in the two corners and bending these down to the groundplane of the transverter pcb and soldering. It is still a good idea to use the lossy rubber (2 x 1" piece) glued in the centre of the bottom lid of the box. With the screen and lossy rubber, no evidence of spurious oscillations were seen in the prototype, and the LO power did not vary much when the lid was fitted.
SMT LO board installed in 23cm transverter (before screen was added to LO board)
SMT LO with screen fitted
goto homepage go to 23cm index