144Mc/s Transverter

28 to 144Mc/s Transverter

The Transverter arrangement I have opted for here is a very cheaply available unit which can be bought from the Ukraine for under £20 and the board is very well constructed. It is fitted inside the SP-38 box with the other ancillary items and mounted on a large finned heat sink. Fortunately there was enough output from the IC-7300 RF unit via the new low pass filter (180mV PP) to drive the Transverter to the stated full 10 watts output, however, unwanted carriers are present at it's output about +/- 10Mc/s emanating from the local oscillator due to a total lack of on board filtering and also the harmonic output at 288Mc/s is excessive and these need to be addressed before it can be put on the air.

The pictures below show the Transverter Board and Heat Sink.

Here are the original spectrum analyser results from the 38.666Mc/s local oscillator and the resultant output of the transverter showing the spurious signals at about 133 and 154Mc/s so it can be seen that there is a requirement for a "clean" 116Mc/s local oscillator.

The top picture shows the local oscillator output with the 38.666Mc/s crystal oscillator signal on the far left and the wanted 116Mc/s signal on the far right, the unwanted signals in between are at about 54 and 76Mc/s.

This picture shows the output of the transverter, the centre is 144Mc/s and the spurious carriers at about 133 and 154.4Mc/s

These spurious signals are completely unacceptable so I decided to make a separate 116Mc/s local oscillator and filter unit utilising a TCXO which is of course very frequency stable matching the high stability of the transceiver, this is followed by a high "Q" band pass filter. I now have a very clean 116Mc/s local oscillator signal which is simply connected to the base of Q2 (original 38.666Mc/s crystal location). Additionally in order to improve the Q of L3/L5 the coupling capacitor C13 has been reduced to 0.5pF. The second harmonic at 288Mc/s can be greatly reduced by fitting a small 30pF trimmer across C40 and this is tuned to minimum output at that frequency.
The traces below show the output of the new 116Mc/s LO board and the final clean output of the Transverter at 144Mc/s.

TCXO 116Mc/s oscillator before the band pass filter

TCXO 116Mc/s signal after the band pass filter

Circuit

Board showing the TCXO module and the band pass filter with the 5v regulator and the pot VR1 which is adjusted to give 3.3v at pin No 14 of the TCXO module.

Parts List
R1 47K
R2 22R
VR1 220R min pre set pot (3.3v set)

C1 2.2mfd tant
C2 2.2mfd tant
C3 0.001mfd disc cer
C4 27pF disc cer
C5 3pF disc cer
C6 3pF disc cer
C7 3pF disc cer
C8 10pF min trimmer

IC1 7805 regulator
TCXO 116Mc/s oscillator module (Rojon)
D1 LED red

L1 Toko MC111 pink
L2 Toko MC111 pink

L1 and L2 are enclosed within a Toko double screening can

The final Transverter output is now "clean"

A bar graph power output indicator has been fitted which uses the same module as that used in the SWR indicator for HF use and it is interfaced to the output of the transverter using the circuit below negating the need for an external power meter and also serves as a drive level indicator when feeding an external linear amplifier.

The final assembly showing the Transverter and new 116Mc/s local Oscillator Board fitted.

The precise frequency of the 116Mc/s TCXO can be adjusted with the small pre-set in the module, see below

As I am using the Transverter to drive an external linear amplifier, the output has been reduced to 4 watts which is all that is needed and this of course also means that the board remains cool due the large heat sink it is mounted on.

When received the board had it's standing PA current, measured at "JP1" of 140mA, I have increased this to 250mA to ensure very linear operation of the stage.

DC Supply to board 13.8v
Receive only 30mA
Transmit 700mA at 4 watts output.