Receive 3cm converter suitable for narrow-band modes 


This is intended to be a simple low cost converter project, and was put together as something suitable for narrow band monitoring following the switch-on of GB3CAM, a couple of miles to the west of Cambridge. Used in conjunction with a scanner, it could also form part of a WBFM set-up, giving much more accurate calibration of the accompanying Gunn or puck controlled transmitter. Part of the cost saving results from using material to hand (in particular the 14.4 MHz oscillator module, the Telonic RF-35 pcb material and the ever useful satellite LNB), assisted by a relaxed attitude to spurious and large signal handling performance.

Choice of IF

Using a 5 or 10 MHz OCXO and double conversion can provide 48 MHz or 18 MHz 2nd IFs with good frequency accuracy. However, it was suggested that 144 MHz equipment was more readily available, so more number crunching was required. Fortunately, there is a good supply of 14.4 MHz TCXO  locally - they were used on the early Simoco Tetra equipment. The multiplication from this (x 710) for a single IF converter is not very useful, but also, if a satellite LNB is going to be used as the front end, a high IF (above 700 MHz) is necessary to avoid butchering the usual IF high pass filter that they all seem to employ. Using 1296 MHz as the first IF works out nicely for anyone with a scanner, and the same 14.4 MHz LO source can also generate low side injection to give a second IF of 144 MHz. For anyone without 1296 MHz or 144 MHz  receiving gear, there is also the opportunity of using a third IF at 28.8 MHz! (which would be fine for narrow-band use).

A stand-alone receiver was also considered, since a 1296 MHz 1st IF could have been followed by a direct conversion detector, similarly supplied from the 14.4 MHz TCXO. However, the tuning pin on the TCXO is only able to tune across 350 - 400 KHz of the 3cm band. This would have been OK for the SSB/CW portion, but would not have covered any of the beacon section. For a stand-alone Tx unit, though, the concept would be fine.

Short term stability of the 14.4 MHz TXCO is not as good as the equipment based on say a Racal 9420 5 MHz OCXO, but is good enough, if boxed,  for SSB/CW use. Long term stability and frequency calibration is not a problem locally either, since GB3CAM is now available to line up with, and it stays within a few hundred Hz of its nominal frequency.

block diagram

Front end

There are several ways that an ex-satellite LNB can be used as the front end. The simplest way is to dissable the internal puck controlled Local Oscillator, and inject a much more stable source. If the new LO assembly is located reasonably close (say less than 1m) to the LNB, then no other mod is required - simply feed volts via the coax IF feed and add a dc block to the receiver rf input.
One problem with this route is that the LO assembly will be outside and much more prone to oscillator frequency shift due to temperature fluctuations.  To avoid this, there are (at least) two other options:

i).   A dual or quad output LNB can be sought. These have two independant antenna/pre-amp paths, one intended for vertical pick-up, the other for horizonal. The horizontal path can remain unmodified, but the vertical one reconfigured to provide 20 dB gain to the LO input path. This corresponds to the loss in about 4m of RG223 quarter-inch feed cable.

ii).  As above, but with extra LNBs modified to act as  20dB line amps to allow longer LO feed runs.

iii).  The inbuilt LNB antenna can be replaced with a connector, allowing the LNB to built into the same assembly as the LO. A second LNB can then be modified to operate as an antenna-with-integral-preamp, allowing a long (up to 3 or 4m) RF interconnect, if using standard RG223 feed.

Since LNBs always have integral 30 - 40 dB IF gain blocks, loss at the IF should not be a problem.

1296/144 MHz converter section 

1296/144 conv

The LNB output at 1296 MHz is fed via the sma socket on the left. This pcb outputs at 144 MHz to the small pcb on the right, which converts to 28.8 MHz and otputs via the sma plug. The only board missing here to the complete 3cm converter is the 9 GHz multiplier pcb - however, it's useful to have a 23cm converter in it's own right...

Multiplier circuit:

multiplier circuit

1296 to 144 MHz converter circuit:

1296 mixer

144 to 28.8 MHz converter circuit:

144 MHz mixer

******************* Page under construction: last updated 30th Dec 2007 *********************