SETI LEAGUE UK - Technical scrapbook

 

The FR5000 Tx driver module - and its use as a L.O. drive source for an integral 1420MHz down-converter

In the main article it is assumed that a separate 1420MHz down-converter is obtained to drive the receiver section of a modified mobile radio equipment. However, where an FX/FR5000 Base Station equipment is being used, it is possible to use part of the transmitter (the driver module) as the local oscillator source for the down-converter. In this equipment, the VHF Tx driver module will supply 1.25W of power - comfortably more than is necessary to drive a diode multiplier to final frequency. Also, if an FR5000 (single channel, non synthesised unit) is used, the resulting signal will have a very low phase noise content due to its direct crystal controlled oscillator stage.

A x6 diode frequency multiplier, followed by a good bandpass filter is all that is needed in the down-converter to complete the local oscillator signal. The output spectrum is very clean (see pic). This pcb is fitted within its own screening can.

The rest of the down-converter (ie, mixer, if amplifier, rf amplifier signal bandpass filter) is easily fitted within another screening can, the two assemblies fitting either side of the FR driver housing castings as shown below:

 

Block diagram of integral down converter

 

The mixer is a diode half balanced type - etched directly onto the pcb as part of an extra interdigital pair as the circuit below shows. On the basis that an antenna mounted pre-amp would be used, the RF amplifier uses a simple MMIC amplifier block of adequate, but not brilliant noise figure (sensitivity is about 0.2uV for 12dB SINAD). The MMICs and the mixer diode pair can all be retrieved from an old 'bullet' or 'bluecap' satellite LNB assembly (see link).

The board layout is shown on the following link. See also link for response of the interdigital filter.

Again, the choice of the 212.666MHz source frequency is that required to convert a 1420MHz signal to 144MHz (ie, 1420 - [6 x 212.666] = 144)

 See link for layout.

There is also a photo of this converter fitted to its intended FX5000 shelf, providing a complete 1420MHz recever.

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