22 February 2012

23cm downconverter from analogue Sat-TV receiver

If you take apart an old analogue satellite TV receiver you might find leaking electrolytic capacitors and what looks like a shielded RF module.

Well, it actually IS a tuneable intermediate frequency of some sort, working across the 23cm HAM band: 900 to 2000 MHz, more or less. It outputs video baseband, so somewhere between 0 and 5.5 MHz.

Why not take control of the module and turn it into a 23cm-to-HF downconverter? With just one setting of the PLL you would be able to tune around 5.5 MHz of SHF. Yeah, sure.

My IF module contains a TSA5055 "2.65 GHZ Bidirectional I²C-bus Controlled Synthesizer" (PLL), and I²C can be easily mastered with modern microcontroller. But but but ...

The whole IF module has been designed to work with strong broadband signals: the sensitivity of my specimen is -60 dBm (pretty deaf!) and there are no info on phase noise. I would need a powerful RF preamp in front of it, and it is not suitable for SSB/CW work. Project idea discarded.

I am left with the PLL system, tuneable in 125 kHz steps between 1 and 2.65 GHz. It might work as a signal generator, so I will not throw it away, in case one day I will get into the microwave mindset... too bad the prescaler output is not available on one pin, otherwise it could be placed in front of a frequency counter.