How stable is a satellite LNB LO?


It had never occured to me to check how stable the puck controlled LO was in any of the many satellite LNBs that had come my way. The assumption had been that they would be so unstable and rough that it wouldn't be possible to obtain an audible beat note from any of them. Then Mike, G8VCN, showed me his portable 3cm receiver, using an LNB fed scanner portable. Good for wideband, I thought, still not fully comprehending the shock about to come my way. Even the request for a 3cm carrier didn't really register. Only the words 'It's a nice scanner - it's got ssb. Several of us at the club have got them. We've been listening to the beacon with them' did I start to understand.

Mike's LNB was an old C-120 waveguide flange type. We had only just connected 12v to it, so the drift was quite bad, but you could still tell that the tone of the beat note was quite good, as it drifted through the receiver passband.
Within a few minutes, stability was good enough to catch the beacon call-sign fully. The LNB had not been selected for good stability, and a day or so later I thought it would be interesting to go through the stock of my unopened LNBs to see how they compared. All the units found had 9.75 GHz LO frequencies, so the IF was going to be about 618 MHz. I have no receiver at this frequency, so to bring this down to HF, a connectorised ring mixer was put together and the HP8640 sig gen switched on. Most LNB units tested had a similar note/stability but the odd one was pretty awful. However, two types had noticeably better note. These were samples of the early Cambridge digi-dish unit and a couple of the LNBs that Lidle were selling two years ago at £3.99.

   cambridge_lnb                       lidle_lnb

The Cambridge LNBs had noticeable drift, even when left on for half an hour, but the the Lidle unit was much better. This can both be heard in this wav file and seen on this waterfall display (the main signal is the standby beacon GB3CAM running in the shed, whilst the weaker one to be left is the main unit operating 12 miles away at the QTH of G4AKD):

lnb_lo1