Project: 3.4 GHZ C-Band LNB Modification (1)

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From earlier ATV experiments, I still had some C-Band LNBs available which I would like to use for DATV and SSB experiments.
This cheap LNB uses a dielectric resonator pill oscillator (DRO) and has a poor frequency stability. Not really usable for SSB operation.
The first modification step was to remove the DRO pill and try to inject an external local oscillator frequency (LO) in the way that the receiving band, which falls into the 9 cm amateur radio band (3.40 – 3.475 MHz), is mixed down to the 23 cm amateur radio band
(1.24 – 1.30 GHz). This would require a 4.70 GHz reference instead of the origin 5.15 GHz.

C-Band LNB before modification:

LNB opened. On the upper side the metal cap with the frequency adjustment screw. Below there is the DRO:

And here it is. I was a bit surprised that there were no mixing diodes, as expected. By analyzing the circuit, it seems like the mixing functionality is been done by the amplification transistor stage above the pill. Really a cheap solution.

After pill removing, a F-connector was mounted with a termination resistor to avoid reflections. The coupling was done with an open cable and was moved and fixed to the optimum coupling position.

And finally, it works! At the spectrum analyzer you can see that the signal is now within the 23 cm amateur radio band at 1.30 GHz (by zooming in, the signal is exactly on 1,30 GHz).

Next step would be to optimize the conversion gain and to have an external GPS locked reference frequency synthesizer for the 4,70 GHz LO.

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