LNB REF signal (TCXO inside the LNB & Ref fed by the duplex) has an effect on the output signal.

1st Official Post

    LNB REF signal (TCXO inside the LNB & Ref fed by the duplex) has an effect on the output signal.

    Recently, I measured and compared the spectrum of the output signal of LNB using different REF modes.

    First measure the TCXO installed inside the LNB.The signal looks clean.Low base noise, good signal-to-noise ratio


    The measurement then provides a REF signal to the LNB through a diplexer.




    The output signal becomes very noisy.The signal-to-noise ratio is significantly lower.Has anyone done the same measurement?

    I hope everyone can see what I post with translation software :) 73 DE BG0AUB

    • Official Post
    Quote from BG0AUB

    The measurement then provides a REF signal to the LNB through a diplexer.

    Can you explain this in a bit more detail: where do you get the new external REF from? Many wideband measurement devices use YIG oscillators due to their wide tuning range but their phase noise is not comparable to a good TCXO and hence should not be used as reference directly.

    Yes - that's expected if the reference is not of very high quality or it's not being fed in at the right level. Incidentally that LNB looks more like a DRO based one than a PLL based one but I assume it is a PLL. The quality of the 25MHz is critical. Any noise on it is multiplied many times. For DATV use you are often better with a standard unmodified PLL LNB. This often shows up in the MER on the beacon.


    Mike

    Feng showed me the inside of his LNB. It has an RDA3566 chip, which doesn't have a 'symmetrical oscillator' (aka kalitron), so 'balanced' feeding does not work. It works perfectly for the NXP chips like I revealed/published in 2019 (or so).

    For the RDA-chips there is an 'input' and 'output' pin (where injected levels differ around +20 dB (relative to the input).

    Quote from PA3FYM

    Feng showed me the inside of his LNB. It has an RDA3566 chip, which doesn't have a 'symmetrical oscillator' (aka kalitron), so 'balanced' feeding does not work. It works perfectly for the NXP chips like I revealed/published in 2019 (or so).

    For the RDA-chips there is an 'input' and 'output' pin (where injected levels differ around +20 dB (relative to the input).

    I tested this balanced method only this year. Here the chip is RT320M.