Effects of high LNB phase noise

  • I was having problems with a Gilat Ka band D LNB which I had modified to use with an external reference. I was using a reasonable signal generator at Flo/256 locked to a GPSDO as the reference. However, the constellation plots showed significant phase noise. Ignoring any synthesiser noise addition, the phase noise increases with frequency multiplication by 20 log10 (N) dB where N is multiplication factor. I changed the division ratio in the Gilat synthesiser from 64 to 16 (the mixer doubles again so the actual multiplication is twice those figures). The reference is now 625MHz for a 20GHz LO. The constellations show the reduction in spreading and improvement in SNR and BER on 16E, 21538H, 8PSK 1054ks/s.

    In this plot, the magenta trace is before mods (/64) and blue is after the mods (/16). It's hard to read accurately on that plot but the improvement is about 12dB

    1.2m Prodelin 1224 az-el with one of:
    S: Helical feed, 0.8dB NF LNA, X: 1.2dB NF LNA, conical feed,
    Ku 12-18GHz: corrugated conical feed, 1.5dB NF LNA, Ku (Standard): Octagon OSLO mod for C120 & ext ref Prodelin feed. Ku (Extended): 11.3GHz LO SMW & Prodelin Feed Ka: Gilat ex-ODU PLLs Gilat feeds, Ka 18-26.5GHz: Patriot feed, home made 4dB NF LNA, Ka DSN: Corrugated conical feed, home made 3dB NF Down-Converter

    1.5m IRTE, Polar mount with Octagon OSLO mod. C120, IRTE feed
    40km East of London, UK

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