By modifying the LNB ref frequency output to HF?

  • My friend put forward a request.On the recommended frequency converter using 28 m intermediate frequency.I did some test for UP2400 seems to be no problem.But the rx ran into trouble.LNB ref no response signal of above ref 26.5 M+.

    28.55 up2400.05 it looks good

    The test environment.Simple frequency converter + 1 w PA

    Someone made similar test?My frequency calculation:

    RX (10489.55 28.55) / 390 = 26.8230xxxx

    28.55 + 2371.5 = 2400.05

  • I'm not sure this is a good idea.

    For reception, you now require the mixer to give an output signal on 28 MHz. The typical LNB output circuit is not designed for this; I'm not sure if the chip works correctly this low. If the chip does work, the LNB circuit certainly won't; it needs to split between DC power and IF output (normally 950-2150 MHz) and the circuit is not designed for 28 MHz - the coils printed on PCB have too low inductance. I would replace that with a parallel-resonant circuit resonant on 28.675 MHz.

    (an argument can be made for the noise from the image frequency but perhaps that also applies for those who mod their LNB for 432 MHz out)

    For transmit, you will find it extremely difficult to filter out the image frequency, 2371.5 - 28.55 Mhz (to use your example). You need extensive filtering, very hard to archieve. In general, try to make your IF at least 10% of the output frequencey (minimal 240 Mhz). That is why many choose 432 MHz.

    Even a transverter 28 -> 432 Mhz is difficult, again because of the 10% rule.

    I'm sure others can give additional suggestions but perhaps this already gives you something to consider.

  • It's possible, but required one more convertion for RX. if standard LNB run sat at 739,xx MHz, Si590 burned for 711,5 MHz can reduce IF frequency to HF range of 28 MHz. Sample of PCB from G0MRF.

    Also, see picture. Not finished device, need only filter and attenuator. Si590 burned for HF out (f LO= 711,5MHz.)

    One more device like this from DF1QE.

  • Quote from BG0AUB

    Someone made similar test?My frequency calculation:

    RX (10489.55 28.55) / 390 = 26.8230xxxx

    28.55 + 2371.5 = 2400.05

    In your frequency calculation is BIG mistake, because you are take RX downlink frequency (10,489) as final. Division by 390 is opposite of multiplier clock XTAL of 25 MHz, to get LO=9750 MHz. Even you are use in HF radio as basic XTAL frequency, not clear what is RX channel frequency. Were is your downconverter in formula? LO freq is demissed. Take basicaly LO=9,75GHz (standard LNB value) to get IF=739,xx MHz. in that case you can convert your RX to any range UHF,VHF or HF.