Lnb with 25 MHz dds Input goes bad.

  • One thing to consider is that by adding a reference frequency input via the coax, we make the voltage of the coax available via a series resonant circuit. So, any alien voltage (think ESD voltage) now finds it's way to the sensitive ports of the oscillator of the LNB chip.


    The standard F connector connects inner pin first before ground connection is made. This is like the audio tulip plug; the hum of connecting an audio source to an amplifier via tulip has blown many speakers.
    With the F connector, the inner pin can launch ESD voltage to the inner oscillator before ground is made.


    In my LNB's, I have modifications to make them run on low-noise 5V (for the PE1CMO kit) and to avoid connecting 13/18V to the 5V input I permanently added an F->BNC converter, tied ans secured through heat schrink tube. This has the advantage that on connect, the connection is made to ground first, so I don't have the ESD issue.


    Perhaps something to ponder,


    Geert Jan

  • Hi, I have same problem with 25Mhz generated by Si 5351 by GPSDO. Is there already a solution to the problem? I'll be glad if you can share it.


    Best regards

    Rumen LZ1JH

    Well, NO..

    I left completely the path of GPSDO and am using a 25Mhz TCXO with an extra 5 stage 25MHz Xtall-filter.

    This gives me pretty stable results finalizing with the beacon lock.

    Still have the GPSDO with DDS and SI5351 but not using them..


    73es

    Benno

    Hi, I have same problem with 25Mhz generated by Si 5351 by GPSDO. Is there already a solution to the problem? I'll be glad if you can share it.


    Best regards

    Rumen LZ1JH

  • Thanks Benno,

    I understand that I lost a few weeks with this project. Eventually I was able to get a relatively clean signal at 10mhz, but I needed 25mhz to stabilize the LNB. Lastly, I would like some advice if someone has done well with a BPF filter or something?

    73 Rumen

  • Rumen,


    The Si5351 is able to generate a quite clean 25MHz signal if you consider some basic requirements:

    - you need a clean 10MHz input signal.

    - the Si5351 is sensitive to supply voltage noise. Use linear voltage regulators and low esr decoupling caps.

    - most important: use even integer divider ratios, as well for the pll stage as for the multisynth divider.


    Usage of even integer divider ratios is essential for a clean output signal, a fractional-n divider leads to a messy signal with lots of spurs.

    The output signal is a square wave, so you may want to add some low-pass filtering.


    73 Jean