What is causing Lnb wobbling?

  • Hi all,

    What is causing the signal wobbling? What I’ve been listening, signals start wobbling after one hour or so, not so accurate temp vs. time measurements. Is there something to do with that wobbling?


    br ismo - oh1nhw

  • Temperature might a reason, I use a Polin GM-201. It worked fine but past days where it start getting sunny and hot I had suddenly problem that LNB start to Jitter heavily and sometimes jump suddenly multiple KHz that the software stabilization from SDR console wasn't able to solve this. I modified the LNB to connect it to a BaMaTech Feed. For weather protection I put it into a IP54 junction box. When I open the box the LNB was very very hot, it burnt my fingers. So I leave the box open over day and it went back to an acceptable range where software stabilization works.

  • I think we are talking about wobble not temperature affects ? The slow wobble occurs in a 21C temperature controlled lab over a a minute or two.

    Dave Cawley | Ex G8EAO & G6ANG/T |
    (when you had to have a separate TV license !)

  • Excuses for 'warming up' this old thread, but maybe this helps someone with similar issues:


    I have experienced this 'jumpy' behaviour of crystals in oscillators when their allowed drive level (in terms of RF power) was exceeded in homebrew QRP equipment. If the drift isn't slow and sinusoidal, the crystal may be overloaded because the two "capacitors to ground" in the Colpitts configuration are not properly selected, or (possibility two) the supply voltage of the oscillator (in this case the synthesizer) changes by a few millivolts, causing a change of the semiconductor's input capacitance by a few femto-farads (like a parasitic varicap diode).

    Hoewever, so far none of the cheap PLL LNBs I have has exhibited this behaviour, so no there was no opportunity to see if modifying the Colpitts-C's near the crystal (to reduce the crystal drive level by a few microwatts) would cure the wobbling. I remember datasheets for 32 kHz 'clock' resonators even suggest a series resistor between the oscillator output and the crystal, to reduce the power dissipated in the crystal (optimum resistance depends on the crystal's individual Q factor).


    There will be some tradeoff between phase noise (less noise with higher crystal drive power) and drift (less drift with less drive, less stress on the crystal).


    Has anyone on the group unsoldered a crystal from an wobbling/jumping LNB and tried to use it in a *low power* oscillator configuration (like the one with two antiparallel Schottky diodes to limit the voltage across the crystal, as shown in older ARRL handbooks) ? It would be interesting to see if they don't wobble in such a configuration, e.g. with a UHF receiver tuned in CW to an overtone of the crystal to hear it drifting / wobbling / jumping around in frequency.


    Well, some food for thought.

    Hope to meet you on QO-100 with the BU-500 carried home from Friedrichshafen soon. But that's OT here.


    Cheers,

    Wolf DL4YHF .

  • Not sure if 'jumpy behaviour' is the correct technical term but yes, I have experienced this myself with a Vivanco STL US3K-N "Universal Single LNB", wrapped with foam rubber and aluminium tape to slow down thermal drift and reflect sunlight (the sun didn't illuminate the LNB radome, only the side of the LNB).

    I don't have a screenshot of the spectrogram, but the initial drift looked like a slow exponential decay, as expected. Then, for a couple of times, the frequency jumped by several hundred Hz within a second, i.e. not a wobble (*) but unpredictable steps, up or down in frequency, at random intervals. Similar effect when gently knocking on the LNB with a single finger, after which the frequency doesn't always (but usually) return to the old value.

    I had posted a photo of the opened LNB - see "receiving system not based on octagon lnb" . The crystal is a through-hole device, but unfortunately on the opposite side of the PCB (not visible in the photo). Otherwise I had replaced it with another 25 MHz crystal already, to see if it makes a difference. But it will be modified by a 10 MHz OCXO anyway (multiplied by five then divided by two, filtered signal sent to the LNB via frequency splitter).


    Cheers,

    Wolf .

  • When I was testing out some TCXOs for LNB-modification I found some close-by effects like wobbeling.

    At some TCXOs the received signals (for example the beacon) became "wided up". But I think this occured because of the jitter (and noise) of some TCXOs.


    Another effect occured when I used a "dirty" powersupply for powering up the LNB. A test showed that there is also a big drift in frequency when the voltage drifts. This may have the reason in using very simple series regulator ICs in the LNB.


    If we are talking about "wobbeling" I assume a "wobbeling frequency".

    What frequency are we talking about ? Some Hz ? Some kHz ?


    73s

    Armin DF1QE

  • For me this looks like a bad x-tal or a noisy powersupply.

    It's a bad xtal I think. The power supply is the Diamond "trafo supply" with the AMSAT-DL rx-converter in between. But I don't see your problem. If you have time and pleasure you can break away all PLL LNB's and modify it. ;)

  • @DJ7GP has done measurements with wobbly LNBs. Once he extracted the crystal and supplied the crystal frequency from a clean external signal generator, the wobbling was gone. Sources tell me that there are many really cheap (as in bad) quartz crystals out there...

  • Hi all,

    on June 16th I posted the following:

    ______

    Hi All,

    I tested a number of LNBs regarding the quality of their ozillators. To do this I mounted the LNBs in the feed holder of a FUBA DAA 850 A offset dish, monitored the CW beacon with an SDRplay RSP2pro and the software SDRconsole and evaluated the signal in the waterfall diagram.

    Out of 50 single LNB's GM-201 the oscillators of 7 pieces have wobbled.

    Out of 10 single LNB's goobay 67269 the oscillators of 2 pieces have wobbled.

    At 10 single LNB's OPTICUM LSP-02G none of the oscillators wobbled.

    Of other LNB types, I only have one or two pieces to test, so I can not say anything about their Oszillaors. With a total of more than 80 LNBs examined, one was included with a remarkably frequency stable oscillator. So there are also outliers for the good side ...

    73! Peter, DJ7GP

    An LNB useful for narrowband applications is shown in Figure 1. An LNB (wobbeling) unusable for narrowband applications is shown in Figure 2.
    I tested all wobbling LNBs for satellite TV reception. Result: No problems!

    But also an insufficient stability of the LNB holder can lead to unsightly instability of the received signal. If you knock on the LNB holder, as you will knock on a door, there are vibrations (frequency modulation) as shown in Figure 3.
    73! Peter, DJ7GP

  • I can confirm that X-tals are a source for wobbling. Since I replaced the 27 MHz X-tal in my HD-BP2 LNB by external GPSDO, the signals are stable and clear! No longer an aurora sound...


    And the HD-BP2 is known for heavy wobbling.