Interpretation of the quality of WB signals ..

    Hello everyone,

    I keep noticing that strong signals with MiniTioune on the WB transponder have a very low MER. Sometimes up to 50% are missing. Other signals are just as high in the MER as can be seen on the BATC WB monitor.
    How can I explain that?


    examples:

    BATC Monitor near 10dB, MER near 10dB


    BATC Monitor bit over 8 dB, MER 7,9 dB


    BATC Monitor near 6 dB, MER 3,4 dB


    BATC Monitor around 8 dB, MER 7,4 dB


    BATC Monitor near 7 dB, MER 5,8 dB


    Why are there sometimes such large differences between BATC signal strength and my MER ?

    RX at the same time and weather conditions ...


    73´s de Robert, DD4YR

    Several possibilities - the MER is not an exact analogue to the SNR


    Either the transmitted signals are distorted:


    No modulator is perfect - DC offsets, IQ imbalance

    Low number of samples compared to the symbol rate

    Unwanted phase modulation in amplifiers,

    Issues with the roll/off and filtering - inter-symbol interference,

    Constellation compression for APSK

    Phase jitter


    Or the received signals are distorted: See above +

    local oscillator phase noise

    other signal interference / intermodulation


    Most probably both transmitted and received signal distortions are happening and for some signals worse than others.


    Quite a few narrow band signals are 8PSK or 16APSK and the distortions become more important. If you can't decode low SR but you can decode higher SR then it's probably the LNB not being stable enough. DRO LNBs won't generally be good enough for low SR. Some PLLs are not all that good either.


    You seem to get lower MER on wider signals in the examples so that might be your phase noise. If you are reference locked a different reference frequency, even only slightly different may make a difference.


    The other thing to look for are spurious signals - I have an LO spur that's quite large on one channel and receiving there is harder than in adjacent channels. You will not see this on the Goonhilly display if it is a function of your own RX and it's well worth looking at a wideband sweep of the whole transponder - e.g. with a LimeSDR.


    Mike

    Oh, thanks for the explanation, Mike.
    There are a lot of possibilities to influence this jungle of dependencies ...

    I have to define a goal so I don't get sidetracked...


    73´s de Robert

    Everything as Mike said....but...
    I've done a lot of experiments to have cleanest output possible and it is a problem more (&more) complicated than You might think.

    1st...Web monitor shows spectrum of your signal in Goonhilly, not power, not MER.
    To measure how good your MiniTiouner decodes your signal compared to power You need something to compare to.

    Only way (I think) is to transmit 1M or 2M signal and control its power to be exactly at same level as beacon and using same rolloff.
    then You have beacon MER for example 7.0db and Your signal MER 6.2 in my case.
    So my signal decodes 0.8db worse than beacon.

    I could write pages what I've tried to fix that gap and I still didn't discovered it yet. But I've made a progress.
    Mike's list is not complete.

    What I see on transponder most have problem with non linear amplifiers.

    Bitrate on QO-100 puts power over quality.
    QPSK and 8PSK works good with horribly overdriven amplifiers, and we (almost) all like to use more bitrate.
    Would You use Your amplifier at 20% of its maximum power capabilities?
    Or just solve 1db "signal quality problem" with that 6db more power available :D

    Second problem is modulator quality (SDR software & hardware). It is also not easy to make clean signal in digital domain, and as Mike said SDR hardware is not perfect (I think right words are "it is barely usable").


    Hello Tom,

    thanks for your hints. I did not unterstand all off it yet, but step by step.

    I see sometimes horrible signals and try to understand the reason.

    I assume that my tx signal meanwhile is ok, my ANDREW pa is used at around 10dB less than max power...

    But we need still more common indicators for good, overdriven and anything else quality feature on signals for RST.

    MER and D is not all as Mike explained.

    Dont know how... but we will find a solution ...


    73´s de Robert

    I understand a liitle "how it's made" because of my many failed attemts to build something.
    I've got spectrum analyzer from the begining...and seeing DIY hamradio on spectrum analyzer is like seeing cheap fast food :D
    There is dirty everywhere, awfully dirty :D

    Still there is common that somone says "my switching power supply does not make noise at all"...
    He just didn't measured it...
    And those switching power supplies are powering our PAs and LNAs, gain blocks...and LDOs are not better.
    In my BladeRF I had oscillating (xtra low noise :D) LDO! It added +-250khz harmonics to signal output.
    And there comes phase noise..,Just count how many generators our signal "pass" from modulator to demodulator...

    But dirty signals on WB seems to be no problem to anyone...this is just "esthetics" problem I think.
    Some are more "pointy" and "eyes hurt" on them...but whats wrong if they are there just for our fun.