Posts by oh2aue

  • 2,4GHz transmission check

    Hi All,


    thought I would share my plans for station monitoring in case somebody finds this useful. My approach is maybe a bit heavy duty for some, but this is how I am ;)


    I have been scrounging bits and pieces from my junk box to put together a "Loop Test Translator". This is essentially a box that takes a sample of the 2.4 GHz uplink and downconverts it directly back to the downlink band on 10 GHz. For Es'hail-2 this means a local oscillator of 8089.5 MHz. This was something I already did many years ago when we (AMSAT-OH) were still developing the P3E/P5A linear/ranging transponder - it made testing the linear transponder a breeze.


    So far I have put together a 40dB RF sample port on a 2.4 GHz load that will absorb my uplink power (you could simply use a directional coupler in the antenna line and point the antenna to zenith for safe testing). This sample is further attenuated and fed to a surplus microwave mixer (Anaren 74129) and the LO is a modified PLL multiplier "brick" that I have retuned for 8.1 GHz output and uses a reference crystal of about 96 MHz that I found in my junk box (this experimental crystal is still about 3 MHz off at X band). Ideally, this should be 96.30357 MHz (84x) or preferrably 101.11875 MHz (80x, in my particular case) to achieve 8089.5 MHz exactly. The upconverted 10 GHz signal is then filtered, attenuated further and coupled to the input of my modified PLL LNB (SPC Electronics) using a waveguide directional coupler, just after the antenna feedhorn.


    In this way, it is possible to make a very quick test for overall station end-to-end performance, also on the WB transponder.

    Michael, OH2AUE

  • Demodulating the QO-100 BPSK Beacon (AO40rcv)

    James, G3RUH developed and add-on ADC for his demodulator enabling FEC decoding already in 2003.


    Back then I also used the AE4JY demodulator for FEC, but you will recognize I am an old-fashioned Physical Layer guy, because I prefer hardware demodulators (logic chips and OP amps!) with real-time displays (analogue oscilloscopes!) of Eye-Height and Constellation ;)


    Regards,

    Michael, oh2aue

  • Demodulating the QO-100 BPSK Beacon (AO40rcv)

    OK, so I have been annually dusting my G3RUH 400 bit/s modem and testing my AE4EY AO40rcv SW with my library of AO-40 audio files. My favourite one is:


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    Is anyone aware of any efforts in writing a suitable decoder for P4A telemetry to match the older AO-40 demodulators?

  • Octagon Oslo PLL LNB with external Reference

    10493 MHz Weak Signal Source


    Calling all tinkerers,


    I put together YAWSS, Yet Another Weak Signal Source. This one is based on simple 54.090 MHz crystal oscillator multiplied by 194 to 10493 MHz using a regular microwave Schottky diode. Here is a short video demonstrating this test transmitter with the Octagon Oslo/KiwiSDR receiver described earlier:


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    Cheers - Michael

  • Octagon Oslo PLL LNB with external Reference

    Hi Remco,


    I played around with this for a while, even running very high RF levels directly coupled to the IF coax just to get an idea if there would be sufficient leakage to injection lock the crystal oscillator - and totally amazingly this IS possible, but the locking range is like tens of Hz. Also experimented with your suggestion early on too, but the replication success of injection locking a crystal oscillator is not very good and I would not recommend it. Injection locking is quite OK, but it can be very touchy and you need practically continuous instrumentation to make sure the locking is OK. One thing you can easily encounter with poor locking is ending up with two references: the crystal running on it's own frequency and the injection signal on it's own. Even worse, the system can break up into parametric oscillation which can result in a horrendous spectrum, though this is unlikely with this particular circuit.


    Michael, OH2AUE

  • Octagon Oslo PLL LNB with external Reference

    Hi Peter!


    Well, initially I will rotate the PLL LNB to + or - 45 degrees, lose 3 dB, but be able to see both transponders superimposed. What I am hoping here is to be able to see both polarisation spectra simultaneously. As the passband noise of each transponder is band limited, there will not be any cross polarisation, co-channel noise penalty and assuming that there is no serious out of band, cross polarisation IMD either, then this concept should work.


    If there are any issues however, then it is a simple matter of raising the feed voltage to e.g. 17Vdc to switch polarisation, just cannot see both spectra simultaneously. There is also a control add-on for the KiwiSDR allowing such remote switching from the Web SDR interface (I am working on another similar receiver for HF and planning for polarisation switching - I use circular polarisation on HF these days...).


    Not quite sure how your DISEQ connection works, but the L band IF cable you are using has to carry the 22 kHz band selection signal to the LNB too, so it is difficult to imagine why 27 MHz would not pass all the way to the LNB too.


    Looks you have much more space inside your LNB to fiddle with ;)


    Btw. you must tighten them Torx screws evenly and properly, as the PCB is not RF coupled to the bottom casting in any other way than even pressure of the top cover. Even one loose screw can cause oscillation of the high gain circuitry!


    73 Michael

  • Octagon Oslo PLL LNB with external Reference

    Greetings to all AMSAT tinkerers.


    Modifying the simple Octagon Oslo PLL LNB for external referencing is rather simple.


    So far I have successfully modified three units (that use the RDA3560 and 27 MHz reference crystal) to use a 27 MHz external reference that is fed via the L band IF cable. A modified LNB appears externally identical to the original, no mechanical modifications are needed.


    The plastic casing may be pried off in one piece if you are careful. Remove the rubber sealant glue with a sharp scalpel etc. and undo the three Torx screws to remove the top cover.


    Remove the crystal, solder a series LC bandpass filter of 2.7uH and 12pF directly at the F connector and using a short jumper of single strand wrap wire from the far end to the RF processor chip pin 20 shown in this photograph:


    https://73.fi/oh2aue/27MHz_BPF_Octagon_Oslo_sized.jpg


    The three I modified will readily lock with reference frequencies between 26 and 28 MHz with an input level of 0dBm and 27 MHz locking is reliable all the way down to -10 dBm, though I would not recommend using such a low level.


    At the shack end I have a simple triplexer for separating +9V to the LNB, L band IF and 27 MHz reference.


    In my first application I filtered and mixed the L band IF further down to HF (10 - 21 MHz) to feed to a KiwiSDR web radio, but I have also experimented using my Barlow Wadley XCR-30 as a portable battery operated system, which I think is rather cool :)


    For inspiration, here is a block diagram of my current prototype:


    https://73.fi/oh2aue/eshail2_kiwisdr.gif


    More info:


    https://73.fi/oh2aue/amsat_oh_…nlink_pll_lnb_kiwisdr.htm


    Michael, OH2AUE


    P.S. photo of system before boxing:

    https://73.fi/oh2aue/kiwisdr_eshail2_receiver_29_12_2018.jpg