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:


    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:


    More info:


    Michael, OH2AUE

    P.S. photo of system before boxing:



    "If you have data, you have something, if you do not, you have nothing." (Bengt Hultqvist, SK 24.02.2019)

    Edited 3 times, last by oh2aue: New website URL. ().

    • Official Post

    Hi Michael !

    this is a pretty cool concept, I really like it. In particular to use the Triplexer for L-Band IF, 27 MHz Reference and Power supply. I was thinking to modify my Octagon TWIN LNB for TCXO, but your concept makes *much* more sense.

    Actually I want to use the TWIN-LNB to be able to receive the NB and WB in parallel. I have mounted it using a DISEQ motor, so that I can also receive normal DTV and point to Es'hail-2 before it's moved to its final position. I guess the motor will not let the 27MHz signal pass, but I could still use the other cable to connect directly using the Triplexer.

    What do you think?

    I opened my LNB, never did this before, and it was much easier than I thought. As you mentioned, only a T-8 Torx is required... Anyway it's fun to look inside the LNB.

    73s Peter

  • 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


    "If you have data, you have something, if you do not, you have nothing." (Bengt Hultqvist, SK 24.02.2019)

    Edited once, last by oh2aue ().

  • Good morning to y'all!

    I've modified my Octagon Optima OTLSO according to G0MRF design. It is all within the unit and there is no need to feed it from the shack.

    Few passive components, a voltage regulator and a TCXO.

    Should you need assistance, do not hesitate to contact me via email or @luciangasparini

  • Claudio,

    Email me, please. I'll help you as much as I can.

    The G0MRF mods were mentioned on an AMSAT-UK Journal.


    I've fully understood your point-of-view. I'm building another project which offers better frequency stability and I'm quite satisfied with the results I've achieved so far.

    Last but not least, this seems to be the proper place to exchange information. As y'all may be aware of, I'm located in such a place that the satellite will be at Elev.: 0.6°. I must get every little bit of signal to hear the NB transponder.

    73 Luciano PT9KK

  • Luciano,

    I know the challenge and the feeling of success. Just for fun, here is an old photo of my portable 90cm L/S band setup I used a few times at the seaside for working the Americas on AO-40:


    So looking forward to hearing you ;)

    73 Michael OH2AUE


    "If you have data, you have something, if you do not, you have nothing." (Bengt Hultqvist, SK 24.02.2019)

    Edited 2 times, last by oh2aue: New URL. ().

    • Official Post

    Please note that the "all-in-one" solution MKU LNC 10 OSCAR P4-A from DB6NT had a fundamental design change.

    According to the manufacturer, a version with IF at 432 MHz was originally planned, but due to component availability, especially the corresponding TCXOs, there is now only one version with IF at 1129 MHz.

    Frequency range (RF) 10489.. 10500 MHz
    Frequency range (IF) 1129..1140 MHz
    LO frequency 9360 MHz

    This is also not compatible with the external downconverter from from Achim, DH2VA. So you will need an SDR...

  • Hi all.

    FYI. I measured the warm up drift of a standard Octagon LNB and in 30 minutes it drifted 34kHz. - OK, after 20 mins it was probably 'usable'.

    After removing the 27MHz crystal and replacing it with a Conner Winfield TCXO the drift was down to 900Hz in 30 mins, with no drift (<50Hz) in the final 5 mins.

    It may be worth avoiding some of the Fox TCXOs as they have internal digital correction which causes the frequency to jump in steps of 100Hz + at a time.

    73 David G0MRF

  • @Michel OH2AUE just curious, in your beautiful

    detailed photo you removed the 27 MHz crystal.

    Did you try your 'BPF mod' with the crystal in place?

    The reason for asking is that some publications

    claim that removing the crystal is not necessary,

    however they inject the external reference not

    into the feedline and use an additional cable/connector,

    or sacrifice one halve of a 'two-connector LNB'.

  • 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


    "If you have data, you have something, if you do not, you have nothing." (Bengt Hultqvist, SK 24.02.2019)

  • 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


    "If you have data, you have something, if you do not, you have nothing." (Bengt Hultqvist, SK 24.02.2019)

  • Tinkerer replies: I use a Baofeng UV-B5 (or UV-5R)

    tuned to 437.065 MHz. Place the Baofeng in front

    (or a few meters away) from the LNB to receive the

    24th harmonic. The signal is not 'spot on frequency'

    but the frequency stability is remarkable.