Modification of Octagon Oslo PLL LNB for External Frequency Reference

  • Hello,

    I'm also using single output LNB's - Octagon and other types - modified for LO injection through the downlead coax cable. I was puzzeled by the amount of LO power needed for the PLL to lock. In my case something like +3 dBm to + 7 dBm. It turns out that with single output LNBs, where the 25 MHz shares F-connector with DC supply voltage, the LO signal is loaded down by a capacitor - 0.1 to 0.33 uF - placed at the input terminal of the 78L06 regulator. Only a meander line RF-choke prevents a total short. This circuit is used on all LNB I have seen.

    My solution was to put a 10 ohm SMD resistor in series with the meander line. It reduces the loading effect of the 0.1uF? capacitor and increases LO injection sensitivity by 15 dB. So now only -10 dBm is necessary. It also gives a 1 V voltage drop, but with 12 V supply to a 78L06 this is not a problem.

    The LNB is not an Octagon, but a cheap 7 Euro PLL type bought locally.

    Diplex filter in the shack to separate IF, 25 MHz and 12 V DC is homemade. Just a few components in a metal box.

    73 Ole

  • That is what the Bias-T reference injector I designed for BATC is for…ence-injection-blank-pcb/


    Nice board!

    This is what is needed in the "Bias/Triplexer" box in the lower left corner of:

    The LO reference channel low pass function is necessary in case the synthesizer reference signal source isn't pure sine - any weak harmonics in the IF band could be quite annoying in waterfall analysis.

    I see the board might also be fitted with a low pass/band pass combination for the reference signal path in case you are using a 22 kHz DiSEqC signal. This signal is also why I use a simple series circuit in my modified LNB's; the band pass function reduces chances of the DeSEqC signal modulating 22 kHz sidebands onto the synthesizer LO.

    /Michael, oh2aue


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

  • Ole,

    on the Octagon Single this was not necessary - to understand why, I really need to take closer look at the IF DC Bias/Regulator circuit next time I have one open.

    But with the 3€ GM201 I had to apply about +16dBm to achieve PLL Lock and this is precisely due to the same reason, there is the L band printed choke, followed by a 0402 270nH chip inductor and then a huge capacitor to ground at the input of the 5V regulator/switch chip.

    /Michael, oh2aue


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

  • Michael,

    did you describe somewhere which components with which values you added? Would be interesting for me to know!




    just made some practical listening tests with two modified GM201's and they work very well indeed. Used both my old jittery FRG9600 with built-in biasing and my older Funcube Dongle with Spectravue.

    I am referencing the LNB PLL with a Leo Bodnar GPSDO programmed for 25 MHz exactly. As noted, it was important to sufficiently increase the impedance of the regulator line so that it does absorb the reference signal as Ole noticed in his experiments. I replaced the ~270nH 0402 inductor with an 11 ohm 0603 resistor and added a second one (11ohm 0603) at the beginning of the meantered choke line - you need to cut the trace to use the two pads on the PCB (makes you wonder, doesn't it?). The LC series circuit is 12pF + 0,47 uH + 2,2 uH, all size 0603.

    The larger chips make it possible to build the circuit as a sturdy "bridge" as you can see in the photo. All five chips are visible in the same photo.

    /Michael, oh2aue


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

  • Ok, thank you, this is a smart solution. I will read it carefully.

    With the third generation off satellites system on board of vessels we had a similar system. Only one single coaxcable between the antenna system and the below deck unit. Via this cable was running the power for the antenna, reference frequency, IF frequency, transmit frequency and a serial communications line. The processor inside the antenna radom pointed the parabolic dish always accurate to the geostationary Inmarsat satellite no matter what the ship below the dish is doing.

  • Diplex filter in the shack to separate IF, 25 MHz and 12 V DC is homemade. Just a few components in a metal box.

    73 Ole


    can you add the values of the components, if they are not visible in the picture, add in the photo? (in particular the choke wire diameter and winding diameter as well as the concealed mapped capacitors)
    That would be great.

    73´s de Robert

  • Robert DD4YR,

    Here is a schematic. I have changed the design from a Butterworth ( picture in my previous posting) to a Chebyshev - the schematic below. Both works fine, but Chebyshev has slightly better cutoff characteristics. I have also changed the attenuator to allow for two receivers. Both designs have 20 dB attenuation. Component tolerances are quite uncritical. Coil diameter is inner diameter.

    This have no relevance for QO-100, but I have set the split frequency at 110 MHz. The lowpass will pass any frequency from DC to 70 MHz and highpass will pass signals from 1 - 2 GHz down to 144 MHz. So this LP/HP filter can also be used for feeding reference signal to a mast mounted microwave transverter with 144 MHz IF.

    73 Ole

  • Hallo Community

    Bin neu und erst seit dem 6. März QRV auf dem OSCAR-100.

    Möchte nun meinen LNB OCTAGON OTLSO (siehe Bild)

    so umbauen, dass ich mit der externen GPSDO Referenz (Leo Bodnar, 2-Port Version) den LNB stabilisieren kann.

    Wie viele verschiedene Umbau-Versionen gibt es dazu?

    Eine genau Anleitung für Hochfrequenz-Anfänger wäre toll inkl. Print Screen der GPSDO Software in Bezug auf die eingestellte Frequenz.

    73 de HB9RYZ


  • Wolfgang, i use the same type of octagon, but only the board, i cut off the horn. It is modified for external ref input 25,......Mc to lock on GPS. I cutted one of the 2 F-conn tracks on the board, and with a dc blocking cap the ref sig is going in, workes FB. Also using Bodnar dual gpsdo with 10mhz and between 25-26mhz outputs. Look for G4JNT mods on lnb.


  • I am using here the same Octagon Twin OTSLO with 2nd output modified for GPSDO input 27,841846MHz just using a second coax and producing an IF of 435MHz with kHz the same as on the SAT downlink. All experimental setup with GPSDO outside next to the RX-dish (weatherproofed).

    Modification after all was very easy. No problem when you even remove the x-tal without de-soldering. In my case it ripped-off when trying to use a Dremel tool. The large solderpads are not used and very simple adding a 1nF chip-C for DC blocking, a 51 ohm load* and a short enamelled (isolated) copper CUL wire as shown.

    A small notch needs to be made in the cover part, preventing the wire getting pinched-off....

    *IMPORTANT UPDATE: My modified LNB lately suffered from sensitivity issue at startup. I finally did find out that the 50 ohms load was placed wrongly (as seen on last picture). The RDA3565 PLL circuit was possibly loaded too much with the 50 ohm (DC-)connected directly to the PLL-pin. Solution was to remove the 50 ohm, because the feeding coax is very short in my case. See the second picture with only extra C = 10nF that provides correct operation!.

    When a 50 ohm load is needed, please change the place of the DC-decoupling C AFTER the load, seen from the input. This is visible in several other modifications.

    Apologies, to whom following my initial pictures for modification...

    73 Ed PA1EW

  • ... and talking about GPSDO's. I had a look at the spectrum from my mini-GPSDO Leo Bodnar and decided it would be good to add some low-pass filtering.

    The programmable LO unit works perfectly and I did not test without the 30MHz filter.

    As I use only a very short coaxial connection I also added a 10dB attenuator, so the LO-input with lowest setting of 8mA to the LNB is abt -3dBm.

    EDIT: This now proofed to be too much attenuation as the lock sometime was lost.

    I found Andy's G4JNT recommandation for 27MHz LO-output level at 0dBm ... +5dBm, so for test I took out the 10dB attenuator --> now abt +7dBm injected at the 2nd port of the Octagon with the lowest setting of 8mA in the mini-GPSDO.

    73 Ed PA1EW