Modification of Octagon Oslo PLL LNB for External Frequency Reference

  • Michael, at which freq are you receiving? I am injecting a twin octagon with has 25MHz in it, but i did a small mod and i use the xtal as filter and i inject 25.769230...in it, 439.800 is 10489.800 here, how is yours?


    Jerry

  • Jerry,


    the series resonance of a crystal is extremely narrow and will be very selective.


    Telefunken used to use this idea in an old 4 GHz microwave radio link crystal multiplier chain LO to reduce sidebands and very close-in phase noise. This solid state LO replaced the original reflex klystron LO, that was locked to mains 50 Hz by a mechanical cavity modulator modulated by AC - a bit of interesting trivia ;-) Maybe the patents can be found online, around 1950'ies ?


    My model of Octagon Oslo synthesizer uses 27 MHz originally, So far, for 10489.550 MHz receive, I have amongst others successfully used:


    Ref LO IF
    27.000000 MHz 9750.000 MHz 739,550 MHz FRG-9600, AR-5000)
    27.851400 MHz 10057,450 MHz 432.100 MHz (IC-402)
    28.648938 MHz 10345,450 MHz 144.100 MHz (IC-202S), unreliable PLL


    There are some '60 second videos' on my YouTube channel.


    The 144 MHz case is just on the limit of locking and is obviously pushing things too far. I have also experimented downwards, 26 MHz and even lower and the PLL/VCO locks fine (only one specimen tested at room temperature).


    There appear to be several other models and brands on the market that might be useable, but the true availability (older models) and image rejection/noise figure (cheaper models) needs further investigation. Most seem to use 25 MHz as the reference crystal and based on the NXP family of synthesizers.


    /Michael, oh2aue

    ---

    "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: Cosmetics. ().

  • My solution for LNB modification. I tried several different capacitor values, but 15pF capacitor works the best for me.



    I also noticed that the LNB sensitivity drops down for about 3dB. It is stlll acceptable for NB, but not for WB on a 80cm dish. I don't know why yet. For WB receiving, I just remove the SMA cable from LNB and it works without external 25MHz reference.


  • Thanks Michael, i think if you inject 27.84184701 you are on 435,

    27.8501547 should be 432 if i'm correct. It's nice to see the kHz on the 70cm are identical to that of the dwnlink, hihi. I didn't try these 2 freq yet to see what i got, but i wil after the coming weekend.


    S59MZ, nice to see how you put the coax thru,i gonna try this just to see how easy it is, but i prefer the F, i have no interests in WB 'yet'.


    Jerry

  • S59MZ are you sure the oscillator stops working when you inject the external 25 MHz signal?

    Check if you maybe see 2 copies of the beacon, one at correct (GPS-stabilized) frequency and the other at the (drifting) crystal frequency.

    It could explain your 3dB drop as you may be receiving everything twice.

  • S59MZ what is your drive level for the reference? 15pF on 25 MHz is fairly high value of reactance. But, of course if you have a low value of capacitance (high reactance) as you have with 15pF, you will affect the already mounted xtal less than if using a 1nF cap (1nF is the value that I tested). 1nF on 25 MHz gives a quite low reactance.

  • S59MZ in the first picture there seems to be two capacitors? Is this in unmodified state?


    In the second picture, there seems that you have moved one of the caps to the other side (moved to upper side in picture) so it is in series with the injected signal there. But there seems to still be the one capacitor there in addition still?


    Can draw a quick schematic of your mod and how you changed the capacitors as well as where you did inject the signal? What PLL chip is on that LNB?

  • pe1chl I tested what hapens when I add the external reference and what when I remove it. The beacon shifts for about 230 kHz lower on the spectrum and stops drifting. The previous beacon disapears completely. For the test beacon I used a 24. harmonic signal from my Baofeng ham radio tuned to 432 MHz. There was only one beacon in both cases.


    I also measure the signal on the crystal at both ends with an oscilloscope and compare it with the 10MHz from GPS in the XY-mode. I got nice and stable Lissajousse curve which means that the 25MHz signal was phase locked to the GPS.


    Mike5000 The signal level is about 3Vpp. I experimented with 1nF at first, but I got very big phase noise. I don't know why exactly, probably because the reference signal level was too high. Then I tried with several smaller capacitances and got much better results. I found the optimal value 15pF for my case.


    In the picture above there are two added capacitors in serial, because I experimented with different values - just added lower capacitance to existed 1nF. In the final version I have only one capacitor 15pF (not shown on picture).

  • PA3FYM I am testing a Ublox 6 GPS that I had in the "junk box". The PPS signal seems to be settable to 24MHz but not above. Unfortunately when I tested with this as a reference to a Si5351 there was a quite unclean signal out (from the Si5351). Is this also your final summary of the situation with Si5151? (That it cant be used as a reference for the LNB 25 MHz XTAL input?)

  • PA3FYM when you say working on that ... do you mean that another PLL design than Si5351 has to be synchronized to the GPS pulses from the uBlox? One with a VCXO or similar?

  • My idea is to make a 10 MHz GPS locked source with a (cheap) Ublox module as general reference.


    'Working on that' means with me that, besides other projects, I'll have to fiddle how to improve the phase noise performance. I didn't have time because another project had to be finished first (i.e. before this weekend). See here.

  • PA3FYM A bit more on the Si5351a. I had been using some code I found on the web to program the registers in the Si5351a. At 27MHz the output looked very clean (and that is what I reported earlier). However, when I generated 25MHz the output was awful. Then I used Silabs 'ClockBuilder' Windows program to produce a register map for 25MHz and the result was clean!

    I tried injecting this into my Quad Octagon (25MHz) and the QO-100 CW beacon sounded good (no raspyness). Then I tried 23.817,949MHz to put the bottom of the transponder on 1,200,550MHz. Again the CW beacon was clean. I am guessing that there is more than one way to produce a frequency from the Si5351a but only the 'ClockBuilder' method gives a clean output. Having said all this, I could not receive DATV on my Minitiouner with the Si5351a input. Probably the phase noise was the problem. I have ordered a sample Si5332E to have a play with (much newer device). 73 Russ.

  • G4SAQ OK, that is interesting news. I know that e.g. ADF PLL's have more than one route to Rome, but the Si5351a too. Worth a try! : -)


    Thanks for the experiments : -)


    BTW Is the difference between sleep mode yes or no also gone?

  • I designed a PCB for a TCXO with Remcos' crystal filter and PSU. The phase noise was OK but that particular TCXO was not stable enough. A GPS locked source from LEO Bodnar is a much better solution.

  • Thanks Michael, i think if you inject 27.84184701 you are on 435,

    27.8501547 should be 432 if i'm correct. It's nice to see the kHz on the 70cm are identical to that of the dwnlink, hihi.


    Hi Jerry,


    yes, I have been experimenting with quite a few different LO frequencies, depending on the capabilities and crystals in my Old Skool radios ;-)


    /Michael, oh2aue

    ---

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

  • My solution for LNB modification. I tried several different capacitor values, but 15pF capacitor works the best for me.


    I also noticed that the LNB sensitivity drops down for about 3dB. It is stlll acceptable for NB, but not for WB on a 80cm dish. I don't know why yet. For WB receiving, I just remove the SMA cable from LNB and it works without external 25MHz reference.


    Hi,


    in my modified LNB's I inject the LO reference frequency into the IF cable via a very simple series resonance circtuit and extract it in the LNB directly from the F connector via a similar series LC circuit, routing it to the synthesizer with a short length of wrap wire and the original crystal removed:




    It is very important the series LC circuit is directly at the F(f) connector as in the photo to ensure stability of the LNB.


    Your LNB uses a very different design regarding frequency response, where selectivity is largely arranged by the RF bandpass filter. You can shift this down reasonably easily, but loading it dielectrically with a small piece of PTFE or Rexolite (Trolitul), about 2 - 3 mm thick. It is best if you can at least monitor the noise response with a spectrum analyser.


    In the models I use (the Octagon OSLO, Single output), is seems there is an image reject mixer in the IF processor (very difficult to find data on this, though something in one of the Chinese PLL LNB patents). Hence a bandpass filter is not required for image selectivity (and it can be argued how necessary it is in the first place, especially if the transponder noise is dominating your sensitivity anyway). Without the bandpass filter, the only frequency selectivity comes from the front end matching circuitry, waveguide cutoff, IF matching circuitry and from my own highpass filter in the triplexer at my receiver end of the IF coax.


    Here is quick measurement I made to figure out the IF response (in my version, the sensitivity is the same at IF's of 432/435, 739 and 950 MHz):




    As you can see, it is quite amazing that 144 MHz works for me: the gain is down considerably, BUT the noise figure is still about the same.


    /Michael, oh2aue


    ---

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