QO-100 Station Build Journal

  • Hi everyone,

    I've started to put together a station for the QO-100, and since I don't have a personal blog, or a personal web page, I'll try to document my progress here. Hopefully it's OK with everyone.

    The setup will be based on an Adalm Pluto.

    I have couple useful LNB's at hand, that I will modify for an external 25MHz reference.

    First step, I made a board that serves three functions.

    1) Bias-T injection for the LNB DC

    2) 740 MHz RX filter

    3) Source for the 25MHz LNB reference

    Just as a personal preference, I didn't choose to use GPS locked PLL. Instead, I decided to use a OCXO for the LNB 25 MHz, and another OCXO for the Pluto.

    Picture of my bias-T/OCXO/filter board.

    Frequency response of the 25 MHz LPF, and the resulting waveform.
    (notice that in the oscilloscope FFT, the y-axis is 20dB per division, so most harmonics are about -70dBc)

    Mikko, AB6RF

  • It was the Thanksgiving holiday here in the US, so I had time again to work on this stuff.

    Today I tested my “EBay amplifier”.

    Very low cost, and has a nice and solid machined aluminum housing, but no heatsink.

    So my first step was to find suitable heatsink stock and drill the mounting holes.

    I don’t have metal machining tools, just hand saws and drills etc. so the heatsink machining is not pretty but it works.

    The amp works fine, and puts out good power.

    With 26 volt VDD, I measured 30 W out at -8dBm in.

    The amplifier is not tuned for 2.4 GHz. It’s best at 2.25 GHz, where it has 3.5dB more gain.

    But it’s fine, this is already more than I’ll need, so I'm not going mess the with the matching.

    (The amp also has an internal isolator that is not designed for 2.4 GHz, it would be easy to remove the isolator and get more gain, but there’s no need)

  • Some progress.

    I got two LNBs to modify for an external 25 MHz Reference.

    One of the LNAs uses 1x RT320M, and the other one uses 2x NXP TFF1014.

    Looking at the internal oscillator signals on an oscilloscope, it's clear that what's been said on this board already many times is correct.

    The RTM320M uses a normal CMOS inverter oscillator, and the crystal can be replaced by feeding the external Ref signal into the Xtal input pin.

    But the TFF1014 is different, and I follow the guidance to feed both Xtal pins with balanced signal.

    (as recommended in the NXP app note)


    I'll use a small balun to make the balanced signal from the 50ohm external Ref.


  • Today I worked on the antenna feed(s).

    I'm going to make two feeds, and when I come to Europe, I'll test which one works better and use that.

    I'll make a normal dual-band POTY, and a helix + LNB.

    Today I started to work on the helix.

    I used 2mm diameter (US 12 AWG) wire for the helix, little bit thinner than some have used, but I'm trying to minimize the shadowing on the 10GHz feed.

    But with that thin wire the helix is pretty wobbly if not supported well.

    So I made a support using Acrylic, or Plexiglas or what ever you want to call it.

    As far as I know, the Acrylic has a low dielectric constant and very low loss tangent.

    Here's a picture of just the parts coming together.

    I will have to fit the LNB (with feed horn) into the helix reflector before I permanently mount and tune the helix.

  • ... You don't need to wait for your trip to the EU, you can see and measure the attenuation on 3cm by the helix on every TV satellite. .... and optimize if necessary.


    Thank you for the comment.

    Yes, that's true, I might do that testing here.

    What is causing me some confusion is the material choice for the helix support.

    Some sources say that Acrylic has very low loss tangent, but other sources do not agree.

    I might make another support using Polypropylene.

    One observation about the helix.

    If I don't have a support structure on "both sides" of the helix, the copper wire wants to spring back to a larger diameter.

    Only way to keep the helix at 42mm diameter is to use this kind of support that holds the wire on both sides of the turns.

  • Yes, I understand.

    In my experiments I used warm rolled copper tube from model making. The tube is softer and stays in the shape as you bend it.

    The support structure will have little effect on 2.4 GHz. On 10.5 GHz, however, depending on the phase position, it will have an extreme effect, since the signal must enter the dielectric and exit it again, experiencing refraction twice. And that on each side.

    You may be able to rotate the support construction so that the lowest attenuation errors occur with the polarization used (H or V).

    You can also try this with TV-Sat at home.

  • That is a good suggestion, I will try to get some small copper tube.

    And yes, I have been thinking about the orientation of the Plexiglas support.

    From a mechanical perspective, it would be best to have the support in vertical orientation, but that puts the support in same orientation with the V polarization of the NB transponder.

    That sounds like it could be a problem.

  • That sounds like it could be a problem.

    Yes, in the worst case this transforms from linear to circular with a loss of 3dB :)

    I prefer to use as less as possible dielectrical material in the radiation pattern.

    There is loss enough by the copper tube, need not more....

    73 de Robert

  • I ordered some Polypropylene (PP) for a better support for the helix.

    But in the mean time, today I made the POTY feed.

    (I think it's a right of passage for all QO-100 users to make at least one POTY, hi)

    I had the parts made out of 1,5mm copper.

    After testing it, I think it would be better to use thinner material for the feed element.

    With 1,5mm material, the resonances are not very pronounced. Maybe a thinner element would give sharper resonances.

    I did have to use some dielectric (foam PVC) to tune my POTY, but the end result seems OK.

    The two symmetrical resonances are there, on both sides of 2,4 GHz.

    And on 2,4 GHz the S11 is about -15dB. Not great but OK.

    Interestingly, by loading the antenna with a Plexiglas sheet directly in front of it, it gets even better.


  • Today redid the helix.

    I have to say the tuning of the matching strip is incredibly sensitive. I'm glad I have a nice VNA, without one it would be simply impossible to tune one of these.

    I used 2mm diameter wire in an effort to try to minimize the 10GHz shadowing.

    4 turns, 42mm helix inside diameter.

    The support arm is made of 0.25 inch Polypropylene.

    S11 = -30dB

    At some point in the coming weeks I'm hoping to measure the radiation pattern (and ellipticity) of my POTY and the helix.


  • While I was planning the transceiver system, I realized I will need several different power supply rails.

    I also wanted to keep it pretty compact and efficient, so I made a custom board that fits my needs.

    The board takes a 24V input from a AC/DC power supply, adds some filtering and protection circuits.

    It provides the following outputs:

    - Relay switched 24V output for PA. Can be "always ON" or controlled by a GPIO

    - 5V (SMPS) Out on USB connector for Raspberry Pi

    - 5V (LDO) Out on USB for Pluto

    - 5V (LDO) Out for 25MHz and 38.8MHz OCXO

    - 5V (LDO) Out for PA Bias

    - 12V (LDO) Out for the LNB

    I used separate LDOs for the "sensitive" blocks to provide clean voltage and have isolation between the different blocks.


  • I have few used OCXO modules, and I made boards to use them for a 25MHz reference for the LNB, and a 38.88MHz reference for the Pluto.

    But like it has been discussed, the reference must have very low phase noise, so I tried to measure the phase noise of these OCXOs.

    I ran into a problem.

    I'm using a very nice R&S FSV Spectrum Analyzer to do the PN measurement, but even it has too high internal PN.

    Every module I measure shows the the same PN plot, it's the noise floor of the Spectrum Analyzer.

    So for the Pluto, what I need to do next is make a 2.4GHz test tone out of the Pluto, and then measure the PN of that with the internal TCXO and with my OCXO.

    Hopefully at the output frequency I can actually see the PN effect of the reference clock.


  • More phase noise testing.

    Today I measured the PN of the Pluto output signal at 2.4GHz.

    Looks like my used 38.8800MHz OCXO (= unknown history, could be damaged etc.) produces a slightly worse PN than the internal TCXO in Pluto.

    In the plot there are three lines.

    Black = measure PN with the OCXO

    Blue = smoothed PN with OCXO

    Green = smoothed PN with internal TCXO

    Below 300Hz, and above 100kHz offset the TCXO PN is better.

    Between 300Hz and 100kHz the PN is equal.

    It's also possible that the LTC6957 ext clock buffer in Pluto causes the PN degradation.

    It does certainly add little bit of clock jitter.

    Now I'm tempted to purchase a different 38.88 or 40.00MHz OCXO to make a comparison with the one I have.


  • Not sure why you make this such a massive "science' project! I have Pluto, a laptop with SDR console, headphones, my preamp and SG labs final amp, a small power supply, and a POTY with a modified LNB. All on a 40+cm travel dish and will be online in 3 weeks from Italy. It really is not that difficult.