uBITX Transceiver semi-kit build

Discussion in 'Homebrew and Kit Projects' started by KD8UYQ, Aug 14, 2018.

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  1. KD8UYQ

    KD8UYQ Ham Member QRZ Page

    Hi Mark,

    I haven't added a mic amplifier yet and if I do it will probably be limited to a 3dB max gain. I am more interested in a mic gain control.

    Yes sir, I have spent a little bit of time over at the bitx groups.io site and looking at the output of my uBITX transceiver on the spectrum analyzer I have noticed that the 80 meter third order harmonic is only 39dB down so that needs to be addressed. On the other hand all of the other bands are clean. IMD is also not an issue on my radio probably because I haven't done anything to the microphone amplifier circuit.

    The thing that I love about this uBITX platform are the on line support groups. You don't have to reinvent the wheel. Whatever issue or question that you might have there are others that have had the same problem and figured out the solution.

    KK5R and W1TRY like this.
  2. N7EKU

    N7EKU Ham Member QRZ Page

    That sounds good,

    Was the testing done with SSB and a two-tone signal, or with CW? There were different problems depending on the mode and the band. Some required bandpass filters and others (the upper bands) needed external LPF's. I'm surprised no problems are showing up as most bands needed power to stay below 2W in order to stay legal.


  3. KD8UYQ

    KD8UYQ Ham Member QRZ Page

    Hello Mark,

    Good question. I tested it in CW mode because it is easy to do. I will take out my two tone generator and see what that looks like on the scope and spectrum analyzer. Testing via CW all the bands are well filtered when measured at max power except for 80 meters.

  4. KD8UYQ

    KD8UYQ Ham Member QRZ Page

    Hello All,

    I made a user error testing the uBITX transceiver's 2nd and 3rd order harmonic compliance on my spectrum analyzer. I redid the testing this morning and found that both the 80 and 40 meter bands have 3rd order harmonics above -43dB. 80 meters is -38.8dBs and on 40 meters it is -33.1dB.
    20 meters on up is fine with 2nd and 3rd order harmonics meet or exceed the -43 threshold.

    So I need to figure out what to do to fix the lower bands.

  5. N1OOQ

    N1OOQ Ham Member QRZ Page

    If it's just harmonics, lowpass filters are easy to implement.
  6. KD8UYQ

    KD8UYQ Ham Member QRZ Page

    Yup. I will have have to look at the low pass filter section on the uBITX main board.

  7. W0VRA

    W0VRA Ham Member QRZ Page

    The bummer is that this is a board layout issue. The LPFs in the µB work fine, but the LPF output traces are coupled enough to the input traces that they pick up some of the harmonics that the LPFs had stripped out.

    It may be that some of those traces could be replaced with shielded conductors?
  8. N1OOQ

    N1OOQ Ham Member QRZ Page

    Or just build the LPFs on a separate board with some shielding. $u¢ks, but you can't cheat physics.
  9. W0VRA

    W0VRA Ham Member QRZ Page

    Yeah, that's an option.

    It may be possible to use QRP Labs' LPFs and their relay board, and use the signals going to the µB relays to switch the relays on the QRP board, so that you wouldn't have to rewrite the code.

  10. N6ALT

    N6ALT Ham Member QRZ Page

    According to some on the Bitx group, the problem(s) starts even further up the chain in the 45mhz IF filter. I'm not 100% convinced there is a huge problem here at reasonable output levels, but the seeds of doubt have been planted. I still use mine almost every night on 40 meters and nobody has knocked at my door yet, and I don't want them too either. It's a shame really I had high hopes for the uBitx:-(

    KC8SBV likes this.

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