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Ensemble RXTX- I vs Q voltage

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  • William Moyes
    I finished assembling the receiver stage, but when I tested the voltage of the I and Q opamp outputs there is difference of up to 0.890v between them. The DC
    Message 1 of 2 , Nov 6, 2012
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      I finished assembling the receiver stage, but when I tested the voltage of the I and Q opamp outputs there is difference of up to 0.890v between them. The DC voltage difference is a non-trivial  function of the LO frequency, especially within the target 20-30-40m passband.


      I looked over the board several times and I don't see anything obviously wrong. The only thing suspicious is the clock output from the 74AC74 is ugly (+7v overshoot peak and -2v undershoot peak).


      Being new to RF I am not sure if the I/Q DC component difference is really a problem, or if I am just being paranoid. When I tried it out by plugging my 2M rubber ducky into the antenna jack (hey, it was within arms reach) I was able to hear the voice announcements at WWV@10Mhz, as well as someone operating SSB out of Las Vegas on 20m. But I would really like to make sure there is no problems with the board before I go onto build the TX portions.



    • vbifyz
      DC offset and its difference between channels is normal, as long as it doesn t get too close to 0 or 5V in either channel (1V margin is a rule of thumb, i.e.
      Message 2 of 2 , Nov 6, 2012
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        DC offset and its difference between channels is normal, as long as it doesn't get too close to 0 or 5V in either channel (1V margin is a rule of thumb, i.e. 1..4V on the opamp output).

        I wonder if one can reduce it by adding some kind of adjustment at the divider stage. For example, two variable pullup or pulldown resistors about 10k on QSD CLK 0 and QSD_CLK 1 signals. They should slightly change the relative clock edge timing as seen by the mixer.

        73, Mike

        --- In softrock40@yahoogroups.com, William Moyes <william_moyes@...> wrote:
        >
        >
        >
        >
        > I finished assembling the receiver stage, but when I tested
        > the voltage of the I and Q opamp outputs there is difference of up to 0.890v
        > between them. The DC voltage difference is a non-trivial function of the LO frequency, especially
        > within the target 20-30-40m passband.
        >
        >
        > I looked over the board several times and I don't see
        > anything obviously wrong. The only thing suspicious is the clock output from
        > the 74AC74 is ugly (+7v overshoot peak and -2v undershoot peak).
        >
        >
        > Being new to RF I am not sure if the I/Q DC component
        > difference is really a problem, or if I am just being paranoid. When I tried it
        > out by plugging my 2M rubber ducky into the antenna jack (hey, it was within
        > arms reach) I was able to hear the voice announcements at WWV@10Mhz, as well as
        > someone operating SSB out of Las Vegas on 20m. But I would really like to make
        > sure there is no problems with the board before I go onto build the TX
        > portions.
        >
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