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Re: Ensemble RXTX- I vs Q voltage

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  • 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 1 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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