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Soft Rock RxTx Image adjustment

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  • Terry
    So, I sent this long note about how to adjust image rejection, and at the end, Yahoo asked for my password again, and the message never showed up here on the
    Message 1 of 1 , Nov 2, 2006
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      So, I sent this long note about how to adjust image rejection, and at
      the end, Yahoo asked for my password again, and the message never
      showed up here on the group. I am assuming that message went into
      oblivion somewhere. Pardon me if this ends up as a duplicate.

      Tony has fixed the transmit low power issue so more SR RxTx units
      should be heard on the air. My RxTx is around 1/2W. Anyone want to
      have a QSO, SR-to-SR?

      Measurements I have taken show the transmit image rejection of my unit
      being 40db, the FCC rules are 43db down. I was able to adjust the
      image rejection in PowerSDR for SSB, but I cannot get them to "take"
      for CW. Fortunately, there is a simple fix: adjust the audio mixer
      line or speaker outs to compensate for gain variances. After making
      the simple adjustment outlined below, my image was down better than
      53db.

      I am using a laptop on the bench with a Creative Audigy 2 ZS notebook
      PCMCIA audio card. It has a Creative audio mixer instead of the normal
      Windows sound mixer, but the basic procedure will work in both cases.

      Begin by transmitting about 2-5kHz off center/LO frequency. My RxTx
      unit is around 7,055,000MHz, so I often use 7,052,000MHz for testing.
      You did set the SR LO frequency in the PowerSDR software, right?
      Adjust both line out channels down a tiny bit, I use 95% level for both
      left and right speakers on the Creative mixer advanced tab under
      speaker volume controls. Under Windows mixer, you can adjust the
      sideways "balance" control for the line out control under playback. It
      only takes a very small adjustment to drop the image down - mine is a
      1% difference between left and right.

      While in transmit, key down, look at the image with a spectrum analyzer
      and adjust only ONE speaker level (or the balance control) slider for
      minimum image level. If you don't have a spectrum analyzer, you can
      use another receiver - see below. This minor adjustment should not
      affect total power output. You might want to write down the positon or
      level, as sometimes PowerSDR plays with the mixer level adjustments
      From what I have seen). Your analyzer needs to ahve a fairly good RBW
      and VBW - I used 300Hz to see the separate signals.

      If you do not have a spectrum analyzer, you can use a separate receiver
      with a meter. First, you must figure out what frequency the image will
      be on.

      Find the RxTx LO on the receiver and note it's frequency. This should
      not be hard, as I can hear the LO from my unit even when it's into a
      dummy load.

      Next, move the SR Tx frequency to a know frequency, about 2-5kHz off
      the LO. Again, I use 3kHz roughly - the PowerSDR adjusts the freq for
      sidetone, etc. I use 7,052,000Mhz. Briefly transmit on that frequency
      (into a dummy load), and tune the signal in on the other reeiver. Not
      it's frequency and level. Now, subtract the actual transmit frequency
      from the LO freq. That is the offset between the two. Add that offest
      amount to the LO frequency to find the image frequency. For example,
      if the LO is at 7,055 and the Tx is 7,053, then 7,055,000-
      7,053,000=3,000. Then for the image: 7,055,000+3,000=7,058,000.

      Now, tune in the image frequency on the other receiver, start
      transmitting on the RxTx again, and tweak the mixer adjustment (as
      mentioned above) for lowest image signal level on the other receiver.
      Once you have it minimized, check the actual transmit leve to make sure
      it did not change much.

      You can verify the mixer adjustment by trying this nulling of the image
      signal at several frequencies that the RxTx covers. Remember, that if
      you move the transmit frequency to above the LO, reverse the math
      (subtract the LO freq from the tx freq, the subtract the difference
      from the LO to find the image freq).

      If your setup is stable (signals don't change as you walk around the
      shack or other strange things happen), you can make rough measurements
      of the image rejection, LO leakage, etc, by using the S meter on the
      other receiver. I haven't done this, as I have an analyzer. There was
      a message a while back on this groutp about how to calculate the
      various signal amounts by using the S meter (6db per S unit, I think).

      You can use a higher Tx frequency, maybe around 10-15kHz, if your
      analyzer does not have a narrow VBW or RBW. Keep in mind that not all
      sound cards work as good at higher frequencies, however. Using a
      better card, this should not be an issue. Using a motherboard sound
      chip may have some problems at higher freqs.

      The above should make your RxTx meet FCC specs for image rejection.
      There shouldn't be any problem meeting 2nd and 3rd harmonics either,
      they are way down.

      Again, before I made this simple adjustment, my image rejection was
      only about 40db down. After adjusting only the differential gain
      between channels, image rejection went to over 53db.

      BTW, this same adjustment can be used for gain errors in receive as
      well, but not phase. Either feed a signal generator into the RxTx in
      at about 3-5kHz off center/LO freq, or try to find a signal already
      there. Then adjust the Line-in balance control to null out the image
      signal.

      Pardon any spelling errors, my keyboard skills seem to be going
      downhill.

      Hope this makes out of my computer, and I hope this helps the RxTx
      users out there.

      73s
      Terry
      WB4JFI
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