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72837Re: Daft question re RXTX

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  • warrenallgyer
    Mar 18 7:28 AM
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      And there it is!

      30 MHz into the RXTX at -73dBm. It shows up at 10 MHz at -79 and at 6 MHz at -86. Not the theoretical 9.5 dB but the principle is clear.

      I wish I had a way to easily low pass the LO to confirm your explanation but I don't and it makes sense on its' face.

      I guess the overall point is: The Softrocks receivers will work pretty well in receiving signals below their intended operating bands. However,the signals you hear may be real or they may be images from 3 and 5 times the LO above where you hear them.

      This is valuable for calibrating against WWV and maybe for nulling the IQ image on another transmitter.... but for serious listening it is not very satisfactory. At 6 and 12 dB down those images are really strong.

      Warren Allgyer - W8TOD

      --- In softrock40@yahoogroups.com, "warrenallgyer" <allgyer@...> wrote:
      >
      > What a great explanation! I think I even understand it! Thank you for setting me straight and for bringing some facts to replace witchcraft and rumor.
      >
      > So we would expect the fifth harmonic to be 9.5 dB below the third?
      >
      > When I input a 30 MHz signal and measure the recovered signal at 10 MHz I find it down about 13 dB..... so if I look for one at 6 MHz it should be down about 23 dB...right?
      >
      > Heading for the test bench.......
      >
      > Warren Allgyer - W8TOD
      >
      > --- In softrock40@yahoogroups.com, "KF4BQ" <mikecol@> wrote:
      > >
      > >
      > > --- In softrock40@yahoogroups.com, "warrenallgyer" <allgyer@> wrote:
      > > >
      > > > This is because the Softrocks LO has significant third harmonic energy and produces very strong images. An easy way to determine if you are listening to an image is increment the KHz digit on the LO display. If the signal moves 3KHz the it is a 3x image.
      > > >
      > >
      > > Hi Warren,
      > >
      > > Although widely stated on this forum: the Softrock LO harmonic energy really has nothing to do with it. You can filter the LO and the radio will work the same (maybe a bit more phase noise since the digital mixer chip is expecting a digital signal (fast edge speed crossing the threshold).
      > >
      > > The reason the radio responds to harmonics is that the mixer samples the input signal. This is somewhat like a strobe light on a rotating wheel.
      > >
      > > The mixer holding cap peaks when integrating a half wavelength for a fundamental input signal. When using 3rd harmonic input: the cap integrates 3 half wavelenths. The next effect is 1/3 the voltage over time. This gives the 20*log(1/3)= 9.5dB attenuation of the 3rd harmonic. This follows for higher harmonics too.
      > >
      > > The even harmonics are cancelled with the transformer 180 deg phasing.
      > >
      > > Have fun!!
      > >
      > > Mike Collins KF4BQ
      > >
      >
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