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Re: [softrock40] Re: TX Image Question PLZ?

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  • Jim
    Thanks for the through explanation, it really clears things up. I’ve got “reasonable” attenuation of the unwanted frequency. I need to get PowerSDR
    Message 1 of 5 , Jun 30, 2012
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      Thanks for the through explanation, it really clears things up. I’ve got “reasonable” attenuation of the unwanted frequency.  I need to get PowerSDR working.  I’ve had an issue with it now allowing me to select my second soundcard as an output device to feed audio to the Ensemble. I might need to resort to RTFM (Reading The Fine Manual) as the only option left Winking smile
      DE N8XYN Jim
      From: g8voip
      Sent: Saturday, June 30, 2012 4:54 PM
      Subject: [softrock40] Re: TX Image Question PLZ?

      Hi Jim,

      HDSDR much like Rocky is far from ideal for transmitting as it uses a fixed LO frequency.

      The TX image will be equally offset on the opposite side of the centre frequency (LO) to the point you are transmitting.

      The major problem with this technique is whilst you can optimise the TX image for that one point, move closer or further away from the centre frequency, due to the soundcard characteristics the image rejection can vary tremendously, certainly by 10 -20dB or more across the whole spectrum when using 96kHz sampling rate in my experience, and that is using a decent sound card.

      With the fixed LO system the TX image can easily fall outside the ham band if you are operating near the bottom or top edges of a band.

      For transmitting PowerSDR is technically a far better option, the frequency agility of the Si570 is fully utilised so that the LO tracks the receive frequency and transmissions always take place at the same limited audio spectrum, i.e DC to 3kHz etc. Once the TX image has been optimised, this remains constant regardless of where in the band you transmit.

      The other advantage is even with no TX image rejection, the only effect on the signal is it becomes a double side band signal as the image is the opposite sideband so unlikely to affect other band users.

      73, Bob G8VOI

      --- In mailto:softrock40%40yahoogroups.com, "Jim" <jimawhitaker@...> wrote:

      > No easy
      > In respect to the centre frequency the image will be exactly
      opposite the fundamental.
      > Is that what you are looking at?
      > Not sure how to answer that. I just tuned up and down the band
      listening for images. I found one at 25 kHz above the transmitters 14.100 KHz frequency. I didn’t notice its nature other then it contained the transmitted audio.
      > "Necessary adjustment" includes setting the TX
      balance in HDSDR while in TX. Is that what you mean?
      > If it is are you
      sure of your RX calibration?
      > First I did the
      RX adjustment, then made the TX adjustments while transmitting of course. I swapped the IQ lines which reduced it quite a bit, then adjusted the phase and amplitude to get the smallest output of the unwanted signal. I cant eliminate it completely but my guess is it’s of no consequence at it’s current level. I just loaded spec lab and it appears to be 60+ dB lower then the wanted (Tuned) signal. Of course I probably wouldn’t want to feed that to an amplifier
      > 73 Alan G4ZFQ
      > DE N8XYN K

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