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TX Image Question PLZ?

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  • Jim Whitaker
    I ve just changed soundcards and making necessary adjustments using HDSDR and Ensemble RXTX 20, 30, 40. I ve found a slight signal 25 kHz above the wanted USB
    Message 1 of 5 , Jun 30 12:40 PM
      I've just changed soundcards and making necessary adjustments using HDSDR and Ensemble RXTX 20, 30, 40. I've found a slight signal 25 kHz above the wanted USB one. Using a second receiver with its gain set to maximum, auto off, a dummy load for an antenna, the main signal is S8 the image doesn't have an S meter reading, would I be safe to "assume" the image is 48 db or more below the wanted signal?


      DE N8XYN K
    • Alan
      ... Subject: [softrock40] TX Image Question PLZ? ... No easy answer. In respect to the centre frequency the image will be exactly opposite the fundamental. Is
      Message 2 of 5 , Jun 30 1:10 PM
        ----- Original Message -----
        Subject: [softrock40] TX Image Question PLZ?


        > I've just changed soundcards and making necessary adjustments using HDSDR and Ensemble RXTX 20, 30, 40. I've found a slight
        > signal 25 kHz above the wanted USB one. Using a second receiver with its gain set to maximum, auto off, a dummy load for an
        > antenna, the main signal is S8 the image doesn't have an S meter reading, would I be safe to "assume" the image is 48 db or more
        > below the wanted signal?
        >

        No easy answer.
        In respect to the centre frequency the image will be exactly opposite the fundamental.
        Is that what you are looking at?
        "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?

        73 Alan G4ZFQ
      • Jim
        No easy answer. 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
        Message 3 of 5 , Jun 30 1:33 PM


          No easy answer.
          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 Winking smile



          73 Alan G4ZFQ

           

          DE N8XYN K



           

        • g8voip
          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
          Message 4 of 5 , Jun 30 1:54 PM
            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 softrock40@yahoogroups.com, "Jim" <jimawhitaker@...> wrote:
            >
            >
            > No easy answer.
            > 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
            >
          • 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 5 of 5 , Jun 30 3:48 PM
              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
              answer.
              > 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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