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Newbie Question - Carrier Suppression?

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  • barockteer
    Just finished building my RXTX and I think it s working properly. However, it seems to have carrier feedthru of only about 25db down from a half watt single
    Message 1 of 11 , May 16 5:03 PM
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      Just finished building my RXTX and I think it's working properly. However, it seems to have carrier feedthru of only about 25db down from a half watt single tone output. This is the level of carrier output when no modulation is present. Is this normal? Newbie here, not sure what to expect. 


      Tony K1KP


    • Shirley Márquez Dúlcey
      You need to adjust the I/Q balance in your software. A built RXTX rarely has perfect balance between the two channels; component variation will keep that from
      Message 2 of 11 , May 16 5:41 PM
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        You need to adjust the I/Q balance in your software. A built RXTX rarely has perfect balance between the two channels; component variation will keep that from happening. The software can make adjustments to the amplitude and phase to compensate. For the transmitter, you can maximize the carrier suppression by listening to your signal on a receiver while you make the adjustments; if you have access to a spectrum analyzer that will also let you check opposite sideband suppression.


        On Fri, May 16, 2014 at 8:03 PM, barockteer@... [softrock40] <softrock40@yahoogroups.com> wrote:
         

        Just finished building my RXTX and I think it's working properly. However, it seems to have carrier feedthru of only about 25db down from a half watt single tone output. This is the level of carrier output when no modulation is present. Is this normal? Newbie here, not sure what to expect. 


        Tony K1KP



      • Tony Brock-Fisher
        Thanks for your reply. However, the carrier suppression is not affected by the IQ balance. This is evidenced by the fact that the -25db carrier feedthru is
        Message 3 of 11 , May 16 5:49 PM
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          Thanks for your reply. However, the carrier suppression is not affected by the IQ balance. This is evidenced by the fact that the -25db carrier feedthru is present even with no IQ input. IQ balance affects undesired side and suppression, but not carrier feedthru.

          As another example, the transmit IQ signals are all AC coupled to the mixer, so no amount of tweaking at DC at the IQ inputs will affect the mixer.

          What is the expected/typical level of carrier feedthru for the RXTX?

          Thanks,

          Tony

          Sent from my iPad

          On May 16, 2014, at 8:41 PM, "Shirley Márquez Dúlcey mark@... [softrock40]" <softrock40@yahoogroups.com> wrote:

           

          You need to adjust the I/Q balance in your software. A built RXTX rarely has perfect balance between the two channels; component variation will keep that from happening. The software can make adjustments to the amplitude and phase to compensate. For the transmitter, you can maximize the carrier suppression by listening to your signal on a receiver while you make the adjustments; if you have access to a spectrum analyzer that will also let you check opposite sideband suppression.


          On Fri, May 16, 2014 at 8:03 PM, barockteer@... [softrock40] <softrock40@yahoogroups.com> wrote:
           

          Just finished building my RXTX and I think it's working properly. However, it seems to have carrier feedthru of only about 25db down from a half watt single tone output. This is the level of carrier output when no modulation is present. Is this normal? Newbie here, not sure what to expect. 


          Tony K1KP



        • warrenallgyer
          Hi Tony I normally see 60 dB or more of carrier suppression on the RXTX. I happened to have my RXTX 15/12/10 on the test bench when I saw your post and I ran
          Message 4 of 11 , May 16 6:19 PM
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            Hi Tony

            I normally see 60 dB or more of carrier suppression on the RXTX. I happened to have  my RXTX 15/12/10 on the test bench when I saw your post and I ran a quick test to get 61 dB of carrier suppression.

            Let's be sure we are using the same terms: My test was using HDSDR and setting the rig up for maximum CW carrier out. Then, with the gains remaining the same, switch to USB mode and transmit an unmodulated SSB signal and measure the carrier.

            The IQ signal in the RXTX transmit section is applied to the QSE in four different phases, 90 degrees apart, on pins 3/10, 6/13, 4/11, and 5/12. My guess is that one of these phases is missing, a different level, or not exactly 90 degrees different from its' neighbor.

            Hope that helps.

            Warren Allgyer
            9V1TD
          • Dave Miller
            Are you talking opposite sideband suppression or at you talking LO leak through? I agree that 50 plus sideband suppression is very doable. I don t think I have
            Message 5 of 11 , May 16 6:26 PM
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              Are you talking opposite sideband suppression or at you talking LO leak through?
              I agree that 50 plus sideband suppression is very doable. I don't think I have seen 61 on any of the 10 RXTX's I have tested the year.  
              LO leak through is definitely there.  Can't remember the number but is is definitely more than 25 DB down

              Dave
              VE7PKE

              Sent from my iPhone

              On May 16, 2014, at 18:19, "allgyer@... [softrock40]" <softrock40@yahoogroups.com> wrote:

               

              Hi Tony

              I normally see 60 dB or more of carrier suppression on the RXTX. I happened to have  my RXTX 15/12/10 on the test bench when I saw your post and I ran a quick test to get 61 dB of carrier suppression.

              Let's be sure we are using the same terms: My test was using HDSDR and setting the rig up for maximum CW carrier out. Then, with the gains remaining the same, switch to USB mode and transmit an unmodulated SSB signal and measure the carrier.

              The IQ signal in the RXTX transmit section is applied to the QSE in four different phases, 90 degrees apart, on pins 3/10, 6/13, 4/11, and 5/12. My guess is that one of these phases is missing, a different level, or not exactly 90 degrees different from its' neighbor.

              Hope that helps.

              Warren Allgyer
              9V1TD

            • warrenallgyer
              I was talking carrier suppression. What I wasn t was fully awake yet and I misread the spectrum analyzer. Second cup of coffee and I agree, 51 dB. Sorry to
              Message 6 of 11 , May 16 8:13 PM
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                I was talking carrier suppression. What I wasn't was fully awake yet and I misread the spectrum analyzer. Second cup of coffee and I agree, 51 dB.

                Sorry to mislead!

                Warren Allgyer
                9V1TD
              • Alan
                Original Message ----- Subject: [softrock40] Newbie Question - Carrier Suppression? ... Tony, The local oscillator feedthrough is normally at a lower level
                Message 7 of 11 , May 16 11:06 PM
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                  Original Message -----
                  Subject: [softrock40] Newbie Question - Carrier Suppression?


                  > Just finished building my RXTX and I think it's working properly. However, it seems to have carrier feedthru of only about 25db
                  > down from a half watt single tone output. This is the level of carrier output when no modulation is present. Is this normal?
                  > Newbie here, not sure what to expect.
                  >

                  Tony,

                  The local oscillator feedthrough is normally at a lower level than that.
                  There is no adjustment.

                  There was some discussion about RX LO feedthrough. It was suggested that variations of internal capacitance of the FST3253(?) mixer
                  caused it.
                  I think I proved that by soldering very short lengths of wire on two of its pins and that, with adjustment of their positions,
                  reduced the feedthrough.

                  So, maybe your mixer has slightly wider variations than normal?

                  I cannot find the original thread. I'd have to find the Softrock I modified to tell more.

                  73 Alan G4ZFQ
                • Alan
                  Original Message ----- Subject: [softrock40] Newbie Question - Carrier Suppression? ... Tony, Looking further I found this
                  Message 8 of 11 , May 16 11:17 PM
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                    Original Message -----
                    Subject: [softrock40] Newbie Question - Carrier Suppression?


                    > Just finished building my RXTX and I think it's working properly. However, it seems to have carrier feedthru of only about 25db
                    > down from a half watt single tone output. This is the level of carrier output when no modulation is present. Is this normal?
                    > Newbie here, not sure what to expect.
                    >

                    Tony,

                    Looking further I found this https://groups.yahoo.com/neo/groups/softrock40/conversations/messages/76051 so I did the checks on the
                    TX mixer.
                    The original discussion was earlier.

                    This Yahoo is a nuisance for this kind of discussion! Click "Show message history"....

                    73 Alan G4ZFQ
                  • Tony Brock-Fisher
                    TNX all - I found a couple bad solder joints at U8. Fixing these improved carrier suppression to maybe 40 db. Better, but still not 61db... -Tony
                    Message 9 of 11 , May 17 4:46 AM
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                      TNX all - I found a couple bad solder joints at U8. Fixing these improved carrier suppression to maybe 40 db. Better, but still not 61db...

                      -Tony

                      On 5/16/2014 9:19 PM, allgyer@... [softrock40] wrote:
                       

                      Hi Tony

                      I normally see 60 dB or more of carrier suppression on the RXTX. I happened to have  my RXTX 15/12/10 on the test bench when I saw your post and I ran a quick test to get 61 dB of carrier suppression.

                      Let's be sure we are using the same terms: My test was using HDSDR and setting the rig up for maximum CW carrier out. Then, with the gains remaining the same, switch to USB mode and transmit an unmodulated SSB signal and measure the carrier.

                      The IQ signal in the RXTX transmit section is applied to the QSE in four different phases, 90 degrees apart, on pins 3/10, 6/13, 4/11, and 5/12. My guess is that one of these phases is missing, a different level, or not exactly 90 degrees different from its' neighbor.

                      Hope that helps.

                      Warren Allgyer
                      9V1TD


                    • warrenallgyer
                      I misled you Tony and I mis-read my instruments. I get about 50-51 dB carrier suppression on SSB Warren Allgyer 9V1TD
                      Message 10 of 11 , May 17 5:02 AM
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                        I misled you Tony and I mis-read my instruments. I get about 50-51 dB carrier suppression on SSB

                        Warren Allgyer
                        9V1TD
                      • warrenallgyer
                        I misled you Tony and I mis-read my instruments. I get about 50-51 dB carrier suppression on SSB Warren Allgyer 9V1TD
                        Message 11 of 11 , May 17 10:53 AM
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                          I misled you Tony and I mis-read my instruments. I get about 50-51 dB carrier suppression on SSB

                          Warren Allgyer
                          9V1TD
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