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Re: v19.3.15 TX Image rejection

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  • wa9cgz
    All, The subject needs to be changed to carrier suppression since the image can be nulled under software (phase and gain) and stored band by band however as
    Message 1 of 25 , Feb 28, 2010
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      All,
      The subject needs to be changed to carrier suppression since
      the image can be nulled under software (phase and gain) and stored band by band however as someone else said a second shielded SDR
      receiver or a GOOD spectrum analyzer is required to make this adjustment.
      Another issue with the TX image is using the AM or FM modes
      since the carrier is generated at 11khz here the image is 22khz away. I found that the image suppression using these modes are not exactly the same as for SSB/CW and need to be re-entered when changing modes.
      I have a Flex Clone but notice the same problem with the Softrock.

      Joe wa9cgz

      --- In powersdr-iq@yahoogroups.com, "skarbows" <dskarbow@...> wrote:
      >
      > Christos,
      >
      > Makes me wonder if my home measurements are accurate. I could take my SR and laptop into work and perform the measurements with the proper equipment but that would be cheating. After all, I'm an amateur!
      >
      > I'm sure there's some variation from SR design to SR design and then throw in part tolerance variation on top of that. I would think that the building of the toroid transformers is a big variable. It would easy to build one that worked but was not symmetrical, especially at higher frequencies. If I was having LO or unwanted emission problems, that would be one place I'd look. I would not think the switch would be a big issue here, especially since it's swamped with external series resistances. The addition of pots to the design probably is nulling out something else other than the switches. Not sure.
      >
      > I don't think anyone should be overly concerned about transmitting with a SR as long as they make a reasonable effort to adjust and monitor the performance. On the other hand, I also wouldn't try to drive a pair of 3-500's as someone else here mentioned.
      >
      > My 750mW signal was heard quite nicely last night on 75M....
      >
      > Dave
      > kx3dx
      >
      > --- In powersdr-iq@yahoogroups.com, "sv1eia" <sv1eia@> wrote:
      > >
      > > Hi Dave,
      > >
      > > As I said, it didnt work for me with the plain stock sr's.
      > > I had to modify them with additions in the QSE to perform better in LO carrier suppression.
      > > On the other hand, I've seen that when I changed the band, problems re-occured so new calibrations (h/w) were needed.
      > >
      > > 73,
      > > Christos SV1EIA
      > >
      > >
      > >
      > >
      > > --- In powersdr-iq@yahoogroups.com, "skarbows" <dskarbow@> wrote:
      > > >
      > > > I didn't find it too hard to achieve -45dBc LO bleed and opposite sideband suppression with a RXTX6.3. It does require some careful assembly of all the parts, cables, settings, etc. For instance, I have found 1/8 stereo plugs and jacks to be noisy, lossy and intermittent. Don't use them - use solder connections. 1.12.20 seems to give me better nulls.
      > > >
      > > > And yeah, keep this context of a 1 watt signal.....
      > > >
      > > > Enjoy.
      > > > Dave
      > > > kx3dx
      > > >
      > > > --- In powersdr-iq@yahoogroups.com, "n1rx" <bruce.beford@> wrote:
      > > > >
      > > > >
      > > > >
      > > > > --- In powersdr-iq@yahoogroups.com, "sv1eia" <sv1eia@> wrote:
      > > > > >
      > > > > > I do not think it is adequate either but IMHO its way too difficult to achieve that carrier suppression as it is right now, no hardware calibration dials exists and no software either in softrocks (up to now).
      > > > > > If you have a narrow band spectrum analyzer (or another sdr) then you can do a check or anyone that can have two softrocks simultaneously and verify its Tx LO leakage of one another.
      > > > > > I've seen that it has level deviation from one band to another too and at least in my case I could never reach anything better than ~35dBc.
      > > > > >
      > > > >
      > > > > I agree Christos. I believe that -most- simple phasing transceivers of this type do NOT meet current FCC specs for spectral purity.
      > > > >
      > > > > Unfortunately, even though as you say, it is "way too difficult" to achieve this suppression in these simple designs, that does not absolve the operator from the responsibility to do so.
      > > > >
      > > > > This is one reason why although I enjoy playing with the simple SoftRock style radios as a receiver or spectrum display, I DO NOT use them in their current TX form.
      > > > >
      > > > > 73,
      > > > > Bruce N1RX
      > > > >
      > > >
      > >
      >
    • rodbrink15
      I ve never seen the schematic of the Flex 5000 but am told by someone there that they use programmable I2C pots connected to the QSE switch. For Tx carrier
      Message 2 of 25 , Mar 1, 2010
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        I've never seen the schematic of the Flex 5000 but am told by someone there that they use programmable I2C pots connected to the QSE switch. For Tx carrier suppression these pots are "tuned" at the factory on a band-by-band basis and the values are stored. Probably a bit too sophisticated for us who homebrew our SDRs, but still worth considering...

        73 de Rod KQ6F
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