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Re: [softrock40] Switches vs Mixers

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  • Chris Albertson
    I think it depends on the designer s goals. If you have a $10 budget and want to limit the PCB to 2 inches square you do one thing but if cost, board space
    Message 1 of 19 , Jun 1, 2007
      I think it depends on the designer's goals. If you have a
      $10 budget and want to limit the PCB to 2 inches square
      you do one thing but if cost, board space and power
      are not limited you'd do something else.

      Enginerring is about cost effective solutions to problems.
      In general it takes much less skill to design an expensive,
      over built product then to design a smaller and lighter
      product that does the same job.

      --- Loren Moline <DSNman@...> wrote:

      > Hello...Can anyone tell me what the advantages of using switches or
      > conventional mixers for obtaining the I & Q signals for SDR's. I
      > believe that with mixers you feed the 2 with the LO 90 deg different
      > in phase. I believe that using mixers may overcome the frequency
      > limits of switches but must be reason why switches are used alot.
      > Thanks!
      >
      >


      Chris Albertson
      Home: 310-376-1029 chrisalbertson90278@...
      Office: 310-336-5189 Christopher.J.Albertson@...



      ____________________________________________________________________________________
      Choose the right car based on your needs. Check out Yahoo! Autos new Car Finder tool.
      http://autos.yahoo.com/carfinder/
    • Kees & Sandy
      I believe the main difference is that the switches allow implementation of a discrete time sampling detector .......which is not really a mixer . Mixer is
      Message 2 of 19 , Jun 1, 2007

        I believe the main difference is that the switches allow implementation of a "discrete time sampling detector" .......which is not really a "mixer". "Mixer" is a "dirty word" in many SDR oriented groups. 

        The results are: 1) much lower noise (if you get the switching right), 2) less overall signal loss, 3) fewer undesired mixer products. 

        When combined with the high speed digital processing (as it evolves) the result is being able to pick weak signals out of the mud or solid copy with strong nearby interference. Both cool things to have when trying to communicate via the ether. 

        Best thing is that simple implementations using the switches are quite inexpensive, to the point where Tony can supply $10 SortRock Lites and have over 2500 interested members of this group.     

        It does have it's limitations due to 4x, or even 2x, Fo requirements, available switch characteristics, noise created by digital switching, DDSs, PLLs, etc ....but that's where the experimentation fun starts.

        73 Kees K5BCQ

        -- "Loren  Moline" <DSNman@...> wrote:

        Hello...Can anyone tell me what the advantages of using switches or
        conventional mixers for obtaining the I & Q signals for SDR's. I
        believe that with mixers you feed the 2 with the LO 90 deg different
        in phase. I believe that using mixers may overcome the frequency
        limits of switches but must be reason why switches are used alot. Thanks!

      • Chris Albertson
        ... It s a mixer but really we are getting into semantics. I my opinion Mixer means a device that takes advantage of the famous trigonometric identity
        Message 3 of 19 , Jun 1, 2007
          --- Kees & Sandy <windy10605@...> wrote:

          > I believe the main difference is that the switches allow
          > implementation of a "discrete time sampling detector" .......which is
          > not really a "mixer". "Mixer" is a "dirty word" in many SDR oriented
          > groups.

          It's a "mixer" but really we are getting into semantics. I my opinion
          "Mixer" means a device that takes advantage of the famous trigonometric
          identity sin(x)cos(y)= 1/2 (cos(x-y) - cos(x+y)) If it computers that
          function in real time that it's a mixer.

          If we assume sin(x) to be the RF signal and cos(y) to be the LO
          then even a switch or diode does in fact compute a multiply.
          The way it does it is to "round" the LO signal to either *1 or -1
          and then the "multiply" is easy to compute: for +1 do nothing,
          for -1 simply invert the phase. The switch does this nicely,
          so do diodes. So it's not "switch vs. mixer" it's "what kind
          of mixer"

          Switches just happen to make good mixers but only in the low
          bands. You can't get them to work up in UHF and maybe not
          even in 10 meters

          Chris Albertson
          Home: 310-376-1029 chrisalbertson90278@...
          Office: 310-336-5189 Christopher.J.Albertson@...



          ____________________________________________________________________________________
          Choose the right car based on your needs. Check out Yahoo! Autos new Car Finder tool.
          http://autos.yahoo.com/carfinder/
        • jabauzit
          After reading the original post and the different answers it seems that there is some confusion and I want to contribute to this thread. I do not pretend to be
          Message 4 of 19 , Jun 2, 2007
            After reading the original post and the different answers it seems
            that there is some confusion and I want to contribute to this thread.
            I do not pretend to be 100% right and any criticism will be welcome:
            one learn by his mistakes.

            --- In softrock40@yahoogroups.com, "Loren Moline" <DSNman@...> wrote:
            >
            > Hello...Can anyone tell me what the advantages of using switches or
            > conventional mixers for obtaining the I & Q signals for SDR's....

            The title of the thread should have been Tayloe Mixer-Detector/QSD vs
            conventional mixers. The QSD, which uses a bus switch chip, is a
            mixer. Among the conventional mixers the H-mode mixer or the DBM are
            commutation mixers: commutation -> commutator -> switch. For example
            in a DBM the diodes are used as switches and the harder you drive them
            the more they behave like switches, the best is to drive them with a
            square signal.

            Using a bus switch chip as a mixer does not imply using a QSD: the
            Pic-A-Star originally used a SBL-1 as a mixer which has been replaced
            by a commutation mixer using a bus switch chip.

            --- In softrock40@yahoogroups.com, "Kees & Sandy" <windy10605@...> wrote:
            >
            > I believe the main difference is that the switches allow
            implementation of a "discrete time sampling detector" .......which is
            not really a "mixer". "Mixer" is a "dirty word" in many SDR oriented
            groups....

            It is a mixer, not a conventional one but nevertheless it is a mixer.
            I do not think that "mixer" is a "dirty word" in many SDR oriented
            groups, the "dirty word" would be "preamplifier". Personally I do not
            think that there is any "dirty word": the original SDR-1000 did not
            have any preamplifier but Flex had to introduce the RFE board for
            performance reasons. It is true that a preamp should be avoided for
            best dynamic range and IMD characteristics but with the components
            available today compromises are unavoidable.

            > It does have it's limitations due to 4x, or even 2x, Fo requirements ...

            The 4x or 2x Fo requirement is not a requirement of the QSD, it is
            just a convenient way to generate 2 signals in quadrature over a wide
            frequency range: the Time Machine which uses 2 DBMs uses a LO at 4x Fo
            which is fed to a PLD and the PLD outputs 2 signals in quadrature. The
            SoftRock V5 which uses a QSD uses a LO at Fo which is fed to a RC
            phase shift network producing the 2 signals in quadrature. A
            quadrature hybrid could also be used. The negative side of the RC
            phase shift network and quadrature hybrid is the limited range of
            frequencies for which the 2 output signals are in quadrature.

            --- In softrock40@yahoogroups.com, Chris Albertson
            <chrisalbertson90278@...> wrote:
            >
            >
            > It's a "mixer" but really we are getting into semantics. ....
            > The switch does this nicely,
            > so do diodes. So it's not "switch vs. mixer" it's "what kind
            > of mixer"
            >
            > Switches just happen to make good mixers but only in the low
            > bands. You can't get them to work up in UHF and maybe not
            > even in 10 meters
            >
            This confirms my thinking.

            Besides the confusion between switches/mixers and LO 4x requirement
            the answers regarding the differences between conventional mixer/QSD
            are right on the mark. The QSD is superior for HF using available
            components but to make a VHF SDR today you must use conventional
            mixers: see the R2 series of receivers or the Time Machine for example.

            To use the bus switch chip in a QSD up to 10m and may be 6m one should
            look at the excellent work done by Giancarlo- i7swx:

            --- In softrock40@yahoogroups.com, "Giancarlo" <i7swx@...> wrote:
            >
            > Hi all,
            >
            > Kees is right.
            > I have a SoftRock V6 driven by my 2xLO digital quadrature generator
            > with manual adjust (see Files). It uses a 74ACT74 and the SR is OK up
            > to nearly 30MHz. The squarer uses a 74AC86 which limit is just over
            > 60MHz.
            >
            > There are some new fast ICs to be tried and certainly will work OK
            > and permit the FST3253 to reach 50MHz. I expect to try them as soon
            > as possible (soon ???).
            >
            > For squarer SN74LVC2G86 or 74LVC2G86 (2x XOR);
            > For FF SN74LVC2G74 or 74LVC2G74 Fc = 280MHz typ (2x FF)
            > timings seem to be very balanced (datasheet).
            > Another possible improvement should be the replacement of FST3253
            > with a FST3125 (best Pericom PI5C3125 Ton=Toff= 0.5nS) driven by
            > 74ACT00 to get 25%T on each phase.

            Jean-Claude PJ2BVU
          • Kees & Sandy
            Good discussion on mixers there are still some disagreements, but we re all learning. Since some of the analog switches have t-plh and t-phl, break before
            Message 5 of 19 , Jun 2, 2007

              Good discussion on "mixers" there are still some disagreements, but we're all learning.

              Since some of the analog switches have t-plh and t-phl, break before make, etc problems and those which do have break before make have other time constraints ....has anyone looked into (and tested) gating the switches by driving the OE lines ? For example, set up the select on S0/S1 at 90 degree intervals and gate the select using OE. Since you would like this to be "fixed", maybe potentiomenter adjustable, make it 5-10ns? "gate off" during select transitioning. At lower frequencies the analog sample would still be close to 90 degrees and drop off as the frequency increases.....but hopefully you get away from the switch overlap, at least to some extent.

              73 Kees K5BCQ 

            • N0UU
              A little different view on mixing here based upon reading early writing on mixers. With traditional diode mixers the signal and switching waveform shared the
              Message 6 of 19 , Jun 2, 2007
                A little different view on mixing here based upon reading early
                writing on mixers. With traditional diode mixers the signal and
                switching waveform shared the same path. You wanted the rise time of
                the switching (LO) signal to be as rapid as possible because the RF
                signal rode up and down on the LO signal and undesired things happened
                there. So the 'stronger' mixers used higher power to get to the more
                vertical part of the LO signal quicker and avoid that area as much as
                possible. When the diode was off or saturated not much happened
                unless the RF was very strong in comparison...rare.

                With the FET mixers or switches, the LO is not in the signal path
                directly. Also because these devices are designed for fast switching,
                for relatively low frequencies the rise and fall times are pretty
                vertical with low switching drive (LO). I SUSPECT that one of the
                reasons that they start to fail at 10 and 50 MHz is that the switching
                turn on/turn off starts to look more like a sine wave and the nice
                sharp edges are gone.

                Hopefully some new switches (for the Gigahertz LAN for example) will
                be faster, but I suspect that like other new products, they will be
                buried in some complex chip and not useful to us.

                Enjoy Tony's products on the low bands and not too bad on the higher
                ones with some reduced performance.

                Can't wait to see what front end the new SDR5000 uses.

                Flames?

                N0UU
              • Kees & Sandy
                That s my understanding. The actual LO signal is never combined with or part of the RF signal with the FET switches. The end result, however, is basically the
                Message 7 of 19 , Jun 2, 2007

                  That's my understanding. The actual LO signal is never combined with or part of the RF signal with the FET switches. The end result, however, is basically the same but with some pros and cons. It's all a matter of what goals you are after with the design. 

                  I would also be very interested in what the SDR5000 uses, especially since Gerald continues to do a fantastic job and those guys are right down the street.

                  73 Kees K5BCQ

                  -- "N0UU" <stoskopf@...> wrote:

                  A little different view on mixing here based upon reading early
                  writing on mixers. With traditional diode mixers the signal and
                  switching waveform shared the same path. You wanted the rise time of
                  the switching (LO) signal to be as rapid as possible because the RF
                  signal rode up and down on the LO signal and undesired things happened
                  there. So the 'stronger' mixers used higher power to get to the more
                  vertical part of the LO signal quicker and avoid that area as much as
                  possible. When the diode was off or saturated not much happened
                  unless the RF was very strong in comparison.. .rare.

                  With the FET mixers or switches, the LO is not in the signal path
                  directly. Also because these devices are designed for fast switching,
                  for relatively low frequencies the rise and fall times are pretty
                  vertical with low switching drive (LO). I SUSPECT that one of the
                  reasons that they start to fail at 10 and 50 MHz is that the switching
                  turn on/turn off starts to look more like a sine wave and the nice
                  sharp edges are gone.

                  Hopefully some new switches (for the Gigahertz LAN for example) will
                  be faster, but I suspect that like other new products, they will be
                  buried in some complex chip and not useful to us.

                  Enjoy Tony's products on the low bands and not too bad on the higher
                  ones with some reduced performance.

                  Can't wait to see what front end the new SDR5000 uses.

                  Flames?

                  N0UU

                • Bill Dumke
                  A diode mixer wants a high level LO source to keep it out of the active region. It is desirable to have it be either on or off but not in the active region
                  Message 8 of 19 , Jun 2, 2007
                    A diode mixer wants a high level LO source to keep it out of the active
                    region. It is desirable to have it be either on or off but not in the
                    active region where there is loss. This permits in transmit mode, more
                    output power, and in the receive mode less intermodulation, in other
                    words it is more linear with high input level received signals in a
                    mixer application.

                    However, for best noise figure in a receiver, diode mixers want to be
                    run at less LO power. High LO power will cause the junction temperature
                    to rise, resulting in worse noise factor, since the noise factor (noise
                    temperature) is proportional with temperature. So there is a compromise
                    in a receiver that much be reached with respect to LO power.

                    Diodes have an exponential VI curve. By playing with various
                    coefficients in the fabrication of a diode for use in a low noise mixer,
                    it is possible to get almost a square knee, and therefore much less
                    loss, and a better noise figure. One of the professors at the
                    University of Virginia in Charlottesville used to build these for use in
                    low noise figure millimeter and sub millimeter wave radio telescope
                    mixers for the National Radio Astronomy Observatory. in the 1970's. In
                    this case they almost behave like a true switch, only in this case
                    operating even into the submillimeter wave frequency range.

                    But an active mixer like a JFET at HF or GaAsFET at the UHF and
                    microwave frequencies is always better than a diode mixer, due to its
                    gain which helps overcome the noise temperature of the IF amplifier
                    following the mixer, and it's lower noise temperature. With a
                    conventional diode mixer the loss adds directly to the noise figure of
                    the IF amplifier. But an active mixer has to be biased correctly and
                    have just the right LO power to perform well.

                    Diodes are usually just poor switches, but they have the big advantage
                    of being usable as mixers well into the UHF and microwave frequency ranges.

                    The switches used in the SoftRock series mixers are designed for digital
                    use, not for this application. They are used because they are very
                    inexpensive. The sound card limits the dynamic range of the receiver.

                    Bill wa9pwr

                    Loren Moline wrote:
                    > Hello...Can anyone tell me what the advantages of using switches or
                    > conventional mixers for obtaining the I & Q signals for SDR's. I
                    > believe that with mixers you feed the 2 with the LO 90 deg different
                    > in phase. I believe that using mixers may overcome the frequency
                    > limits of switches but must be reason why switches are used alot. Thanks!
                    >
                    >
                    >
                    >
                    > Yahoo! Groups Links
                    >
                    >
                    >
                    >
                    >
                    >
                    >
                  • Marco IK1ODO
                    ... Not only, Bill. They have an almost ideal switching characteristic, and the resultant dynamic range is excellent. The standard SoftRock is close to 100dB
                    Message 9 of 19 , Jun 2, 2007
                      At 20.46 02/06/2007, Bill wa9pwr wrote:

                      >...
                      >The switches used in the SoftRock series mixers are designed for digital
                      >use, not for this application. They are used because they are very
                      >inexpensive.

                      Not only, Bill. They have an almost ideal switching characteristic,
                      and the resultant dynamic range is excellent. The standard SoftRock
                      is close to 100dB of 3rd order IMD dynamic range in 2.4kHz, if used
                      with an high performance audio interface. With some modifications
                      (substitution of the original opamps with one LT6231) the sensitivity
                      is -125dBm (always in 2.4kHz). Similar results are difficult to reach
                      with a traditional diode mixers, and are one of the reasons for the
                      success of the SR series.

                      The QSD mixer could be further improved; I'm working (currently very
                      part-time) on this topic. See
                      http://www.spin-it.com/sdr/ik1odo_sdr1.html and
                      http://www.spin-it.com/sdr/ik1odo_sdr2.html

                      73 - Marco IK1ODO / AI4YF
                    • Bill Dumke
                      Marco, Thanks for the info.. That is very good work you have done on the QSD. I have a couple of questions. Do you believe the Johnson counter is essential
                      Message 10 of 19 , Jun 2, 2007
                        Marco,

                        Thanks for the info.. That is very good work you have done on the QSD.

                        I have a couple of questions.

                        Do you believe the Johnson counter is essential for proper operation of
                        the QSD? Kees brought this up in an earlier e-mail. The SoftRocks
                        don't use Johnson counters at present.

                        What ham band were you doing your tests on? I don't see it mentioned on
                        the web page, SDR-1. Also you didn't mention it in your reply to me
                        about dynamic range.

                        Have you seen your MDS degrade as the frequency is raised above 10 MHz?

                        Do you believe your QSD will ever work well on 10 meters? A lot of us
                        would like to use it on the higher frequency bands. I am particularly
                        interested in 15 meters myself, as well as 10 meters.

                        Bill wa9pwr
                      • N0UU
                        ... driving the OE lines ? For example, set up the select on S0/S1 at 90 degree intervals and gate the select using OE. Since you would like this to be
                        Message 11 of 19 , Jun 2, 2007
                          >....has anyone looked into (and tested) gating the switches by
                          driving the OE lines ? For example, set up the select on S0/S1 at 90
                          degree intervals and gate the select using OE. Since you would like
                          this to be "fixed", maybe potentiomenter adjustable, make it 5-10ns?
                          "gate off" during select transitioning. At lower frequencies the
                          analog sample would still be close to 90 degrees and drop off as the
                          frequency increases.....but hopefully you get away from the switch
                          overlap, at least to some extent.
                          > 73 Kees K5BCQ


                          Actually, one of the early authors suggested gating off the transition
                          time to get rid of the potential IM there. At LF shouldn't be much of
                          a logic problem, but higher????????

                          The other unmentioned so far this time is the bit about mixer
                          termination: You don't want the unwanted signals and products hitting
                          an imbalance on any of the ports and crashing back to mix again to
                          make more..... 'unless you can use them to boost the signal'. So for
                          linear mixers they use something like diplexers to provide a very
                          broadband load for all signals possible. The other way is to provide
                          an 'open' so that the signal has no current and thus no power, or a
                          short at all non-IF frequencies, thus no voltage and no power. I
                          rather suspect that the QSD with the low pass RC output filter acts as
                          the latter and again, a reason for good performance.

                          Great discussion and review for us all.

                          Since the noise level of good post-QSD amps is near practical noise
                          levels, at (at least at LF) transitions and mixers are very good, that
                          leaves us with matching the input filters inpedance to the QSD and
                          working on better quadrature LO noise, rise times, etc. It's fun to
                          look at the effects of phase and voltage imbalance on sideband
                          supression also.

                          Lots of circuit possibilities here.

                          N0UU
                        • aaintil
                          ... very ... Hi Marco et al, I decided to add some modifications of that development work. See in the Files section the simulation and schematic diagrams
                          Message 12 of 19 , Jun 3, 2007
                            --- In softrock40@yahoogroups.com, Marco IK1ODO <ik1odo@...> wrote:
                            >
                            > The QSD mixer could be further improved; I'm working (currently
                            very
                            > part-time) on this topic. See
                            > http://www.spin-it.com/sdr/ik1odo_sdr1.html and
                            > http://www.spin-it.com/sdr/ik1odo_sdr2.html
                            >
                            > 73 - Marco IK1ODO / AI4YF
                            >
                            Hi Marco et al,

                            I decided to add some modifications of that development work. See in
                            the Files section the simulation and schematic diagrams related to
                            Marco's excellent work:
                            IQSDsim.zip

                            If you have the free simulation program TINA from TI, you can use
                            your creative imagination and may see very interesting results.

                            73, Ahti OH2RZ
                          • Kees & Sandy
                            Ahti, Thank you for sending this informative note. I was not aware of the TINA-TI software and it looks very usefull. The other item is those fast break before
                            Message 13 of 19 , Jun 3, 2007

                              Ahti,

                              Thank you for sending this informative note. I was not aware of the TINA-TI software and it looks very usefull. The other item is those fast break before make switches. Previously I had ordered some TS5A23157DGSR and TS5A3357DCUR switches which have "sample-and-hold" listed as an application (which in my mind is basically "signal gating/switching"). The 74VC1G series actually spells out applications like "signal chopping" and "signal multiplexing for A-D or D-A conversions" (which in my mind implies a design point directed towards what we need for SDR). The specifications support that. 

                              TI actually has a "potential use" category of "SDR" now

                              73 Kees K5BCQ

                              -- "aaintil" <aaintil@...> wrote:

                              --- In softrock40@yahoogro ups.com, Marco IK1ODO <ik1odo@...> wrote:
                              >
                              > The QSD mixer could be further improved; I'm working (currently
                              very
                              > part-time) on this topic. See
                              > http://www.spin- it.com/sdr/ ik1odo_sdr1. html and
                              > http://www.spin- it.com/sdr/ ik1odo_sdr2. html
                              >
                              > 73 - Marco IK1ODO / AI4YF
                              >
                              Hi Marco et al,

                              I decided to add some modifications of that development work. See in
                              the Files section the simulation and schematic diagrams related to
                              Marco's excellent work:
                              IQSDsim.zip

                              If you have the free simulation program TINA from TI, you can use
                              your creative imagination and may see very interesting results.

                              73, Ahti OH2RZ

                            • Jim Miller
                              74VC1G series is this a TI product? is it spelled right? i couldn t find any info on it. 73 jim ab3cv ... From: Kees & Sandy To: softrock40@yahoogroups.com
                              Message 14 of 19 , Jun 4, 2007
                                74VC1G series
                                 
                                is this a TI product? is it spelled right? i couldn't find any info on it.
                                 
                                73
                                 
                                jim ab3cv
                                 
                                ----- Original Message -----
                                Sent: Sunday, June 03, 2007 5:48 PM
                                Subject: Re: [softrock40] Re: Switches vs Mixers

                                Ahti,

                                The 74VC1G series actually

                              • Kees & Sandy
                                74LVC1G3157DBV ...to be exact (DBV is the package and can vary) 73 Kees K5BCQ -- Jim Miller wrote: 74VC1G series is this a TI product? is
                                Message 15 of 19 , Jun 4, 2007

                                  74LVC1G3157DBV ...to be exact (DBV is the package and can vary)

                                  73 Kees K5BCQ

                                  -- "Jim Miller" <jim@...> wrote:

                                  74VC1G series
                                   
                                  is this a TI product? is it spelled right? i couldn't find any info on it.
                                   
                                  73
                                   
                                  jim ab3cv
                                   
                                  ----- Original Message -----
                                  Sent: Sunday, June 03, 2007 5:48 PM
                                  Subject: Re: [softrock40] Re: Switches vs Mixers

                                  Ahti,

                                  The 74VC1G series actually

                                • aaintil
                                  ... on it. ... Jim, Try this: Ahti OH2RZ
                                  Message 16 of 19 , Jun 5, 2007
                                    --- In softrock40@yahoogroups.com, "Jim Miller" <jim@...> wrote:
                                    >
                                    > 74VC1G series
                                    >
                                    > is this a TI product? is it spelled right? i couldn't find any info
                                    on it.
                                    >
                                    > 73
                                    >
                                    > jim ab3cv

                                    Jim,
                                    Try this:
                                    <http://focus.ti.com/docs/prod/folders/print/sn74lvc1g3157.html>

                                    Ahti OH2RZ
                                  • Jim Miller
                                    kees thanks. i thought i was missing another new part in the alphabet soup of 74xx offerings. 73 jim ab3cv
                                    Message 17 of 19 , Jun 5, 2007
                                      kees
                                       
                                      thanks. i thought i was missing another new part in the alphabet soup of 74xx offerings.
                                       
                                      73
                                       
                                      jim ab3cv
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