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Re: SDR Questions

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  • drmail377
    1. You are correct, withing the limits of the commutating switches and other parts as stated by WB6DHW you can theoretically receive or transmit on any
    Message 1 of 11 , Oct 1, 2008
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      1. You are correct, withing the limits of the commutating switches and
      other parts as stated by WB6DHW you can theoretically receive or
      transmit on any frequency covered by the Si570 output divided by four.
      As far as I know, any SDR software will work in terms of receive. I
      don't know if any of the SDR programs automatically limit going into
      transmit mode (toggle PTT for example) only for frequencies within the
      Ham bands - I doubt it as I've never seen an option to select which
      regional band plan to use, for-example.

      2. WB6DHW has answered this in-part. With respect to filtering: From a
      strict point of view, you would need to have filters on the front end.
      It is arguable if a receive filter is required at UHF/VHF as an SDR
      design like the Softrock has inherently good 2nd order rejection (not
      third order rejection). So 220MHz reception would be fairly immune to
      70cm signals. But that would not be the case for receiving on 2m and
      rejecting 70cm signals. For transmit at really usable power levels I
      would venture to guess output filtering is going to be required. Good
      transmit filters if used for receive solve the receive filter problem
      but may limit contiguous coverage that would be possible for receive
      only filters (read Item-3 below).

      3. The filter on the front end is a challenge, especially for
      transmitting. If you look at the BPF simulation in Files > Lite+Xtall
      v8.3 of this Group you will see how the four receive filters work for
      the V8.3 softrock which is essentially a genera-coverage receiver, not
      transmitter. The latest version softrock in development is receive +
      transmit with a USB controlled Si570. I haven't seen the proposed
      front end filtering for this radio, but I would venture to guess the
      filters are going to be more selective in order to remove unwanted
      transmit products, therefore if the same filters are used during
      receive, you will not have general-coverage. There is a switchable BPF
      board with six filters available at http://www.wb4dhw.com, have a look
      at the schematic.

      4. Simultaneous transmit receive - Hmmm... I can't answer if any
      single instance of any SDR program can both transmit and receive at
      the same time. You might be able to run two instances of a program at
      the same time and use one for transmit and one for receive. In either
      case, you would need two sound cards or a sound card with multiple
      input/outputs.

      Hope this helps... 73's David WB4ONA

      --- In softrock40@yahoogroups.com, brainerd@... wrote:
      >
      > On 1 Oct 2008 at 2:07, dcameraman wrote:
      >
      > > Hello all,
      > >
      > > I've been reading all of the posts for the Softrock40 group for about
      > > a month now and feel that I know enough to be able to ask some
      questions.
      > >
      > > 1. With the latest RxTx kit, whether or not it comes with a LVDS or
      > > CMOS SI570, you can receive and transmit on all of the frequencies
      > > that the SI570 is capable of. Is that correct? Does it take a special
      > > software to do it? or will Rocky do it?
      > >
      > > 2. I'm guessing that the latest version of the RxTx will transmit on
      > > VHF and UHF, with the right SI570, but it doesn't have the "filtering"
      > > to be "clean". Is that assumption correct?
      > >
      > > 3. If I'm right so far, then the biggest challenge is how to cover the
      > > 34 Mhz of UHF and VHF as far as filtering is concerned. Is that
      correct?
      > >
      > > 4. Is there a software that will simultaneously receive on one SDR and
      > > transmit on another, thereby having a repeater?
      > >
      > > I guess that's about all that I'll bother you all with for now.
      > >
      > > Thank you for your time.
      > > Josh Mathis
      > > K7GVI
      > >
      > The Si570 will cover VHF and UHF, but the RXTX will only go to 30
      MHz or so. The
      > limiatation is the switches in the QSD and the filters. I am laying
      out a PCB for a transceiver
      > that will cover up to 700 MHz using the Si570(it uses diode DBM's
      rather than a QSD).
      >
      > Dave - WB6DHW
      > <http://wb6dhw.com>
      >
    • Josh Mathis
      Thank you all so much for answering. It sounds like the RxTx wasn t designed with the SI570 in mind, at first. But then who would have known how popular it
      Message 2 of 11 , Oct 2, 2008
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        Thank you all so much for answering.

        It sounds like the RxTx wasn't designed with the SI570 in mind, at first. But then who would have known how popular it would be?

        1. It also sounds like I need to learn a lot about filtering. I think I know that the transmitted signal needs to be clean and all that. What I don't understand is how you take a filter and have it "change" depending on what frequency you are transmitting or receiving on!! I work with repeaters a little and I see big "cans" getting used all the time to filter rx and tx. It would be very tedious to have to tune the cans every time you changed frequency. If someone could help me out a little here I would appreciate that.

        2. David WB4ONA mentioned something about the output of the SI570 divided by four. Does that mean that if the SI570 is capable of 1400mhz, then the highest that an RxTx would be able to go would be 350mhz?

        Thank you all for your time.
        I'm trying to learn it.
        Josh Mathis
        K7GVI




        ----- Original Message ----
        From: drmail377 <drmail377@...>
        To: softrock40@yahoogroups.com
        Sent: Wednesday, October 1, 2008 12:55:32 AM
        Subject: [softrock40] Re: SDR Questions


        1. You are correct, withing the limits of the commutating switches and
        other parts as stated by WB6DHW you can theoretically receive or
        transmit on any frequency covered by the Si570 output divided by four.
        As far as I know, any SDR software will work in terms of receive. I
        don't know if any of the SDR programs automatically limit going into
        transmit mode (toggle PTT for example) only for frequencies within the
        Ham bands - I doubt it as I've never seen an option to select which
        regional band plan to use, for-example.

        2. WB6DHW has answered this in-part. With respect to filtering: From a
        strict point of view, you would need to have filters on the front end.
        It is arguable if a receive filter is required at UHF/VHF as an SDR
        design like the Softrock has inherently good 2nd order rejection (not
        third order rejection). So 220MHz reception would be fairly immune to
        70cm signals. But that would not be the case for receiving on 2m and
        rejecting 70cm signals. For transmit at really usable power levels I
        would venture to guess output filtering is going to be required. Good
        transmit filters if used for receive solve the receive filter problem
        but may limit contiguous coverage that would be possible for receive
        only filters (read Item-3 below).

        3. The filter on the front end is a challenge, especially for
        transmitting. If you look at the BPF simulation in Files > Lite+Xtall
        v8.3 of this Group you will see how the four receive filters work for
        the V8.3 softrock which is essentially a genera-coverage receiver, not
        transmitter. The latest version softrock in development is receive +
        transmit with a USB controlled Si570. I haven't seen the proposed
        front end filtering for this radio, but I would venture to guess the
        filters are going to be more selective in order to remove unwanted
        transmit products, therefore if the same filters are used during
        receive, you will not have general-coverage. There is a switchable BPF
        board with six filters available at http://www.wb4dhw. com, have a look
        at the schematic.

        4. Simultaneous transmit receive - Hmmm... I can't answer if any
        single instance of any SDR program can both transmit and receive at
        the same time. You might be able to run two instances of a program at
        the same time and use one for transmit and one for receive. In either
        case, you would need two sound cards or a sound card with multiple
        input/outputs.

        Hope this helps... 73's David WB4ONA

        --- In softrock40@yahoogro ups.com, brainerd@... wrote:
        >
        > On 1 Oct 2008 at 2:07, dcameraman wrote:
        >
        > > Hello all,
        > >
        > > I've been reading all of the posts for the Softrock40 group for about
        > > a month now and feel that I know enough to be able to ask some
        questions.
        > >
        > > 1. With the latest RxTx kit, whether or not it comes with a LVDS or
        > > CMOS SI570, you can receive and transmit on all of the frequencies
        > > that the SI570 is capable of. Is that correct? Does it take a special
        > > software to do it? or will Rocky do it?
        > >
        > > 2. I'm guessing that the latest version of the RxTx will transmit on
        > > VHF and UHF, with the right SI570, but it doesn't have the "filtering"
        > > to be "clean". Is that assumption correct?
        > >
        > > 3. If I'm right so far, then the biggest challenge is how to cover the
        > > 34 Mhz of UHF and VHF as far as filtering is concerned. Is that
        correct?
        > >
        > > 4. Is there a software that will simultaneously receive on one SDR and
        > > transmit on another, thereby having a repeater?
        > >
        > > I guess that's about all that I'll bother you all with for now.
        > >
        > > Thank you for your time.
        > > Josh Mathis
        > > K7GVI
        > >
        > The Si570 will cover VHF and UHF, but the RXTX will only go to 30
        MHz or so. The
        > limiatation is the switches in the QSD and the filters. I am laying
        out a PCB for a transceiver
        > that will cover up to 700 MHz using the Si570(it uses diode DBM's
        rather than a QSD).
        >
        > Dave - WB6DHW
        > <http://wb6dhw. com>
        >


      • Jeff Kashinsky
        The important information is: within the limits of the commutating switches and other parts as stated by WB6DHW . Tony s design is good to around 30 MHz. This
        Message 3 of 11 , Oct 2, 2008
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          The important information is: "within the limits of the commutating
          switches and other parts as stated by WB6DHW".

          Tony's design is good to around 30 MHz. This is based on the design,
          layout, and the components chosen.

          That is not to say that a SDR can't be designed to operate at higher
          frequencies.

          There are faster parts that would probably allow 70 MHz but I'm not
          sure the layout would allow it.

          There are other SDR designs that would allow even wider coverage.
          Look at the Collins 95S-1(A) at about $9k. 5KHz-2000MHz SDR.

          Just remember that the RXTX is one implementation of a Software
          Defined Radio, but not the only one. It has its limitations but for
          its price and ease to build it can't be beat.

          The WB6DHW designs offer high performance but at a higher cost in
          both price and ease to build.

          There are commercial products such as the Ettus Research USRP that
          cover up to 2.9GHz in the $1.5K range.
        • k5nwa
          ... By a copy of the ARRL handbook they have a section on filters and how to calculate the values of the components, it s way more involved than a paragraph or
          Message 4 of 11 , Oct 2, 2008
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            At 10:53 AM 10/2/2008, you wrote:
            >Thank you all so much for answering.
            >
            >It sounds like the RxTx wasn't designed with the SI570 in mind, at
            >first. But then who would have known how popular it would be?
            >
            >1. It also sounds like I need to learn a lot about filtering. I
            >think I know that the transmitted signal needs to be clean and all
            >that. What I don't understand is how you take a filter and have it
            >"change" depending on what frequency you are transmitting or
            >receiving on!! I work with repeaters a little and I see big "cans"
            >getting used all the time to filter rx and tx. It would be very
            >tedious to have to tune the cans every time you changed frequency.
            >If someone could help me out a little here I would appreciate that.
            >
            >2. David WB4ONA mentioned something about the output of the SI570
            >divided by four. Does that mean that if the SI570 is capable of
            >1400mhz, then the highest that an RxTx would be able to go would be 350mhz?
            >
            >Thank you all for your time.
            >I'm trying to learn it.
            >Josh Mathis
            >K7GVI
            >

            By a copy of the ARRL handbook they have a section on filters and how
            to calculate the values of the components, it's way more involved
            than a paragraph or two. But the book has tables already calculated
            for common frequencies.

            The answer is no, the SI570 are available in versions that can go to
            1400MHz but that is not the version that Tony sells, in any case that
            is just one component on the radio the other components must be able
            to work that high also. The SoftRock is designed for HF use the
            components are not rated to work at 350MHz or anywhere close to those
            frequencies. About 30MHz is the top end and even there the sensitivity is low.

            Software defined radios similar to the SoftRocks are actually two
            radios with the clocks being offset by 90 degrees, the reason for
            that is that math can be applied in the PC to look at the two signals
            as a Complex number and thereby enabling the removal of signal images.

            The easiest way to generate digital signals that are 90 degrees apart
            is to take a clock that is 4x the frequency of interest and divide it
            by 4, each of the original clocks represents 90 degrees offset from
            each other, by connecting the divider to the switches then you can
            create the two clocks that are 90 degrees apart from each other. The
            common terminology for two signals at 90 degree phase difference from
            is other is called "Quadrature" you will hear that term used often on the list.

            Below is a link to articles that might help you understand;
            < http://www.dspradio.org/SDR_Basics >


            Cecil
            K5NWA
            www.softrockradio.org www.qrpradio.com

            "Blessed are the cracked, for they shall let in the light."
          • Chris Albertson
            ... First off the Softrock is working in HF. Not VHF or UHF. 30Mhz is the practical limit for the SR40 design. How to change the filter? Typically a filter
            Message 5 of 11 , Oct 2, 2008
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              On Thu, Oct 2, 2008 at 8:53 AM, Josh Mathis <dcameraman@...> wrote:

              > 1. It also sounds like I need to learn a lot about filtering. I think I know
              > that the transmitted signal needs to be clean and all that. What I don't
              > understand is how you take a filter and have it "change" depending on what
              > frequency you are transmitting or receiving on!! I work with repeaters a
              > little and I see big "cans" getting used all the time to filter rx and tx.
              > It would be very tedious to have to tune the cans every time you changed
              > frequency. If someone could help me out a little here I would appreciate
              > that.

              First off the Softrock is working in HF. Not VHF or UHF. 30Mhz is the
              practical limit for the SR40 design.

              How to change the filter? Typically a filter is designed for one band,
              say the 40 meter band. It will pass the entire band. In a multi band
              transciever they use switches to switch between a bank of bandpass
              filters. The switches are either diodes or mechanical relays.
              Further filtering withing the band is done by the software on the PC.

              Your repeater needs those very selective filters because it transmits and
              receives simultaneously. An HF radio operates simplex and never transmits
              and receives at the same time so it does not need such a sophisticated filter

              =====
              Chris Albertson
              Redondo Beach, California
            • g8xul
              If you want to play with filter designs try Elsie ( http://tonnesoftware.com/elsie.html ). It s a free download. I found it a great learning tool as well as
              Message 6 of 11 , Oct 2, 2008
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                If you want to play with filter designs try Elsie (
                http://tonnesoftware.com/elsie.html ). It's a free download. I
                found it a great learning tool as well as a design tool. You can
                built Tony's BPF design and see how it performs then tweak to your
                hearts content.

                For reasons given below it might not help you with your current
                problem, but might help understand why :-)

                73,

                Dave

                > > 1. It also sounds like I need to learn a lot about filtering. I
                think I know
                > > that the transmitted signal needs to be clean and all that. What
                I don't
                > > understand is how you take a filter and have it "change"
                depending on what
                > > frequency you are transmitting or receiving on!! I work with
                repeaters a
                > > little and I see big "cans" getting used all the time to filter
                rx and tx.
                > > It would be very tedious to have to tune the cans every time you
                changed
                > > frequency. If someone could help me out a little here I would
                appreciate
                > > that.
                >
                > First off the Softrock is working in HF. Not VHF or UHF. 30Mhz
                is the
                > practical limit for the SR40 design.
                >
                > How to change the filter? Typically a filter is designed for one
                band,
                > say the 40 meter band. It will pass the entire band. In a multi
                band
                > transciever they use switches to switch between a bank of bandpass
                > filters. The switches are either diodes or mechanical relays.
                > Further filtering withing the band is done by the software on the
                PC.
                >
                > Your repeater needs those very selective filters because it
                transmits and
                > receives simultaneously. An HF radio operates simplex and never
                transmits
                > and receives at the same time so it does not need such a
                sophisticated filter
                >
                > =====
                > Chris Albertson
                > Redondo Beach, California
                >
              • brainerd@wildblue.net
                ... The RxTx was designed before the Si570 was available. Using two flip flops in a Johnson Counter arrangement is the most common way of generating 2
                Message 7 of 11 , Oct 2, 2008
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                  On 2 Oct 2008 at 8:53, Josh Mathis wrote:

                  >
                  > Thank you all so much for answering.
                  >
                  > It sounds like the RxTx wasn't designed with the SI570 in mind, at first. But then who would have
                  > known how popular it would be?
                  >
                  > 1. It also sounds like I need to learn a lot about filtering. I think I know that the transmitted signal
                  > needs to be clean and all that. What I don't understand is how you take a filter and have it
                  > "change" depending on what frequency you are transmitting or receiving on!! I work with repeaters
                  > a little and I see big "cans" getting used all the time to filter rx and tx. It would be very tedious to
                  > have to tune the cans every time you changed frequency. If someone could help me out a little
                  > here I would appreciate that.
                  >
                  > 2. David WB4ONA mentioned something about the output of the SI570 divided by four. Does that
                  > mean that if the SI570 is capable of 1400mhz, then the highest that an RxTx would be able to go
                  > would be 350mhz?
                  >
                  > Thank you all for your time.
                  > I'm trying to learn it.
                  > Josh Mathis
                  > K7GVI
                  >
                  The RxTx was designed before the Si570 was available.
                  Using two flip flops in a "Johnson Counter" arrangement is the most common way of
                  generating 2 signals that are 90 degrees apart over a wide bandwidth. That is a divide by
                  four. If you already have 2 signals exactly 180 degrees apart(the LVDS and PECL versions
                  of the Si570 do) you can clock the 2nd FF with the out of phase clock and only need 2X.
                  Using this, the Si570 will give I and Q up to 700 MHz. For a receiver or transmitter, however,
                  you need parts designed for those frequencies. I don't know of any inexpensive IC switches
                  that will do that. Double balanced diode mixers(DBM's), however, are widely available at
                  reasonable prices for that and much higher frequencies. The downside is they have about
                  6dB of loss and are sensitive to mismatches at their ports.

                  Dave - WB6DHW
                  <http://wb6dhw.com>
                • Josh Mathis
                  Thank you all so much for answering. Pretty much I m just getting an idea as to what the Softrock is and isn t capable of doing. You all have been very helpful
                  Message 8 of 11 , Oct 2, 2008
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                    Thank you all so much for answering.

                    Pretty much I'm just getting an idea as to what the Softrock is and isn't capable of doing. You all have been very helpful in clearing that up.

                    Some one said that the filters are of the band pass type. And so a multiband radio has to have several and switch between them.

                    With the SDR radios is that all that's required to meet FCC specs? or how do you get the spurious emissions down to however many db they are supposed to be. It would seem to me that you would need the band pass to be just narrow enough to cover your transmitted signal. The hard part about that is that then you've got to have the filter be tunable . . .

                    Or, maybe, just maybe with the SDR you can filter the audio so steep in the software, there aren't any spurious emissions?

                    Anyway, I was thinking that a bandpass filter wouldn't be enough and that you'd have to have a tunable filter on the transmit side of things just wide enough to cover your transmitted signal.

                    Or, maybe, the 2nd and 3rd and 4th harmonic are outside of the whole band, and you don't need a really tight filter!! Am I getting close????


                    Anyway, thank you all for your comments and helping me out.
                    Josh Mathis
                    K7GVI

                  • Chris Albertson
                    ... That was me who pointed out that band pass filters were used. As an example the ham 40 meter band is from 7.0Mhz to 7.3Mhz so the pass band is only about
                    Message 9 of 11 , Oct 2, 2008
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                      On Thu, Oct 2, 2008 at 2:35 PM, Josh Mathis <dcameraman@...> wrote:

                      > Or, maybe, the 2nd and 3rd and 4th harmonic are outside of the whole band,
                      > and you don't need a really tight filter!! Am I getting close????

                      That was me who pointed out that band pass filters were used.
                      As an example the ham 40 meter band is from 7.0Mhz to 7.3Mhz
                      so the pass band is only about 300Khz wide.

                      --
                      =====
                      Chris Albertson
                      Redondo Beach, California
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