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SDR Questions

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  • dcameraman
    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.
    Message 1 of 11 , Sep 30, 2008
      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
    • brainerd@wildblue.net
      ... 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
      Message 2 of 11 , Sep 30, 2008
        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>
      • 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 3 of 11 , Oct 1, 2008
          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 4 of 11 , Oct 2, 2008
            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 5 of 11 , Oct 2, 2008
              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 6 of 11 , Oct 2, 2008
                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 7 of 11 , Oct 2, 2008
                  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 8 of 11 , Oct 2, 2008
                    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 9 of 11 , Oct 2, 2008
                      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 10 of 11 , Oct 2, 2008
                        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 11 of 11 , Oct 2, 2008
                          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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