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Re: [softrock40] Theory of the Softrock

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  • Cecil Bayona
    ... You have to have a filter because the detector will respond to odd harmonics of the LO, that is why for some frequencies we have been discussing dividing
    Message 1 of 2 , Oct 1, 2006
      racerxgt2 wrote:
      > Ok, so I've been trying to work through the math of the softrock
      > receiver in my quest to build what I discussed in a previous post
      > (multi-band shortwave/amateur receiver). I've read through the
      > document "Software defined radio for the masses" a couple of times,
      > (admitting that I have stopped short of getting the pencil to the
      > paper, probably the best option to completely understand this) and
      > just need a few things clarified. My understanding is that its the
      > system impedance (50ohms) in conjunction with the shunt cap(s) that
      > determines the bandwidth of the reciever, and the xtal determines
      > your center frequency. Can you then, theoretically, open up the BPF
      > on the front end as large as you want? There has to be some
      > limiting factor or else the softrock design wouldn't have included
      > the BPF in the design. The ultimate bandwidth of the receiver is
      > limited by your soundcard. So, my question is, what is the limiting
      > factor of covering a wider bandwidth with a frequency synthesizer of
      > some sort? Please let me know if any of my thoughts on the
      > innerworkings of the softrock are incorrect. I'm just trying to
      > figure out if I need to have a mixer stage with IF or if I can just
      > use a wider front end filter with a synthesizer to realize my design.
      > Thanks, and sorry for my inexperience
      > Kyle
      > KC9JJL
      > Yahoo! Groups Links
      You have to have a filter because the detector will respond to odd
      harmonics of the LO, that is why for some frequencies we have been
      discussing dividing the crystal frequency by 3 to use a crystal that is
      easier to find, it's generally called sub-harmonic sampling.

      The bandwidth is determined by the sound card sampling rate, within
      reason, the cap size, the op-amp that follows need to have the proper
      frequency response. People have successfully used the SoftRock with
      48KHz, 96KHz, and 192KHz. The circuitry is capable of it, the limiting
      factor is the sound card, of course you must have a PC capable of
      handling such a flood of data.

      A AD9851 DDS is capable with a 180MHz reference of outputting 60MHz and
      it's inexpensive.

      AT the higher frequencies the limiting factor can become the switches
      due to leakage and unsymmetrical turn on/off times, that is seen as
      increased noise, at some point the noise is the limit, that problem can
      be helped with a pre-amp.

      However you are not limited to just using switches as the mixer, there
      are a multitude of mixer technologies that will also work, for example
      you could use a Diode Ring Mixer, specially the balanced ones can be
      good into the GHz range and are not expensive, the cost is that they
      need a lot more power than a FET switch mixer. Gilbert Cell mixers can
      be used as well as many others.

      Hope it helps.

      www.qrpradio.com www.hpsdr.com

      "Sacred Cows make the best Hamburger!" Don Seglio Batuna
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