Building a RF-Frontend

hi,

for some simple experiments I want to build a simple
transceiver which works with a soundcard line-in/out.
My current target communicates at 25 kHz with on-off-keying
at a rate of 1600 Hz, but for future experiemnts I want
a RF-Frontend for from 20 kHz up to at least some Mhz (the more
the better).

As I understand it I need a circuit which schematically
looks like this:

      ____       _______       __________
     |    \_    |       |     |          |

/—*–>| AMP |–>| Mixer |---->| Low-Pass |------> Line-In
| ^ | / | -> | | Filter |
| | |
/ || ||
| | ^ ___________
C | | | |
o | *<—| Frequency |
i | | | Generator |
l | | |___________|
| | v
| | | |
| ---------<----| Mixer |<----------------------- Line-Out / Speaker
---- | <- |
– |
____|
(common Ground)

(roughly resembling http://en.wikipedia.org/wiki/Image:Superhet2.png)

my questions are:

  • does this design make sense? Did I leave out an important Stage? (some
    more Amplifier?)
  • which ICs should I use for which Block?
  • is an OpAmp good enough as RF Amplifier?
  • is an passive Low-Pass filter ok?
  • for the mixers: TCA440 or NE612 or something different?
  • for the local Oscillator: I want something adjustable either
    via poti or electronical if I can interface it to a Computer
    somehow.
  • ideally I want to use this circuit with my laptop, but where should
    I draw power from?

thanks for your time & cu,
Maurice Massar

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