Idea for a Vector Network Analyzer


I had the following idea for a home built vector network analyzer project.
The block diagram is shown below. Essentially channels A and B are mixed down to separate 1kHz IF and sampled with a stereo 24bit DACE for the PC. Phase and magnitude of the resulting 1kHz signals are then calculated with DSP methods.
f1-f0 = 1kHz as f0 sweeps from 0 to 20MHz.
I know there will be problems as f0 gets close to 1kHz and again at near 0Hz, due to SA602 not being an ideal mixer.

Anyone think concept won't work ?


                                       To PC
                                         *
                                         |
                                      -------
                ------               |FT245BM|
ch A  -----    |      |              |       |
o----|SA602|---| 2kHz |------.       |USB I/O|
     |mixer|   | LPF  |      |        -------
      -----     ------       |           |
         | LO                |           |
         |                 ------     -------
         o----.           |24bit |   | AVR   |
         |    |           | ADC  |---|       |
         |    |           |stereo|   | uC    |
         |    |            ------     -------
         |    |   ------     |           |
ch B  -----   |  |      |    |           |
o----|SA602|-----| 2kHz |----'           |
     |mixer|  |  | LPF  |                |
      -----   |   ------                 |
              |                          |
              |                          |
              |   ------     ------      |
            f1|  |      |   |AD9834|     |
              '--| 20MHz|---| DDS  |-----o
                 | LPF  |   |      |     |
                  ------     ------      |
                                         |
       /|         ------      ------     |
SIG   / |      f0|      |    |AD9834|    |
OUT__/  |--------| 20MHz|----| DDS  |----'
f0   \  |        | LPF  |    |      |
      \ |         ------      ------
       \|
     BUFFER

[view in monospaced characters]


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