Re: Idea for a Vector Network Analyzer

Chris Jones wrote: 
Adam S wrote:


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] 



Are you already aware of the N2PK network analyser?  (Try google).  I think
your idea of using an IF frequency rather than mixing down to DC might be
preferable but there probably isn't much difference.  Using a non-DC IF,
you might be able to make the receiver digitally filter out responses due
to the third (and other) harmonics of the source frequency, which could
make the signal source purity less difficult.  The third and fifth
harmonics will unavoidably be present in the receive LO (since the mixer
only works well with hard driven LO port, creating odd harmonics), but
since the source and LO frequencies are different by 1* fIF in your system,
the harmonics of the source will be received and downconverted to
frequencies 3fIF and 5fIf etc. which are not the same as the desired IF
signal so you can filter them out.

I would also like to make a VNA, but to be interesting to me I would really
want it to cover the 2.4GHz WLAN frequencies so I could test filters,
antennas etc.  The receiver part looks feasible to me using AD8343 mixers,
or maybe diode samplers, but the signal source and the s-parameter test set
seem like the challenging parts to me.  Rather than a 10kHz to 3GHz signal
generator, I wonder is it easier to generate a very wideband pseudo-random
bit sequence using ECL type comparators or latches, and to use that as the
stimulus, recording the response of the network in the time domain using a
second shifted PRBS generator driving a correlator, and computing the
frequency response.  I fear that this would take too long for a very wide
sweep however.

Chris 


That last part paragraph flew over my head. I've heard of PRBS
generators used as a stimulus for audio speaker frequency analysis.
Maybe there is someway of correlating two of these generators and use
digital sampling (ones and zeros) at super high speeds to somehow
compute the frequency response. Could just be my imagination.

I have seen the renowned N2PK network analyzer published on the web. I
would of though dynamic range could be seriously improved by using AC,
as DC would have drift problems. Once the 1kHz is inside the PC then
there are no noise/drift sources to speak of. There is also the other
advance as you mentioned, about harmonics of the source signal being
blocked by the 1kHz since they will not be 1kHz relative to harmonics of
the LO. But the catch is the DDS generates a bunch of non-harmonic
related spurs that shift all over the place as you sweep the frequency.
At some frequencies multiple harmonics will get mixed down to 1kHz and
cause error on the output. The DC mixing method of the N2PK will have
every spur joining together from each DDS and mixing down. Some will
subtract some will add. Needs more thinking..

Adam




.

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