[time-nuts] VNA design

[Attila Kinali]

On Mon, 02 Jun 2014 09:35:41 -0700
"Richard (Rick) Karlquist" <richard at karlquist.com> wrote:


> Julius said that he designed it before
> the era of cheap calibration.  Now that everyone has calibration,
> you don't need a good directional coupler.  You can get away
> with a MiniCircuits coupler.

How about impedance matching issues? Can those be calibrated away?

>  But in fact it is
> even easier to just use a resistive bridge.  Four ordinary
> resistors will easily go to 3 GHz.  Use a differential amplifier
> at the output.  Lots of info on this in the literature.

I somewhat fear that the parasitics of resistors will give me
garbage when going to >1GHz. Maybe i should look into those and
calculate how much performance they would cost.


> Another way to make in effect a directional coupler is to use
> a 180 degree hybrid.

My understanding is, that those are rather narrow band.
Those i've seen were at most one octave in range.

> Don't even think about trying this at home.

You mention that for both Podell's and Botkas design. May i ask what
the reason is? Why do you think it cannot be done at home?

Also, i quickly tried to search for both Botka and Podell. While
Podell gives a couple of matches on ieeexplore, Botka comes back
almost empty. Any hints to for finding references to their work?
 
> BTW, you didn't mention software, but that's a big part of
> the job.

Because i have not had a look into that yet. My guess is that
phase detection of a known frequency (i ignore multi-tone detection
due to harmonics for the moment) is known and should be easy.
So the big part of the softare is getting thins into the PC and doing
a good job of presenting the data, ie GUI stuff. And GUI is, once you
have a graphical design, just implementation work.

Please correct me if i'm mistaken.


			Attila Kinali
-- 
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