[time-nuts] VNA design

Dr. David Kirkby drkirkby at gmail.com
Mon Jun 2 13:45:23 EDT 2014


On 2 Jun 2014 18:14, "Thomas S. Knutsen" <la3pna at gmail.com> wrote:
>
> The design of an VNA is an interesting thing. It requires quite high focus
> on good RF practices and screening.
>
> In the range 0-3GHz there is no low cost devices avaible, not counting the
> copper mountain tech boxes ( http://www.coppermountaintech.com/ ). Up to
> 1.3GHz there is the DG8SAQ VNWA avaible from sdrkits, these can also be
> used with mixers to extend the max frequency. The VNWA is an 2 detector
VNA
> that needs an S-parameter testset in order to get all the 4 S-parameters.
>
> An homebrew alternative would be fun to do, but its a lot of work, both in
> getting reproduceable data from the hardware and in programming. Building
> somthing that is connected to the PC simplifies things a lot.
> Couplers and detectors are not the hardest thing to make, some small SMD
> resistors, an balanced amplifier - detector and things should work to 6GHz
> or higher with some care in the layout.
> Signal generation is perhaps the hardest part, there is AFAIK no single
> solution working from LF to high UHF, one cool alternative is to build an
> generator with an YIG and mixing down, but that requires a lot of work to
> get stable over the range 0-3GHz. In addition you need to keep the signal
> from the generator out of the detector in order to keep the dynamic range
> high.
> If you are building your own VNA, I would build it with 4 detectors and
the
> posibility to re-configure those. It opens for several of the more
advanced
> calibration methods and eliminates some of the errors in the VNA.
>
> If I were to build something, I think I would base it on the N2PK design,
> as there is documentation and programs avaible that makes for some part of
> the work.
>
> There are some IC's avaible that do the detection of the power levels,
> AD8302 comes to mind, the common denominator for these are that they don't
> solve for the phase sign, and thereby are not true vector. In addition,
> those I have tested don't behave to well with
>
> As an alternative, the HP8410 series are avaible here in the EU, sometimes
> quite cheap

I assume you mean 8510.

> The accuracy of the VNA is determined by the calkit used to calibrate it.
> There is no way around obtaining an good calkit, learning how to use it
> without destroying it, and do repeatable calibrations. The calkit is the
> single most important part of the VNA. Do use an calkit for the connectors
> you are going to measure, don't add adaptors or worse, coaxial cable after
> the calibration plane.

I had at one point an HP 8753A (3 GHz) VNA with a full S-parameter test
set. It cost me about 50% of what my HP 85054B 18 GHz N calibration kit
cost me and I think I got the 85054B cheap at about $3800.

I do sell low cost calibration kits for N, SMA and X-band waveguide.

Dave


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