[time-nuts] VNA design

[Thomas S. Knutsen]

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, if you can wait a bit. Mine is mostly used at microwaves, with
some external mixers and testsets. If you are low on cash, this may be the
best approach, but it requires some work.

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.

The book by Joel Dunsmore is excelent, highly reccomended if you are doing
or interested in VNA measurments.

BR.
Thomas.




2014-06-02 16:43 GMT+02:00 Attila Kinali <attila at kinali.ch>:

> Hi,
>
> I know this is not exactly a time-nut question, but i guess this is
> the best place i know to ask about this stuff.
>
> I recently got introduced into the usefullness of a VNA. But these
> things are horribly expensive for home use, even if bought from ebay
> (before you say anything, remember i live in europe, where every
> boat anchor hast to travel a long way). But given that most of the
> designs that are on ebay are from the 80s and early 90s, i thought that
> with todays ICs it should be easy to come up with a design that does
> the same thing but can be build on a kitchen table.
>
> Well, my problem now is, that i don't know how to build a VNA.
> Yes, i understand the basic principle. I can come up with a design
> that should work. But i have no clue about any problems or difficulties
> in building these devices. Ie it's very likely that i fall into a dozen
> traps when i try to build one.
>
> I tried to get information on how to build a VNA, or what kind of trouble
> people had operating one, but beside the VNA book[1] Rick mentioned a
> couple
> of months ago and ko4bb's site (thanks man! your manual collection is a
> gold
> mine!), my searches came out blank. As i'm quite sure that there is
> information of that kind out there, i would like to ask whether someone
> could point me to some documents, webpages, books, papers, etc that would
> show me the detailed design of VNA, the problems people had with some
> designs or anything else that would be of interest in such an endavor.
>
>
> Also, any good resource on how to build a directional coupler that
> does 10-3000MHz without going to exotic materials would be much
> appreciated. All papers i found deal mostly with stuff above 5GHz.
> Seems like "low frequency" couplers are considered "a solved problem".
>
>
>                         Attila Kinali
>
> [1] Handbook of Microwave Component Measurements: with Advanced VNA
> Techniques
> by Dunsmore, 2012
>
> --
> The trouble with you, Shev, is you don't say anything until you've saved
> up a whole truckload of damned heavy brick arguments and then you dump
> them all out and never look at the bleeding body mangled beneath the heap
>                 -- Tirin, The Dispossessed, U. Le Guin
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>



-- 

 Please  avoid sending  me  Word  or  PowerPoint  attachments.
 See  <http://www.gnu.org/philosophy/no-word-attachments.html>
PDF is an better alternative and there are always LaTeX!