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Calibration standards.


Stephen Laurence 2021/02/07 02:00

Dear All,

This is not a new topic but has not been discussed for a while.

Calibration standards can be purchased for £5 to £1500 on Ebay. The “quite good” set bundled with the 2N has been characterised by Dr Poulsen.

However, up to 3ghz, does it actually make any difference to the measurements possible with the saa2 vnas?

It seems the most important factors if you are making your own standards is the electrical length difference between open and short, and careful making of the load with two small sma resistors. Quality of the sma or N connectors being cut up also has a small effect, mainly after a period of use with the female receptacle being worn or damaged. But it may be cheaper to make new ones rather than invest in expensive sacrificial Aphenol ones.

I await the storm. Please note I have never been anywhere near a professional vna or its calibration kit, so I do not know what “really good” is. I have managed to get the noise level on port 2 down to -80db up to 2.5ghz by covering both sides of of the main circuit board with ferrite plates (watch out for them on Ebay) held in place with Gaffer tape. It makes the unit a bit heavier! I have moved zero to the very top of the screen and if I want to gaze at the noise floor at lower frequencies I have to scale it at 12db per grid unit.

Steve L

canale 2021/02/07 09:15

Hi Steve
can you please give more details about covering the circuit board with ferrite plates ?
Thank you.
Regards
IZ2ZNC Gianfranco

Neal Pollack 2021/02/07 10:34

Personally, after getting ripped off for $25 USD for what "looked" like a
good cal set on eBay,
which turned out to be PATHETIC TRASH, I just made my own. I took some
panel mount
SMA females like these:
https://www.ebay.com/itm/10x-SMA-Female-Jack-Chassis-4Hole-Panel-Mount-Post-Terminal-RF-Coax-Connector/333654105073

then with three of them, shaved off the backside pin and insulator flush
with the mounting surface, using a dremel cutting tool.
Next, I left one open, I shorted another with a large ball of solder, and
on the third, I layed down two 100 ohm surface mount
resistors going from the center pin to the panel mounting flange. It is
easy to solder all over these connectors.
The resulting standards have a perfectly equal measurement plane and
perform VERY well. Unlike the crap on ebay that has
a load resistor with longer leads and a longer center pin, an open with no
center pin at all, and a short that is yet a 3rd but
different length, resulting in the reference plane and inductance moving
all over the place.

Make your own. To get a set as good as the above costs at least $100 USD
or more, if purchased new.
You can also get these same panel mount connectors with male connectors, so
you can have both male
and female calibration standards with a precision reference plane.

Neal


On Sun, Feb 7, 2021 at 2:00 AM Stephen Laurence <Gaslaurence@gmail.com>
wrote:

TomC 2021/02/09 09:31

How did you identify the "pathetic trash" cal standards? Visually, or from
performance?

With the female panel mount jacks that you used did you then connect them with a
male to male SMA adapter?

Tom

On 2/7/2021 10:34 AM, Neal Pollack wrote:

Neal Pollack 2021/02/09 21:16

Regarding: "How did I identify the "pathetic trash" cal standards?
Visually, or from
performance?

REF:
https://www.ebay.com/itm/3-5mm-SMA-J-SMA-M-Connetor-Network-Meter-Calibration-Kit-SMA-Calibration-DC-6GHz/254582825337
Look at the length of the cylinder above the connector. I tore it open.
At the time I bought it, it was $25 from a different vendor, but identical
look.
Inside that long cylinder was a standard 1/8 watt resistor with long
leads. The "open" was open right above the pin. The "load" had much
longer
wire leads on the resistor, which skewed the reference plane. When each
was tried, on the smith chart, they were not on the horizontal line, like
a good quality 50 ohm load would be. It had enough inductance to move it a
1/4 inch off the horizontal line. Garbage!

By comparison, the method I described for making my own, put the reference
plane for all 3 (open, short, load) at precisely the same position,
with the same inductance, and was FAR more accurate than anything I could
afford to buy. Total cost: $5 USD in parts, maybe another $20 in time.

Regarding: "With the female panel mount jacks that you used did you then
connect them with a
male to male SMA adapter?"

No, I don't like to have any unneeded adaptors when taking VNA
measurements, unless I have to.
So I made a cal kit with the female panel mounts, and it was so easy, I
then made another set with
male panel mounts. But I use the female panel mounts much more.
Explanation: You always want to calibrate a VNA right up to the point
(plane) where you will connect
your DUT (Device Under Test). So, for example, if I am testing a SMA
bandpass filter, it will typically have
female (threads exposed) connectors mounted on it (just like the nanoVNA
does. A standard SMA patch
cable with male connectors (larger rotating shell with threads on the
inside) is used to hook up to it. Well, you
want to calibrate right at the end of those Male to Male SMA patch cables,
so that the cables are not considered
in the display, only the device you are testing. So you hook your
calibration standards onto the end of those
patch cables. Well, if the SMA patch cable is a male connector, then your
calibration standards would be
female panel mounts (just like what you have on the nanoVNA box).

Does that help a little?

Neal



On Tue, Feb 9, 2021 at 9:32 AM TomC <tomc@viclink.com> wrote:

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