Hi Alan,
I just want to point out that the polynomial + delay + impedance method for
characterising standards is itself just an approximation.
In this day and age (of cheap storage) it is simpler to just sweep the
standards on a calibrated VNA, get an s-param file for each one and use
that data in the calcs - with some interpolation for the in-between
frequencies.
This gives the highest accuracy as there is no modelling error.
It also means you can use any old standard you like. The polynomial models
don't fit poorly made standards very well over a wide frequency range,
hence the need for high quality expensive parts.
However I'm not sure if this device has the storage available to do it this
way, if not the old school approach might be the best way.
Regards,
Roger
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On Tue, 30 Jul 2019 at 08:49, alan victor <avictor73@...> wrote:
Thanks! That is helpful. The fact that the polynomial code to describe the
open is present implies he was aware of the situation and at this point
elected to put it aside. So for all intent and purpose, the loads are
assumed to be ideal and that tends to agree with the final sweep to check
the loads after cal. The check returns them to be IDEAL... And they are
anything but ideal as that is the electrical-physical nature of a SMA
connector.
Alan
________________________________
From: [email protected] <[email protected]> on behalf of
Roger Henderson <hendorog@...>
Sent: Monday, July 29, 2019 8:32 PM
To: [email protected] <[email protected]>
Subject: Re: [nanovna-users] Correct button?
I checked the code repo's posted and this is what I think the situation is
with the standards:
The load is assumed to be perfect - i.e. the assumption is that nothing is
reflected from the load.
The open is assumed to have a single C0 capacitance term of 50e-15F
The short is either assumed to be perfect, or is assumed to be 180degrees
away from the open. I am not quite sure on that.
There is some unused code which looks to be intended to create a
C0,C1,C2,C3 model of the open, but it is not used.
Roger
On Mon, 29 Jul 2019 at 14:37, alan victor <avictor73@...> wrote:
You are correct and I confirmed on the big box, that is the function
performed.
This turns CAL ON or OFF. Again, I suspect hp as well others, added this
function as an aid to be sure the correction data sets make sense as a
corrective measurement is conducted on a component. Its much easier to do
this then preset the instrument and have to do a complete re calibration.
Alan
________________________________
From: [email protected] <[email protected]> on behalf of
alan
victor <avictor73@...>
Sent: Monday, July 29, 2019 1:20 AM
To: [email protected] <[email protected]>
Subject: Re: [nanovna-users] Correct button?
Good suggestion and it does appear to toggle in that manner.
Now, I have to think why I would toggle corrections ON and OFF, other
than
to see if the corrections ON make sense.
I am not sure this is a good place to bring up the fact that the
STANDARDS
provided with the nanoVNA are taken, I believe, by the device and the
firmware as IDEAL. Point in fact, they are NOT. Not an issue at lower
frequencies unless you want to split hairs. But as you approach 900 MHz,
bottom line, when measuring a component, you are not going to get the
correct answer.
I emailed the developer on this topic but as of now, no response.
Alan
________________________________
From: [email protected] <[email protected]> on behalf of
Roger Henderson <hendorog@...>
Sent: Monday, July 29, 2019 1:10 AM
To: [email protected] <[email protected]>
Subject: Re: [nanovna-users] Correct button?
I would guess it is a mode switch between: 'Apply calibration
corrections'
and 'Don't apply calibration corrections'
This feature is present in other VNA's. Am not in front of a device to
confirm it though.
Regards,
Roger
On Mon, 29 Jul 2019 at 13:06, alan victor <avictor73@...> wrote:
Good question Ernst. Let me take a look and see if I can figure it
out. I
have a guess that it may have to do with the fact that the standards,
open,
short, load are NOT perfect. But... the softkey selection does not lead
to
additional menu selection. So far now, it appears to be a PLACE HOLDER
for
further work.
Alan
________________________________
From: [email protected] <[email protected]> on behalf of
dk1vi <dk1vi@...>
Sent: Sunday, July 28, 2019 11:44 PM
To: [email protected] <[email protected]>
Subject: [nanovna-users] Correct button?
Hi,
what is the purpose of the "CORRECT" button in the calibration menu?
I have not read anything about it or did I miss something?
Ernst
