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On 7/15/21 5:54 AM, Richard Clemens wrote:

Given my limited work with a NanoVNA I ask:

It appears the calibration routine (so far I have only needed for one port) is done without regard to any parameters being set.
Once it is done and saved you can reload it later and use it again.

Questions:
When would be a case that requires a new calibration vs just reloading one that has been saved?
Is it better to do a new calibration each time you fire up the nano rather than used a saved one?
What makes a calibration worth saving? ie. how should they be labeled?

Reasons to recalibrate (as opposed to use a saved cal set):

1) A different test setup, where the "reference plane" for the measurements is different.? For instance, if you have test port cables with different length, and you do the cal at the end of the cables.

2) If the frequency span of the measurements is different than the frequency span of the calibration.? The NanoVNA does some interpolation, but if you did a cal from 0-50 MHz and now you're measuring a DUT from 200-300 MHz, the measurement might be iffy. Likewise if you did a cal from 0-900 MHz, and the measurement is from 20-30 MHz.

3) You can't remember what you did for a cal before

4) The temperature or something else has changed significantly. I don't know how temperature sensitive the NanoVNA is, but things like the bridges and detectors will change somewhat with temperature.? The oscillator frequency also changes, although I would expect that to be a 10s of ppm sort of thing, so unless you're measuring crystals or 2kHz wide filters at 30 MHz, you probably wouldn't see it.

Think of the NanoVNA as being somewhat like a RF Ohmmeter.? When you turn on the ohmmeter you check to see that it reads infinity with the leads apart, and zero with the leads shorted.? Same sort of thing.? Check to see that it reads appropriately with open and load, and if it looks ok, it probably is.? (If you store it with the load on CH0, then when you turn it on, you'll see the good match, and then when you disconnect the load to connect the DUT, you'll see the reflection match, so that's a easy way to check.

Given my limited work with a NanoVNA I ask:

It appears the calibration routine (so far I have only needed for one port) is done without regard to any parameters being set.
Once it is done and saved you can reload it later and use it again.

Questions:
When would be a case that requires a new calibration vs just reloading one that has been saved?
Is it better to do a new calibration each time you fire up the nano rather than used a saved one?
What makes a calibration worth saving? ie. how should they be labeled?

TNX

How does this differ from "nanovna-saver" and is it better or just different. I find the <saver> to be extremely useful and helpful.

On Wed, Jul 14, 2021 at 07:17 PM, Anne Ranch wrote:

How about putting horse before cart and give link to the software before
telling how to use it ?

I would assume it can be found at the same place you got the version you are using now (if you are already using SimSmith). Try here:



Scroll down to download.

On 15/07/2021 06:49, David J Taylor via groups.io wrote:

Many thanks for that! Having fun getting it on my iPad for ChromeCasting to
the TV.
I see Part 2 there, but not Part 1. Am I missing something?

Fixed - file name was confusing!

David
--
SatSignal Software - Quality software for you
Web:
Email: david-taylor@...
Twitter: @gm8arv

On 14/07/2021 19:39, Barry K3EUI wrote:

This is the most recent video on my presentation:
Nano VNA Part I The Basics (not much math)
Nano VNA Part II The Under the Hood Approach (Advanced topics)
I'll also include the latest PDF of the slides - enough for a few hours of discussion
The video listed below was recorded a few days ago by the Leicester Radio Society (UK)
TU G8PGO David Carter.
I'm learning as I am playing, so here are the latest two videos
The Part I is just the "basics" without any of the math. That video was also recorded.
If you spot any errors on either video please let me know. This is rather technical stuff.
The ending on both presentations was a bit abrupt since I ran out of time.
!AtLJBaL57qLbg5sz6VcyxJdPeQaCrw?e=RkNpYx
have fun watching
de k3eui Barry

Barry,

Many thanks for that! Having fun getting it on my iPad for ChromeCasting to the TV.

I see Part 2 there, but not Part 1. Am I missing something?

73,
David GM8ARV
--
SatSignal Software - Quality software for you
Web:
Email: david-taylor@...
Twitter: @gm8arv

This is really great. Thank you

Martin
N6QLH

How about putting horse before cart and give link to the software before telling how to use it ?

The link that Barry posted is not complete. Try this one




--
*Don - W3DRM*

When I click on the link it goes to outlook live.

Charles Miller

For some reason I’m having trouble downloading your updated files.

Charles Miller

?A new utility has been introduced in SimSmuth.

This utility can be used to connect to, calibrate, initiate measurements, write touchstone files, and import them for further analysis.

A quick overview is here



Ward
Ae6ty

On 7/14/21 12:02 PM, WB2UAQ wrote:

This is a long thread so this might have been mentioned already. I bet I am way out of line. Sorry.

Why can't the measurement be made with a 50 ohm network analyzer? We're just looking at impedance so it can easily be converted to SWR or other formats mathematically referencing 75 ohms. Maybe this is what the formulas show above are doing?

It is not easy to fabricate a 75 ohm OSL cal kit using discrete components. If this is an HF app maybe.

Yes, you could do it with a 50 ohm analyzer and mathematically convert.

I'm not sure a 75 ohm OSL kit would be super difficult. You'd have to choose a 75 ohm connector family (i.e. BNC, TNC, F, I don't know what others are available, probably there's a 75 ohm N) and then do the open and short and load - probably use a pair of 150 ohm or a quad of 300 ohm SMT resistors.? You could probably *buy* a 75 ohm termination that would be "good enough" (e.g. video systems and cable TV use them by the gazillion- surely there are higher and lower quality available).


I don't know of any 75 ohm systems above L-band (CATV and DBS satellite LNB outputs).

This is a long thread so this might have been mentioned already. I bet I am way out of line. Sorry.

Why can't the measurement be made with a 50 ohm network analyzer? We're just looking at impedance so it can easily be converted to SWR or other formats mathematically referencing 75 ohms. Maybe this is what the formulas show above are doing?

It is not easy to fabricate a 75 ohm OSL cal kit using discrete components. If this is an HF app maybe.

This is the most recent video on my presentation:
Nano VNA Part I The Basics (not much math)
Nano VNA Part II The Under the Hood Approach (Advanced topics)

I'll also include the latest PDF of the slides - enough for a few hours of discussion

The video listed below was recorded a few days ago by the Leicester Radio Society (UK)
TU G8PGO David Carter.

I'm learning as I am playing, so here are the latest two videos
The Part I is just the "basics" without any of the math. That video was also recorded.
If you spot any errors on either video please let me know. This is rather technical stuff.
The ending on both presentations was a bit abrupt since I ran out of time.


!AtLJBaL57qLbg5sz6VcyxJdPeQaCrw?e=RkNpYx

have fun watching
de k3eui Barry
k3euibarry@...

I test ojisankoubou Z normalization code, adaptate it to NanoVNA

Calibrate NanoVNA by 50 Om load
Measure 25 Om (SWR = 2 and dot at left smith)

After add DUT impedance as 25 Om and calculate how it should look
And see SWR = 1.0 and dot at center of smith

Screenshot from my H4

And quite a number of special applications use something entirely different
than either 50 or 75-ohms, not to mention 30-Ohms. Take the 'flash' lasers
at Los Alamos and Phillips lab, to name a couple I'm a bit familiar with.
They use slab lines to establish a low impedance in the vicinity of 2-Ohms,
or less. Why? That's the easiest way they can obtain the fast fise times
at full power.

Dave - W?LEV

On Wed, Jul 14, 2021 at 3:25 AM Jim Allyn - N7JA <jim@...>
wrote:

Actually, there is a good reason for 75 ohm coax. If you plot cable loss
versus impedance, you will find that minimum loss occurs at 75 ohms. When
you've got miles and miles of coax strung everywhere, as cable TV companies
do, you want to minimize loss to reduce the number of booster amplifiers
needed. In addition, since 75 ohm coax has lower capacitance per unit
length, it has better high frequency response than 50 ohm cables. Best
power handling ability occurs at 30 ohms impedance. I don't know that any
industry uses a 30 ohm system impedance, but - 50 ohms is a good compromise
between minimum loss at 75 ohms and best power handling at 30 ohms. (All
this is the best I remember it, there might be a bit more to it that I have
forgotten over the years.)

--
*Dave - W?LEV*
*Just Let Darwin Work*

On 7/14/21 12:25 AM, Uwe Lange wrote:

On Tue, Jul 13, 2021 at 03:27 PM, David Eckhardt wrote:

So what if the "bull's eye" on the Smith Chart is not appropriately placed
in the center and appears to the right of center halfway between the center
and where the 100-ohm circle intersects the real axis? I know that is
75-ohms. So, what is the problem??

There are two issues:
First the one from OP: he built at 50/75ohm pad and calibrated the NanoVNA with
a 75ohm load. The pad transforms every impedance on its 75ohms side into the
50ohms world of the NanoVNA and the reported impedances are then off which
confuses the OP.

A resistive pad does NOT transform from a reference impedance of 75 ohms to a reference impedance of 50 ohms.? A transformer would (over some frequency range).

All a resistive pad (of whatever loss) does is provide a 50 ohm impedance on port A when port B is terminated in 75 ohms, and provide a 75 ohm impedance, when port A is terminated in 50 ohms. It does not, for instance, provide a 100 ohm impedance when Port B is terminated in 150 ohms.

There are several solutions to this: The probably easiest is to not use the pad
in the first place and calibrate with a 50ohm load. This would also avoid the
5.6dB loss of dynamic range incurred by the pad.

Right, and then you can just change the way the smith chart displays and VSWR, etc. are computed so they turn the 50 ohm measurement into some other reference impedance.

Another option is to keep the pad and calibrate with a 75ohm load but *inform*
the NanoVNA that the cal load was 75ohm. The NanoVNA then has a chance to
display correct impedance values. My patch to NanoVNAsaver achieves this.

Sure, you could do this.? But why not just calibrate with a 75 ohm load, leaving out the pad (which in this case would reduce dynamic range).

And then, you'd need to change some other code to make sure that displays and the calibration use the 75 ohm reference (and calibration) impedance.

Here is a good explanation






On Wed, Jul 14, 2021 at 5:25 AM Jim Allyn - N7JA <jim@...>
wrote:

Actually, there is a good reason for 75 ohm coax. If you plot cable loss
versus impedance, you will find that minimum loss occurs at 75 ohms. When
you've got miles and miles of coax strung everywhere, as cable TV companies
do, you want to minimize loss to reduce the number of booster amplifiers
needed. In addition, since 75 ohm coax has lower capacitance per unit
length, it has better high frequency response than 50 ohm cables. Best
power handling ability occurs at 30 ohms impedance. I don't know that any
industry uses a 30 ohm system impedance, but - 50 ohms is a good compromise
between minimum loss at 75 ohms and best power handling at 30 ohms. (All
this is the best I remember it, there might be a bit more to it that I have
forgotten over the years.)





--

Kenneth Hansen
Owner
ScanAm Technical Services, LLC

Actually, there is a good reason for 75 ohm coax. If you plot cable loss versus impedance, you will find that minimum loss occurs at 75 ohms. When you've got miles and miles of coax strung everywhere, as cable TV companies do, you want to minimize loss to reduce the number of booster amplifiers needed. In addition, since 75 ohm coax has lower capacitance per unit length, it has better high frequency response than 50 ohm cables. Best power handling ability occurs at 30 ohms impedance. I don't know that any industry uses a 30 ohm system impedance, but - 50 ohms is a good compromise between minimum loss at 75 ohms and best power handling at 30 ohms. (All this is the best I remember it, there might be a bit more to it that I have forgotten over the years.)