PopularFX
Home Help Search Login Register
Welcome,Guest. Please login or register.
2021-01-18, 14:29:52
News: If you have a suggestion or need for a new board title, please PM the Admins.

Pages: 1 ... 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 [24] 25 26 27 28
Author Topic: Smudge proposed NMR experiment replication.  (Read 20666 times)

Group: Moderator
Hero Member
*****

Posts: 2490

Have put off "validate data" in the saver programm and now it continues.

Below is the same layout data as with the nylon spacers output shown a few posts above  (serial measurement):




Itsu
   

Group: Moderator
Hero Member
*****

Posts: 2490

Below screenshot is with the H-Field probe.

Now i see a strong 5Mhz peak
   

Group: Professor
Hero Member
*****

Posts: 1913
Below screenshot is with the H-Field probe.
Now i see a strong 5Mhz peak
Yeah!, I see it too. It bothers me because it means that for some reason the H-field has over 7x higher amplitude at 5MHz compared to 1MHz - that's 50x more power, where we are expecting less power !
According to MPTT, in order to obtain maximum power transfer to a reactive load, the reactance of the source should be of equal magnitude but opposite sign. ..but the VNA's transmitter source impedance is 50Ω+0j, isn't it ?

There wasn't anything unusual going on at 5MHz when you were doing the purely electric series measurement before.
What was the H-probes's orientation and position when you acquired that signal ?

Does the H-probe have a flat response according to the VNA?
You can check it by clamping the i-Probe on a piece* of Litz wire, which shorts the VNA's transmitter.

* as short as possible !
   

Group: Professor
Hero Member
*****

Posts: 1913
Have put off "validate data" in the saver program and now it continues.
So you turned off the error detection and are ignoring the data transmission errors.

Anyway, is the green graph the Real and the blue graph the Imaginary component on your S21 Real/Imaginary plot ?
   

Group: Moderator
Hero Member
*****

Posts: 2490
Quote
Yeah!, I see it too. It bothers me because it means that for some reason the H-field has over 7x higher amplitude at 5MHz compared to 1MHz - that's 50x more power, where we are expecting less power !
According to MPTT, in order to obtain maximum power transfer to a reactive load, the reactance of the source should be of equal magnitude but opposite sign. ..but the VNA's transmitter source impedance is 50Ω+0j, isn't it ?

I guess the problem is the calibration on "thru" with the H-Field probe.
I again calibrated it in thru in the middle, but could it be that the H-field is distributed differently without the nylon?

Yes, the VNA transmitter suppose to be 50 Ohm.

 
Quote
There wasn't anything unusual going on at 5MHz when you were doing the purely electric series measurement before.
What was the H-probes's orientation and position when you acquired that signal ?

No, nothing unusual going on, no equipment other then the nano and PC was on.
H-probe was as in the last video at 2:23, see picture below.

Quote
Does the H-probe have a flat response according to the VNA?
You can check it by clamping the i-Probe on a piece* of Litz wire, which shorts the VNA's transmitter.

I will check again later today the "serial measurement", including the flat response of the VNA.


Quote
So you turned off the error detection and are ignoring the data transmission errors.

Yes, seems like it

Quote
Anyway, is the green graph the Real and the blue graph the Imaginary component on your S21 Real/Imaginary plot ?

Well, thats not clear from the Graph, as it does not specify, but looking at the nanoVNA itself, the blue is the real, the green the imaginary.

Itsu
   

Group: Moderator
Hero Member
*****

Posts: 2490

Quote
Does the H-probe have a flat response according to the VNA?
You can check it by clamping the i-Probe on a piece* of Litz wire, which shorts the VNA's transmitter.

Below graphs show the sweep from 10KHz to 100MHz, so fairly flat.


   

Group: Professor
Hero Member
*****

Posts: 1913
I guess the problem is the calibration on "thru" with the H-Field probe.
Yes, I have the same problem.
I just do the H-probe "thru" calibration by clamping the i-Probe on a very-very short bundle of 4 thick Litz wires, that short the VNA's transmitter through a custom N fixture.

If anyone knows a better way to convert a constant voltage amplitude signal to a constant amplitude H-field -  speak up.

   

Group: Professor
Hero Member
*****

Posts: 1913
Below graphs show the sweep from 10KHz to 100MHz, so fairly flat.
The │S21│ vertical scale is too coarse (1.25U / div.) to see any variations but the logarithmic scale shows <2dB variation up to 50MHz, so not bad.
   

Group: Moderator
Hero Member
*****

Posts: 2490
The │S21│ vertical scale is too coarse (1.25U / div.) to see any variations but the logarithmic scale shows <2dB variation up to 50MHz, so not bad.

I had to manually adjust the Data portion to get some resolution.
The funny thing is i had to use a comma as decimal point instead of the usual american period as that would "stick" to a max scale of 1  :D

Itsu
   

Group: Professor
Hero Member
*****

Posts: 1913
It's great to have that vertical scaling under control.
The i-Probe's response is still too flat to explain that H-field's amplitude peak at 5MHz.

Does the S21 Impedance in the purely electric Series Measurement have its complex component go to zero at 5MHz ?
   

Group: Moderator
Hero Member
*****

Posts: 2490
It's great to have that vertical scaling under control.
The i-Probe's response is still too flat to explain that H-field's amplitude peak at 5MHz.

Does the S21 Impedance in the purely electric Series Measurement have its complex component go to zero at 5MHz ?


Hmmm,  not sure what you mean by "the S21 Impedance", but there is nothing going to zero at 5Mhz in the impedance related graphs.

See below the possible S21 graps i can select (the bottom one is TDR), so only real and imaginary.
The possible 3 S11 inpedance graphs i have selected next to this S21 one are shown in the screenshot.

Its a freshly calibrated Series Measurement sweep, marker 1 (red) is at 5MHz.


Itsu
   

Group: Professor
Hero Member
*****

Posts: 1913
Hmmm,  not sure what you mean by "the S21 Impedance",
The "S11 R+jX (Ω)", is what I would call "S11 Impedance".  You have that plot displayed, but we don't need it in the Series Measurement.
What we need is "S21 R+jX (Ω)", which is what I would call "S21 Impedance".

"S21 R+jX (Ω)" plot is different from the "S21 Real/Imaginary" plot because the latter depicts a ratio of voltage amplitude at the receiver divided by voltage amplitude at the transmitter.
Notice, that the units of that ratio are V/V or just Unity ( often written as: U or mU or kU ), not (Ω) like for Impedance.
Internally, the VNA makes the same measurement for both, it just converts it to the real Ohms of the resistance (R) and imaginary Ohms of the reactance (jX), which are components of the complex impedance.
   

Group: Moderator
Hero Member
*****

Posts: 2490

Right,  thanks, 

so it seems the Saver program, does not have the option to show this "S21 R+jX (Ω)" impedance plot.

The nanoVNA itself has something called "resistance" and "reactance" for both S11 as S21.

The below screenshot shows them for S21, is that useable?
Nothing at 5Mhz either though.

Itsu
   

Group: Professor
Hero Member
*****

Posts: 1913
so it seems the Saver program, does not have the option to show this "S21 R+jX (Ω)" impedance plot.
Nooooo, it can show "S11 R+jX (Ω)" but not "S21 R+jX (Ω)" ?!!!!   WTF ?!!   If I were you, I would gripe about this lack of symmetry to the developers of this program.

The nanoVNA itself has something called "resistance" and "reactance" for both S11 as S21.
As any VNA should.

The below screenshot shows them for S21, is that usable?
Yes, it shows that the DUT acts capacitively below 10MHz and above 32MHz.  I find the former hard to believe*.
Between 10MHz and 32MHz it shows inductive behavior (positive reactance).
Also, the reactance increases monotonically from 0-32MHz - as it should.

When dealing with reactance [X] plots (phase plots, too), you should place the markers where it crosses the ZERO or the horizontal axis. On that plot it would be around 10MHz and 32MHz.

What values of X and R do you get when you zoom 0-10MHz and place the markers at 5MHz ?


* This is most likely due to the Series Measurement having a huge error for impedances around zero. The Shunt-Thru measurement has a smaller error at low impedances.  Only a splice of these two measurements would give you a complete and true plot of the DUT's impedance.
   

Group: Moderator
Hero Member
*****

Posts: 2490

Quote
Nooooo, it can show "S11 R+jX (Ω)" but not "S21 R+jX (Ω)" ?!!!!   WTF ?!!   If I were you, I would gripe about this lack of symmetry to the developers of this program.

There are 3 PC programms i can find dealing with the nanoVNA, but none is able to display that "S21 R+jX (Ω)" graph  :(

Quote
What values of X and R do you get when you zoom 0-10MHz and place the markers at 5MHz ?

See screenshot:
   

Group: Professor
Hero Member
*****

Posts: 1913
Yeah, it is highly unlikely that the reactance is negative (capacitive) at 5MHz with these 1mm air gaps and the real resistance is so close to 50Ω.

If the inductance of this coil is still 16µH after the removal of the nylon spacers, then its inductive reactance at 5MHz should be above +500Ω (because XL=2πfL).
Below 50Ω, the Shunt-Trough Measurement is more accurate. Above 50Ω the Series Measurement is more accurate. At 5 MHz we are expecting more than 500Ω.

We have a huge measurement error somewhere and the amplitude of the H-field at 5MHz supports it...
   

Group: Moderator
Hero Member
*****

Posts: 2490

I measured the coil still to be 16uH without the nylon spacers using my LCR meter.

New calibration and using still the "series measurement" shows the same above (10KHz-10Mhz) output, using the nanoVNA itself, so not the PC programm with data validate disabled.

Itsu
   

Group: Moderator
Hero Member
*****

Posts: 2490

Quote
Nooooo, it can show "S11 R+jX (Ω)" but not "S21 R+jX (Ω)" ?!!!!   WTF ?!!   If I were you, I would gripe about this lack of symmetry to the developers of this program.

When i posted the request for the S21 R+jX graph on the nanoVNA forum, this is the first response i got just now:


S21 is a measure of device transmission characteristics.  R+jX is an impedance measurement and that is why it can be displayed as a S11 option but not S21.

I supposed if you used an external bridge then S21 could be used to measure R+jX, but no program currently available for the NanoVNA has an external bridge option.


Itsu
   

Group: Professor
Hero Member
*****

Posts: 1913
S21 is a measure of device transmission characteristics.
Impedance also affects transmission when the DUT is in series.  See:
https://youtu.be/0JglLhrisOs

I supposed if you used an external bridge then S21 could be used to measure R+jX,
Yet the NanoVNA displays this plot.

Here is a proof from one of the big boys (Keysight) that Transmission Impedance (ZT) is a thing and it can be calculated ONLY from the S21 scattering and Z0 (system impedance. Usually 50Ω).
In other words: If you can measure S21 and know Z0, then you can calculate the S21 Impedance.
   
Group: Ambassador
Hero Member
*****

Posts: 3447
Please remove if inappropriate

Here a comment from fellow I have great appreciation for [many years]and has some thoughts/experience ? [good] on topic here.
Turbo
Quote
That last pic is good but to drive like that is critical in terms of timing even the connecting leads have to be cut to match half and quarter wavelengths and its a key element in separating and extracting and decoupling the in and out coming signals from each other without overloading the pre amp.
Both should be close to exact opposites as possible on the Smith chart.
end quote

PS
as mentioned I will refrain from cross posting !

from Mirror topic here
https://overunity.com/18502/nuclear-magnetic-resonance-nmr/msg550686/#new

   

Group: Professor
Hero Member
*****

Posts: 1913
That last pic is good but to drive like that is critical in terms of timing even the connecting leads have to be cut to match half and quarter wavelengths
Do you know which pic he is referring to ?

Anyway, he is correct about the lengths of the interconnects, but I will counter that with the statement, that the influences of these lengths are calibrated out by the VNA.

Both should be close to exact opposites as possible on the Smith chart.
He is correct that the maximum power transfer occurs when the impedance of the source and load are complex conjugates...but we need to know them first before we can calculate the conjugate and right now Itsu is getting bogus reactance (X) and resistance (R) values at the 5MHz.

The matter of the maximum power transfer is further complicated by the fact that in NMR we are NOT after the maximum power dissipation in the coil/antenna (as it would be in a Ham radio).
Instead, we are after the maximum H-field amplitude in the radial direction and that means that near fields and reactive currents count towards that goal.
« Last Edit: 2020-09-18, 20:07:00 by verpies »
   

Group: Professor
Hero Member
*****

Posts: 1913
Maybe one of the RF connectors is open (inside or outside of the VNA). An Ohmmeter might be able to detect it.
   

Group: Moderator
Hero Member
*****

Posts: 2490

I measured the paths from the nanoVNA up to the coil with a DMM and all connectors measure OK (0 Ohm through, infinity inner to outer).
The nanoVNA TX port measures 58 Ohm, the RX port 50 Ohm.

I used several different (length of) coax from nanoVNA to the coil, but the screenshot signals does not change from the last one shown above.

Itsu
   

Group: Professor
Hero Member
*****

Posts: 1913
I measured the paths from the nanoVNA up to the coil with a DMM and all connectors measure OK (0 Ohm through, infinity inner to outer).
The nanoVNA TX port measures 58 Ohm, the RX port 50 Ohm.
No easy answers then  >:(
I surmise that, when you measure between the center conductors of the two semi-rigid coax cables (the ends that plug into the VNA) then you get 0.5Ω.

How does it look when you solder a 510Ω SMD resistor in place of your coil ?
   

Group: Moderator
Hero Member
*****

Posts: 2490

A 510 Ohm smd resistor he!     I will take one from my stack of 510 Ohm smd resistors  :D

   
Pages: 1 ... 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 [24] 25 26 27 28
« previous next »


 

Home Help Search Login Register
Theme © PopularFX | Based on PFX Ideas! | Scripts from iScript4u 2021-01-18, 14:29:52