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I'm measuring the group delay of some 60in cables with a NanoVNAH4 and I am getting outliers in the data. I am using nano-vna saver and I have tried both with on board calibration disabled and calibrated from 50kHz to 1.5GHz, the outliers are present still. I also have another H4 and I had the same issue just at a lower frequency ~150MHz. In the attached plot the points go to about +- 20 ns at just under 700MHz. I have also changed the sweep to have a lower df width but the same outliers were there at the same frequency. My sweep here is 1Mhz to 701Mhz 1212 pts 3avg df=578kHz

Is there any explanation or way to minimize this? Or is the best path forward just to remove the outliers manually when processing the data.

Thanks!

On Mon, Jun 26, 2023 at 01:23 AM, Mike wrote:

This is my test fixture, calibrated at the croc clips. It's not ideal but I'm
limited by the length of the coil. Should be OK at my measurement frequency of
70kHz though!

Calibrating with the crocodile clips will not give you a good reference plane. The reason I say this is if you keep them the same distance apart when you cal with an open, short and load you will have considerable inductance in the short and 50 ohm "cal loads". If you calibrate with the clips close together and then spread them the reference plane has changed. Neither is a good option.

I suggest you calibrate right at the screw terminations on the green block with the alligator clip leads removed. Then attach the leads and make your measurement. From the photo it looks like the leads are about 4" long and each one will add about 100 nH of inductance (total 200 nH or 0.2 uH). That extra .2 uH when you are measuring 110 uH is not significant. However you should get a better estimate of the SRF and be able to calculate the parasitic capacitance to more accuracy.

Try it and see what you find...

Roger

On 6/26/23 7:26 AM, Fran?ois wrote:

I knew about the phone lines. My problem is that if we calculate the S11 with complex values, it leads, for example, to a ROS which can be negative. S11 is no longer in a circle of radius 1.
Is this normal or am I mistaken?

It's possible, that with some active systems, you could get a reflection that is bigger than the incident wave (i.e. if the Zload were negative). But I think that for an entirely passive load, the reflected wave cannot be greater than the incident wave.

My study (amateur) concerns the cascading of 2 adapters in 'L' one high pass and the other low pass to carry out two adaptations of impedance at two different frequencies.

To generalize the brought back impedance can be different for the two frequencies and complex.

It works easily with a purely real impedance but when the impedance is complex, I tried to optimize the ROS by looking for its minimum value which I thought was ONE it does not work because the ROS varies from - infinity to + infinity.

I had to change the search criteria to subtraction
double optimizer = Complex.Subtract(Zouth_, Application.Zcibleh).Magnitude;

And now it works. What is curious is that when the solution adapter is found, the ROS is indeed equal to ONE. A crazy story.
--
F1AMM
Fran?ois

I knew about the phone lines. My problem is that if we calculate the S11 with complex values, it leads, for example, to a ROS which can be negative. S11 is no longer in a circle of radius 1.

Is this normal or am I mistaken?
--
F1AMM
Fran?ois

On 6/26/23 06:32, Jim Lux wrote:

On 6/26/23 2:22 AM, Fran?ois wrote:

Hello

In the attached formula (ROS.png), as long as Zo is real, even if Zl is complex, everything is fine and S11 remains in a circle of radius 1. and the ROS is between 1 and infinity.

If Zo is complex, this is no longer the case.

While writing C# code looking for a (complex) adaptation by dichotomy, I was looking for a ROS of 1. It doesn't work. I had to use the modulus of the difference between my target and the current value.

double optimiseur = Complex.Subtract(Zouth_, Application.Zcibleh).Magnitude;

Is it legitimate to talk about S11 when Zo is complex?

73

Sure it is: S11 is just a representation of the reflection coefficient on port 1 of the UUT.? Generally people use just the log magnitude in dB, but it has a phase, too.
Nothing prevents the Z0 being complex.

Definitely so. When working with telephone cable pairs at voice frequencies, Zo is always complex, generally with a phase angle approaching -45 degrees. Various graphical aids were published by the Bell System for solving transmission line problems with complex Zo, but today it's more common to simply solve the appropriate expressions using complex math in software.

Various schemes were developed for "loading" cable pairs for voice transmission, the most common being the addition of inductors at intervals of a few thousand feet. The most common loading scheme in the Bell System added 88 mH every 6000 feet, but there were quite a few other loading schemes in use. Loading a pair made it's Zo close to, but not exactly, real across the frequency span of the load scheme, reduced the loss of the pair within that frequency span, and acted as a sharp cutoff low-pass filter, reducing significantly transmission at frequencies above the cutoff frequency.

73,

Maynard
W6PAP

On 6/26/23 2:22 AM, Fran?ois wrote:

Hello
In the attached formula (ROS.png), as long as Zo is real, even if Zl is complex, everything is fine and S11 remains in a circle of radius 1. and the ROS is between 1 and infinity.
If Zo is complex, this is no longer the case.
While writing C# code looking for a (complex) adaptation by dichotomy, I was looking for a ROS of 1. It doesn't work. I had to use the modulus of the difference between my target and the current value.
double optimiseur = Complex.Subtract(Zouth_, Application.Zcibleh).Magnitude;
Is it legitimate to talk about S11 when Zo is complex?
73

Sure it is: S11 is just a representation of the reflection coefficient on port 1 of the UUT. Generally people use just the log magnitude in dB, but it has a phase, too.

Nothing prevents the Z0 being complex.

Hello

In the attached formula (ROS.png), as long as Zo is real, even if Zl is complex, everything is fine and S11 remains in a circle of radius 1. and the ROS is between 1 and infinity.

If Zo is complex, this is no longer the case.

While writing C# code looking for a (complex) adaptation by dichotomy, I was looking for a ROS of 1. It doesn't work. I had to use the modulus of the difference between my target and the current value.

double optimiseur = Complex.Subtract(Zouth_, Application.Zcibleh).Magnitude;

Is it legitimate to talk about S11 when Zo is complex?

73
--
F1AMM
Fran?ois

This is my test fixture, calibrated at the croc clips. It's not ideal but I'm limited by the length of the coil. Should be OK at my measurement frequency of 70kHz though!

--
Mike

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On Sun, Jun 25, 2023 at 03:15 PM, Roger Need wrote:

The VNA can only measure R + jX or R//jX (with later firmware versions). It
calculates inductance by dividing X by 2*pi*frequency and this ONLY gives an
estimate of the true coil L if the coil is air wound and the frequency is low
enough that the skin effect is not having much effect on underlying
inductance.

Roger, the method I suggested requires inductance calculated this way. It yields an accurate coil model over a narrow frequency range. To my surprise, it seemed good enough over the whole 3.5-4 MHz band. But the wideband model suggested in the writeup noticeably improved accuracy over the somewhat wider 88-108 MHz band.

Brian

On Sun, Jun 25, 2023 at 11:04 PM, Roger Need wrote:

On Sun, Jun 25, 2023 at 02:27 PM, Mike wrote:

Thank you Roger. The coil is 95 turns of 0.9mm enamelled copper wire close
wound on a 36mm PVC former.

Mike,

Attached is an analysis of your coil using Coil64. The numbers are close to
what you measured. For an air wound coil the actual L will not vary much in
the frequency range of up to 12 MHz. Note the following:

--> ESR is increasing with frequency and simulation shows .299 ohms at DC,
.422 at 1 MHz. and 9.233 ohms at 5 MHz.
--> Self capacitance is calculated at 1.61 pF which is very small. You
estimated 2 pF based on your SRF measurement. Any stray capacitance in your
test setup will significantly affect your self resonant frequency so you need
a good test jig if this is an area of concern.

Note: For air wound coils assuming that apparent inductance at low frequencies
is equal to actual inductance L at higher frequencies is a reasonable
approximation. Therefore the method of calculating parasitic capacitance
based on using this value of L and the SRF to calculate parasitic capacitance
gives a decent estimate. BUT this method does not work if the inductor is a
ferrite core design.

Roger

Perfect! Thanks for that explanation Roger.

--
Mike

On Sun, Jun 25, 2023 at 02:55 PM, Brian Beezley wrote:

Mike, measure the inductance and series resistance at the model frequency. I
believe recent VNA firmware versions can provide these values directly.
Otherwise calculate them from R and X.

The VNA can only measure R + jX or R//jX (with later firmware versions). It calculates inductance by dividing X by 2*pi*frequency and this ONLY gives an estimate of the true coil L if the coil is air wound and the frequency is low enough that the skin effect is not having much effect on underlying inductance. If the coil is wound on a ferrite core you can't use this method to estimate L at higher frequencies.

Roger

On Sun, Jun 25, 2023 at 02:27 PM, Mike wrote:

Thank you Roger. The coil is 95 turns of 0.9mm enamelled copper wire close
wound on a 36mm PVC former.

Mike,

Attached is an analysis of your coil using Coil64. The numbers are close to what you measured. For an air wound coil the actual L will not vary much in the frequency range of up to 12 MHz. Note the following:

--> ESR is increasing with frequency and simulation shows .299 ohms at DC, .422 at 1 MHz. and 9.233 ohms at 5 MHz.
--> Self capacitance is calculated at 1.61 pF which is very small. You estimated 2 pF based on your SRF measurement. Any stray capacitance in your test setup will significantly affect your self resonant frequency so you need a good test jig if this is an area of concern.

Note: For air wound coils assuming that apparent inductance at low frequencies is equal to actual inductance L at higher frequencies is a reasonable approximation. Therefore the method of calculating parasitic capacitance based on using this value of L and the SRF to calculate parasitic capacitance gives a decent estimate. BUT this method does not work if the inductor is a ferrite core design.

Roger

On Sun, Jun 25, 2023 at 10:55 PM, Brian Beezley wrote:

Mike, measure the inductance and series resistance at the model frequency. I
believe recent VNA firmware versions can provide these values directly.
Otherwise calculate them from R and X. Then create a simple load with the
inductance and resistance in series. This works fine over a single ham band.
To create a wideband model, see this:



Brian

Thanks for the link Brian.

--
Mike

Mike, measure the inductance and series resistance at the model frequency. I believe recent VNA firmware versions can provide these values directly. Otherwise calculate them from R and X. Then create a simple load with the inductance and resistance in series. This works fine over a single ham band. To create a wideband model, see this:



Brian

On Sun, Jun 25, 2023 at 10:25 PM, Jim Lux wrote:

ESR is the ohmic resistance *at RF* which will be higher than the DC
resistance (skin effect).

What you should be able to do is measure the Z (both X and R) far away
from self resonance, and get a rough estimate.

Thank you Jim.

--
Mike

Thank you Roger. The coil is 95 turns of 0.9mm enamelled copper wire close wound on a 36mm PVC former.

--
Mike

On 6/25/23 1:26 PM, Mike wrote:

I have wound a 110uH coil for an antenna system and I want to create a model of the inductor that I can use in a simulation program. In other words, I need to know the inductance, parasitic capacitance and ESR.
Using my NanoVNA-H4 I measured the inductance on a Smith chart at a low frequency (around 70kHz) where the reactance is about 50 ohms. I then measured the self resonant frequency (10.5MHz) and calculated the parasitic capacitance as 2pF.
Is that correct so far?
What about ESR? Is that the ohmic resistance of the coil or is it more complicated than that?
Thanks!

ESR is the ohmic resistance *at RF* which will be higher than the DC resistance (skin effect).

What you should be able to do is measure the Z (both X and R) far away from self resonance, and get a rough estimate.

On Sun, Jun 25, 2023 at 01:26 PM, Mike wrote:

I have wound a 110uH coil for an antenna system and I want to create a model
of the inductor that I can use in a simulation program. In other words, I need
to know the inductance, parasitic capacitance and ESR.

Using my NanoVNA-H4 I measured the inductance on a Smith chart at a low
frequency (around 70kHz) where the reactance is about 50 ohms. I then measured
the self resonant frequency (10.5MHz) and calculated the parasitic capacitance
as 2pF.

Is that correct so far?

What about ESR? Is that the ohmic resistance of the coil or is it more
complicated than that?

Yes it is more complicated than that. The inductance will vary with frequency and so will the ESR. In the case of an air wound coil the underlying inductance will slightly change as you increase frequency due to the "skin effect" which forces current to the outer perimeter of the conductor. If the coil is wound on a powdered iron or ferrite core there will be considerable change of inductance with frequency due to the permeability decreasing with frequency. The ESR will increase with frequency due to core losses and the skin effect which increases the RF resistance of the coil windings.

When you try to measure the inductance of an inductor using a VNA the firmware or PC application will calculate the "apparent inductance" by simpling dividing the measured reactance by 2*pi*frequency. This is not the same as the actual inductance L. The reason is that the parasitic capacitance is in parallel with the inductor and you now have capacitor reactance in parallel with the inductor reactance which results in a higher reactance than that of the inductor alone. This is shown in the attached diagram.

So if you tell us what type of inductor you are measuring (air wound, powdered iron or ferrite) more specific information can be provided.

Roger