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Re: Basic Output Definitions
Andy,
Here is a link to an excellent paper by Olivier Pilloud HB9CEM which describes the relationship between the reflection coefficient Gamma (¦£). complex impedance (R+jX), Return Loss and VSWR. He also shows how the rectangular plot, polar plot and Smith chart are overlaid. He finishes up with an example and plots the results. IHope it helps you - Roger |
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Re: Basic Output Definitions
Andy,
Below is a copy /paste of my answer to another old thread. It could be of interest for your question 3. REAL and IMAG parameters are not the samething as RESISTANCE and REACTANCE parameters. REAL and IMAG apply to reflection coefficient ¦£, in its complex form (a+j.b). That's why values are always in the [-1,1] interval, without any associated unit. When REAL=-1 and IMAG=0, it is the Short circuit situation. When REAL=1 and IMAG=0, it is the Open circuit situation. When REAL=0 and IMAG=0, it is the normal Loaded (50 ohms) situation. LINEAR is the ¦£ modulus form of combined REAL and IMAG values, and finally POLAR is the geometric representation of REAL, IMAG and PHASE values. When POLAR is displayed by the NanoVNA, and even if data values are exactly displayed as for Smith Chart, results must not be read in the same way. Have a try by displaying two CH0 traces, POLAR and SMITH. SWR and LOGMAG (Return Loss) are derivated from ¦£ modulus (LINEAR). For educational purposes I have created an ods file (see below), showing and calculating NanoVNA parameters. You can play with it by entering values in the blue fields, and also checking what are the arithmetic relations behind the different results. Here Group Delay is not relevant as calculations are done for a discrete (CW) frequency. A last word about the use of REAL and IMAG parameters. The following case (see attachment) is an opened coaxial cable (length 2 meters), creating a quarter wave stub (at red marker). An opened coaxial cable remains a good use case for education and increase of knowledge. On the NanoVNAsaver snapshot we see clearly that displayed values between RESISTANCE/REACTANCE and REAL/IMAG do not allow an immediate comparison. For example at red marker, R+jX or Smith Chart highlight a value of 0+j0 ohms (short circuit situation), and checking this with REAL/IMAG chart you get -1 (REAL) and 0 (IMAG) which is the same thing. Be careful with REAL and IMAG curves which follow sinus and cosinus rules, it reflects simply a monotonous variation of PHASE. REAL is also interesting if you want to measure a coaxial cable length, thanks to advanced TDR function. 73 from Jean-Roger / F6EGK |
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Re: Nano VNA Classic/H upgrade
#firmware
The group has a very well built wiki. Please read it there as well. If this seems like a lot at first reading because your problem is hard to put on any description, set it aside for a while, read it again, and understand what you need to do. It's not such a big deal, believe me.
You should not believe all the answers of your computer regarding the driver. Think and act. When I first installed the DefuSe Demo, I also installed the driver and later there was never an issue with the lack of a driver. Yes, in old age, the human brain has a hard time understanding what others are doing out of routine at a young age. Maybe that¡¯s why they¡¯re looking for template descriptions who run into errors. Each operating system uses a file manager. This file manager should be used to find the required drivers at the DefuSe Demo installation site. Not only does a ¡°good priest learn to his death,¡± we also need to continually learn in an ever-changing world to understand and follow change. Gyula HA3HZ -- *** If you are not part of the solution, then you are the problem. ( ) *** |
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Re: Basic Output Definitions
1. ¡°Phase is the relative phase difference between source and received
signals.¡± What does this really mean? Is it current to voltage phase, or change in that as influenced by the DUT, or simply phase change of voltage alone, or¡? The phase represents the voltage/current offset produced by the DUT (Device Under Test) as measured by the NANOVNA. Generally, voltage is 180-degrees out of phase with current - high current ===> low voltage and low current ===> high voltage. So the two are 180-degrees, or ¦Ð-radians, out of phase with each other. The 180-degree phase difference between current and voltage applies strictly to resistive loads with no reactance - the ¡À jX term in the complete impedance expressioin: Z = R ¡À jX. Z = total impdeance, R = resistive term, ¡À jX = reactive term. Introduction of a complex portion of the impedance measurement skews the voltage/current phase relationship to something other than the resistive 180-degree offset. [image: image.png] 2. ¡°Polar is same as Smith chart but with complex reactance rather than L or C shown.¡± Yes, the trace is identical to Smith, but why are the real and imaginary numbers reported nowheres near the R and X reported by Resistance and Reactance outputs? The following two charts from a course I once taught express the mapping between ¡À jX and their equivalent lumped equiivalent form. As noted above and below, they are both frequency dependent, can not dissipate power by themselves (in the ideal model), and can be associated with external fields. The resistance term of the impedance expression is frequency independent, can and does dissipate power, and is not associated with external fields. Yet another difference: 1) resistance can not modify or change phase relationships between current and voltage, and 2) reactance indeed can and does modify or change the phase relationships between current and voltage. [image: image.png] [image: image.png] 3. Likewise, Real and Imaginary have no similarity to Resistance and Reactance; shouldn¡¯t they be the same? Absolutely not the same !!! Consider the above two charts. They should make it clear why resistance and reactance are not the same. iI've also pointed out the difference between resistance and reactance - the ¡À jX term of the total impedance expression. 4. Everyone says that the VNA reports S11 and S21; I get that these are reflection and transmission, but what are the units (or dimensions)? Is it voltage, or power, or current¡? S-Parameters in their pure form are vector power measurements. There are a number of ways they can be reported. For S11: 1) a simple numerical ratio (in its simplest form) of input vs reflected powers, 2) that numerical ratio expressed in dB, 3) numerical ratio of the reflection coefficient, and 4) numerical ratio of the reflection coefficient expressen in dB. dB is all base 10 as we are dealing with power. For S22: 1) a simple numerical ratio of input power vs. transmitted power, 2) that numerical ratio expressed in dB. Note that S22 as measured on the "output" side of the DUT is the equivalent of S11 on the "input" side of the DUT, both with the opposite port properly terminated. The transmission measurement for impedance, S21, really has no meaning (no Smith Chart applicability). The measurement for the output port of impedance is S22. Dave - W?LEV On Thu, Sep 9, 2021 at 3:11 PM Andrew Kurtz via groups.io <adkurtz= [email protected]> wrote: I have been through the Wiki and got some answers, but here are some-- *Dave - W?LEV* *Just Let Darwin Work* |
Roger Need
Gyula Molnar
