Network theory: travelling waves vs power waves

[pancho_hideboo]

For example when I'm seeing a talk on RFPA design and someone suggests that reflection coefficient Γ at the gate of a FET should be zero,
and shows the equation Γ=(Zs-Zin)/(Zs+Zin), they're just wrong.

See https://www.designers-guide.org/Forum/YaBB.pl?num=1266403928/#10

Gamma_V=(Zs-Zin)/(Zs+Zin)
Gammp_P=(Zs-conj(Zin))/(Zs+Zin)

Gamma_V=0 means non-distortion matching.
Gamma_P=0 means power matching(=conjugate matching) condition, however output waveform is distorted unless input signal is single frequency sinusoidal.

 

[volker@muehlhaus]

@mtwieg, do you speak German? I would have an RF textbook chapter on that reflection factor at conjugate match topic.

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@mtwieg, do you speak German? I would have an RF textbook chapter on that reflection factor at conjugate match topic.

From Frank Gustrau's textbook "Hochfrequenztechnik. Grundlagen der mobilen Kommunikationstechnik"

 

[mtwieg]

Gamma_V=(Zs-Zin)/(Zs+Zin)
Gammp_P=(Zs-conj(Zin))/(Zs+Zin)

Never seen the distinction made in this way, can you provide an example in literature or text?

Gamma_V=0 means non-distortion matching.

This is the first time I've heard that impedance matching implied anything about distortion. How can that be since these network parameters are all based on linear assumptions?

Another ambiguity I've seen popping up is the symbol ρ used to sometimes refer to reflection coefficients. For example the wiki article on s parameters uses ρ_in and ρ_L in this equation, but the agilent article calls them S'₁₁ and Γ_L.

 

[pancho_hideboo]

Never seen the distinction made in this way,
can you provide an example in literature or text?

Sorry, I don't have international books about these.

However concept of "Gamma_V" and "Gamma_P, that is, "non-distortion-matching" and "maximum-power-deliver-matching" are decribed in my bachelor level text books on basic electric circuit and pulse circuit.

My nationality is same as "Kaneyuki Kurokawa".

If your purpose is signal processing of modulated signal or broad band signal, satisfying Gamma_V=0 is important.

If your purpose is amplifying narrow band signal, satisfying Gamma_P=0 is important.

**broken link removed**
https://en.wikipedia.org/wiki/Reflection_coefficient

This is the first time I've heard that impedance matching implied anything about distortion.
How can that be since these network parameters are all based on linear assumptions?

I can not undestand what you want to mean.

When Gamma_V is not 0, you can see linear distortion in waveform.
TDR(Time Domain Reflectometer) is a typical example of linear distortion by reflection.
https://en.wikipedia.org/wiki/Time-domain_reflectometer

Do you surely read https://www.designers-guide.org/Forum/YaBB.pl?num=1266403928/#10

The following can be explained by Gamma_V.
https://www.tiepie.com/en/classroom/Frequently_Used_Terms/Probe

Another ambiguity I've seen popping up is the symbol ρ used to sometimes refer to reflection coefficients.

That is no more than preference.
Generally, "r", "R", "s", "S", "ρ", "Γ" are used to mean reflection coefficient.

However "ρ" is also often used for meaning VSWR.

 

[volker@muehlhaus]

However concept of "Gamma_V" and "Gamma_P, that is, "non-distortion-matching" and "maximum-power-deliver-matching" are decribed in my bachelor level text books on basic electric circuit and pulse circuit.

May I jump in again for a second?

Defining a "gamma value for minimum distortion" is an unexpected concept to me. Gamma is frequency dependent, and distortion means we have to deal with harmonics. For all the RF power amplifier stuff, we look at the matching (return loss) at each harmonic frequency individually.

 

[pancho_hideboo]

Harmonic distortion is nonlinear distortion.

I mean linear distortion.
For example, group delay deviation is linear distortion.

 

[volker@muehlhaus]

I mean linear distortion.
For example, group delay deviation is linear distortion.

Linear distortion is not a common term in RF, but I see what you are talking about. But what would be a "gamma value for minimum distortion", where gamma itself is already freqency dependent? And would you agree that group delay is calculated from transmission rather than reflection?

 

[pancho_hideboo]

"Linear Distortion" is very common term in transmission theory.
https://en.wikipedia.org/wiki/Distortion

"1+Gamma_V" can be transfer function where input and output are same place.

When Gamma_V=0, Voltage Wave has no reflection.

When Gamma_P=0, Power Wave has no reflection, although Voltage Wave has reflection.

 

[volker@muehlhaus]

You are a master in web search with hyperlinks that are only remotely related to the actual topic.

 

[volker@muehlhaus]

For example when I'm seeing a talk on RFPA design and someone suggests that reflection coefficient Γ at the gate of a FET should be zero, and shows the equation Γ=(Zs-Zin)/(Zs+Zin), they're just wrong.

Coming back to this ...

For this reflection coefficient math, how do you like the approach in #22? Use an arbitrary location in the network and then look at the reflection coefficient towards source ri and towards load ra, both calculated with a real reference impedance ZL.

We then have power matching where ra=ri* and impedance matched cases are a subset of this.

 

[pancho_hideboo]

You are a master in web search with hyperlinks that are only remotely related to the actual topic.

Simply terminologies I use are all very standard, so we can easily find them in Wikipedia without exception.