S
Stutzer
Guest
Tom Bruhns wrote:
How does
How does
D.Stutzer
P
Peter O. Brackett
Guest
Stutzer:
[snip]
Yep, I'm sure...
There are basically two methods to "measure" VSWR. Both methods are
based upon so called "directional couplers" and do not measure VSWR directly
but rather measure a reflected wave and compute the VSWR as a function of
the ratio "rho" of the reflected wave to the incident wave. "rho" is the
reflection
coefficient and recall that rho = (reflected wave)/(incident wave) and...
VSWR = (1 - |rho|)/(1 + |rho|):
The first method [Which is usually not very broadbanded.] uses distributed
directional coupler technology [distributed throughout space] like slotted
lines,
etc... and is based upon the directional characteristics of "coupled
transmission
lines" which basically operate by "looking" at a travelling waveform spread
out
across many points in space. Such techniques do not work very well at low
frequencies, in fact they fail completely at DC!
I agree that these distributed techniques for measuring VSWR are difficult
to simulate with Spice.
The second method [Which is usually quite broadbanded.] computes the
refelected wave from two "measurements" at the same point in space
[one of current "i" and one of voltage "v"].
In modern times this latter technique has been called the Bruene technique
because of Warren Bruene's patent [Warren was/is a great radio Engineer
who worked for Collins Radio and the patent is ssigned to Collins Radio]
but Charles Wheatstone and Oliver Heaviside those great British
"electricians"
from the nineteenth century would roll over in their graves since they never
met Warren and Warren Bruene's invention is merely a subtle form of
Wheatstone's famous bridge circuit that was widely used to separate forward
and backward waves on single wire telegraph lines way back in the mid
1800's!
Contrary to popular opinion Sam Morse was not the only inventor active in
telegraphy. And Wheatstone had his telegraph running in Britain before
Morse
got his going in the US.
The Wheatstone bridge was invented by Wheatstone to enable full duplex
telegraph transmission and it allowed old time telegraph operators to send
and recieve signals in both directions simultaneously on the same telegraph
line.
i.e. the bridge separates the forward and reverse "waves". Today we do this
in literally millions of locations all day long with full duplex dial up
modems
and we call it "echo cancelling".
In this latter "single point" measurement technique [Wheatstone/Bruene] for
determining VSWR two variables namely the current "i" and voltage "v" at
the measurement point on a transmission line are measured and the "incident
voltage wave", sometimes called "a" is [within a constant of
proportionality]
given by: a = v + ir and the "reflected voltage wave", sometimes called "b"
is
given by: b = v - ir. Recall that the reflection coefficient rho is simply
rho = b/a = (v - ir)/(v + ir) and VSWR is VSWR = (1 - |rho|)/(1 + |rho|).
A simple circuit analysis of a bridge circuit, an active Op Amp bridge
circuit
will do as well as a passive bridge, shows that the bridge output
voltage is "b" where b = v - ir. Where "r" is the "reference impedance".
Often the reference impedance "r" is simply 50 Ohms resistive, and is the
"known" element in the bridge while the transmission line impedance is the
unknown, but in general r can be a complex impedance say the complex
Zo of the transmission line, etc...
Both techniques, "coupled space distributed lines" or "point bridges", give
the same results for the reflected wave and for VSWR.
The coupled line techniques appear to be more intuitive, but suffer from
"bandwidth" effects which must be "corrected" for varying wavelengths much
more so than the point "bridge" techniques simply because the fixed physical
length of the sampling lines of such directional couplers results in
amplitude distortion of the results as the wavelength of the signal changes.
[The
length of a space sampling line is physically fixed and so it's output
response
varies in amplitude with wavelength complicating the measurement of VSWR.]
This is because it is practically impossible to make a physical sampling
line
whose length changes automatically with the wavelength of the signal it is
sampling.
And so since the sampling line is physically fixed in length the reflected
wave output of the
distributed line sampler changes with changing frequency. While
Wheatstone's
and Bruene's bridge measurement techniques can be made to work over many
decades of frequency and even down to DC!
What is more... the bridge technique can be simulated by Spice, while in
general
the coupled lines cannot because Spice is not a PDE solver!
This is exactly how I do VSWR and directional wave measurements in Spice all
the time...
I just simply model or simulate a bridge in Spice [Only takes a few (4-5)
simple
components to do this!] and I place the bridge subcircuit in the Spice
circuit where
I want to measure the VSWR or forward and reflected waves. In fact I often
put
several, or even a whole bunch, of such directional couplers in many of my
Spice simulations.
I even use Spice simulations to help me with the direct analysis and design
of
VSWR bridges themselves!
These are fun analog design projects [Designing good VSWR bridges] and I've
done
a lot of these designs using Spice over the years... and put literally
millions of my bridges into
production. You are probably using them right now as you type, there are
at least two in every analog modem! I used to specialize in the design of
various
kinds of return loss bridges and I always use Spice when I do this.
In these cases the VSWR bridge subcircuits are not used to measure the VSWR
in
another Spice cirucuit rather they are the "whole" circuit that I am
simulating in Spice!
Wonderful! Wave simulation in Spice! [smile]
Once you have a "good" Spice simulation of a VSWR bridge, simply make it
into a subcircuit and use it as a component to plug into your other circuits
wherever you want a reflected wave or VSWR measurement.
--
Peter
[snip]
Yep, I'm sure...
There are basically two methods to "measure" VSWR. Both methods are
based upon so called "directional couplers" and do not measure VSWR directly
but rather measure a reflected wave and compute the VSWR as a function of
the ratio "rho" of the reflected wave to the incident wave. "rho" is the
reflection
coefficient and recall that rho = (reflected wave)/(incident wave) and...
VSWR = (1 - |rho|)/(1 + |rho|):
The first method [Which is usually not very broadbanded.] uses distributed
directional coupler technology [distributed throughout space] like slotted
lines,
etc... and is based upon the directional characteristics of "coupled
transmission
lines" which basically operate by "looking" at a travelling waveform spread
out
across many points in space. Such techniques do not work very well at low
frequencies, in fact they fail completely at DC!
I agree that these distributed techniques for measuring VSWR are difficult
to simulate with Spice.
The second method [Which is usually quite broadbanded.] computes the
refelected wave from two "measurements" at the same point in space
[one of current "i" and one of voltage "v"].
In modern times this latter technique has been called the Bruene technique
because of Warren Bruene's patent [Warren was/is a great radio Engineer
who worked for Collins Radio and the patent is ssigned to Collins Radio]
but Charles Wheatstone and Oliver Heaviside those great British
"electricians"
from the nineteenth century would roll over in their graves since they never
met Warren and Warren Bruene's invention is merely a subtle form of
Wheatstone's famous bridge circuit that was widely used to separate forward
and backward waves on single wire telegraph lines way back in the mid
1800's!
Contrary to popular opinion Sam Morse was not the only inventor active in
telegraphy. And Wheatstone had his telegraph running in Britain before
Morse
got his going in the US.
The Wheatstone bridge was invented by Wheatstone to enable full duplex
telegraph transmission and it allowed old time telegraph operators to send
and recieve signals in both directions simultaneously on the same telegraph
line.
i.e. the bridge separates the forward and reverse "waves". Today we do this
in literally millions of locations all day long with full duplex dial up
modems
and we call it "echo cancelling".
In this latter "single point" measurement technique [Wheatstone/Bruene] for
determining VSWR two variables namely the current "i" and voltage "v" at
the measurement point on a transmission line are measured and the "incident
voltage wave", sometimes called "a" is [within a constant of
proportionality]
given by: a = v + ir and the "reflected voltage wave", sometimes called "b"
is
given by: b = v - ir. Recall that the reflection coefficient rho is simply
rho = b/a = (v - ir)/(v + ir) and VSWR is VSWR = (1 - |rho|)/(1 + |rho|).
A simple circuit analysis of a bridge circuit, an active Op Amp bridge
circuit
will do as well as a passive bridge, shows that the bridge output
voltage is "b" where b = v - ir. Where "r" is the "reference impedance".
Often the reference impedance "r" is simply 50 Ohms resistive, and is the
"known" element in the bridge while the transmission line impedance is the
unknown, but in general r can be a complex impedance say the complex
Zo of the transmission line, etc...
Both techniques, "coupled space distributed lines" or "point bridges", give
the same results for the reflected wave and for VSWR.
The coupled line techniques appear to be more intuitive, but suffer from
"bandwidth" effects which must be "corrected" for varying wavelengths much
more so than the point "bridge" techniques simply because the fixed physical
length of the sampling lines of such directional couplers results in
amplitude distortion of the results as the wavelength of the signal changes.
[The
length of a space sampling line is physically fixed and so it's output
response
varies in amplitude with wavelength complicating the measurement of VSWR.]
This is because it is practically impossible to make a physical sampling
line
whose length changes automatically with the wavelength of the signal it is
sampling.
And so since the sampling line is physically fixed in length the reflected
wave output of the
distributed line sampler changes with changing frequency. While
Wheatstone's
and Bruene's bridge measurement techniques can be made to work over many
decades of frequency and even down to DC!
What is more... the bridge technique can be simulated by Spice, while in
general
the coupled lines cannot because Spice is not a PDE solver!
This is exactly how I do VSWR and directional wave measurements in Spice all
the time...
I just simply model or simulate a bridge in Spice [Only takes a few (4-5)
simple
components to do this!] and I place the bridge subcircuit in the Spice
circuit where
I want to measure the VSWR or forward and reflected waves. In fact I often
put
several, or even a whole bunch, of such directional couplers in many of my
Spice simulations.
I even use Spice simulations to help me with the direct analysis and design
of
VSWR bridges themselves!
These are fun analog design projects [Designing good VSWR bridges] and I've
done
a lot of these designs using Spice over the years... and put literally
millions of my bridges into
production. You are probably using them right now as you type, there are
at least two in every analog modem! I used to specialize in the design of
various
kinds of return loss bridges and I always use Spice when I do this.
In these cases the VSWR bridge subcircuits are not used to measure the VSWR
in
another Spice cirucuit rather they are the "whole" circuit that I am
simulating in Spice!
Wonderful! Wave simulation in Spice! [smile]
Once you have a "good" Spice simulation of a VSWR bridge, simply make it
into a subcircuit and use it as a component to plug into your other circuits
wherever you want a reflected wave or VSWR measurement.
--
Peter