Measuring transitor impedances
 

Hi chaps,

Is there a way of determining what basically amounts to S-parameters
for BJTs without resorting to a hard-to-get VNA? Some way of doing the
job just with the more usual test equipment you have around?
(scope/sig-gen etc.)

p.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

"Paul Burridge" a écrit dans le
message de ...
Hi chaps,

Is there a way of determining what basically amounts to
S-parameters
for BJTs without resorting to a hard-to-get VNA? Some way of
doing the
job just with the more usual test equipment you have around?
(scope/sig-gen etc.)

p.
--

"What is now proved was once only imagin'd." - William Blake,
1793.
You can use a Vector Voltmeter like the HP8405A (up to 1300 MHz),
and a signal generator. The test will be done on a single
frequency but the 8405 support some low manually sweep of the
frequency to cover the bandwith you test.
A 8405 can be found at cheap prices and is cheapest than a VNA.

"Paul Burridge" wrote in message
...
Hi chaps,

Is there a way of determining what basically amounts to S-parameters
for BJTs without resorting to a hard-to-get VNA? Some way of doing the
job just with the more usual test equipment you have around?
(scope/sig-gen etc.)
p.
--
"What is now proved was once only imagin'd." - William Blake, 1793.

I'd consider one of the common antenna impedance analyzers, but I don' know
what the output level is. Scope & sig gen is asking a lot unless you think
you can measure RF voltage and current magnitudes and phase while driving
the device with its "normal" signal level.
If you are talking about RF power transistors, the pros use matching devices
like shorted stubs and stretch lines to get power out, then remove the
device and measure the matching networks with the available equipment. Good
luck.
--
Steve N, K,9;d, c. i My email has no u's.

On Tue, 10 Aug 2004 01:53:44 GMT, "Joe Rocci" wrote:

Here's an idea that's just whacky enough that it might work...has anyone
tried it?

If you have a dual-trace scope with enough bandwidth, you might be able use
Lissajous patterns. The idea would be to drive the complex load through a
directional coupler with fairly good directivity. A sample of the incident
signal would go into the scope X input and a sample of the reflected signal
would go into the Y input. With a pure reactance as a calibration load,
adjust the X and Y scope gain for a perfect circle on the display. Replace
the test load with a complex load, and the magnitude and inclination of the
line/oval display can tell you amplitude and phase of the reflected signal,
from which any other metric can be calculated.

Nice idea, Joe! If it could only be made to work, you'd be a genius.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

Joe,

Interesting. A splitter as directional coupler for the reverse
direction.... Hmmm Yea... and since the device 'probably' won't be near 75
ohms, that directivity won't be a big issue. What about the phase response
bandwidth? That's important too. Unfortunately the phase response starts
changing well into the passband, right?...
--
Steve N, K,9;d, c. i My email has no u's.


"Joe Rocci" wrote in message
news:T0eSc.4297$Kv2.4186@trndny09...
Steve,
A decent (not the dollar store variety) CATV splitter has directivity of
about 30 dB from 5Mhz to over 500 MHZ. I think this would do the job.

Joe
W3JDR


Steve Nosko wrote in message
...

"Paul Burridge" wrote in message
...
On Tue, 10 Aug 2004 01:53:44 GMT, "Joe Rocci" wrote:

Here's an idea that's just whacky enough that it might work...has
anyone
tried it?

If you have a dual-trace scope with enough bandwidth, you might be
able
use
Lissajous patterns. The idea would be to drive the complex load
through
a
directional coupler with fairly good directivity. A sample of the
incident
signal would go into the scope X input and a sample of the reflected
signal
would go into the Y input. With a pure reactance as a calibration
load,
adjust the X and Y scope gain for a perfect circle on the display.
Replace
the test load with a complex load, and the magnitude and inclination
of
the
line/oval display can tell you amplitude and phase of the reflected
signal,
from which any other metric can be calculated.

Nice idea, Joe! If it could only be made to work, you'd be a genius.
--


OK Actually, this looks good as long as you have a directional coupler
for
the frequency of interest! Paul didn't say. (SWR Bridge, actual
bridge,
for lower frequencies) You'll also have to allow for the difference in
location of the two samples by adjusting the line lengths to the two
scope
probes intil they are both at the same "distance" from the load.
Judging
phase shift from a circle can't be as good as simply looking at the two
waveforms. The scope horiz gain can be adjusted for some nice number of
divisions for each half cycle, say 9...
I've got a 100MHz. dual trace storage scope, but any directional
couplers around here only will be down to 130 Mhz at best... I do also
have
a good bridge for down to 5 MHz., (also about 1 meter worth of stretch
line...so it is possible as long as the power level is within the bridge
capability.
--
Steve N, K,9;d, c. i My email has no u's.

On Wed, 11 Aug 2004 13:30:01 -0500, "Steve Nosko"
wrote:

Joe,

Interesting. A splitter as directional coupler for the reverse
direction.... Hmmm Yea... and since the device 'probably' won't be near 75
ohms, that directivity won't be a big issue. What about the phase response
bandwidth? That's important too. Unfortunately the phase response starts
changing well into the passband, right?...

Well it was a neat idea in principle, certainly. Let's see if there's
a workaround for the issues you raise...

--

"What is now proved was once only imagin'd." - William Blake, 1793.

Most people think of a directional coupler as a directional coupler, and a
splitter as a splitter. In reality, the splitter is simply the extreme case
of a directioanl coupler where the coupling factor is 50%.

The fact that it was designed for 75 ohms is somewhat incidental. The
impedance on any of the ports is a function of the impedance on the other
ports. As to phase, it has to be well-behaved or you wouldn't have good
directivity.

Joe
W3JDR


Steve Nosko wrote in message
...
Joe,

Interesting. A splitter as directional coupler for the reverse
direction.... Hmmm Yea... and since the device 'probably' won't be near
75
ohms, that directivity won't be a big issue. What about the phase
response
bandwidth? That's important too. Unfortunately the phase response starts
changing well into the passband, right?...
--
Steve N, K,9;d, c. i My email has no u's.


"Joe Rocci" wrote in message
news:T0eSc.4297$Kv2.4186@trndny09...
Steve,
A decent (not the dollar store variety) CATV splitter has directivity of
about 30 dB from 5Mhz to over 500 MHZ. I think this would do the job.

Joe
W3JDR


Steve Nosko wrote in message
...

"Paul Burridge" wrote in message
...
On Tue, 10 Aug 2004 01:53:44 GMT, "Joe Rocci"
wrote:

Here's an idea that's just whacky enough that it might work...has
anyone
tried it?

If you have a dual-trace scope with enough bandwidth, you might be
able
use
Lissajous patterns. The idea would be to drive the complex load
through
a
directional coupler with fairly good directivity. A sample of the
incident
signal would go into the scope X input and a sample of the
reflected
signal
would go into the Y input. With a pure reactance as a calibration
load,
adjust the X and Y scope gain for a perfect circle on the display.
Replace
the test load with a complex load, and the magnitude and
inclination
of
the
line/oval display can tell you amplitude and phase of the reflected
signal,
from which any other metric can be calculated.

Nice idea, Joe! If it could only be made to work, you'd be a genius.
--


OK Actually, this looks good as long as you have a directional
coupler
for
the frequency of interest! Paul didn't say. (SWR Bridge, actual
bridge,
for lower frequencies) You'll also have to allow for the difference
in
location of the two samples by adjusting the line lengths to the two
scope
probes intil they are both at the same "distance" from the load.
Judging
phase shift from a circle can't be as good as simply looking at the
two
waveforms. The scope horiz gain can be adjusted for some nice number
of
divisions for each half cycle, say 9...
I've got a 100MHz. dual trace storage scope, but any directional
couplers around here only will be down to 130 Mhz at best... I do
also
have
a good bridge for down to 5 MHz., (also about 1 meter worth of stretch
line...so it is possible as long as the power level is within the
bridge
capability.
--
Steve N, K,9;d, c. i My email has no u's.

"Joe Rocci" wrote in message
news:EUwSc.9258$dG.759@trndny02...
Most people think of a directional coupler as a directional coupler, and a
splitter as a splitter. In reality, the splitter is simply the extreme
case
of a directioanl coupler where the coupling factor is 50%.

The fact that it was designed for 75 ohms is somewhat incidental. The
impedance on any of the ports is a function of the impedance on the other
ports. As to phase, it has to be well-behaved or you wouldn't have good
directivity.

Joe
W3JDR

Sure Joe,

you are right. I just never thought of it on that light.

Lets think about this phase thing, however. As long as there is the proper
phase cancellation _in_ the splitter, it will balance. This doesn't mean
that it is flat, does it? You'll be sampling the forward power somehow and
the reverse power with one of these and you need to have them both constant
with respect to each other--otherwise you cant measure phase.. If the fwd
sampler is the same thing do you get matching characteristics (ignore
secondary imperfections for now). I don't know how good ones are made, so I
have know first hand knowledge.
--
Steve N, K,9;d, c. i My email has no u's.

On Thu, 12 Aug 2004 15:32:00 -0500, "Steve Nosko"
wrote:


Lets think about this phase thing, however. As long as there is the proper
phase cancellation _in_ the splitter, it will balance. This doesn't mean
that it is flat, does it? You'll be sampling the forward power somehow and
the reverse power with one of these and you need to have them both constant
with respect to each other--otherwise you cant measure phase.. If the fwd
sampler is the same thing do you get matching characteristics (ignore
secondary imperfections for now). I don't know how good ones are made, so I
have know first hand knowledge.

Let's assume for one moment the splitter idea would function
acceptably as a directional coupler for this purpose. What's next in
the block diagram? Phase comparitor? DC amp?

--

"What is now proved was once only imagin'd." - William Blake, 1793.

Paul & Steve,

Steve,
I'm pretty confident that the phase shift will be constant and can be
calibrated out. If not, it can be made irrelevant by using two splitters,
one for the forward path sample and one for the return path sample.

Paul,
The idea is to use the splitter 'backwards"; drive the RF into one of the
splitter legs and feed the load through the common port. Assuming good
directivity, any RF coming out of the other splitter leg must be reflected
energy. If you put a sample of the forward energy into a scope's X input
(horizontal) and a sample of the reflected energy into the Y input
(vertical), you will get an elliptical display called a Lissajous pattern.
If X and Y are equal in magnitude and exactly 90 deg out of phase, this
will be a perfect circle. Any other phase angle will result in a elliptical
pattern whose inclination angle relative to the X axis (or Y axis) is a
function of the phase angle. The length of the ellipse is a function of the
magnitude. This is classic stuff...look it up if you're not familiar with
it.

Joe
W3JDR


Paul Burridge wrote in message
...
On Thu, 12 Aug 2004 15:32:00 -0500, "Steve Nosko"
wrote:


Lets think about this phase thing, however. As long as there is the
proper
phase cancellation _in_ the splitter, it will balance. This doesn't mean
that it is flat, does it? You'll be sampling the forward power somehow
and
the reverse power with one of these and you need to have them both
constant
with respect to each other--otherwise you cant measure phase.. If the
fwd
sampler is the same thing do you get matching characteristics (ignore
secondary imperfections for now). I don't know how good ones are made,
so I
have know first hand knowledge.

Let's assume for one moment the splitter idea would function
acceptably as a directional coupler for this purpose. What's next in
the block diagram? Phase comparitor? DC amp?

--

"What is now proved was once only imagin'd." - William Blake, 1793.

Check out the article 'A Low-Cost 100 MHz Vector Network Analyzer with USB
Interface' in the July/August 2004 issue of QEX, for measuring S pars.

Al, w6wqc

On Thu, 12 Aug 2004 23:38:00 GMT, "Joe Rocci" wrote:

Paul & Steve,

Steve,
I'm pretty confident that the phase shift will be constant and can be
calibrated out. If not, it can be made irrelevant by using two splitters,
one for the forward path sample and one for the return path sample.

Paul,
The idea is to use the splitter 'backwards"; drive the RF into one of the
splitter legs and feed the load through the common port. Assuming good
directivity, any RF coming out of the other splitter leg must be reflected
energy. If you put a sample of the forward energy into a scope's X input
(horizontal) and a sample of the reflected energy into the Y input
(vertical), you will get an elliptical display called a Lissajous pattern.
If X and Y are equal in magnitude and exactly 90 deg out of phase, this
will be a perfect circle. Any other phase angle will result in a elliptical
pattern whose inclination angle relative to the X axis (or Y axis) is a
function of the phase angle. The length of the ellipse is a function of the
magnitude. This is classic stuff...look it up if you're not familiar with

THanks for the explanation, but I doubt this idea has enough accuracy
for determining the parameters within any acceptable degree. Neat
concept, though!
--

"What is now proved was once only imagin'd." - William Blake, 1793.

Paul,
I agree. That's why I started out by saying it was a wacky idea.

Things like this make you think though. Sometimes those types of musings pay
off later down the road. One of my mentors once said: "A good engineer
remembers every good idea he ever heard. The only thing he forgets is who he
heard it from".

Joe
W3JDR


"Paul Burridge" wrote in message
...
On Thu, 12 Aug 2004 23:38:00 GMT, "Joe Rocci" wrote:

Paul & Steve,

Steve,
I'm pretty confident that the phase shift will be constant and can be
calibrated out. If not, it can be made irrelevant by using two splitters,
one for the forward path sample and one for the return path sample.

Paul,
The idea is to use the splitter 'backwards"; drive the RF into one of the
splitter legs and feed the load through the common port. Assuming good
directivity, any RF coming out of the other splitter leg must be
reflected
energy. If you put a sample of the forward energy into a scope's X input
(horizontal) and a sample of the reflected energy into the Y input
(vertical), you will get an elliptical display called a Lissajous
pattern.
If X and Y are equal in magnitude and exactly 90 deg out of phase, this
will be a perfect circle. Any other phase angle will result in a
elliptical
pattern whose inclination angle relative to the X axis (or Y axis) is a
function of the phase angle. The length of the ellipse is a function of
the
magnitude. This is classic stuff...look it up if you're not familiar with

THanks for the explanation, but I doubt this idea has enough accuracy
for determining the parameters within any acceptable degree. Neat
concept, though!
--

"What is now proved was once only imagin'd." - William Blake, 1793.

On Fri, 13 Aug 2004 12:02:26 GMT, "W3JDR" wrote:

Paul,
I agree. That's why I started out by saying it was a wacky idea.

Things like this make you think though. Sometimes those types of musings pay
off later down the road. One of my mentors once said: "A good engineer
remembers every good idea he ever heard. The only thing he forgets is who he
heard it from".

Too true!
Well don't forget this one; there's certainly a seed of possibility
there, bar a few inevitable hurdles to overcome.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

"Joe Rocci" wrote in message
news:sbTSc.7003$BS3.1582@trndny04...
Paul & Steve,

Steve,
I'm pretty confident that the phase shift will be constant and can be
calibrated out. If not, it can be made irrelevant by using two splitters,
one for the forward path sample and one for the return path sample.

Paul,
The idea is to use the splitter 'backwards"; drive the RF into one of the
splitter legs and feed the load through the common port. Assuming good
directivity, any RF coming out of the other splitter leg must be reflected
energy. If you put a sample of the forward energy into a scope's X input
(horizontal) and a sample of the reflected energy into the Y input
(vertical), you will get an elliptical display called a Lissajous pattern.
If X and Y are equal in magnitude and exactly 90 deg out of phase, this
will be a perfect circle. Any other phase angle will result in a
elliptical
pattern whose inclination angle relative to the X axis (or Y axis) is a
function of the phase angle. The length of the ellipse is a function of
the
magnitude. This is classic stuff...look it up if you're not familiar with
it.
Joe
W3JDR


You guys seem too negative. I know I was pointing out difficulties, but
only to keep the limitations in mind. Actually, it ain't that wacky. It is
EXACTLY how the Professional equipment does it, just with really good
couplers etc... As long as the phase and magnitude can either be controlled
or known and calibrated out, it should be pretty good.

If you have a scope that can go to the frequency of interest, you have a
really good basis for success.
The one thing I couldn't get fixed in my mind is the phase considerations of
using one splitter for forward sampling and one for reverse. Two things to
consider a
1- As the freq gets higher, the phase of the FWD sample and REV samples get
"further apart" since they can not be at exactly at the same point on the
t-line, namely the load. Therefore, you have to adjust the line lengths to
'move' them to the same virtual point. Perhaps at HF this is a don't care.
I'm used to doing this @ 150 MHz.
2- If you are in a region where there is a phase change with frequency in
the splitter/coupler, are you sure that it is the same in both directions,
or does it not work that way and will it work to increase the error rather
than both of them tracking together. I just can't model this in my brain
without putting some things on paper... I guess, with the scope, you can
examine that and make adjustments for it by using "calibration loads" just
like the calibration kits used by the pros.

I still believe the Lissajous (I'm sure glad you can spell that) method is
not as good as simply measuring phase shift between the two signals directly
in normal scope mode. With the Lissajous, it is harder to get the
magnitudes equal and then measure random angles from the "tilt" of the oval.
Don't the Lissajous formulas all assume equal magnitude sine waves? You'd
have to compare amplitudes in normal mode and adjust the vert gain to set
them equal anyway, so why not just set the horizontal sweep speed for a
'nice' speed and then the shift is on a calibrated scale. 9 divisions for a
1/2 cycle and you have 20 degrees per cm and the little divisions are 4 and
you estimate from there. This way the phase measurement is almost 100%
independent of amplitude 'cuz you're looking only @ the zero crossings. I
don't think you can estimate 4 degrees on a Lissajous that easily.

I still wonder WHAT kind of transistors the OP wanted to measure...

--
Steve N, K,9;d, c. i My email has no u's.

On Fri, 13 Aug 2004 10:03:51 -0500, "Steve Nosko"
wrote:

I still wonder WHAT kind of transistors the OP wanted to measure...

Nothing toooo special: 2N5109s and that ilk.

--

"What is now proved was once only imagin'd." - William Blake, 1793.

John Miles wrote:

The fly in the ointment is that scopes typically don't have very high
bandwidth (more than a few MHz) in X-Y mode. The X-axis amplifier
section of even a relatively-fast analog scope like the 2467 is only
rated at 3 MHz, and you'll probably get significant phase shift long
before that point is reached

-- jm

For sure, you shouldn't use an analog scope for this, for the reason
John gives. In an analog scope, both the vertical amplifiers and the
vertical CRT deflection system are optimized for speed, but the
horizontal aren't. Even if you use identical amplifiers for X and Y, the
horizontal CRT deflection structure will limit the X bandwidth, and give
you considerable phase shift well below the cutoff frequency.

Modern digital scopes, on the other hand, should work equally well on
the two axes.

Roy Lewallen, W7EL

Use a direction coupler for forward and reverse measurement.
Use a 2 Chan. Scope great bandwith not nessesary, it is the amplitude that
is to be measured.
reflection coefficient = Vreflected/Vincident = polar magn. and angle
Chris Bowick Book page 104, reflection coefficient may be plotted directly
in the Smith Chart, and the corresponding impedance read off the chart.
Or did I missed something?


"Joe Rocci" schreef in bericht
news:T0eSc.4297$Kv2.4186@trndny09...
Steve,
A decent (not the dollar store variety) CATV splitter has directivity of
about 30 dB from 5Mhz to over 500 MHZ. I think this would do the job.

Joe
W3JDR


Steve Nosko wrote in message
...

"Paul Burridge" wrote in message
...
On Tue, 10 Aug 2004 01:53:44 GMT, "Joe Rocci" wrote:

Here's an idea that's just whacky enough that it might work...has
anyone
tried it?

If you have a dual-trace scope with enough bandwidth, you might be
able
use
Lissajous patterns. The idea would be to drive the complex load
through
a
directional coupler with fairly good directivity. A sample of the
incident
signal would go into the scope X input and a sample of the reflected
signal
would go into the Y input. With a pure reactance as a calibration
load,
adjust the X and Y scope gain for a perfect circle on the display.
Replace
the test load with a complex load, and the magnitude and inclination
of
the
line/oval display can tell you amplitude and phase of the reflected
signal,
from which any other metric can be calculated.

Nice idea, Joe! If it could only be made to work, you'd be a genius.
--


OK Actually, this looks good as long as you have a directional coupler
for
the frequency of interest! Paul didn't say. (SWR Bridge, actual
bridge,
for lower frequencies) You'll also have to allow for the difference in
location of the two samples by adjusting the line lengths to the two
scope
probes intil they are both at the same "distance" from the load.
Judging
phase shift from a circle can't be as good as simply looking at the two
waveforms. The scope horiz gain can be adjusted for some nice number of
divisions for each half cycle, say 9...
I've got a 100MHz. dual trace storage scope, but any directional
couplers around here only will be down to 130 Mhz at best... I do also
have
a good bridge for down to 5 MHz., (also about 1 meter worth of stretch
line...so it is possible as long as the power level is within the bridge
capability.
--
Steve N, K,9;d, c. i My email has no u's.

Read the whole thread....

Joe
W3JDR



bviel wrote in message
...
Use a direction coupler for forward and reverse measurement.
Use a 2 Chan. Scope great bandwith not nessesary, it is the amplitude that
is to be measured.
reflection coefficient = Vreflected/Vincident = polar magn. and angle
Chris Bowick Book page 104, reflection coefficient may be plotted directly
in the Smith Chart, and the corresponding impedance read off the chart.
Or did I missed something?


"Joe Rocci" schreef in bericht
news:T0eSc.4297$Kv2.4186@trndny09...
Steve,
A decent (not the dollar store variety) CATV splitter has directivity of
about 30 dB from 5Mhz to over 500 MHZ. I think this would do the job.

Joe
W3JDR


Steve Nosko wrote in message
...

"Paul Burridge" wrote in message
...
On Tue, 10 Aug 2004 01:53:44 GMT, "Joe Rocci"
wrote:

Here's an idea that's just whacky enough that it might work...has
anyone
tried it?

If you have a dual-trace scope with enough bandwidth, you might be
able
use
Lissajous patterns. The idea would be to drive the complex load
through
a
directional coupler with fairly good directivity. A sample of the
incident
signal would go into the scope X input and a sample of the
reflected
signal
would go into the Y input. With a pure reactance as a calibration
load,
adjust the X and Y scope gain for a perfect circle on the display.
Replace
the test load with a complex load, and the magnitude and
inclination
of
the
line/oval display can tell you amplitude and phase of the reflected
signal,
from which any other metric can be calculated.

Nice idea, Joe! If it could only be made to work, you'd be a genius.
--


OK Actually, this looks good as long as you have a directional
coupler
for
the frequency of interest! Paul didn't say. (SWR Bridge, actual
bridge,
for lower frequencies) You'll also have to allow for the difference
in
location of the two samples by adjusting the line lengths to the two
scope
probes intil they are both at the same "distance" from the load.
Judging
phase shift from a circle can't be as good as simply looking at the
two
waveforms. The scope horiz gain can be adjusted for some nice number
of
divisions for each half cycle, say 9...
I've got a 100MHz. dual trace storage scope, but any directional
couplers around here only will be down to 130 Mhz at best... I do
also
have
a good bridge for down to 5 MHz., (also about 1 meter worth of stretch
line...so it is possible as long as the power level is within the
bridge
capability.
--
Steve N, K,9;d, c. i My email has no u's.

"bviel" wrote in message
...
Use a direction coupler for forward and reverse measurement.
Use a 2 Chan. Scope great bandwith not nessesary, it is the amplitude that
is to be measured.
reflection coefficient = Vreflected/Vincident = polar magn. and angle
Chris Bowick Book page 104, reflection coefficient may be plotted directly
in the Smith Chart, and the corresponding impedance read off the chart.
Or did I missed something?

Yup, much of what was said. You simply restated it, to some degree. We
were MUCH deeper...way past the basics.

The scope has to work reasonably well at the frequency of interest. They
can die pretty fast abofe their useful frequency.

You also have to measure *phase* between the two signals.
--
Steve N, K,9;d, c. i My email has no u's.





"Joe Rocci" schreef in bericht
news:T0eSc.4297$Kv2.4186@trndny09...
Steve,
A decent (not the dollar store variety) CATV splitter has directivity of
about 30 dB from 5Mhz to over 500 MHZ. I think this would do the job.

Joe
W3JDR


Steve Nosko wrote in message
...

"Paul Burridge" wrote in message
...
On Tue, 10 Aug 2004 01:53:44 GMT, "Joe Rocci"
wrote:

Here's an idea that's just whacky enough that it might work...has
anyone
tried it?

If you have a dual-trace scope with enough bandwidth, you might be
able
use
Lissajous patterns. The idea would be to drive the complex load
through
a
directional coupler with fairly good directivity. A sample of the
incident
signal would go into the scope X input and a sample of the
reflected
signal
would go into the Y input. With a pure reactance as a calibration
load,
adjust the X and Y scope gain for a perfect circle on the display.
Replace
the test load with a complex load, and the magnitude and
inclination
of
the
line/oval display can tell you amplitude and phase of the reflected
signal,
from which any other metric can be calculated.

Nice idea, Joe! If it could only be made to work, you'd be a genius.
--


OK Actually, this looks good as long as you have a directional
coupler
for
the frequency of interest! Paul didn't say. (SWR Bridge, actual
bridge,
for lower frequencies) You'll also have to allow for the difference
in
location of the two samples by adjusting the line lengths to the two
scope
probes intil they are both at the same "distance" from the load.
Judging
phase shift from a circle can't be as good as simply looking at the
two
waveforms. The scope horiz gain can be adjusted for some nice number
of
divisions for each half cycle, say 9...
I've got a 100MHz. dual trace storage scope, but any directional
couplers around here only will be down to 130 Mhz at best... I do
also
have
a good bridge for down to 5 MHz., (also about 1 meter worth of stretch
line...so it is possible as long as the power level is within the
bridge
capability.
--
Steve N, K,9;d, c. i My email has no u's.

Yes, I did read the whole thread, how could a restate whitout reading the
whole thread.
The "restate" is also the way a do measure S-parameters.
And I did say you need an angle also.
reflection coefficient = Vreflected/Vincident = polar magn. and angle
Whit angle I mean Phase Shift.
If you can measure polar magn. and angle, there is no need to go MUCH
deeper.
The fact you did go much deeper is? Use of analog equipment.
The question was, instead of use VNA, use oscilloscoop?
Lissajous figures measurement method is very basic stuff, its stated in my
scopes
manuel.
Work with complex numbers and Smith Chart is a hobby of me, so I know too
what is a "complex load".
How to construct a direction coupler, stripline, any impedance, frequency,
even a
direction coupler for every frequency if you only have to measure phase
shift.
To measure the magn. I use the bridge of Wheatstone, with a zero biassed
Schottky detector diode DVM and some formules.
To give 40,000 $ for a VNA is one way, do it with "things" around you with
the
same results, Hey isn't that homebrew?
I don't think there is anything I can't measure, even noise density ! If it
can't be
measured, I can calculate it.

Now the purpose for my reaction was, that you where going to think, hey
dummy
HOW did you do it?
Instead of that you did reply, we where MUCH deeper....
That's the way (maybe a little unfair) I did reply, did I miss something?
I learnt that you should not trough your self away, someone else will do it
for you !

BTW the scope is limited to 1Ghz, digital, full of wave calculations,
anything you name.

Man... did you hurt me MUCH !

Would this be the end of this thread, a would not hurt anyone.
Maybe I should have not reply at all, I choose the wrong "entry".





"Steve Nosko" schreef in bericht
...

"bviel" wrote in message
...
Use a direction coupler for forward and reverse measurement.
Use a 2 Chan. Scope great bandwith not nessesary, it is the amplitude
that
is to be measured.
reflection coefficient = Vreflected/Vincident = polar magn. and angle
Chris Bowick Book page 104, reflection coefficient may be plotted
directly
in the Smith Chart, and the corresponding impedance read off the chart.
Or did I missed something?

Yup, much of what was said. You simply restated it, to some degree. We
were MUCH deeper...way past the basics.

The scope has to work reasonably well at the frequency of interest. They
can die pretty fast abofe their useful frequency.

You also have to measure *phase* between the two signals.
--
Steve N, K,9;d, c. i My email has no u's.





"Joe Rocci" schreef in bericht
news:T0eSc.4297$Kv2.4186@trndny09...
Steve,
A decent (not the dollar store variety) CATV splitter has directivity
of
about 30 dB from 5Mhz to over 500 MHZ. I think this would do the job.

Joe
W3JDR


Steve Nosko wrote in message
...

"Paul Burridge" wrote in message
...
On Tue, 10 Aug 2004 01:53:44 GMT, "Joe Rocci"
wrote:

Here's an idea that's just whacky enough that it might work...has
anyone
tried it?

If you have a dual-trace scope with enough bandwidth, you might
be
able
use
Lissajous patterns. The idea would be to drive the complex load
through
a
directional coupler with fairly good directivity. A sample of the
incident
signal would go into the scope X input and a sample of the
reflected
signal
would go into the Y input. With a pure reactance as a calibration
load,
adjust the X and Y scope gain for a perfect circle on the
display.
Replace
the test load with a complex load, and the magnitude and
inclination
of
the
line/oval display can tell you amplitude and phase of the
reflected
signal,
from which any other metric can be calculated.

Nice idea, Joe! If it could only be made to work, you'd be a
genius.
--


OK Actually, this looks good as long as you have a directional
coupler
for
the frequency of interest! Paul didn't say. (SWR Bridge, actual
bridge,
for lower frequencies) You'll also have to allow for the difference
in
location of the two samples by adjusting the line lengths to the two
scope
probes intil they are both at the same "distance" from the load.
Judging
phase shift from a circle can't be as good as simply looking at the
two
waveforms. The scope horiz gain can be adjusted for some nice
number
of
divisions for each half cycle, say 9...
I've got a 100MHz. dual trace storage scope, but any directional
couplers around here only will be down to 130 Mhz at best... I do
also
have
a good bridge for down to 5 MHz., (also about 1 meter worth of
stretch
line...so it is possible as long as the power level is within the
bridge
capability.
--
Steve N, K,9;d, c. i My email has no u's.