"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.