On Tue, 06 Dec 2005 11:37:04 -0700, Wes Stewart
wrote:


Maybe I misunderstand, but what you seem to be saying is that if I put
Probe A at point X on a transmission line and Probe B at point X+Y
(Y0), the phase difference with respect to frequency will not change.

The directional coupler is no different, it is sampling the main line
at two physically different locations.

With respect, I think you two guys are talking about different kinds
of couplers.

Wes, your earlier description makes is clear that samples in the
coupler you described are located at equidistant from their nearest
ends, but not in the centre, so at different positions in the coupler.
I think the coupled lines type of coupler for lower frequency use
might fit this category.

Frank assumes a coupler where the samples for both ports are taken at
the same physical location on the main line. I think a crossed
waveguide coupler might fit this category (depending on the way the
coupling holes are implemented).

Owen
--

On Tue, 06 Dec 2005 19:18:50 GMT, Owen Duffy wrote:

On Tue, 06 Dec 2005 11:37:04 -0700, Wes Stewart
wrote:


Maybe I misunderstand, but what you seem to be saying is that if I put
Probe A at point X on a transmission line and Probe B at point X+Y
(Y0), the phase difference with respect to frequency will not change.

The directional coupler is no different, it is sampling the main line
at two physically different locations.

With respect, I think you two guys are talking about different kinds
of couplers.

Wes, your earlier description makes is clear that samples in the
coupler you described are located at equidistant from their nearest
ends, but not in the centre, so at different positions in the coupler.
I think the coupled lines type of coupler for lower frequency use
might fit this category.

Frank assumes a coupler where the samples for both ports are taken at
the same physical location on the main line.

May be, but his link:

http://cp.literature.agilent.com/lit.../5952-8133.pdf

shows exactly what I'm talking about in Figure 3. Note the line
stretcher on one sample port. Because I haven't seen the inside of one
of these I don't know where the coupled arms reside with respect to
each other but the fact that H-P shows the line stretcher tells me
that they must have some (unavoidable, if not purposeful) asymmetry.
They claim 4 degree phase tracking but it's unclear to me whether they
mean the coupled arms with respect to the main line or to each other.


I think a crossed
waveguide coupler might fit this category (depending on the way the
coupling holes are implemented).



Owen

"Wes Stewart" wrote in message
...
On Tue, 06 Dec 2005 19:18:50 GMT, Owen Duffy wrote:

On Tue, 06 Dec 2005 11:37:04 -0700, Wes Stewart
wrote:


Maybe I misunderstand, but what you seem to be saying is that if I put
Probe A at point X on a transmission line and Probe B at point X+Y
(Y0), the phase difference with respect to frequency will not change.

The directional coupler is no different, it is sampling the main line
at two physically different locations.

With respect, I think you two guys are talking about different kinds
of couplers.

...Owen's coupler location discussion...
May be, but his link:

http://cp.literature.agilent.com/lit.../5952-8133.pdf

shows exactly what I'm talking about in Figure 3. Note the line
stretcher on one sample port. Because I haven't seen the inside of one
of these I don't know where the coupled arms reside with respect to
each other but the fact that H-P shows the line stretcher tells me
that they must have some (unavoidable, if not purposeful) asymmetry.
They claim 4 degree phase tracking but it's unclear to me whether they
mean the coupled arms with respect to the main line or to each other.


I have done this type of measurement and the coupler is somewhat
un-important in regard to where on the line its samples are taken from. The
line stretcher does the compensation to place both samples at the same
point on the main line. It is set in a calibration process to get to that
point (seems to mee only a short is required). Memory is starting to fade
here, but if you want the samples to be "at the plane of the DUT" then you
either must use two line stretchers or use the vector voltmeter capability
to compensate for the difference in sample location vs. DUT location. If I
recall correctly, this is simply a phase offset and the old HP vector
voltmeter has such an offset capability.

Look at it this way. You want the FWD and REF phases to be at the DUT. To
do this you must put an equal (electrical) length of line in all three
sections of line - the desired, the FWD sample and the reverse sample. I
admit the reflected part has an intuitive glitch that I can't resolve at
this time since it is always longer, but I know this works. You can sweep
the set-up and the phase of the two samples will sit right there on that of
a short . This rells you that the samples are "at the DUT".

Right??

73, Steve, K,9.D;C'I

On Tue, 06 Dec 2005 14:22:31 -0700, Wes Stewart
wrote:


May be, but his link:

http://cp.literature.agilent.com/lit.../5952-8133.pdf

shows exactly what I'm talking about in Figure 3. Note the line
stretcher on one sample port. Because I haven't seen the inside of one
of these I don't know where the coupled arms reside with respect to
each other but the fact that H-P shows the line stretcher tells me
that they must have some (unavoidable, if not purposeful) asymmetry.
They claim 4 degree phase tracking but it's unclear to me whether they
mean the coupled arms with respect to the main line or to each other.

I could be wrong, but I think that 4 deg seems part of the tolerance
of the phase alignment of the two sample ports wrt each other over the
frequency range. Perhaps this is a coupler where the ports are
approximately in phase, and perhaps the line strether is to adjust
phase of forward and reflected ports to create a new reference plane
where it is needed or convenient to the measurement.

Owen
--

On Tue, 06 Dec 2005 23:13:35 GMT, Owen Duffy wrote:

On Tue, 06 Dec 2005 14:22:31 -0700, Wes Stewart
wrote:


May be, but his link:

http://cp.literature.agilent.com/lit.../5952-8133.pdf

shows exactly what I'm talking about in Figure 3. Note the line
stretcher on one sample port. Because I haven't seen the inside of one
of these I don't know where the coupled arms reside with respect to
each other but the fact that H-P shows the line stretcher tells me
that they must have some (unavoidable, if not purposeful) asymmetry.
They claim 4 degree phase tracking but it's unclear to me whether they
mean the coupled arms with respect to the main line or to each other.

I could be wrong, but I think that 4 deg seems part of the tolerance
of the phase alignment of the two sample ports wrt each other over the
frequency range.

I tend to agree, but as someone who used to write specs and then sit
back and watch vendors try to skirt them, I'm always dubious. :-)

Perhaps this is a coupler where the ports are
approximately in phase, and perhaps the line strether is to adjust
phase of forward and reflected ports to create a new reference plane
where it is needed or convenient to the measurement.

I think what's being missed in this discussion is this: The 8405 has
a phase offset adjustment that can make -any- relative phase read 180
degrees on the meter. So you could put a short on 100 feet (30.48m)
and adjust the phase offset to make the meter read 180 deg. This
would be fine until you changed frequency then you're in trouble.
That same effect is the reason for adding a line stretcher; it's to
make that 180 degree relationship track with respect to frequency.

Paragraph 2 of the document I provided explains this nicely.

I've built reflectometers (many times) where dual couplers were not
available and two back-to-back singles were used. (Sometimes, three
were used with the added one in a feedback loop used to improve the
source match of the generator) I can guarantee you that the two
coupled arms didn't phase track and that is the general case that I
was trying to put forth at the outset of this discussion.

Wes,

Thank you. Two couplers configured "in to in" from a tee, to set up isolated "in
phase" coupler signals, do seem to maintain very close to a relationship across
frequencies. This was observed with an oscilloscope

After much scrounging around to make the connectors a constant length and number
the 8405a does not maintain a constant phase across the same inputs. The 20kc IF
output shows a significant phase change across the same frequency range.

Its back to the drawing board to re-tune the 8405a. Somewhere in the process I
missed something. The phase angle should not vary that much.

Dan

Wes Stewart wrote:
On Tue, 06 Dec 2005 23:13:35 GMT, Owen Duffy wrote:


On Tue, 06 Dec 2005 14:22:31 -0700, Wes Stewart
wrote:



May be, but his link:

http://cp.literature.agilent.com/lit.../5952-8133.pdf

shows exactly what I'm talking about in Figure 3. Note the line
stretcher on one sample port. Because I haven't seen the inside of one
of these I don't know where the coupled arms reside with respect to
each other but the fact that H-P shows the line stretcher tells me
that they must have some (unavoidable, if not purposeful) asymmetry.
They claim 4 degree phase tracking but it's unclear to me whether they
mean the coupled arms with respect to the main line or to each other.

I could be wrong, but I think that 4 deg seems part of the tolerance
of the phase alignment of the two sample ports wrt each other over the
frequency range.


I tend to agree, but as someone who used to write specs and then sit
back and watch vendors try to skirt them, I'm always dubious. :-)


Perhaps this is a coupler where the ports are
approximately in phase, and perhaps the line strether is to adjust
phase of forward and reflected ports to create a new reference plane
where it is needed or convenient to the measurement.


I think what's being missed in this discussion is this: The 8405 has
a phase offset adjustment that can make -any- relative phase read 180
degrees on the meter. So you could put a short on 100 feet (30.48m)
and adjust the phase offset to make the meter read 180 deg. This
would be fine until you changed frequency then you're in trouble.
That same effect is the reason for adding a line stretcher; it's to
make that 180 degree relationship track with respect to frequency.

Paragraph 2 of the document I provided explains this nicely.

I've built reflectometers (many times) where dual couplers were not
available and two back-to-back singles were used. (Sometimes, three
were used with the added one in a feedback loop used to improve the
source match of the generator) I can guarantee you that the two
coupled arms didn't phase track and that is the general case that I
was trying to put forth at the outset of this discussion.

"Owen Duffy" wrote in message
news
On Tue, 06 Dec 2005 11:37:04 -0700, Wes Stewart
wrote:


Maybe I misunderstand, but what you seem to be saying is that if I put
Probe A at point X on a transmission line and Probe B at point X+Y
(Y0), the phase difference with respect to frequency will not change.

The directional coupler is no different, it is sampling the main line
at two physically different locations.

With respect, I think you two guys are talking about different kinds
of couplers.

Wes, your earlier description makes is clear that samples in the
coupler you described are located at equidistant from their nearest
ends, but not in the centre, so at different positions in the coupler.
I think the coupled lines type of coupler for lower frequency use
might fit this category.

Frank assumes a coupler where the samples for both ports are taken at
the same physical location on the main line. I think a crossed
waveguide coupler might fit this category (depending on the way the
coupling holes are implemented).

Owen

The type of coupler I am thinking of is indeed where the coupling lines for
forward and reflected are in the same physical region. The point I was
making refers particularly to the HP 778D; where it is specified in:
http://cp.literature.agilent.com/lit.../5952-8133.pdf
"Data can be read from the two meters of the vector voltmeter and
transferred directly to a Smith Chart". Indicating that there is no
significant phase error over the nominal bandwidth of the coupler. It is
certainly something I have always taken for granted, but in recent years
have been spoiled by Agilent's VNAs. Even Matthaei, Young, and Jones'
classic text does not get into specifics of the phase response of TEM
couplers. If I get a chance in the near future, I will run a phase response
of an HP778D coupler, and/or also run an Eagleware simulation of a co-planar
WG coupler.

Just read Wes' comments on the line stretcher on the above pdf. I must
admit I also wondered about that, but assume it is intended as a cal
adjustment for a short/open standard. Even VNAs, in their non calibrated
state, show a pretty good dot on the right/left of the Smith Chart for an
open/short in the lower hundreds of MHz.

Incidentally I have tried to access the
http://www.qsl.net/n7ws/Pages%20from%20AN77-3.pdf which is unbelievably
slow, but I keep getting error messages saying the file is corrupted. I am
using a 2.5 Mb ADSL, which is normally pretty fast.

Frank

"Frank" wrote in message
news:6Pnlf.138047$y_1.50980@edtnps89...


The type of coupler I am thinking of is indeed where the coupling lines
for
forward and reflected are in the same physical region. The point I was
making refers particularly to the HP 778D; where it is specified in:
http://cp.literature.agilent.com/lit.../5952-8133.pdf
"Data can be read from the two meters of the vector voltmeter and
transferred directly to a Smith Chart". Indicating that there is no
significant phase error over the nominal bandwidth of the coupler.

The old vector voltmeter has a phase offset ability to compensate.


Just read Wes' comments on the line stretcher on the above pdf. I must
admit I also wondered about that, but assume it is intended as a cal
adjustment for a short/open standard.

See my previous post on this. It IS used to get both samples "At the
measurement plane."

73, Steve, K,9.D;C'I

The type of coupler I am thinking of is indeed where the coupling lines
for
forward and reflected are in the same physical region. The point I was
making refers particularly to the HP 778D; where it is specified in:
http://cp.literature.agilent.com/lit.../5952-8133.pdf
"Data can be read from the two meters of the vector voltmeter and
transferred directly to a Smith Chart". Indicating that there is no
significant phase error over the nominal bandwidth of the coupler.

The old vector voltmeter has a phase offset ability to compensate.


Just read Wes' comments on the line stretcher on the above pdf. I must
admit I also wondered about that, but assume it is intended as a cal
adjustment for a short/open standard.

See my previous post on this. It IS used to get both samples "At the
measurement plane."

73, Steve, K,9.D;C'I

Ok, Steve, makes sense. Anyway, have re-visited Matthaei et al, and also
"Foundations for Microstrip Circuit Design", by T. C. Edwards (Edwards does
appear to contain an error referring to "j-omega", which should simply be
"omega"). Both texts agree on the expression for coupling on a single
section, quarter wave, TEM mode, coupled transmission line. Also realized
that the equation does contain a frequency parameter.

Running an analysis in MathCAD, for a 20 dB coupler with design center at
150 MHz, produces some interesting results:

Coupling at 100 MHz, and 200 MHz = -22.5 dB;

Maximum phase error -- at band edges = +/- 2.5 degrees.

Increasing the frequency range of analysis from 50 to 250 MHz shows a
dramatic drop in coupling amplitude, at these frequency limits, to 32 dB.
Phase error, however, does not seem to be effected very much; peaking at +/-
2.8 degrees, and following what appears to be a sinusoidal curve.

From experience I know that coupler directivity degrades significantly
beyond the design bandwidth. Edwards does state: " Values of directivity,
on microstrip, beyond 12 to 14 dB are difficult to achieve". Co-planar
structures are much better, and can easily be analyzed with Genesys' 2.5D EM
simulations. Without access to HFSS, or similar FEM programs, I doubt
directivity could be calculated for coaxial structures.

The main problem, with operation of a coupler beyond its design bandwidth,
appears to be its loss of coupling. This, combined with degradation of
directivity, would certainly account for very large phase errors.

Frank

Let's try again... I've placed the file he

http://users.triconet.org/wesandlind...rom_AN77-3.pdf