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
--

"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

Wes,

The file reports an error. Can you load it?

Thanks very much. Dan

Wes Stewart wrote:
On Tue, 06 Dec 2005 16:48:32 GMT, "Frank"
wrote:


Sort of, but not exactly. As I described in another thread, the
pitfall is the fact that the phase lengths of the coupler are
different. You could set the phase offset to zero at frequency X but
when you change to frequency Y, there is a built-in error.

I should have added that you don't do this with a 50 ohm load, but a
short circuit instead. You want a big reflection with a known phase
for a calibration standard.

The first step is to establish the accuracy of the test equipment. A
short/open at the output port of the directional coupler should produce a
frequency independant phase shift between the forward and reverse coupled
port.


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.



If you are not getting 180/0 deg, then there is something
fundamentally wrong with the test set up. Connecting both probes of the
8405A to the same source should establish if any errors exist in the vector
voltmeter. Using the BNC/probe adapters is essential to maintain
repeatability in your measurements.

Reference HP's 778D coupler at
http://cp.literature.agilent.com/lit.../5952-8133.pdf :

"
Impedance Measurements

The 778D is also well suited for measurements of impedance when used with

the Agilent 8405A vector voltmeter. The technique is described in
Application

Note 77-3, Measurement of Complex Impedance, available at your nearest

Agilent sales office. Again, a reflectometry technique is used. With the
vector

voltmeter, however, both magnitude and phase angle of the reflection
coefficient

can be measured. This setup is shown in Figure 3.

Data can be read from the two meters of the vector voltmeter and transferred

directly to a Smith Chart to provide impedance of such devices as antennas
or

other passive components.


"It is many years since I have seen AN77-3, but I seem unable to find a copy
on the web.

Frank


The pertinent pages are he

http://www.qsl.net/n7ws/Pages%20from%20AN77-3.pdf

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

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

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

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.