dansawyeror wrote:
Do the measuring ports of a bi-directional coupler accurately represent
or preserve the relative phases of the signal?

Let's look at a typical SWR meter sampling circuit. The current is
sampled by a one turn primary on a ferrite toroid. The voltage is
sampled by a tap on the line close to the point at which the toroid
is mounted. At HF frequencies, a wavelength is so long compared to
that configuration that physical sample point errors are usually
considered to be negligible. That obviously changes at UHF+.

No coupler 100% preserves the relative phases. The question is:
What is the accuracy? For any configuration, a worst-case accuracy
can be specified. At 4 MHz, it's not a problem. At 4 GHz, it's a
big problem. At visible light frequencies, most don't even try.
--
73, Cecil http://www.qsl.net/w5dxp

Frank,

The bi-directional coupler is a machined block about 1 x 3 x 5. The inside is a
straight through line, the pickups are simply terminated one loop lines. It is a
UHF coupler that works reasonably down to 2 meters. When I configure this to
look at the forward and reflected 'open' circuit case they are not in phase.
Reflected lags forward by about 40 degrees. (I checked the connection delay and
this is not a cable issue.) This is frequency independent. Shorting the output
reverses this relationship. The outputs are terminated in 50 Ohms so I conclude
it is a 50 Ohm device. When I terminate the device in 50 Ohms the forward and
reflected outputs are out of phase by about 140 degrees.

What is the significance a non frequency dependent phase shift between forward
and reflected? This shift is frequency independent.

Thanks - Dan kb0qil


Frank wrote:
Your answer to the question about bidirectional couplers was they do not
compensate for phase shift. Let me ask it again:

Do the measuring ports of a bi-directional coupler accurately represent or
preserve the relative phases of the signal?

To put it another way is the phase shift of the driving and reflected
signals changed by the same about?

Thanks - Dan kb0qil


The phases seen at each coupled port should be identical to the phase of the
forward and reflected signals. This is easily verifiable, and frequency
independant, as follows:

No load -- forward and reverse amplitudes equal, and in phase;
Short circuit at output -- forward and reverse amplitudes equal, and 180
degrees phase difference;
50 ohm load -- reverse than forward by = specified coupler directivity,
and phase difference can 0 theta +/ 180.

This is only true if the frequencies are low enough such that the standards
do not require quantification by the use of "Standard definitions" -- see
www.vnahelp.com.


Regards,

Frank

Paul Burridge wrote:
On Sat, 03 Dec 2005 21:13:36 -0800, Roy Lewallen
wrote:


As I mentioned in my earlier posting, most people overestimate their
ability to make accurate RF measurements. It's not at all trivial. Be
sure to check your results frequently by measuring known load impedances
close to the values being measured. How do you find the values of those
"known" load impedances? Well, welcome to the world of metrology!


Roy, I've seen your postings hereabouts over the years and you've
always struck me as one of the most knowledgeable posters on this,
*the* most technically-challenging of all hobbies.

Thanks for your vote of confidence. But on the topic of network analyzer
measurements, I gladly defer to Wes Stewart, Tom Bruhns, and other
posters who have spent much more time making real-life measurements with
them than I have. I've used them from time to time, and for some really
challenging measurements, but not by any means as much as those folks have.

I've recently bought a VNA and am going about the laborious process of
setting it up with precisely-cut interconnects to the T/R bridge. Next
thing I need to know is...
Say I have a mica capacitor (for example) that I want to check for its
SRF. How should I mount this component so as to minimize stray L&C
from anything other than the component itself? IOW, what 'platform'
(for want of a better word) do I need to construct to permit accurate
measurements of this cap's RF characteristics in isolation?

In general, you minimize stray inductance by keeping leads short, and
capacitance by keeping conductors apart. The ideal setup is a coaxial
environment right up to the DUT, but even that is subject to coupling
around the DUT, both from one terminal to the other and from each
terminal to ground. If possible, the best plan is to calibrate out the
strays. That's a science and art in itself, and I'll have to yield to
people with more experience than mine for practical information about
how best to do this.

The effect of the strays depends heavily on what you're measuring. For
example, if you're measuring a low impedance, you can get by with more
shunt C than if you're measuring a high impedance. If you're measuring a
high impedance, you can tolerate more series inductance than when
measuring a low impedance. So when you inevitably find that you have to
make tradeoffs in designing a fixture, the trades you make will depend
on what you expect to measure.

Roy Lewallen, W7EL

Dan,

your original posting says the shift you are getting is frequency dependent.
Your last posting says it is not. Which one I read wrong?

Thks
Ivan

"dansawyeror" wrote in message
...
Frank,

The bi-directional coupler is a machined block about 1 x 3 x 5. The inside
is a
straight through line, the pickups are simply terminated one loop lines.
It is a
UHF coupler that works reasonably down to 2 meters. When I configure this
to
look at the forward and reflected 'open' circuit case they are not in
phase.
Reflected lags forward by about 40 degrees. (I checked the connection
delay and
this is not a cable issue.) This is frequency independent. Shorting the
output
reverses this relationship. The outputs are terminated in 50 Ohms so I
conclude
it is a 50 Ohm device. When I terminate the device in 50 Ohms the forward
and
reflected outputs are out of phase by about 140 degrees.

What is the significance a non frequency dependent phase shift between
forward
and reflected? This shift is frequency independent.

Thanks - Dan kb0qil


Frank wrote:
Your answer to the question about bidirectional couplers was they do not
compensate for phase shift. Let me ask it again:

Do the measuring ports of a bi-directional coupler accurately represent
or
preserve the relative phases of the signal?

To put it another way is the phase shift of the driving and reflected
signals changed by the same about?

Thanks - Dan kb0qil


The phases seen at each coupled port should be identical to the phase of
the
forward and reflected signals. This is easily verifiable, and frequency
independant, as follows:

No load -- forward and reverse amplitudes equal, and in phase;
Short circuit at output -- forward and reverse amplitudes equal, and 180
degrees phase difference;
50 ohm load -- reverse than forward by = specified coupler
directivity,
and phase difference can 0 theta +/ 180.

This is only true if the frequencies are low enough such that the
standards
do not require quantification by the use of "Standard definitions" --
see
www.vnahelp.com.


Regards,

Frank

The posts refer to two different couplers, the first posting is in reference to
a Mini-circuits ZFDC-1-3. The last posting is in reference to a bi-directional
coupler as described. At this point the objective is to 'learn' as much as
possible about the operation of couplers.


Ivan Makarov wrote:
Dan,

your original posting says the shift you are getting is frequency dependent.
Your last posting says it is not. Which one I read wrong?

Thks
Ivan

"dansawyeror" wrote in message
...

Frank,

The bi-directional coupler is a machined block about 1 x 3 x 5. The inside

is a

straight through line, the pickups are simply terminated one loop lines.

It is a

UHF coupler that works reasonably down to 2 meters. When I configure this

to

look at the forward and reflected 'open' circuit case they are not in

phase.

Reflected lags forward by about 40 degrees. (I checked the connection

delay and

this is not a cable issue.) This is frequency independent. Shorting the

output

reverses this relationship. The outputs are terminated in 50 Ohms so I

conclude

it is a 50 Ohm device. When I terminate the device in 50 Ohms the forward

and

reflected outputs are out of phase by about 140 degrees.

What is the significance a non frequency dependent phase shift between

forward

and reflected? This shift is frequency independent.

Thanks - Dan kb0qil


Frank wrote:

Your answer to the question about bidirectional couplers was they do not
compensate for phase shift. Let me ask it again:

Do the measuring ports of a bi-directional coupler accurately represent

or

preserve the relative phases of the signal?

To put it another way is the phase shift of the driving and reflected
signals changed by the same about?

Thanks - Dan kb0qil


The phases seen at each coupled port should be identical to the phase of

the

forward and reflected signals. This is easily verifiable, and frequency
independant, as follows:

No load -- forward and reverse amplitudes equal, and in phase;
Short circuit at output -- forward and reverse amplitudes equal, and 180
degrees phase difference;
50 ohm load -- reverse than forward by = specified coupler

directivity,

and phase difference can 0 theta +/ 180.

This is only true if the frequencies are low enough such that the

standards

do not require quantification by the use of "Standard definitions" --

see

www.vnahelp.com.


Regards,

Frank

On Sun, 04 Dec 2005 07:57:55 -0800, dansawyeror
wrote:

Wes,

Your answer to the question about bidirectional couplers was they do not
compensate for phase shift. Let me ask it again:

Do the measuring ports of a bi-directional coupler accurately represent or
preserve the relative phases of the signal?

To put it another way is the phase shift of the driving and reflected signals
changed by the same about?

Thanks - Dan kb0qil

I'm not sure I understand the question(s) but in the case of a vector
reflectometer using a dual directional coupler maybe this will help.

Here is a dual directional coupler.


Reverse Forward
| |
| |
|----------R R ---------|
X X
Input --A-----------------------B--C Load


Let's say that at frequency, F, the coupling factor (X) is -10 dB with
no phase shift between point B and the forward port and between point
A and the reverse port to keep it simple.

So a wave propagating in the forward direction (Input -- Load)
induces a signal at the forward port that is 10 dB below the input at
0 degrees phase with respect to point B. A wave propagating in the
opposite direction has the same relationship at the reverse port; 10
dB down and 0 degrees phase with respect to point A.

A to Reverse and B to Forward -might- track reasonably well in both
magnitude and phase, but in this case, it's immaterial.

Because B-A and C-B 0 there will be a frequency dependent phase
difference between A, B and C.

When we calibrate using a short on the load port here's what happens.

The signal at the forward port becomes the reference, i.e., unity
amplitude and 0 degrees phase.

The short creates a 100% reflection and -180 degree phase shift. This
signal propagates back down the main line to the source, which is
assumed to be a perfect match, so there is no re-reflection. A -10 dB
sample (by definition: unity) is coupled to the reverse port, with a
phase shift, theta(F), determined by the electrical length of the line
C - B - A.

Unless we are lucky enough to be Lotto winners, the signal at the
reflected port -will not- be 1 @ ang-180 deg. So our calibration
routine must do whatever math is necessary to make the ratio B/A = 1 @
ang-180. This fudge factor is then applied to all subsequent
measurements to "correct" the data.

Now to address (I think) your question. If we change frequencies,
theta(F) changes and the fudge factor no longer corrects for it.
While the coupling factors might track, it is of little consolation
because the calibration is good only at the frequency where it was
performed. Automatic network analyzers perform calibration at each
test frequency, or at least enough points to interpolate between.

Wes,

Thanks. If I read the gist of your reply the physical dimensions are the root
cause of the phase difference between the forward and reflected signals. Is this
true?

Thanks again - this is very helpful. Dan - kb0qil

Wes Stewart wrote:
On Sun, 04 Dec 2005 07:57:55 -0800, dansawyeror
wrote:


Wes,

Your answer to the question about bidirectional couplers was they do not
compensate for phase shift. Let me ask it again:

Do the measuring ports of a bi-directional coupler accurately represent or
preserve the relative phases of the signal?

To put it another way is the phase shift of the driving and reflected signals
changed by the same about?

Thanks - Dan kb0qil


I'm not sure I understand the question(s) but in the case of a vector
reflectometer using a dual directional coupler maybe this will help.

Here is a dual directional coupler.


Reverse Forward
| |
| |
|----------R R ---------|
X X
Input --A-----------------------B--C Load


Let's say that at frequency, F, the coupling factor (X) is -10 dB with
no phase shift between point B and the forward port and between point
A and the reverse port to keep it simple.

So a wave propagating in the forward direction (Input -- Load)
induces a signal at the forward port that is 10 dB below the input at
0 degrees phase with respect to point B. A wave propagating in the
opposite direction has the same relationship at the reverse port; 10
dB down and 0 degrees phase with respect to point A.

A to Reverse and B to Forward -might- track reasonably well in both
magnitude and phase, but in this case, it's immaterial.

Because B-A and C-B 0 there will be a frequency dependent phase
difference between A, B and C.

When we calibrate using a short on the load port here's what happens.

The signal at the forward port becomes the reference, i.e., unity
amplitude and 0 degrees phase.

The short creates a 100% reflection and -180 degree phase shift. This
signal propagates back down the main line to the source, which is
assumed to be a perfect match, so there is no re-reflection. A -10 dB
sample (by definition: unity) is coupled to the reverse port, with a
phase shift, theta(F), determined by the electrical length of the line
C - B - A.

Unless we are lucky enough to be Lotto winners, the signal at the
reflected port -will not- be 1 @ ang-180 deg. So our calibration
routine must do whatever math is necessary to make the ratio B/A = 1 @
ang-180. This fudge factor is then applied to all subsequent
measurements to "correct" the data.

Now to address (I think) your question. If we change frequencies,
theta(F) changes and the fudge factor no longer corrects for it.
While the coupling factors might track, it is of little consolation
because the calibration is good only at the frequency where it was
performed. Automatic network analyzers perform calibration at each
test frequency, or at least enough points to interpolate between.