On Sun, 01 Jan 2006 14:01:18 -0700, Wes Stewart
wrote:

On Sun, 01 Jan 2006 19:15:12 -0000, (Dave Platt)
wrote:

In article ,
Wes Stewart wrote:

The measuring setup is a standard signal generator feeding a tee. One branch
feeds the A input of the 8405A and the other a 20 db directional configured in
reverse, that is feeding the out port. The assumption is the reflected signal
will be read correctly on the in port. The in port is then terminated or
connected to an antenna.

It would also be nice it you had a 6 - 10 dB pad between the generator
and the directional coupler (DC); located right at the DC. You want
the source match to be set right there and the A probe to sample right
there.

The 8405A manual indicates the use of a power divider, and then a pair
of equal-value pads. One side goes to the probe T for the A
(reference) probe and thence to the termination, and the other goes to
the probe T for the B probe and thence to the device-under-test.

Actually, my manual does not show this. Although I have the full kit
of a resistive tee, two 50 ohm "N" sampling tees and appropriate
terminations, I don't believe Dan does.

The manual is quite clear that the A and B probes need to be connected
to points which are isolated from one another.

I've read this someplace, but again my version of the manual (unless
I'm really missing something) doesn't say it. Nevertheless, the
directional coupler provides the isolation between probes.

You really don't want
the oddities of the load connected to the B side to affect the
voltage/phase of the reference signal seen by the "A" probe - it'd
certainly wreck the measurement.

Using a power divider, and resistive pads for isolation is one way to
do this.

Actually, you do want to measure (include) the effects of the load on
the source. Although you ideally want the source to be unchangable
with respect to changes in the load (good source match). That is the
point of my suggestion to pad the source at the input to the coupler.
(Just as does paragraph 2.3 of Owen's reference)

But if the load -does- pull the source you damn sure want to know it
and account for it. The desired ratio is that between the incident
signal and the reflected signal. Sampling somewhere off in isolated
space via a tee and additional padding does -not- yield the incident
signal.


Using a pad followed by a dual directional coupler (as in
the experiment page to which Owen posted a link - thanks!) is another.

My point exactly. With the dual coupler the forward coupled arm -is-
measuring the incident signal -regardless- of what effects load pull
have on the source output.

I've got to clarify this a bit if I can...

If you have the full set of parts per figure 11 in AN77-3 and you are
using them as shown, then with equal loads on the two ends, the
circuit is essentially a resistive Wheatstone bridge in balance with
the null detected by the difference between probes A and B.

In this case, the "incident" signal -is- measured by the A probe and
the effects to the source by a changing load are incorporated into the
measurement.

In the case at hand, at least as I imagine it, there is no longer an
nice tidy resistive Wheatstone bridge, but some cabling and a
directional coupler in the mix. In this case, the generator is no
longer the "source", the source is the signal at the input to the
coupler. It is my belief (unless I change my mind later) that a
sample derived from a resistive divider remote from the input to the
directional coupler is not a true measure of the incident signal.

In article ,
Wes Stewart wrote:

The 8405A manual indicates the use of a power divider, and then a pair
of equal-value pads. One side goes to the probe T for the A
(reference) probe and thence to the termination, and the other goes to
the probe T for the B probe and thence to the device-under-test.

Actually, my manual does not show this. Although I have the full kit
of a resistive tee, two 50 ohm "N" sampling tees and appropriate
terminations, I don't believe Dan does.

The manual is quite clear that the A and B probes need to be connected
to points which are isolated from one another.

I've read this someplace, but again my version of the manual (unless
I'm really missing something) doesn't say it.

The BAMA copy mentions it in paragraph 3-14. Later text indicates
that attaching the two probes to a single point is an appropriate way
to set phase-zero.

Nevertheless, the
directional coupler provides the isolation between probes.

I see the issue, and I think I was conflating two different sorts of
measurement regimes.

The splitter/isolator/pad arrangement I was referring to appears on
page 3-3 of the 8405A manual available at BAMA. It's what's
appropriate for doing an in-line test of a transmission line or other
network, where you want to see the effect of the network itself and
can measure (via probe B) at the network's output.

Page 3-4 shows a somewhat similar hookup, which doesn't include the
resistive pads... I presume because the device-under-test (an
amplifier) is assumed to have high isolation as part of its design.

Neither of these hookups wouldn't work for measuring an antenna, since
you can't measure at the antenna's output. Instead, using a
directional coupler provides the necessary isolation, and (as you
point out) lets you determine the incident and reflected signals
accurately.

I've got to clarify this a bit if I can...

If you have the full set of parts per figure 11 in AN77-3 and you are
using them as shown, then with equal loads on the two ends, the
circuit is essentially a resistive Wheatstone bridge in balance with
the null detected by the difference between probes A and B.

In this case, the "incident" signal -is- measured by the A probe and
the effects to the source by a changing load are incorporated into the
measurement.

In the case at hand, at least as I imagine it, there is no longer an
nice tidy resistive Wheatstone bridge, but some cabling and a
directional coupler in the mix. In this case, the generator is no
longer the "source", the source is the signal at the input to the
coupler. It is my belief (unless I change my mind later) that a
sample derived from a resistive divider remote from the input to the
directional coupler is not a true measure of the incident signal.

Hmmm. In the general case, I believe you're correct.

I suspect that the setup shown in the 8405A manual sets up a specific
special case, though. The diagrams and text seem to be defining a
case in which:

- there is a physical and electrical symmetry in the T arrangement -
that is, the power splitter is symmetrical, and the pair of
attenuator pads between the splitter and the (A probe tap) and
(device under test) are matched. The manual makes a point of this
issue.

- The pads being used are matched to the system's transmission line
impedance, so that any reflected signal coming back from the
DUT/coupler sees a proper termination by the source (the pad and
signal generator, in this case) and is not re-reflected.

In this particular situation, I believe that the incident signals
reaching the DUT (the input to the coupler, in this case) and the "A"
probe, would be identical... would they not? The proper termination
of the reflected wave will mean that it won't re-reflect off of the
generator and alter the incident wave. The "A" probe signal (off on
its side of the "T") and a signal read out via the incident-wave tap
on the directional coupler ought to be the same, once the coupling
coefficient is taken into account... no?

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On Sun, 01 Jan 2006 22:10:09 -0000, (Dave Platt)
wrote:


In article ,
Wes Stewart wrote:

The 8405A manual indicates the use of a power divider, and then a pair
of equal-value pads. One side goes to the probe T for the A
(reference) probe and thence to the termination, and the other goes to
the probe T for the B probe and thence to the device-under-test.

Actually, my manual does not show this. Although I have the full kit
of a resistive tee, two 50 ohm "N" sampling tees and appropriate
terminations, I don't believe Dan does.

The manual is quite clear that the A and B probes need to be connected
to points which are isolated from one another.

I've read this someplace, but again my version of the manual (unless
I'm really missing something) doesn't say it.

The BAMA copy mentions it in paragraph 3-14. Later text indicates
that attaching the two probes to a single point is an appropriate way
to set phase-zero.

Bingo. Didn't seem like an "Electrical feature" to me :-)

Nevertheless, the
directional coupler provides the isolation between probes.

I see the issue, and I think I was conflating two different sorts of
measurement regimes.

The splitter/isolator/pad arrangement I was referring to appears on
page 3-3 of the 8405A manual available at BAMA. It's what's
appropriate for doing an in-line test of a transmission line or other
network, where you want to see the effect of the network itself and
can measure (via probe B) at the network's output.

Page 3-4 shows a somewhat similar hookup, which doesn't include the
resistive pads... I presume because the device-under-test (an
amplifier) is assumed to have high isolation as part of its design.

No, that's very similar to figure 11 in AN77-3 that I mentioned below.
Without inserting the device under test per Fig 3-3, but removing the
right hand 50 ohm load and connecting the device there, reflection
measurements can be performed. That's a technique described in
AN77-3. The configuration is that of a Wheatstone bridge as mentioned
earlier.

But I still submit that when you separate the bridge, insert a DC and
some cabling, you lose the symmetry and the signal measured by the A
probe is not necessarily the same as the signal incident at the input
to the DC. Close maybe, but not something I would rely on.

Remember, when doing the calibration there is a 100% reflection. This
can have a huge perturbing effect on the incident signal at the
coupler input if the source is not well matched. That's why I
originally suggested a pad right at the coupler input, especially if
there is some cabling between the generator (or power splitting tee)
and the DC.


Neither of these hookups wouldn't work for measuring an antenna, since
you can't measure at the antenna's output.

Not so, see above.

Instead, using a
directional coupler provides the necessary isolation, and (as you
point out) lets you determine the incident and reflected signals
accurately.

I've got to clarify this a bit if I can...

If you have the full set of parts per figure 11 in AN77-3 and you are
using them as shown, then with equal loads on the two ends, the
circuit is essentially a resistive Wheatstone bridge in balance with
the null detected by the difference between probes A and B.

In this case, the "incident" signal -is- measured by the A probe and
the effects to the source by a changing load are incorporated into the
measurement.

In the case at hand, at least as I imagine it, there is no longer an
nice tidy resistive Wheatstone bridge, but some cabling and a
directional coupler in the mix. In this case, the generator is no
longer the "source", the source is the signal at the input to the
coupler. It is my belief (unless I change my mind later) that a
sample derived from a resistive divider remote from the input to the
directional coupler is not a true measure of the incident signal.

Hmmm. In the general case, I believe you're correct.

I suspect that the setup shown in the 8405A manual sets up a specific
special case, though. The diagrams and text seem to be defining a
case in which:

- there is a physical and electrical symmetry in the T arrangement -
that is, the power splitter is symmetrical, and the pair of
attenuator pads between the splitter and the (A probe tap) and
(device under test) are matched. The manual makes a point of this
issue.

- The pads being used are matched to the system's transmission line
impedance, so that any reflected signal coming back from the
DUT/coupler sees a proper termination by the source (the pad and
signal generator, in this case) and is not re-reflected.

In this particular situation, I believe that the incident signals
reaching the DUT (the input to the coupler, in this case) and the "A"
probe, would be identical... would they not? The proper termination
of the reflected wave will mean that it won't re-reflect off of the
generator and alter the incident wave. The "A" probe signal (off on
its side of the "T") and a signal read out via the incident-wave tap
on the directional coupler ought to be the same, once the coupling
coefficient is taken into account... no?

No. The B probe, in the single directional coupler arrangement, is
not measuring -incident-, but reflected signal.

In any event, Dan has stated that he doesn't have all of this stuff
and is stuck using the DC only. My suggestion holds, put a pad at the
DC input, measure the incident at the DC input and of course, the
reflected at the coupled port.

In article ,
Wes Stewart wrote:

Bingo. Didn't seem like an "Electrical feature" to me :-)

Yeah. I'm not sure just why there would be substantial interaction if
the two points are connected to the same test point, since the rated
impedance is pretty high even without 10:1 isolators.

But I still submit that when you separate the bridge, insert a DC and
some cabling, you lose the symmetry and the signal measured by the A
probe is not necessarily the same as the signal incident at the input
to the DC. Close maybe, but not something I would rely on.

The signal on the other side of the T-and-attenuator setup wouldn't be
the same as the signal at the input to the DC, certainly, since the
signal at the input of the DC would be affected by the reflected
signal. I don't disagree with you there.

What I suggest, though, is that the signal on the "A" probe (at the
other side of the T from the DC), and a signal as seen at the output
of the DC's "forward" coupler line, ought to be very closely
correlated. They'd differ by the coupler's coupling factor, of
course, and there's be a bit of phase shift from the coupler
(dependent on the coupler line length and the frequency). However,
the loading at the coupler output from the load (or the calibration
short) ought not to affect the signal appearing at the 'forward' tap
on the coupler.

Remember, when doing the calibration there is a 100% reflection. This
can have a huge perturbing effect on the incident signal at the
coupler input if the source is not well matched.

Agreed, and I don't suggest that measuring the incident at the coupler
input is a good idea.


That's why I
originally suggested a pad right at the coupler input, especially if
there is some cabling between the generator (or power splitting tee)
and the DC.

Agreed.

No. The B probe, in the single directional coupler arrangement, is
not measuring -incident-, but reflected signal.

True. I was assuming a double directional coupler, and asserting that
the "forward" output on the coupler will produce a signal equivalent
(except for scaling and perhaps a tad of phase shift) to a signal
taken from the far side of the splitter-and-pads "T".

In any event, Dan has stated that he doesn't have all of this stuff
and is stuck using the DC only. My suggestion holds, put a pad at the
DC input, measure the incident at the DC input and of course, the
reflected at the coupled port.

Yes, that should work quite well, and I think it'd give results pretty
much equivalent to [1] a dual directional coupler or [2] the
splitter-and-two-pads isolation arrangement.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Wes,

I only have copies of a few pages from the app note showing a bi-directional
coupler. Is your reference that the note shows a bridge?

Thanks - Dan

Wes Stewart wrote:
On Sun, 01 Jan 2006 22:10:09 -0000, (Dave Platt)
wrote:


In article ,
Wes Stewart wrote:


The 8405A manual indicates the use of a power divider, and then a pair
of equal-value pads. One side goes to the probe T for the A
(reference) probe and thence to the termination, and the other goes to
the probe T for the B probe and thence to the device-under-test.

Actually, my manual does not show this. Although I have the full kit
of a resistive tee, two 50 ohm "N" sampling tees and appropriate
terminations, I don't believe Dan does.

The manual is quite clear that the A and B probes need to be connected
to points which are isolated from one another.

I've read this someplace, but again my version of the manual (unless
I'm really missing something) doesn't say it.

The BAMA copy mentions it in paragraph 3-14. Later text indicates
that attaching the two probes to a single point is an appropriate way
to set phase-zero.


Bingo. Didn't seem like an "Electrical feature" to me :-)

Nevertheless, the
directional coupler provides the isolation between probes.

I see the issue, and I think I was conflating two different sorts of
measurement regimes.

The splitter/isolator/pad arrangement I was referring to appears on
page 3-3 of the 8405A manual available at BAMA. It's what's
appropriate for doing an in-line test of a transmission line or other
network, where you want to see the effect of the network itself and
can measure (via probe B) at the network's output.

Page 3-4 shows a somewhat similar hookup, which doesn't include the
resistive pads... I presume because the device-under-test (an
amplifier) is assumed to have high isolation as part of its design.


No, that's very similar to figure 11 in AN77-3 that I mentioned below.
Without inserting the device under test per Fig 3-3, but removing the
right hand 50 ohm load and connecting the device there, reflection
measurements can be performed. That's a technique described in
AN77-3. The configuration is that of a Wheatstone bridge as mentioned
earlier.

But I still submit that when you separate the bridge, insert a DC and
some cabling, you lose the symmetry and the signal measured by the A
probe is not necessarily the same as the signal incident at the input
to the DC. Close maybe, but not something I would rely on.

Remember, when doing the calibration there is a 100% reflection. This
can have a huge perturbing effect on the incident signal at the
coupler input if the source is not well matched. That's why I
originally suggested a pad right at the coupler input, especially if
there is some cabling between the generator (or power splitting tee)
and the DC.


Neither of these hookups wouldn't work for measuring an antenna, since
you can't measure at the antenna's output.


Not so, see above.


Instead, using a
directional coupler provides the necessary isolation, and (as you
point out) lets you determine the incident and reflected signals
accurately.


I've got to clarify this a bit if I can...

If you have the full set of parts per figure 11 in AN77-3 and you are
using them as shown, then with equal loads on the two ends, the
circuit is essentially a resistive Wheatstone bridge in balance with
the null detected by the difference between probes A and B.

In this case, the "incident" signal -is- measured by the A probe and
the effects to the source by a changing load are incorporated into the
measurement.

In the case at hand, at least as I imagine it, there is no longer an
nice tidy resistive Wheatstone bridge, but some cabling and a
directional coupler in the mix. In this case, the generator is no
longer the "source", the source is the signal at the input to the
coupler. It is my belief (unless I change my mind later) that a
sample derived from a resistive divider remote from the input to the
directional coupler is not a true measure of the incident signal.

Hmmm. In the general case, I believe you're correct.

I suspect that the setup shown in the 8405A manual sets up a specific
special case, though. The diagrams and text seem to be defining a
case in which:

- there is a physical and electrical symmetry in the T arrangement -
that is, the power splitter is symmetrical, and the pair of
attenuator pads between the splitter and the (A probe tap) and
(device under test) are matched. The manual makes a point of this
issue.

- The pads being used are matched to the system's transmission line
impedance, so that any reflected signal coming back from the
DUT/coupler sees a proper termination by the source (the pad and
signal generator, in this case) and is not re-reflected.

In this particular situation, I believe that the incident signals
reaching the DUT (the input to the coupler, in this case) and the "A"
probe, would be identical... would they not? The proper termination
of the reflected wave will mean that it won't re-reflect off of the
generator and alter the incident wave. The "A" probe signal (off on
its side of the "T") and a signal read out via the incident-wave tap
on the directional coupler ought to be the same, once the coupling
coefficient is taken into account... no?


No. The B probe, in the single directional coupler arrangement, is
not measuring -incident-, but reflected signal.

In any event, Dan has stated that he doesn't have all of this stuff
and is stuck using the DC only. My suggestion holds, put a pad at the
DC input, measure the incident at the DC input and of course, the
reflected at the coupled port.