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Measure Z with Vector Voltmeter properly



 
 
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  #1  
Old April 28th 04, 11:18 PM
The other John Smith
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Posts: n/a
Default Measure Z with Vector Voltmeter properly

I now have an HP 8405A Vector Voltmeter. I also have a Narda dual
directional coupler. Here is how I plan to set up and measure things at
about 440 MHz (antennas, etc) so I can determine their impedance (view in
fixed-width font):


50 Ohm Coax 50 Ohm Coax
.----------. .-----------. .--.
| | | | | |
| | \ | Dual | \ | |
| RF Gen |------------|T Coupler L|-------------| | Load
| | / | | / | |
| | | R F | | |
'----------' '-----------' '--'
| |
.----------. | | .---------.
| |--| |--| |
'----------' | | '---------'
50 Ohms | | 50 Ohms
| |
\|/ \|/
.----------------------------------.
| B A |
| |
| |
| |
| HP 8405A Vector Voltmeter |
'----------------------------------'
created by Andy┬┤s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de


T is the transmitter port, L is the load port, F is the -30 dB forward
coupled port, and R is the -30 dB reverse coupled port.

The HP voltmeter is able to zero out the phase angle between channels A and
B. So, I'll put a 50 Ohm dummy load out there for the Load and set the phase
angle offset to zero.

Then I'll put the actual load out there for the Load and read the channel A
voltage, the channel B voltage, and the phase angle between them.

Then I'll calculate the impedance with:

Z = 50(1+Er/Ef)/(1-Er/Ef)

where Ef is the forward voltage and Er is the reflected voltage.

For my tests, I will assume the coax really is 50 Ohms and non-reactive.
Since A is the reference, it will appear to be a real number. Ef will be a
complex number.

I'm not looking extreme accuracy. About plus or minus 10 to 15 percent will
be acceptable. Will this work? Your thoughts and advice will be appreciated.

Thanks,
John


  #2  
Old April 29th 04, 12:37 AM
Richard Clark
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Default

On Wed, 28 Apr 2004 22:18:19 GMT, "The other John Smith"
wrote:

Will this work? Your thoughts and advice will be appreciated.

Thanks,
John


Hi John,

The topology is good, however there is work to do to justify your
confidence in the numbers (even if you are willing to discount them to
10 to 15%; which, by the way, is quite optimistic).

I presume you have three 50 Ohm loads, or so I am lead to believe by
the nature of your description. If so, it would do well to rotate
them all through each port and confirm NOTHING changes. This is no
simple expectation. It also pays immensely to have calibrated
mismatch loads on hand to confirm your measurements of an unknown (the
system may fail unbeknownst to you otherwise).

Further, you should also swap the dual coupler input/output and the
side arm ports to confirm it is in fact operating at fixed ratio (this
says nothing of the presumed ratio, but is still a necessary step).
Then repeat the paragraph above concerning the 50 Ohm load rotation.
You can then proceed to confirming the coupling coefficient which may
surprise you (you haven't given the pedigree of this particular
beastie).

I presume you have undamaged leads for your meter (I won't even
presume they are calibrated, but for these purposes, testing should
reveal problems if they are not sufficient to the task).

I presume your source is powerful enough to present at least 1mV of RF
to the meter. I know the meter is more sensitive, but you need head
room to simple measure the return loss (or Z or SWR or any derived
characteristic).

I presume your source is free of spurs and harmonics at that power.
It hardly is worth the effort to measure out of band products.

I presume your source will not pull when presented with a large
mismatch.

I presume your source offers a 50 Ohm output Z. There is nothing like
a mismatched source looking at a mismatched load to increase confusion
by the square.

So, sure, it'll be a snap. ;-)

73's
Richard Clark, KB7QHC
  #3  
Old April 29th 04, 03:17 AM
The other John Smith
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Posts: n/a
Default

"Richard Clark" wrote in message
...
On Wed, 28 Apr 2004 22:18:19 GMT, "The other John Smith"
wrote:

Will this work? Your thoughts and advice will be appreciated.

Thanks,
John


Hi John,

The topology is good, however there is work to do to justify your
confidence in the numbers (even if you are willing to discount them to
10 to 15%; which, by the way, is quite optimistic).

I presume you have three 50 Ohm loads, or so I am lead to believe by
the nature of your description. If so, it would do well to rotate
them all through each port and confirm NOTHING changes. This is no
simple expectation. It also pays immensely to have calibrated
mismatch loads on hand to confirm your measurements of an unknown (the
system may fail unbeknownst to you otherwise).



A good tip and one I'll adopt. I do have three 50 Ohm loads. I don't have
mismatched loads, but I can parallel 50 Ohm loads to make a 25 Ohm load.
Come to think of it, 75 and 93 Ohm terminators are available through Mouser.
Perhaps I should invest in some.


Further, you should also swap the dual coupler input/output and the
side arm ports to confirm it is in fact operating at fixed ratio (this
says nothing of the presumed ratio, but is still a necessary step).
Then repeat the paragraph above concerning the 50 Ohm load rotation.
You can then proceed to confirming the coupling coefficient which may
surprise you (you haven't given the pedigree of this particular
beastie).



Yes. I did not mention that I would do that, but my plan was to compare the
forward and reverse coupled outputs and record the ratio of the two. I don't
think the absolute value of coupling (supposedly -30 dB) is needed for now.
The Narda coupler model is not listed at Narda's Web site, so I'm going by
the marking on the case.

I have no way of calibrating the Vector Voltmeter.


I presume you have undamaged leads for your meter (I won't even
presume they are calibrated, but for these purposes, testing should
reveal problems if they are not sufficient to the task).



They are undamamged as best as I can tell. Both channels read the same and
agree with a homebrew power meter. I realize the power meter is probably
less accurate than the voltmeter.


I presume your source is powerful enough to present at least 1mV of RF
to the meter. I know the meter is more sensitive, but you need head
room to simple measure the return loss (or Z or SWR or any derived
characteristic).



I can use an amateur transceiver with or without attenuators if need be.


I presume your source is free of spurs and harmonics at that power.
It hardly is worth the effort to measure out of band products.



The Vector Voltmeter has a 1 kHz bandwidth and a limited-range PLL whose
frequency band is manually settable. HP says it will lock to an incoming
square wave fundamental without problems. However, I also have the
transceiver and an HP signal generator both of which will have fairly clean
outputs.


I presume your source will not pull when presented with a large
mismatch.



The transceiver will not. I can't testify to the stability of the HP
generator at this time. In any case, the voltmeter will track it. My only
problem may be that I am unsure of the exact frequency if it gets pulled
away from the preset value. I guess I can use the receiver to verify
frequency until I gain confidence in the generator's stability.


I presume your source offers a 50 Ohm output Z. There is nothing like
a mismatched source looking at a mismatched load to increase confusion
by the square.



What is the output impedance of an amateur transmitter? I see arguments all
the time about this. And even so, what will be the result if it is not
matched? If there are reflections from the source, it will show up in the
composite forward voltage which will be the reference anyway. Is this not
so?


So, sure, it'll be a snap. ;-)

73's
Richard Clark, KB7QHC



Is my equation correct and are the comments associated with it correct?

It appears that you have given this some thought. Thank you.

John KD5YI


  #4  
Old April 29th 04, 04:18 AM
Richard Clark
Guest
 
Posts: n/a
Default

On Thu, 29 Apr 2004 02:17:12 GMT, "The other John Smith"
wrote:

"Richard Clark" wrote in message
.. .
On Wed, 28 Apr 2004 22:18:19 GMT, "The other John Smith"
wrote:


A good tip and one I'll adopt. I do have three 50 Ohm loads. I don't have
mismatched loads, but I can parallel 50 Ohm loads to make a 25 Ohm load.
Come to think of it, 75 and 93 Ohm terminators are available through Mouser.
Perhaps I should invest in some.


This does not replace testing however. Care should be taken to not
heat the loads as this will cause them to drift in value. Use them
with 1/10 rated power dissipation capacity.

Further, you should also swap the dual coupler input/output and the
side arm ports to confirm it is in fact operating at fixed ratio (this
says nothing of the presumed ratio, but is still a necessary step).
Then repeat the paragraph above concerning the 50 Ohm load rotation.
You can then proceed to confirming the coupling coefficient which may
surprise you (you haven't given the pedigree of this particular
beastie).


Yes. I did not mention that I would do that, but my plan was to compare the
forward and reverse coupled outputs and record the ratio of the two.


This goes further than that, reverse the input/output ports as well.
ALL ports must be tested against every other port. Along with the
loads, this becomes an increasingly larger matrix of readings - it is
all necessary. This is about Orthogonality because as a bridge this
cannot be taken for granted.

I don't
think the absolute value of coupling (supposedly -30 dB) is needed for now.
The Narda coupler model is not listed at Narda's Web site, so I'm going by
the marking on the case.


OK, so it is a Narda. They are typically very good from 10% to 90%
over the band of operation (if you also use the correction factors).

I have no way of calibrating the Vector Voltmeter.


Such is life. As you are only employing it to make relative
measurements, what you DO need to calibrate is the Gain/Attenuator
range control. Construct or obtain a good step attenuator. This, of
course re-introduces the issue of calibrating it, but if you can
reduce variables and check its DC and low AC operation, you are fairly
down the road ahead of bonehead problems. If you have fixed
attenuators that are of quality, you can do the RF side through
substitution.

I presume you have undamaged leads for your meter (I won't even
presume they are calibrated, but for these purposes, testing should
reveal problems if they are not sufficient to the task).


They are undamamged as best as I can tell. Both channels read the same and
agree with a homebrew power meter. I realize the power meter is probably
less accurate than the voltmeter.


So many mistakenly infer the opposite.

I presume your source offers a 50 Ohm output Z. There is nothing like
a mismatched source looking at a mismatched load to increase confusion
by the square.


What is the output impedance of an amateur transmitter?


50 Ohms at rated output - just like the manufacture specifies. Other
useful power settings are not terribly far off (somewhere between 30
and 80 Ohms).

I see arguments all the time about this.


Nonsense is free of charge, and worth every penny spent.

And even so, what will be the result if it is not
matched? If there are reflections from the source, it will show up in the
composite forward voltage which will be the reference anyway. Is this not
so?


In SPADES! A source that is 2:1 mismatched feeding a load that is 2:1
mismatched runs the minimum error of your measurement upwards to 20 or
30% [It could be worse, I'm too encumbered to find my reference to
quote accurately.] This is why I said your low-ball acceptance of
10-15% error was stardust.

Is my equation correct and are the comments associated with it correct?


I'm focusing on technique here. I presume you've done your math
right. The tougher part is realizing it accurately in a world of
error.

It appears that you have given this some thought. Thank you.


Hi John,

I spent a few years as a Metrologist specializing in RF power
measurement. Very, very few know the term (even engineers) - think
National Bureau of Standards (or NIST). In practical terms, this
means I have calibrated every item in front of you (and much more).

So, with all the caveats in place, you can easily step right into it
by abandoning them all and noting some values. Then you can implement
each of these issues and observe how much the data changes. This is
the merit of knowing your equipment and your skill. No measurement is
right the first time, nor the second, and rarely the third time.
Besides, it is more fun to refine your bench and testing and close in
on the actual value by hammering out error. The bench artist will
even disturb the set up with a known error, and observe how it
propagates through to the final determination.

73's
Richard Clark, KB7QHC
  #5  
Old April 29th 04, 03:03 PM
The other John Smith
Guest
 
Posts: n/a
Default


"Richard Clark" wrote in message
...

I spent a few years as a Metrologist specializing in RF power
measurement. Very, very few know the term (even engineers) - think
National Bureau of Standards (or NIST). In practical terms, this
means I have calibrated every item in front of you (and much more).

So, with all the caveats in place, you can easily step right into it
by abandoning them all and noting some values. Then you can implement
each of these issues and observe how much the data changes. This is
the merit of knowing your equipment and your skill. No measurement is
right the first time, nor the second, and rarely the third time.
Besides, it is more fun to refine your bench and testing and close in
on the actual value by hammering out error. The bench artist will
even disturb the set up with a known error, and observe how it
propagates through to the final determination.

73's
Richard Clark, KB7QHC




Thanks for all the good info, Richard.

John


  #6  
Old April 29th 04, 05:46 PM
Steve Nosko
Guest
 
Posts: n/a
Default


"The other John Smith" wrote in message
link.net...
I now have an HP 8405A Vector Voltmeter. I also have a Narda dual
directional coupler.



It's been a long time since I've used a VVM. I see 'em in the "send to
scrap" piles around here. Richard points out much more than I could and
certainly has a more jaundiced eye than I, but this one thing cought my eye

The HP voltmeter is able to zero out the phase angle between channels A

and
B. So, I'll put a 50 Ohm dummy load out there for the Load and set the

phase
angle offset to zero.


I forget how it offsets the phase reading, but I'd start by making the cable
lengths so that the two measurement points are "at the same pointy along the
line". (of course I have a line stretcher to make it easier)

It sure seems to me that the Zero angle 50 ohm load must be a bad
assumption. The 50 could be anywhere angle-wise and is the hardest to
measure since the reflected is so small...Wouldn't it be "better" to use a
short (of course I'm looking at a couple of Narda shorts at my operating
position every night) for a primary 180 degree reference (the open being
"harder" to obtain)? I think you mentioned using several other loads as well
(parallel 50s=25, etc). I would use several different loads like this to
"measure" to see how the measurements look compared to expected (cal-kit
style). I'd measure opens, shorts and several other mismatches to make sure
all available "knows" looks close, then proceed.
I'd also pad the Tx output. Long lengths of miniature coax in lew of
actual power attenuators.
OOPS! gotta re-boot, 73

--
Steve N, K,9;d, c. i My email has no u's.


  #7  
Old April 30th 04, 05:56 PM
Wes Stewart
Guest
 
Posts: n/a
Default

On Wed, 28 Apr 2004 22:18:19 GMT, "The other John Smith"
wrote:

|I now have an HP 8405A Vector Voltmeter. I also have a Narda dual
|directional coupler. Here is how I plan to set up and measure things at
|about 440 MHz (antennas, etc) so I can determine their impedance (view in
|fixed-width font):
|
[snip]


You need a copy of HP app note 77-3.
  #8  
Old April 30th 04, 10:41 PM
John
Guest
 
Posts: n/a
Default


"Wes Stewart" wrote in message
news
On Wed, 28 Apr 2004 22:18:19 GMT, "The other John Smith"
wrote:

|I now have an HP 8405A Vector Voltmeter. I also have a Narda dual
|directional coupler. Here is how I plan to set up and measure things at
|about 440 MHz (antennas, etc) so I can determine their impedance (view in
|fixed-width font):
|
[snip]


You need a copy of HP app note 77-3.



Yes, I do.


  #9  
Old April 30th 04, 11:35 PM
Wes Stewart
Guest
 
Posts: n/a
Default

On Fri, 30 Apr 2004 16:41:06 -0500, "John"
wrote:

|
|"Wes Stewart" wrote in message
|news | On Wed, 28 Apr 2004 22:18:19 GMT, "The other John Smith"
| wrote:
|
| |I now have an HP 8405A Vector Voltmeter. I also have a Narda dual
| |directional coupler. Here is how I plan to set up and measure things at
| |about 440 MHz (antennas, etc) so I can determine their impedance (view in
| |fixed-width font):
| |
| [snip]
|
|
| You need a copy of HP app note 77-3.
|
|
|Yes, I do.

I figured you might. I scanned it into pdf, but to get decent quality
the file is huge and I only have dialup service at about 26 kB.
(Where is that BPL [g])

I'll put it on my web site later when I have a couple of hours free if
you want it.

Wes


|

  #10  
Old May 1st 04, 02:41 AM
Wes Stewart
Guest
 
Posts: n/a
Default

On Fri, 30 Apr 2004 15:35:11 -0700, Wes Stewart
wrote:


|| You need a copy of HP app note 77-3.


http://www.qsl.net/n7ws/AN77-3.pdf
 




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