Can anyone post any informationon or web addresses on phase matching and
batch / set matching of coax cables? I am interested in principles and
procedures to phase match coax cables. Phase matching involves cutting coax
cables to the same electrical length. The phase matched cables can then be
analysed and put into batches or sets i.e. cables with similar
characteristics and frequency response are put into a set. What would be
analysed to form a set?

This practice is used on coax cables that feed antennas for satellite
systems. For example, if several antennas receive GPS signals, to
triangulate position, the GPS signals must all arrive at a particular point
at exactly the same time.

It also appears that going for the best quality low loss cable can actually
be worse, because using lower quality coax cable with some loss can dampen
the reflections from in-line connectors.

Stages are 1) phase matching 2) batch matching 3) assemble cables chosen by
batch matching process onto RF system and perform vector generation. A
vector gen file is created which is a calibration snapshot of the system
i.e. records the RF performance in the form of a signature or footprint.

Measuring instrument is a Network Analyser HP8753.

David first let me comment that using lossy coax to make the results
look better is muddy thinking at best... Use the lowest loss feed line
you can get, period!

Next, check the ARRL Antenna Handbook for a diagram of the equipment
and layout for cutting coax to resonance at a given frequency... I have
not worked at GPS frequency but the basics are the same at 435 mhz -
just harder at the higher frequencies...
A signal generator for driving the coax... Put a 3dB pad on the output
of the generator to stabilize the impedence it sees... At the load end
of the coax is a resistive load for terminating the coax in its
characteristic impedence... An oscilloscope pick up across the load for
seeing amplitude and phase... I normally drive the coax at a frequency
that makes that coax length an odd 1/4 wave at the test frequency...
Then I trim the coax for the voltage minimum...
As far as phase batching the the individual coax pieces after they are
cut to frequency I would pick one coax piece as the master..
I would put a splitter on the output of the generator and drive this
master coax from one side of the splitter and also drive a test coax
off the other side of the splitter... At the load ends of the of the
coax pair I would use a dual channel oscilloscope. or vector meter,
etc., to compare phase between the load on the end of the master coax
and test coax, repectively......

Now, having said that let me comment that unless you are using hard
coax, I suspect at GPS frequencies just routing two flexible coax
cables around the innards of a piece of electronic gear will introduce
enough internal mechanical distortion to cause phase variations at the
load ends... I may be wrong but that is my suspicion..

denny / k8do


David wrote:
Can anyone post any informationon or web addresses on phase matching and
batch / set matching of coax cables? I am interested in principles and
procedures to phase match coax cables. Phase matching involves cutting coax
cables to the same electrical length. The phase matched cables can then be
analysed and put into batches or sets i.e. cables with similar
characteristics and frequency response are put into a set. What would be
analysed to form a set?

This practice is used on coax cables that feed antennas for satellite
systems. For example, if several antennas receive GPS signals, to
triangulate position, the GPS signals must all arrive at a particular point
at exactly the same time.

It also appears that going for the best quality low loss cable can actually
be worse, because using lower quality coax cable with some loss can dampen
the reflections from in-line connectors.

Stages are 1) phase matching 2) batch matching 3) assemble cables chosen by
batch matching process onto RF system and perform vector generation. A
vector gen file is created which is a calibration snapshot of the system
i.e. records the RF performance in the form of a signature or footprint.

Measuring instrument is a Network Analyser HP8753.

On 6 Nov 2006 06:26:53 -0800, "Denny" wrote:

A signal generator for driving the coax... Put a 3dB pad on the output
of the generator to stabilize the impedence it sees...

Hi Denny, and student,

This pad should exhibit a 50 Ohm load to the source, and a 50 Ohm load
to the line. This is usually a T-Pad.

At the load end
of the coax is a resistive load for terminating the coax in its
characteristic impedence...

This is good advice, which, unfortunately is thwarted by the advice
for:
An oscilloscope pick up across the load for
seeing amplitude and phase...
Wrong. At frequencies above 50MHz (or 200MHz for the best of
equipment), you should always use a 50 Ohm load to the input of the
O'scope - NOT a probe, or link.

I normally drive the coax at a frequency
that makes that coax length an odd 1/4 wave at the test frequency...

Odd Eighth wave is much better as transformations will be far easier
to measure - and easier is indicative of available accuracy. Quarter
and Half waves do offer convenience when you terminate in opens and
shorts, but with other loads (other than the characteristic Z of the
line), Eighth wave is preferable.

As for Half or Quarter Wave choices, short the far end. Open
terminations are problematic. Measuring a open/short termination
transformed as a short is preferred. Hence, the best configuration is
a shorted Half Wave line.

Then I trim the coax for the voltage minimum...
As far as phase batching the the individual coax pieces after they are
cut to frequency I would pick one coax piece as the master..
I would put a splitter on the output of the generator and drive this
master coax from one side of the splitter and also drive a test coax
off the other side of the splitter... At the load ends of the of the
coax pair I would use a dual channel oscilloscope. or vector meter,
etc., to compare phase between the load on the end of the master coax
and test coax, repectively......

This is taxing the resources of equipment at 1GHz. The method is
informative, but it does not reveal the accumulation of error (and our
student hasn't actually expressed how accurate these matches need to
be).

Now, having said that let me comment that unless you are using hard
coax, I suspect at GPS frequencies just routing two flexible coax
cables around the innards of a piece of electronic gear will introduce
enough internal mechanical distortion to cause phase variations at the
load ends... I may be wrong but that is my suspicion..

Think of quarter wave radius sweeps.

73's
Richard Clark, KB7QHC

David wrote:
Can anyone post any informationon or web addresses on phase matching and
batch / set matching of coax cables? I am interested in principles and
procedures to phase match coax cables. Phase matching involves cutting coax
cables to the same electrical length. The phase matched cables can then be
analysed and put into batches or sets i.e. cables with similar
characteristics and frequency response are put into a set. What would be
analysed to form a set?

This practice is used on coax cables that feed antennas for satellite
systems. For example, if several antennas receive GPS signals, to
triangulate position, the GPS signals must all arrive at a particular point
at exactly the same time.

It also appears that going for the best quality low loss cable can actually
be worse, because using lower quality coax cable with some loss can dampen
the reflections from in-line connectors.

Stages are 1) phase matching 2) batch matching 3) assemble cables chosen by
batch matching process onto RF system and perform vector generation. A
vector gen file is created which is a calibration snapshot of the system
i.e. records the RF performance in the form of a signature or footprint.

Measuring instrument is a Network Analyser HP8753.

I suppose you left out some (rather important) stages...

The first one I'd put down is: select the cable. The physical design
of the cable is important. Some cable changes appreciably when it
flexes, and some changes appreciably as temperature changes. (If it
changes significantly when the humidity changes, you've REALLY picked
the wrong cable...) Cable with properly installed, high quality
connectors should not show significant reflections from the connectors,
and even if it does, if all the cables are assembled the same, the
effect should be the same on all of them.

Cable of a given physical size and general construction should have
close to the same loss. If you want MORE loss, pick SMALLER cable, and
especially pick cable with smaller inner conductor. Stranded
conductors also help increase the loss. But if you have two similarly
designed cables, and one shows appreciably more loss at GHz
frequencies, definitely avoid it. The key reason is that whatever is
causing the loss is likely the result of a poorly controlled
manufacturing process, and that's something you do NOT want to put up
with in precision matched cable assemblies.

What exactly do you want to match? Phase as a function of frequency,
over some particular range of frequencies? Attenuation, also over the
range of frequencies? Something else? Some combination of
characteristics? I would expect it would depend on the application.
If you assume that the cable assemblies are linear, and that they are
adequately represented for your application by a two-port model, then
measure them with your vector network analyzer. Especially since you
are going for MATCHING and apparently don't need absolute accuracy, let
the analyzer (powered on) and the cables stabilize in the stable
environment where you'll measure them, do a reasonable job calibrating
the analyzer, measure each of the cables (S11, S21, S12, S22, over your
range of frequencies), check the calibration of the VNA, remeasure the
cables (or at least spot-check them) to be sure you get the same
results the second time. If all is good, arrange the cables using
whatever measure of match you need for your application. If you need
matching over a range of temperatures, or where one cable is at a
different temperature than the others, measure under those conditions.
Note that if there are measurement errors, if your technique is
consistent, those errors should be very nearly the same for each cable
that measures the same: matching will be better than absolute
accuracy.

Note that if you are measuring completed assemblies over a range of
frequencies, you likely won't have the option of picking an "odd number
of 1/8 wavelengths," or any other particular number of degrees, for
your measurement. Let's say your cables are 10 nanoseconds long (a
couple meters). If you want to check the match from 1.6GHz to 1.7GHz,
a rather narrow range, the cable length varies by 360 degrees. But
that doesn't matter: for matching, it's the repeatability of the
measurement that does matter. If you get to caring about absolute
accuracy, check the accuracy specs of your instrument, and read ap
notes...

Have you checked for HP and/or Agilent and/or Rohde & Schwarz and/or
Anritsu (and perhaps some others) ap notes? I know for sure that there
are a lot of them from Agilent (which may have originally been written
at HP, before Agilent split off). Since you are using an HP8753, give
you local Agilent sales rep a call if you can't find the ap notes on
the web, and ask her/him for applications information.

Cheers,
Tom

What sort of mode should be used on a Network Analyser for phase matching?
Should it be some sort of Time Domain Reflectometer mode?

Input impedance of network analyser is 50 ohms. Many cables are 75 ohms
impedance. For tests on these, a 75 ohm to 50 ohm converter or adaptor is
used. Adaptor name is something like 'min loss pad'.

My application is a system that checks the RF performance of a Unit Under
Test (UUT). The test cables that mate with the UUT are longer than the UUT,
so the aim is to get the cables to be exactly the same. This is reason for
phase and batch matching - to avoid differential error.

Because a lot of cables are analysed, the analysis is performed by computer
which connects to Network Analyser. What data sets e.g. S12 should be
compared by the Network Analyser to batch match the cables?

Cables have different connectors on end, but still have to be batch matched.
Have you any suggestions about this? Cables all have SMA connector at one
end. At other end, the connector can be 1) SMA 2) 50 ohm N type 3) coax
contact in circular connector.

Normally, phase matched cables are all made from same coax drum. Ensure coax
comes in on one big drum, not lots of small drums that may have been made at
different times using different processes (e.g. different operator or
machine setting).

Cables are batch matched and assembled. If several years later, a cable
breaks, how do I get a replacement batch matched cable to repair system?
Although, I would have kept spare cables from original build, the cables on
the system will have been subject to a different environment e.g. different
stresses and humidity.

Another scenario. RF cable is built in slightly more humid environment,
resulting in more moisture in dielectric. Vector gen run and passes. Cable
dries out becoming much better. Vector gen then fails. It appears that a
cable can become too good.

So--is this a homework assignment?

David wrote:
What sort of mode should be used on a Network Analyser for phase matching?
Should it be some sort of Time Domain Reflectometer mode?

Since the VNA operates "native" in the S-domain, I would recommend just
doing S-parameter measurements. I believe you will find that all other
measurements are derived from the measured S parameters. As I wrote
before, if you believe that the cables are adequately represented by a
linear two-port model, then the S-parametes tell you everything you
need to know. (But a two-port model may not be adequate.) Caveat: be
sure to measure them over a wide enough frequency range that you are
sure that there is no phase ambiguity. VNAs typically do not
distinguish among 0 degrees, -360 degrees, +360 degrees, +720 degrees.
But the slope of phase versus frequency is pretty much a give-away
clue.

Input impedance of network analyser is 50 ohms. Many cables are 75 ohms
impedance. For tests on these, a 75 ohm to 50 ohm converter or adaptor is
used. Adaptor name is something like 'min loss pad'.

OK, noted. If you are doing a lot of 75 ohm testing, you may perhaps
want to get a 75 ohm VNA.

My application is a system that checks the RF performance of a Unit Under
Test (UUT). The test cables that mate with the UUT are longer than the UUT,
so the aim is to get the cables to be exactly the same. This is reason for
phase and batch matching - to avoid differential error.

I'm not clear on what you're saying here. If you think that simply
matching the pair of cables that go from your HP8753 VNA's Port 1 and
Port 2 to the UUT will yield accurate test results on the UUT, you are
sadly mistaken. To begin with, there is no need that the cables be
matched in impedance, physical or electrical length, or loss. They may
even have impedance irregularities, and things will still work. What
will work _best_ is if they are low loss and stable. There definitly
IS an Agilent/HP ap note about this sort of thing. What is important,
if you want accurate absolute measurements on your UUT, is that you
CALIBRATE the test system, including the VNA, the cables, and whatever
else you have to connect up to the UUT. Modern VNAs, and in particular
the HP/Agilent 8753, are designed to be calibrated at some plane in
general remote from the VNA's ports, to automatically remove the
effects of whatever cables you use to connect to your UUT.

Because a lot of cables are analysed, the analysis is performed by computer
which connects to Network Analyser. What data sets e.g. S12 should be
compared by the Network Analyser to batch match the cables?

OK, back to matching cables: what you match is up to you. YOU decide
what's important. YOU may wish to put together a "cost function" that
accounts for how badly you want each parameter matched. What is
important in your particular application? You have four
two-dimensional parameters to deal with: S11, S12, S21, S22.
Presumably on a passive line, S11=S22 and S21=S12...

Cables have different connectors on end, but still have to be batch matched.
Have you any suggestions about this? Cables all have SMA connector at one
end. At other end, the connector can be 1) SMA 2) 50 ohm N type 3) coax
contact in circular connector.

And you are trying to match cables with different connectors? That is,
you're trying to match one cable with an SMA and an N, to another with
two SMAs? Good luck! How do you then define where each cable ends?
There are ways to do it, but it's going to depend on just what you are
trying to accomplish in the end.

If you're only trying to match cables of one type with others of the
same type, what's the problem? You can put in whatever adapters you
need, and measure the cables. For matching, what's important is that
they are the SAME, not that they are some absolute value.

Normally, phase matched cables are all made from same coax drum. Ensure coax
comes in on one big drum, not lots of small drums that may have been made at
different times using different processes (e.g. different operator or
machine setting).

And you have control over this? And you'll maintain control over
this when they cables end up being manufactured by a CM overseas? You
may well end up spending all your time just making sure they put the
connectors on the cable properly. Anyway, if it ends up not affecting
your final yield, does it really matter? Maybe you should just buy the
raw cable from a vendor that has taken to heart the idea that process
control is key. That vendor's cable may well be more consistent from
reel to reel than another's is within a single reel.

Cables are batch matched and assembled. If several years later, a cable
breaks, how do I get a replacement batch matched cable to repair system?
Although, I would have kept spare cables from original build, the cables on
the system will have been subject to a different environment e.g. different
stresses and humidity.

You want them matched? Buy a new set. If one broke, and they are all
subject to roughly the same conditions, the others are most likely near
(or beyond) the end of their service life. If there are stresses on
them that can change them, and if matching is important, why have you
not set up a calibration cycle on them? If you kept a spare or spares
of the set, why did you not cycle those through the active set on a
regular basis, if you think that they will be affected by whatever
environment the active set is subject to? If, for example, they are
bundled, why not just include the spare(s) in the bundle, terminated in
dummy connectors, so they are always ready to go? There are solutions
for what you suggest. There are undoubtedly engineering (technical,
cost, ...) tradeoffs among the different solutions. Evaluate them
against the application and pick one. If you discover you picked the
wrong one, learn from the mistake and do it differently the next time.

Another scenario. RF cable is built in slightly more humid environment,
resulting in more moisture in dielectric. Vector gen run and passes. Cable
dries out becoming much better. Vector gen then fails. It appears that a
cable can become too good.

If you're worried about that, why have you not set up a calibration in
the "vector gen" that takes care of it? (The dielectrics and jackets
on cables properly chosen for the application should not have a problem
with humidity. Also, consider where most of the loss in coaxial cable
is...)

Your questions sound a lot like they are spurred by a homework
assignment; they are too broad it seems to be targetting a particular
application. That's all OK, but at some point, you have to get down to
the tradeoffs for a specific application, and make your choices based
on that--on what you have control over, on what the specific needs are,
on the costs ($, time, other resources, ...) associated with various
solutions.

Cheers,
Tom

On 6 Nov 2006 13:50:34 -0800, "K7ITM" wrote:

Normally, phase matched cables are all made from same coax drum. Ensure coax
comes in on one big drum, not lots of small drums that may have been made at
different times using different processes (e.g. different operator or
machine setting).

... That vendor's cable may well be more consistent from
reel to reel than another's is within a single reel.

Hi Both,

This has all the hallmarks of a mutual suicide pact.

The right source for matched lines is from a vendor such as:
http://www.micro-coax.com/pages/prod...axialCable.asp
where a specific example has sub percent tolerances:
http://www.micro-coax.com/pages/prod...asp?ID=UT-390C

And the question remains:
"What is the degree of matching?"

The answer to this spans cables from Radio Shack, to the provider
offered above who specializes in precision.

73's
Richard Clark, KB7QHC

notes...

Have you checked for HP and/or Agilent and/or Rohde & Schwarz and/or
Anritsu (and perhaps some others) ap notes? I know for sure that there
are a lot of them from Agilent (which may have originally been written
at HP, before Agilent split off). Since you are using an HP8753, give
you local Agilent sales rep a call if you can't find the ap notes on
the web, and ask her/him for applications information.

Cheers,
Tom


What you want to do should be in the instruction manual that came with the
network analyser. You may need specially treated cable if it is to be
subjected to changing temprature. Some of the rg214 cable I use is annealed
in a oven.