Vector network analysis question
 

hi guys,

After an hour or more of pushing and twiddling various buttons and
knobs on my VNA I have at least established two standards: I connect a
short and the (polar) spot flys off to the extreme left-hand-side and
sits stably on the central horizontal axis. I then remove the short and
replace it with an open and the spot jumps across 180 degrees to the
extreme right-hand side on the same plane. All well and good so far.
Next, I replace the open with a lab-grade 50 ohm termination and I wind
up with an arc on the outermost circle of the polar chart at about
two-thirds of the way round the circumference (clockwise) from "12
o'clock" as opposed to a spot in the dead center of the display as I
would expect. What have I done wrong? I have checked this with 3
different 50 ohm loads and they all show an arc at the 'south west'
corner of the chart. What's goin' on here, guys??
confused
Paul

Vector network analysis question
 

You haven't fully described your setup, so it's hard to tell. What are
you using for a bridge? If I recall correctly, your VNA is an HP8754A,
which won't do S-parameter measurements by itself; it requires an
external bridge (an S-parameter test set, or a reflection-transmission
test set, for example).

The fact that you see an arc on the outermost circle, assuming that you
have it set as a unit circle on the linear reflection coefficient plane
(the Smith Chart outer circle), implies that you have a (nearly) unity
reflection coefficient. Is something almost disconnected so it looks
like a tiny series capacitance?

Vector network analysis question
 

Okay, thanks. I must admit it is symptomatic of the load's center pin
not quite making contact for some reason, even though the load is fully
screwed home. But the same thing happens with 3 different loads!
I'm using the T/R test set, by the way, although I don't see how that
could explain the problem. I don't have the full S-parameter bridge but
am in the market for one.

Vector network analysis question
 

Okay, thanks. I must admit it is symptomatic of the load's center pin
not quite making contact for some reason, even though the load is fully
screwed home. But the same thing happens with 3 different loads!
I'm using the T/R test set, by the way, although I don't see how that
could explain the problem. I don't have the full S-parameter bridge but
am in the market for one.

If you're using N connectors, check the center pins of the females.
Make sure that they aren't splayed open.

The local repeater association I'm a part of has had repeated problems
with splayed N connectors. They can be caused by a couple of
problems. A botched install of an N male can cause its center pin to
project too far forwards, out of the shell. We've also seen some N
adapters (Chinese-made I believe) whose center pin was larger than
usual in diameter, or was not tapered in the usual way.

In either case, if you screw an N male connector with a bad center pin
into an N female, the male's pin can force the split segments of the
center female pin outwards. Once this happens, a _good_ N male
connector's pin may not make reliable contact with the female's
damaged center pin. Intermittent nastiness can occur.

It's sometimes possible to bend the female's split pin sections back
together with needle-nose pliers, creating a repair which may or may
not be permanent. A better solution is to replace the damaged female
connectors entirely, and inspect all N male plugs and discard any
whose center pins are too large in diameter, not tapered properly, or
project too far out of the shell.

I suppose that similar problems might occur on BNC or SMA connectors
as well, for related reasons.

--
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!

Vector network analysis question
 

Okay, thanks for that guys.
One clue that might assist in tracking down this problem is that the
aforementioned arc that appears at about '37 minutes past the hour' on
the outermost polar circle when the 50 ohm termination is fully screwed
in first begins to appear at just before 12 o'clock and gradually
circumambulates around this outer circle anticlockwise to its final
position as it's fully screwed home. I'd have (intuitively) thought
that once the centre pin makes proper contact with its counterpart, the
trace would suddenly flick to close to the centre. But with no
experience of VNAs, that's just a hunch. Any suggestions?

Vector network analysis question
 

Dave Platt wrote, among other things,
"In either case, if you screw an N male connector with a bad center pin
into an N female, the male's pin can force the split segments of the
center female pin outwards. Once this happens, a _good_ N male
connector's pin may not make reliable contact with the female's
damaged center pin. Intermittent nastiness can occur."

I've seen people suggest that you can mate an N plug with a BNC jack in
a pinch, but what you describe is _exactly_ why you should never do
that. The N plug center pin is a distinctly larger diameter than a BNC
center pin.

Also note that there exist 75-ohm N connectors. Do not try to mate
them with 50 ohm versions. A 75 ohm plug in a 50 ohm jack is likely to
not make good contact, and a 50 ohm plug in a 75 ohm jack may cause
damage to the jack.

Vector network analysis question
 

Suggestions? You bet! Get out your RFSim99 and build a model of what
you are putting on the RT test set test port, including any connecting
cable. Play with the model to see what will get you the sort of
performance you are seeing.

Note that you haven't really told us what you are plotting. I'm
assuming it's a plot of the complex reflection coefficient on a display
that's scaled to a unit circle. Are you using a standard Smith chart
overlay?

The normal interpretation of the result you describe, however, doesn't
make much sense, since the load seems to go from something like a
nearly pure inductance with reactance equal to the reference resistance
at the center of the plot (50 ohms?), and wanders around to become a
very low impedance (at the left edge, -1+j0 reflection coefficient),
and then around to become capacitive with moderate reactance (35
ohms?). Could you possibly be using some other scaling? What happens
if you are trying to display a value beyond full scale on your display?

I suspect something is not what you think it is in the measuring
instrument, and from past experience, I'd say it could be just about
anything. Careful visual inspections often reveal problems.

Cheers,
Tom

Vector network analysis question
 

Oh dear. It appears you may be right.
I've just checked with the suppliers and they tell me the plugs I
bought are 75 ohm ones. This really stinks as in their catalogue,
although they don't specifiy the impedance of these plugs, they do
clearly state that they're suitable for RG214 coax. AFAIK, RG214 is
only available in 50 ohm - if anyone knows otherwise please tell me. It
took me a good deal of time and trouble to make up these patch leads so
all in all, I ain't exactly pleased.
Here's the URL for the catalogue page so you guys can tell who's at
fault he

http://cpc.farnell.com/jsp/endecaSea...=CN06870&N=411

Vector network analysis question
 

Since posting earlier I've been on to the actual manufacturers and they
reckon these plugs are actually 50 ohms. So the supplier's insist
they're 75 and the makers say they're 50! I need to know for sure, as
one or the other is in error. How can one tell, by visual inspection
and or physical measurement, which type is which? There must be
something that's visibly different about the two types.
THanks.

Vector network analysis question
 

Vector network analysis question
 

Thanks, Jeroen.
Well on that basis, all the plugs I have here are indeed 50 ohms, then.
In fact I pulled a centre pin out of a new packet from the same batch
and it fits the sockets in the VNA perfectly. They all accept the pin
with a nice snug fit and what's more the sockets show no signs of ever
sustaining any damage/spreading/flairing/bending and that goes for
those on the T/R bridge, too. On reconnecting everything, however, the
problem still persists, although the 50 ohm load is now showing as very
close to a dead short; virtually the same spot on the Smith chart
overlay as when the socket is shorted, in fact. Once again, the other
loads do likewise and they all check out fine as 50 ohms with a DVM.
Very strange! I wonder if there's some setting I may have got wrong
somewhere. :-/

Vector network analysis question
 

In article . com,
wrote:

On reconnecting everything, however, the
problem still persists, although the 50 ohm load is now showing as very
close to a dead short; virtually the same spot on the Smith chart
overlay as when the socket is shorted, in fact. Once again, the other
loads do likewise and they all check out fine as 50 ohms with a DVM.
Very strange! I wonder if there's some setting I may have got wrong
somewhere. :-/

Paul-

What would be the result of calibrating the system with an inductive
"short"? If it can be done, results of testing with a good 50 Ohm load
might be skewed.

Fred

Vector network analysis question
 

Having had a look in my (junk)box with N-connectors I found that most
silver plated (professional) units have 50 Ohms stamped into the body
,whereas the nickel plated units either have no identification or a
specific manufacturer's code .
The Farnell catalogue you referred says these N-connectors are nickel
plated . Try to find any stamped code on your units . If any I'll be
happy to compare with those on my connectors.

Frank GM0CSZ / KN6WH
==========================================
Since posting earlier I've been on to the actual manufacturers and they
reckon these plugs are actually 50 ohms. So the supplier's insist
they're 75 and the makers say they're 50! I need to know for sure, as
one or the other is in error. How can one tell, by visual inspection
and or physical measurement, which type is which? There must be
something that's visibly different about the two types.
THanks.

Vector network analysis question
 

Good thinking, Fred. That's theoretically possible, I guess. But I'm
carrying out these checks purposely at only around 5Mhz to minimise
such high frequency effects. You may well be on the right lines, though!

Vector network analysis question
 

Jeroen Belleman wrote:
. . .
It's one of my long-time gripes about coaxial connectors that N and
BNC series of the two impedance levels are sufficiently compatible
to mate together, but not enough to do so without damage. In a lab
with someone ignorant of this fact, it's a source of endless trouble.

All the 75 ohm BNC connectors I have, have the same diameter center pin
in the connection region as 50 ohm connectors. The difference is in the
thickness of the dielectric around the pin in the connection region,
being much thinner in the 75 ohm connectors. The pin of the 75 ohm
connector is smaller only where it's in solid dielectric at the bottom
of the connector.

Roy Lewallen, W7EL

Vector network analysis question
 

Roy Lewallen wrote:

Jeroen Belleman wrote:

It's one of my long-time gripes about coaxial connectors that N
and BNC series of the two impedance levels are sufficiently
compatible to mate together, but not enough to do so without
damage. In a lab with someone ignorant of this fact, it's a
source of endless trouble.

All the 75 ohm BNC connectors I have, have the same diameter
center pin in the connection region as 50 ohm connectors. The
difference is in the thickness of the dielectric around the pin
in the connection region, being much thinner in the 75 ohm
connectors. The pin of the 75 ohm connector is smaller only where
it's in solid dielectric at the bottom of the connector.

Roy Lewallen, W7EL

Hi Roy,

There's lots of room for confusion. The Kings tool FAQ implies the
center contact diameter got smaller when the pin was redesigned and
the old crimp die set would not work:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~
Why won't my Kings 2025-X-9 die set work with the 2065-X-9
connectors?

With the redesign of the True 75 Ohm BNC connectors, the center
contact diameter got smaller to maintain a constant 75 Ohm
impedance through the connector. Consequently, the center contact
crimp dimension got smaller also.

The problem with the inability of the die set to crimp this new
connector is probably because it is the older, larger die
dimension. The Kings website has a Cross-Reference Search that
lists the applicable die set for most Kings connector part
numbers.

http://www.kingselectronics.com/Reso...4/Default.aspx

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~
However, another manufacturer claims their new 75 ohm hardware is
interchangeable with 50 ohm connectors:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~
TRUE 75 Ohm BNC & Connectors

Where Controlled Impedance is a Necessity

Emerson Network Power Connectivity Solutions is introducing its
premium line of TRUE 75 Ohm BNC connectors and adapters. These
flexible cable 3 Piece BNC Connectors are essential for analog and
digital high frequency signals.

They have been designed around MIL-STD-202 for up to a 3GHz
frequency range, and are constructed of high performance materials
including a machined nickel plated over brass housing, a teflon
dielectric and gold plated brass contacts. These connectors are
designed to eliminate distortion and impedance mismatching caused
by using 50 ohm connectors on 75 ohm cables, and they safely
inter-mate with standard 50 Ohm BNC connectors.

http://emersonnetworkpower.com/webap.../pdfs/True-75-
Ohm.pdf

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~
If your junkbox is anything like mine, it may have stuff purchased
20 years ago mixed with more recent purchases. So it might be
worthwhile to check carefully before mating different hardware.

Regards,

Mike Monett

Vector network analysis question
 

Mike Monett wrote:

So it might be
worthwhile to check carefully before mating different hardware.

So the thought finally strikes me. How do you check the female socket
dimension? Do you stick a male pin in and see how much friction there is?
That could bend the contacts and damage the pin.

How about checking the male pin diameter when it is already installed on a
connector? There's not enough room for calipers to reach the pin.

I often find the BNC connectors on my scope go intermittent due to a loose
center pin. Often the only choice is to use a pair of pliers to squeeze the
contacts together again. The damage appears to have been caused by some
off-brand 50 ohm terminators that require an unusual mount of force to seat
on the connector. But the repair doesn't last. Once the female pin has been
forced open, it quickly goes intermittent again. This is a big problem
since the scope connector is usually a special item that is difficult to
get.

Regards,

Mike Monett

Vector network analysis question
 

Mike Monett wrote:

Hi Roy,

There's lots of room for confusion. The Kings tool FAQ implies the
center contact diameter got smaller when the pin was redesigned and
the old crimp die set would not work:

. . .

However, another manufacturer claims their new 75 ohm hardware is
interchangeable with 50 ohm connectors:

. . .

If your junkbox is anything like mine, it may have stuff purchased
20 years ago mixed with more recent purchases. So it might be
worthwhile to check carefully before mating different hardware.

For sure. It looks like it's not safe to mate a 75 ohm connector with
anything, even another 75 ohm connector, without checking pin diameter.

Roy Lewallen, W7EL

Vector network analysis question
 

Maybe your "N" female connector has "splayed" leaves on the center
contact. This sometimes happens when someone tries to screw a PL-259 UHF
connector in place by accident.

wrote:

Okay, thanks. I must admit it is symptomatic of the load's center pin
not quite making contact for some reason, even though the load is fully
screwed home. But the same thing happens with 3 different loads!
I'm using the T/R test set, by the way, although I don't see how that
could explain the problem. I don't have the full S-parameter bridge but
am in the market for one.




--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P

Vector network analysis question
 

Roy Lewallen wrote:

For sure. It looks like it's not safe to mate a 75 ohm connector with
anything, even another 75 ohm connector, without checking pin diameter.

Roy Lewallen, W7EL

Or even 50 ohms, for that matter. But how do you check the pin diameter on
an existing connector? A caliper won't go in there. Also, the diameter
could be correct but the pin could be assembled incorrectly and stick out
too far. This will damage the female pin.

Or maybe you know of some other non-destructive method to tell if you can
mate an existing male and female coax connector without damage?

The vulnerable part is the female pin. Once those legs get splayed there's
no way to put them back. All it takes is one event to effectively ruin a
brand-new scope.

Regards,

Mike Monett

Vector network analysis question
 

Mike Monett wrote:
Roy Lewallen wrote:

For sure. It looks like it's not safe to mate a 75 ohm connector with
anything, even another 75 ohm connector, without checking pin diameter.

Roy Lewallen, W7EL

Or even 50 ohms, for that matter. But how do you check the pin diameter on
an existing connector? A caliper won't go in there. Also, the diameter
could be correct but the pin could be assembled incorrectly and stick out
too far. This will damage the female pin.

Or maybe you know of some other non-destructive method to tell if you can
mate an existing male and female coax connector without damage?

The vulnerable part is the female pin. Once those legs get splayed there's
no way to put them back. All it takes is one event to effectively ruin a
brand-new scope.

Well, a scope will have a 50 ohm female connector, which should tolerate
any kind of male without damage. The only vulnerable connector should be
a 75 ohm female connector of the sort which has a smaller pin diameter.
I don't believe I've ever seen one of those, and you'll certainly never
see one on any commercial equipment except only possibly some
video-related units.

Roy Lewallen, W7EL

Vector network analysis question
 

Roy Lewallen wrote:

Well, a scope will have a 50 ohm female connector, which should
tolerate any kind of male without damage.

That's what I was talking about earlier. I ruined the BNC inputs on
some of my older TEK scopes by using off-brand 50 ohm terminators
that were difficult to install. Apparently the male pin extended too
far into the female and spread the pins. Some cheap commercial coax
cable had the same problem. The scope bnc connectors are special and
had the multiplier contact for 10X probes, and were too difficult
and expensive to get and install.

I would often waste time debugging strange waveforms when it turned
out the problem was in the scope connector. Squeezing the pins
gently would make it work for a while, then it would come right
back.

You often see the problem on used equipment like spectrum analyzers,
sig generators and counters. Evidently someone went in with
needlenose pliers to try to tighten the legs, but instead squashed
them into a square instead of a circle. They don't make reliable
connections after that.

After discussing this, I'm inclined to get a handful of female
connectors to test the male coax hardware. If it ruins the connector
and cannot be fixed, toss it in the garbage where it belongs.

The only vulnerable connector should be a 75 ohm female connector
of the sort which has a smaller pin diameter.

I don't believe I've ever seen one of those, and you'll certainly
never see one on any commercial equipment except only possibly
some video-related units.

That's good - I don't think I've ever used or seen any 75 ohm
hardware, but maybe someone working on TV might run into the
problem.

Roy Lewallen, W7EL

Regards,

Mike Monett

Vector network analysis question
 

We've built some equipment with 75 ohm connectors. In those network
analyzers that use a separate S-parameter test set, you can often get
the test set in a 75 ohm version as well as the more common 50 ohm.
But the 75 ohm units are (as far as I know, _always_) labeled with a
caution about using the right impedance connectors.

Cheers,
Tom

Vector network analysis question
 

"K7ITM" wrote in message
oups.com...
We've built some equipment with 75 ohm connectors. In those network
analyzers that use a separate S-parameter test set, you can often get
the test set in a 75 ohm version as well as the more common 50 ohm.
But the 75 ohm units are (as far as I know, _always_) labeled with a
caution about using the right impedance connectors.

Cheers,
Tom

You've built a hell of a lot of it. All the stuff for the Telco's. like my
backup HP-313 that I mod'd back to 50 Ohms.

Regards
W4ZCB

Vector network analysis question
 

Well, yeah, I guess we have. In fact, not long ago I was cleaning up a
lab bench and found some of the old Telco style connectors, all gold
plated and pretty. I couldn't bear to toss them, even though the
chances of us ever using them again are somewhere between slim and
none, toward the none end of that scale. I suppose that any
special-impedance market that was large enough, we built for. Of
course the older audio generators (200CD et al) were 600 ohm.

Cheers,
Tom