Hi all,

I'd like to see what this new dummy load I just bought on e-bay looks
like under test at its purported frequency limit of one Gig.
Problem is, all the documentation I have for my VNA shows devices to be

tested having *two* connectors (an 'in' and an 'out' if you will) like
filters for example. I can only connect to the *one* connection that
dummy loads typically have, since it'a a termination device rather than

a through one like a filter or attenuator. Does that mean I can't
analyse its
characteristics? Or is there a work around/special cable configuration
that would solve
this problem?
Thanks,
clueless paul

wrote in message
oups.com...
Problem is, all the documentation I have for my VNA shows devices to be
tested having *two* connectors (an 'in' and an 'out' if you will) like
filters for example. I can only connect to the *one* connection that
dummy loads typically have, since it'a a termination device rather than
a through one like a filter or attenuator. Does that mean I can't
analyse its
characteristics?

Tell your VNA you want to measure S11; it won't even touch the second port
(feel free to hook it up to a high-gain antenna and point it at the nearest
cell phone tower in your neighborhood...)

Unless your VNA is 20+ years old, it'll be happy to display the S11
measurement in the form of a Smith chart, real/imaginary impedances, VSWR,
etc...

Hello Paul,


I'd like to see what this new dummy load I just bought on e-bay looks
like under test at its purported frequency limit of one Gig.
Problem is, all the documentation I have for my VNA shows devices to be

tested having *two* connectors (an 'in' and an 'out' if you will) like
filters for example. I can only connect to the *one* connection that
dummy loads typically have, since it'a a termination device rather than

a through one like a filter or attenuator. Does that mean I can't
analyse its
characteristics? Or is there a work around/special cable configuration
that would solve
this problem?


A quick and dirty one would be to use a resistor from VNA-out to dummy
load, then another from dummy load to VNA-in. 50 ohms or less. With next
to nothing reactive, of course. All nicely RF fashioned, chip resistors
maybe. It's not going to be matched but you can have the analyzer do the
math to calculate out the dummy load. Mind the power limits.

Sometimes I do this with an HP resistive splitter when the impedance
analyzer is down (like it is right now, again....%*&#!!).

Regards, Joerg

http://www.analogconsultants.com

Hi Paul,

I don't mean to pick on you, but... you really should become more
familiar with VNA measurements in general. You should be able to find
documentation on the web; try the Agilent site for example, and look
for VNA ap notes. I downloaded some recently, so I'm pretty sure they
are there. A VNA is a wonderful piece of test equipment, but it won't
give you answers you can trust if you don't understand it. I would
trust what they tell you (or similar from R&S or other reputable
manufacturer) more than I would trust replies here to be not only
accurate but complete.

For example, you should understand that your dummy load could be
considered a two-port device where S21 and S12 will be very
low--hopefully vanishingly close to zero. If you connect it to port 1,
you should get a valid indication of S11. You should expect S22 to be
unity or very close to it, if that port is left open or shorted. (But
why would you even bother to ask for S22, or S21, or S12?)

But there's another problem here. HOW do you calibrate your VNA?? You
ideally should have a precision short, a precision open, and a
precision load--best if you have calibration data for the loads. If
you measure your load, how will you know whether an imperfect S11 is
due to the load, or to the VNA? Of course, it will be to some extent
both, but it would be nice to know that the VNA is accurate to within
some tolerance.

Cheers,
Tom

Okay guys, thanks.
Yes, as you've surmised, I'm only just beginning to get to grips with
this complex piece of kit and there is *much* to learn indeed. In my
defence, I have to say although I purchased this thing 5 months ago,
I've only now been able to assemble the ancilliary cables, loads,
splitters and whatnot to be able to check it out properly. All I could
do back in December was to switch it on and check for signs of life.
Now, however, things start to get rather more complicated ( but
hopefully stimulating, too).

Hello Paul,


Okay guys, thanks.
Yes, as you've surmised, I'm only just beginning to get to grips with
this complex piece of kit and there is *much* to learn indeed. In my
defence, I have to say although I purchased this thing 5 months ago,
I've only now been able to assemble the ancilliary cables, loads,
splitters and whatnot to be able to check it out properly. All I could
do back in December was to switch it on and check for signs of life.
Now, however, things start to get rather more complicated ( but
hopefully stimulating, too).


Just curious, which VNA are you using?

Tentec has a really nice one. Only goes to 100MHz but for around $700 it
seems like a good deal. Some of my clients could have used these but
unfortunately this VNA does not seem to have an industry-standard
calibration scheme. Wish they did, then I could probably get rid of my
heavy 4191 in the lab.

Regards, Joerg

http://www.analogconsultants.com

It's an HP 8754A (4 to 1300Mhz) and luckily old enough to be reasonably
serviceable and repairable!
This is just as well since there is a fault with the rectangular trace,
which just appears to 'flatline' with nothing visible other than the
sweep + markers. So no nice bod plots of filters, I'm afraid. However,
the polar and phase displays seem absolutely fine. If I can just get
that one fault fixed, it'll be a great piece of test gear to have
around. I've never worked on anything this complicated before, though,
so it'll be something of a challenge!

Hello Paul,


It's an HP 8754A (4 to 1300Mhz) and luckily old enough to be reasonably
serviceable and repairable!
This is just as well since there is a fault with the rectangular trace,
which just appears to 'flatline' with nothing visible other than the
sweep + markers. So no nice bod plots of filters, I'm afraid. However,
the polar and phase displays seem absolutely fine. If I can just get
that one fault fixed, it'll be a great piece of test gear to have
around. I've never worked on anything this complicated before, though,
so it'll be something of a challenge!


That looks like a problem in the digital section. Just hope that none of
the EPROMs suffered a memory loss. I am not sure whether they'd report a
checksum error in the 8754. I had found one that had become unseated on
another analyzer and the instrument produced only garbage output but
reported nothing on self-test. That disappointed me a bit. The only
reason I found the cause was that I opened it and looked at it long enough.

Complicated they are. However, while on the troubleshooting trail in my
HP4191 I sometimes came upon a large conglomeration of discretes and
banged my head, asking myself "Why didn't they do that with a uA733?".
And on the directional bridge they seem to have left about 10dB of
dynamic range on the table for the test channel versus the ref channel.
I'd have put a wee amp before the sampler.

Regards, Joerg

http://www.analogconsultants.com

Thanks, Joerg.
I think this item pre-dates a lot of this digital stuff; I doubt
there's a single eprom in it - and no custom chips, either!
It was built in about 1980 and doesn't go through any self-testing
routines. One has to carry out manual tests oneself. This wonderful
simplicity should hopefully enable me to keep it running indefinitely,
once I can get it fixed. Serviceability is one of my main criteria when
choosing an item of test equipment.

Hello Paul,


I think this item pre-dates a lot of this digital stuff; I doubt
there's a single eprom in it - and no custom chips, either!
It was built in about 1980 and doesn't go through any self-testing
routines. One has to carry out manual tests oneself. This wonderful
simplicity should hopefully enable me to keep it running indefinitely,
once I can get it fixed. Serviceability is one of my main criteria when
choosing an item of test equipment.


The HP4191A is from that era as well. However, it is one heck of a
complicated machine with a computer board as large as a family-size
pizza. Lots of PROMs and stuff :-(

Regards, Joerg

http://www.analogconsultants.com