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on-board measurement of RF parameters: - RF Cafe Forums
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| jinhao
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Post subject: on-board measurement of RF parameters:
Posted: Tue Apr 24, 2007 1:26 pm
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| Lieutenant |
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Joined: Tue Apr 24, 2007
1:10 pm
Posts: 3 |
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Hi,
I have little experience of measurement,
especially on-board input-impedance of an antenna,
or of a filter, etc.
Or, .... any suggessions
are welcome.
like where we can be trained to
perform those test, etc.
thanks a lot.
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rftroubleshooter |
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Post subject:
Posted: Wed May 16, 2007 4:27 pm
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Joined: Wed May 16, 2007
4:16 pm
Posts: 12 |
Well, I can tell you how I do it. I usually work
at 900 MHz, so I don't know how well this method
will scale with frequency, though it should if you
use the right cables. First of all, you need
a vector network analyzer. The 8753D or later model
from HP/Agilent is a nice choice. The important
thing when using a VNA is to calibrate the ports.
More on that later. On your board, you need
a way to disconnect the DUT (device under test)
from the rest of the circuitry. Normally, there
is a DC blocking cap in line with most parts of
an RF circuit, so the easiest thing to do is to
remove the DC blocking capacitor. Next, take a short
section of RG174 cable with a SMA connector on one
end and strip back the other end to expose the shield
and center conductor. You will also need a complete
SMA RG-174 cable with SMA connectors on both ends
for calibration. Solder the shield of the
test cable to a ground point very close to the antenna,
filter, or whatever you are testing. Solder the
center conductor to the pad of the DC block you
just removed so that the cable is now electrically
connected to the DUT. Now, connect the other
cable (the one we are using for calibration) to
one of the ports of the VNA; Lets use port 1 for
this example. Using the instructions for the VNA,
perform a 1-port calibration on port 1. THis will
involved you connected calibration standard loads
to the end of the cable; there will be a 50-ohm,
a short, and an open standard. Once complete, the
VNA is now calibrated, including the length of cable
you are using with the DUT. You should now
be able to connect the cable soldered to the DUT
to port 1 of the VNA and perform your measurements.
You can do the same trick with 2 cables
if you need to do 2 port analysis for filters.
Please post any questions you have if this isn't
clear enough. - Steve Montgomery The RF
Troubleshooter
www.rftroubleshooter.com
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aleksey0131 |
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Post subject:
Posted: Fri May 18, 2007 9:18 am
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Joined: Fri May 18, 2007
9:05 am
Posts: 2 |
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Hi!
I have some questions. Can I similarly to
do measurement of a dipole antenna and input resistance
of a chip? And can I use other 50-Ohm cable (for
example, RG-58 )?
Thanks in advance!
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rftroubleshooter |
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Post subject:
Posted: Fri May 18, 2007 11:02 am
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Joined: Wed May 16, 2007
4:16 pm
Posts: 12 |
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You should be able to mesure dipole antennas that
way.
Testing the input resistance of a chip
can be a little trickier. Most RFIC receiver inputs
are differential and you need a balun to make them
single ended. The balun needs to be arranged such
that it also acts as a DC block. Usually, this balun
is designed to match the input of the RFIC to a
50-ohm source (like an LNA or SAW filter). So what
you are really testing is the match of the balun.
Does that make sense?
You can use RG-58,
but it is very thick and rigid compared to RF-174
and will be difficult to manage if it is soldered
directly to a PCB.
And now a shameless plug.
You should check out my website from time to time.
I just redid it and it now contains an "Ask The
Guru" forum where you can ask questions like this
and it will contain a bunch of new articles that
I am writing. Look at my sig for the address.
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aleksey0131 |
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Post subject:
Posted: Mon May 21, 2007 6:51 am
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Joined: Fri May 18, 2007
9:05 am
Posts: 2 |
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Thank you for the answer. I necessarily will visit
your website, when I will have free time. In this
topic I have else one question. Why it's possible,
to measure the dipole antenna without a balun? If
I am not mistaken, a dipole antenna has a balanced
input.
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rftroubleshooter |
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Post subject:
Posted: Mon May 21, 2007 10:38 am
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Joined: Wed May 16, 2007
4:16 pm
Posts: 12 |
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You are right. Technically a dipole antenna is balanced.
And the proper way to feed it is through a balun.
However, a lot of people just ground one of the
poles and drive it as a single-ended load. You loose
a few dB performance, but it is much easier to deal
with.
So you want to test it the same way
you are going to drive it. If you are using a balun
in your circuit, then use one when you test it.
If you are driving it as a single ended load, don't
use a balun.
When you said dipole in your
last post, I had a standard monopole antenna in
mind. So many people refer to those as dipoles.
Sorry for the confusion.
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Posted 11/12/2012
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