Transmission line Matching network - RF Cafe Forums
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Post subject: Transmission line Matching network Posted: Wed Nov 01, 2006 1:12 am
Joined: Tue Apr 18, 2006 10:40 pm
hi...i have couple of question on basic transmission line
matching network design:
how do you choose what Zo (characteristic impedance) one might want for the
matching series and stubs? since they are matching networks Zo doesn't have to be 50 Ohm then how do you go about
selecting it? does higher Zo mean lower alpha (attenuation loss)? in other words, is Zo proportional to 1/alpha?
what exactly is dispersion and which has less dispersion: microstrip or coplanar? how is diespersion related
to attenuation and Zo. thanks
Post subject: Posted: Thu Nov 02, 2006 2:41 pm
Joined: Mon Jun 27, 2005 2:02 pm
The 50-ohm characteristic impedance is a compromise between the need for minimum attenuation and the ability
to deliver maximal power.
In a system of Zo=77 ohm, there is a minimal attenuation and in a system of Zo=33 ohm
there is maximal power capability. Another consideration is the dimension (Width9 of the line depending on the
substrate and frequency. So 50 ohm was chosen as the best trade-off to all of these constraints.
Dispersion is a different signal velocity versus the frequency - similar to the group delay in filters (After all
a transmission line is a filter). The dispersion is much smaller in Coplanar and Stripline than in microstrip.
In the following line you can find more about the dispersion of different transmission lines:
http://www.microwaves101.com/encycloped ... ersion.cfm
Post subject: Posted: Fri Nov 03, 2006 4:38 am
Fri Feb 17, 2006 12:07 pm
Location: London UK
In addition to the points IR has made, remember
that the input impedance (and therefore the inductance or capacitance) of a short section of line depends on its
intrinsic characteristic impedance, and the terminating impedance.
Zin = Z0 * (Zr + jZ0 *
(Z0 + jZr * TAN(2pi length/lambda))
the intrinsic characteristic impedance, Zr is the terminating impedance, and lambda is the wavelength in the
dielectric of the substrate. The transformer ratio is thus Z0/Zr
The intrinsic characteristic impedance is
dependent on physical dimensions and the substrate permittivity.
So if a matching network calls for a
particular L and C combination, then for a particular center frequency you have to find the right length, width
and junction point for the series and shunt element traces.
Thank goodness we have software these days to
do all this. But at least we need to know what the software is trying to do, in order to understand the answers
and judge if they make sense.
Post subject: Posted: Wed Nov 29, 2006 8:12 pm
Joined: Wed Nov 29, 2006 5:51 pm
how do you choose what Zo
(characteristic impedance) one might want for the matching series and stubs?
It all depends mostly on
what bandwidth you need.
In order to go from from a certain (usually) low impedance to a 50 ohm impedance
you use matching techniques that will transform the impedance from the one you have at the device to a certain
intermediate value from where you will use another element to move again. In most cases for today's higher
frequency designs they are done using microstrip transmission lines and stubs that will rotate the impedance point
in a smith chart around the transmission line's characteristic impedance and the degrees it rotates will be
determined by the length of the transmission line.
There are infinity solutions to a matching network
ranging from a one step to multiple steps using the transmission lines or capacitive stubs that you mentioned
depending which way in the smith chart you want to rotate but they all have very different broadband responses.
The multiple step approach usually allows you to go from point A to point B keeping the broader bandwidth
because the Q of the network will be lower.
Transformations with devices like baluns allow you to get your
point B closer to the match requiring lower Q networks since you're starting from a closer point.
general rule of thumb broader bandwidth matches are better for temperature variations and for manufacturing
because they allow for variations in device impedance with little or no degradation in performance.