Transmission Line Matching Network - RF Cafe Forums
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:
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
Joined: 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 * TAN(2pi*length/lambda))
(Z0 + jZr * TAN(2pi length/lambda))
Z0 is 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
The intrinsic characteristic impedance is dependent
on physical dimensions and the substrate permittivity.
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
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
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
It all depends mostly on what bandwidth you
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.
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.
As a 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.