LimEom
3way power splitters
Updated December
22, 2008
Click
here to go to our main page on couplers and splitters
We now have two examples of
using Lim and Eom's unique splitter to combiner power amplifiers:
Click
here to learn how to use the Lim Eom splitter as a threeway
combiner
Click
here to learn how to use the Lim Eom splitter as a twoway
combiner
This is a truly new power splitter
(a rare event in microwaves!), first detailed by JongSik Lim and
SoonYoung EOM in a 1996 IEEE MTTS paper titled A New 3way
Power Divider with Various Output Ratios. As a threeway splitter,
it fits only niche applications, because of the phase relationships
of the three outputs, as well as the asymmetric layout, but still,
it's a cool splitter that you should consider. One application that
comes to mind in the LO splitter for the classic threechannel monopulse
receiver. By the way, Lim and Eom were at the Seoul National University's
Applied Electromagnetics Laboratory when they wrote the origianl
paper. The AEL posts many of their publications on their web site,
definitely worth checking out! We have a page on SNU go
there if you want to check out the link!
We made a model of Lim and Eom's
sixport neteowrk using Eagleware Genisys. We also entered the equations
into Excel, just to play with the split ratios to see the effect
on line impedances.
The splitter resembles a doublebox
hybrid. Below is the schematic from our Genisys project. Port 1
(top left port) is the common port, while ports 2, 4 and 6 are the
split ports. Note that you have to terminate ports 3 and 5 to operate
it. The line impedances shown will provide an equal threeway split
in a fifty ohm system.
Lim and EOM's paper gives equations
for line impedances that you can use to obtain different power split
ratios. The integers m, n and k are what sets up the power division.
For an equal split, m=n=k=1. The impedances given in the above schematic
are for equal split. Here is how we entered them in Genisys:
m=?1
n=?1
k=?1
delta1=m+n+k
delta2=n+k
Z0=50
Z1=(delta1/delta2)^.5*Z0
Z2=(delta1/m)^.5*Z0
Z3=Z0
Z4=(delta2/N)^.5*Z0
Z5=(delta2/K)^.5*Z0
These are the variables that
Lim and EOM defined in their paper. Note that m, n, k don't have
to be integers (and in Genisys, the question mark is what allows
you to tune them while viewing the response). It is easy to convert
the equations so that you input the ratios in dB (that's the first
thing we did, in an Excel file we'll give you if you ask nicely!)
Note that the impedance Z3 is
has no effect on the power split. We left it at 50 ohms like they
did in their paper. The plot below shows the frequency response
for the equal split case (4.77 dB each arm). Notice that the bandwidths
of the three split arms are all different. The bandwidth isn't all
that good relative to a Wilkinson splitter.
Below is the isolation of ports
3, and 5, along with the input match S11. From this point of view
you could use the splitter over a 20% bandwidth and get 20 dB isolation.
Let's look at the phases of
each path. The longest path is S14 (one wavelength), followed by
S16 (halfwavelength), then S12 (quarterwavelength). We see that
at center frequency that S14 and S16 are +/ 90 degrees out of phase
with S12, and the phase relationship is not very constant over frequency.
This might be a limitation in certain applications, for equal phases
you should consider a threeway Wilkinson.
You might think of the LimEom splitter as a combination of a 90
degree hybrid and a 180 degree hybrid.
If you wanted to
achieve equal phase, equal split, here's a layout for you. The input
port is in the center of the north side, and the west, south and
east sides have transmission lines that give nearly equal phase
outputs. Note the "fan stub" terminations, whoever did
this cool design deserves a fat raise!
Now let's try out an unequal
split, in this case 1:5:5. You might do this if you wanted most
of your power to feed two devices, and the third port going to a
detector to monitor power. We followed the equations and they do
what Lim and EOM claimed! The two facored ports are 7 dB above the
starved port.
More to come! We still need
to look at line impedances versus coupling ratios.
