Lecture 14: Double-Stub Tuning Double

Lecture 14: Double-Stub Tuning
Double-stub tuning
Why we use double-stub tuning?
The single-stub tuner has one limitation: it must be placed at the
proper distance from the load, which is a variable that is difficult to
adjust in practice.
Original circuit
with the load an
arbitrary distance
from the first stub
The double-stub tuner uses two tuning stubs, partially removes the
requirement for variable distance from the load, and is widely used in
laboratory practice as a single frequency matching device.
Equivalent circuit
with load at the first
Shunt are easiest to
implement in practice.
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Smith Chart Solution for finding the matching
parameters: l1, and l2.
Step 4: the transmission line between l1 and l2 will rotate back
to the original 1+jb circle. And y1 (or y1’ ) rotates to y2 (or y2’ )
Step 5: Design l2 so that the susceptance is cancelled out and
impedance is matched to the line (origin in the Smith Chart).
Step 1: Draw the
normalized load
impedance yL in the
Smith Chart.
1. If the load admittance is located inside the shaded area
(forbidden area), it simply cannot be matched by the doublestub tuning.
Step 2: Taking into
account the effect of the
transmission line d
between two stubs --rotated 1+jb circle
Step 3: Design l1 so
that yL moves to y1 (or
y1’ for solution 2).
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How to shrink the forbidden area?
2. In practice, stub spacings are usually chosen as λ/8 or 3λ/8.
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Example 5.4 on Page 237
of Pozar
Solution (cont.)
Match a load impedance
ZL = 60 - j 80 Ω to a 50 Ω
line using a double-stub
tuner. The stubs are opencircuited are spaced λ/8
apart. The match
frequency is 2 GHz.
3. Find the susceptance
of the first stub,
b1 = 1.314
or b1 = −0.114
4. Rotate the 1+jb circle
to the original location,
and have
y2 = 1 − j 3.38
1. y L = 0.3 + j 0.4
2. Construct the rotated
1+jb circle.
Solution 1 has a
much narrower
bandwidth than
solution 2
Read page 238240 of Pozar for
analytic solution.
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b2 = 3.38
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5. Then the susceptance
of the second stub
should be
y2' = 1 + j1.38
b2' = −1.38
6. The lengths of the
open-circuited stubs are
l1 = 0.146λ
l2 = 0.204λ
or l = 0.482λ
l2' = 0.350λ
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