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
stub.
Shunt are easiest to
implement in practice.
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Smith Chart Solution for finding the matching
parameters: l1, and l2.
2
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.
Notes:
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
or
2. Construct the rotated
1+jb circle.
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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
'
Solution:
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5. Then the susceptance
of the second stub
should be
y2' = 1 + j1.38
or
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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