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Lecture 14: Double-Stub Tuning Double-Stub Tuning

Double-stub tuning is used to match a load to a transmission line when the load is located an arbitrary distance from the line. It uses two tuning stubs to partially remove the dependence on the load location. The double-stub tuner is widely used as a single frequency matching device. To design the double-stub tuner, the Smith chart is used to first rotate the load impedance to the first stub using l1, then rotate back to the original point and to the second stub using l2 to cancel the susceptance and match the impedance to the line. The spacing between the stubs is usually chosen as λ/8 or 3λ/8 to minimize the forbidden region where matching is not possible.

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Ranjith Kumar
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222 views2 pages

Lecture 14: Double-Stub Tuning Double-Stub Tuning

Double-stub tuning is used to match a load to a transmission line when the load is located an arbitrary distance from the line. It uses two tuning stubs to partially remove the dependence on the load location. The double-stub tuner is widely used as a single frequency matching device. To design the double-stub tuner, the Smith chart is used to first rotate the load impedance to the first stub using l1, then rotate back to the original point and to the second stub using l2 to cancel the susceptance and match the impedance to the line. The spacing between the stubs is usually chosen as λ/8 or 3λ/8 to minimize the forbidden region where matching is not possible.

Uploaded by

Ranjith Kumar
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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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 Original circuit
proper distance from the load, which is a variable that is difficult to with the load an
adjust in practice. 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.
ELEC4630, Shu Yang, HKUST 1 ELEC4630, Shu Yang, HKUST 2

Smith Chart Solution for finding the matching


Step 4: the transmission line between l1 and l2 will rotate back
parameters: l1, and l2.
to the original 1+jb circle. And y1 (or y1 ) rotates to y2 (or y2 )

Step 1: Draw the Step 5: Design l2 so that the susceptance is cancelled out and
normalized load impedance is matched to the line (origin in the Smith Chart).
impedance yL in the Notes:
Smith Chart.
1. If the load admittance is located inside the shaded area
Step 2: Taking into (forbidden area), it simply cannot be matched by the double-
account the effect of the stub tuning.
transmission line d
between two stubs --- How to shrink the forbidden area?
rotated 1+jb circle 2. In practice, stub spacings are usually chosen as /8 or 3/8.
Step 3: Design l1 so
that yL moves to y1 (or
y1 for solution 2).
ELEC4630, Shu Yang, HKUST 3 ELEC4630, Shu Yang, HKUST 4
Example 5.4 on Page 237
of Pozar Solution (cont.)
Match a load impedance 3. Find the susceptance 5. Then the susceptance
ZL = 60 - j 80 to a 50 of the first stub, of the second stub
line using a double-stub should be
b1 = 1.314
tuner. The stubs are open-
or b1 = 0.114 b2 = 3.38
circuited are spaced /8
'

apart. The match 4. Rotate the 1+jb circle or b2' = 1.38


frequency is 2 GHz. to the original location, 6. The lengths of the
Solution: and have open-circuited stubs are

1. y L = 0.3 + j 0.4 y2 = 1 j 3.38 l1 = 0.146 l2 = 0.204

or y2' = 1 + j1.38 or l = 0.482


'
l2' = 0.350
2. Construct the rotated 1

1+jb circle.

ELEC4630, Shu Yang, HKUST 5 ELEC4630, Shu Yang, HKUST 6

Solution 1 has a
much narrower
bandwidth than
solution 2

Read page 238-


240 of Pozar for
analytic solution.

7
ELEC4630, Shu Yang, HKUST

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