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Document Title: Broadbanding the Shunt Pin Diode SPDT Switch (AN 957-1)
Part Number: 5964-3902E
Revision Date: January 1996
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hH
Broadbanding the Shunt
PIN Diode SPDT Switch
Application Note 957-1
In the design of a stripline or BIAS 1 BIAS 2
RF PORT 3
microstrip SPDT PIN diode
switch, bandwidth and physical L L
RF RF
construction are often important PORT 1
D1 D2 C PORT 2
C
considerations. Three basic de-
sign approaches for single pole,
double throw diode switches are L
COMMON
shown in Figures 1 through 3. JUNCTION
The series diode switch of Figure 1
is capable of very large (multi-oc- Figure 1. Series PIN SPDT Switch.
tave) bandwidth, limited only by
the bias inductors L and capacitors
BIAS 1 BIAS 2
C, and the length of any transmis- RF PORT 3
sion line between the diodes and
the common junction. Etched flat L L
spirals or aircore solenoids produce RF
RF
good broadband lumped-element PORT 1 C C C C PORT 2
Zo Zo
inductors, and MOS capacitors fea- /4 /4
ture self-resonant frequencies
above 18 GHz. This structure is D1 D2
easiest to fabricate with beamlead
diodes on alumina substrate MIC.
In plastic dielectric symmetrical
stripline, difficulty is encountered Figure 2. Shunt PIN SPDT Switch.
in relieving the faces of both boards
to accept packaged diodes, and in BIAS 1 BIAS 2
locating the diode junctions electri- RF PORT 3
cally close to the common arm of
L L RF
the switch. Finally, parasitic capaci- RF
D3 D4 PORT 2
PORT 1 C C
tance gives rise to poor isolation at
microwave frequencies, with a 6 dB
per octave rolloff as a function of L
D1 D2
frequency.
The shunt diode switch, shown
in Figure 2, features high isola- Figure 3. Series/Shunt PIN Switch.
hH
tion, relatively independent of RF PORT 3
frequency. It is an easy structure
to design and fabricate if BIAS 1 BIAS 2
stripline package PIN diodes Z
/4
such as the HP 5082-3040 series
are used. In these products, the L L
RF
diode junction capacitance has PORT 1
RF
PORT 2
been matched out by integration Z Z
into a low-pass filter structure. C C
/4 /4
C C
In an MIC switch, a chip, such as D1 D2
the HP 5082-0001, allows easy
assembly. However, the user
must then provide the matching Figure 4. Broadband Shunt SPDT Switch.
structure. The main drawback of
this type of switch is the band-
2.4:1
width restriction arising from from the common junction. This Z = 50
the use of quarter wavelength switch, however, is complicated, 2.2:1
transmission lines between the and consumes twice the bias power Z = 45
2:1
common junction and each shunt of the shunt switch shown in Figure
Z = 40
diode. At the midband frequency 2. Here, as in Figure 1, it is difficult 1.8:1
SWR
fO, where the transmission lines to mount the D3 and D4 diode junc- Z = 30
are /4 in length, the switch op- tions electrically close to the com-
1.6:1
Z = 35
erates as follows: When Diode D1 mon arm. 1.4:1
is forward biased and Diode D2 is
reverse biased, R.F. power will The bandwidth of the shunt di- 1.2:1
flow from Port 3 to Port 2, and ode switch can be improved by 1.0:1
0.5 0.7 0.9 1.1 1.3 1.5
R.F. Port 1 will be isolated. The the simple impedance matching FREQUENCY, BROADBAND SWITCH,