Application Notes

Philips Semiconductors
Application note
AN10173-01
2.45 GHz T/R, RF switch for e.g. bluetooth
application using PIN diodes
1 Introduction.
One of the most important building blocks for today’s wireless communication equipment is a high
performance RF switch. The switch main function is to switch an RF port (ANT) between the transmitter
(TX) and the receiver (RX).The most important design requirements are, Low insertion Loss (IL), Low
intermodulation distortion,(IMD), High isolation between TX and RX, Fast switching and Low current
consumption especially for portable communication equipment. This application note addresses a
transmit and receive switch for 2.4-2.5 GHz the unlicensed ISM band, in which e.g. the bluetooth standard
operates. The design demonstrates a high performance T-R switch utilising low cost Philips BAP51-02
PIN Diodes as switching elements.
2 PIN diode switch design.
There are a number of PIN diode based, single pole double throw (SPDT) topologies, which are shown
in the figures 1,2 and 3. Al these topologies are being used widely in RF and microwave design. They all
will give good performance, due to their symmetry they will show the same performance in both the RX
and TX mode. The disadvantage of these topologies is the need of a pair of digital control signals, and
in both TX and RX mode bias current is needed.
L4
C2
C6
TX
L3
TL1
λ/4
λ/4
C7
D4
RX
C8
L5
C9
TL2
D3
D2
RF
D1
Vb
C10
C3
L2
C4
TX
Va
C1
RF
L1
Vb
Va
C5
RX
L6
Figure 1. SPDT switch with series diodes
Figure 2. SPDT switch with λ/4 sections to permit
shunt diodes
The topology we used for the design in this application note is shown in fig 4. Typically this is a
combination of figure 1 and 2. The design consists of a series-connected PIN diode, placed between the
transmitter-amplifier and antenna, and a shunt-connected PIN diode at the receiver-port, which is a
quarter wavelength away from the antenna. In the transmit-mode both diodes are biased with a forward
bias current. Both diodes are in the low impedance state. Which means a low-loss TX-ANT path and a
protected RX port from the TX power.
The=λ/4 transmission line transforms the low impedance at the RX port to a high impedance at the
antenna. In the receive mode both diodes are zero biased ( high impedance state), which results in a low
loss path between antenna and receiver and high isolation ANT-TX path. One of the advantages of this
approach is no current consumption is needed in the receive mode.
© PHILIPS Electronics N.V. 2000
Sheet 2 of 7
L10
C12
D7
D8
D6
C16
RX
TX
C18
C17
D9
λ/4
D10
TX
C19
L8
C14
D5
C11
C13
Ant
RF
L7
Vs
Vb
Va
C15
RX
L9
C20 R1
Figure 3. SPDT switch with series shunt diodes
which results in high isolation
Figure 4. SPDT switch with a combination of a
series and a shunt connected PIN diode.
The PIN diodes used in an switch like this should have low capacitance at zero bias(VR=0V), and low
series resistance at low forward current. The BAP51-02 typical shows [email protected];freq=1MHz and 2 Ω
@3mA;freq=100MHz. For the shunt diode also low series inductance is required, for the BAP51-02 this
is 0.6 nH.
3 Circuit design.
Circuit and Layout has been designed with the use of Agilent’s Advance Design System (ADS). The
target performance of the switch is shown in table 1.
Mode
Insertion Loss
Isolation TX/RX
Isolation RX/Ant
Isolation TX/Ant
VSWR RX
VSWR TX
VSWR Ant
Power handling
Current consumption
RX (0V)
< 0.65 dB
>18 dB
>16.5
<1.2
<1.2
+20dBm
TX(3mA)
< 0.8 dB
>14.5 dB
>14.5dB
<1.3
<1.3
+20dBm
3mA @ 3.7V
Table 1
The ADS circuit of the switch is given in figure 5. Notice that D1 is the series connected PIN diode in
the receive path en D2 is connected in shunt in the receive RF path. DC bias current is provided
through inductance L1, and limited to about 3mA by resistor R1=680 Ω. Notice also that the λ/4
microstripline (width 1.136mm, length =16.57mm) is divided into several sections in order to save
some board space. All the footprints for the SMD components have been modelled as a gap and a
piece of stripline in order to approach the actual practice of the design on PCB.
© PHILIPS Electronics N.V. 2000
Sheet 3 of 7
Figure 5 ADS circuit file
The discontinuity effects of the microstrip to coaxial interface have not been taken into account.
4 BAP51-02 model.
The silicon PIN diode of the Philips semiconductors BAP51-02 is designed to operate as a low loss
high isolation switching element, and is capable of operating with low intermodulation distortion.
The model for the BAP51-02 PIN diode for an ADS environment is shown in figure 6. The model
consists of two diodes, in order to achieve a fit on both DC and RF behaviour. Diode1 is used to
model the DC voltage-current characteristics, Diode 2 is the PIN diode build in model of ADS and is
used to model the RF resistance versus DC current behaviour of the PIN diode-model. Both diodes
are connected in series to ensure the same current flow. For RF the PN junction Diode1 is shorted by
an ideal capacitor(DC block), while the portion of the RF resistance, which reflects the residual
amount of series resistance is modelled with R1=1.128 Ω. To avoid affecting the DC performance this
resistor is shunted with the ideal Inductor (DC feed). Capacitance C2 and inductors L2 and L3 reflect
the package parasitics. The here described model is a linear model that emulates the DC and RF
properties of the PIN diode from 6 MHz up to 6 GHz.
© PHILIPS Electronics N.V. 2000
Sheet 4 of 7
Figure 6; BAP51-02 Small Signal Model for an ADS environment
5 Circuit and Layout Description
The circuit diagram for the switch is shown in figure 7 and the PC board layout is shown in figure 8.
The bill of materials for the switch is given in table2.
For the PC board 0.635mm thick FR4 material (εr = 4.6)metalized on two sides with 35 µm thick
copper, 3 µm gold plated was used.=On the test board SMA connectors were used to fed the RF
signals to the design.
Vs=0/3.7V
C2
1nF
Ant
L1
22nH
TL2, 50 Ω
1.14x7mm
C3
6.8pF
C4
6.8pF
D1
C1
2.2pF
TL4, 50 Ω RX input
50 Ω
1.14x6mm
D2
TX output TL1, 50 Ω
50 Ω
1.14x12mm
C5
4.7pF
TL3, 50 Ω
1.14x16.6mm
C6
2.2pF
R1
680Ω
Figure 8; PC board Layout.
Figure 7; circuit diagram
Component
C1
C2*
Value
2.2 pF
1 nF
Footprint
0402
0402
Manufacturer
Philips
Philips
© PHILIPS Electronics N.V. 2000
Sheet 5 of 7
C3
6.8 pF
0402
C4
6.8 pF
0402
C5
4.7 pF
0402
C6
2.2 pF
0402
R1
680 Ω
0402
D1
BAP51-02
SC79
D2
BAP51-02
SC79
L1
22 nH
1005
TL1
λ/4;50 Ω
Table 2 Bill of materials *C2 is optional.
Philips
Philips
Philips
Philips
Philips
Philips
Philips
Taiyo yuden
on the PCB
6 Measurement results.
In table 3 the measured performance of the switch is summarised. In figure 9, both the simulation and
Measurement results in TX mode (3.7V/3mA) is shown, for the RX mode this can be seen in fig.10.
parameter
Insertion Loss @ 2.45GHz
Isolation TX/RX @ 2.45GHz
Isolation Ant/RX @ 2.45 GHz
Isolation TX/Ant @2.45 GHz
VSWR RX @2.45 GHz
VSWR TX @2.45 GHz
VSWR Ant @2.45 GHz
IM3 Pin 0 dBm f1=2.449 GHz f2=2.451 GHz
IP3 Pin 0 dBm f1=2.449 GHz f2=2.451 GHz
IM3 Pin +20 dBm f1=2.449 GHz f2=2.451 GHz
IP3 Pin +20 dBm f1=2.449 GHz f2=2.451 GHz
Power handling
Current consumption
Mode
RX (0V)
TX(3mA)
< 0.57 dB
< 1.0 dB
>20.4 dB
>23.6 dB
>23.5 dB
>19.76 dB
1.24
1.35
1.19
1.29
+39 dBm
+40 dBm
+43.8 dBm
+44.8 dBm
+38.5 dBm
+39.5 dBm
+43.3 dBm
+44.3 dBm
+20 dBm
+20 dBm
3mA @ 3.7V
Table 3 measured switch performance.
Intermodulation distortion measurements were performed as follows. In both RX and TX state, first
the measurements were done with two input-signals, each at 0 dBm and second each signal at +20
dBm. In transmit state these signals were applied to the TX port, distortion was measured at the
antenna port, while the RX port was terminated with 50Ω. In receive state the two signals were
applied to the ANT port, distortion was measured at the RX port, with the TX port terminated.
According to reference 2, the third order harmonic distortion product is 9.54 dB less than the third
order Intermodulation product, the third order harmonic intercept point IP3 is 9.54/2 higher than the
third order Intermodulation intercept point IM3.
© PHILIPS Electronics N.V. 2000
Sheet 6 of 7
Figure 9; Results in TX mode; red curves are measurements, blue curves are the simulated ones.
Remark: Loss and Isolation results are all including approximately 0.2 dB loss of the SMA
connectors which were used to fed the RF signals through the design. this has a great effect on the
Insertion-Loss results.
© PHILIPS Electronics N.V. 2000
Sheet 7 of 7
Figure 10; Results in RX mode; red curves are measurements, blue curves are the simulated ones
Remark: Loss and Isolation results are all including approximately 0.2 dB loss of the SMA connectors
which were used to fed the RF signals through the design. this has a great effect on the InsertionLoss results.
Recommendations.
1
In this design the BAP51-02 was used because it’s designed for switching applications related
to Insertion Loss and Isolation. When for instance a better IM distortion is recommended it’s
better to use the BAP64-02 of Philips Semiconductors.
2
As you can see the λ/4 section still needs a lot of boards space. This section could be replaced
by a lumped element configuration, which results in an extra boardspace reduction.
References:
1; Gerald Hiller, “Design with PIN diodes”, App note APN1002 Alpha industries inc.
2; Gerald Hiller, “Predict intercept points in PIN diode switches”, Microwaves & RF, Dec. 1985.
3; Robert Caverly and Gerald Hiller, “Distortion in PIN diode control circuits” IEEE Trans.Microwave
theory tech.,May 1987.
© PHILIPS Electronics N.V. 2000
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