1. Duplexer design project
Introduction
A duplexer is a three port network that allows bi-directional communication over a single frequency channel which is composed of transmitting and receiving band.
One channel
transmitting
receiving
Power spectrum
frequency
antenna
Receiving
circuit
Duplexer
Transmitting
circuit

2. A simple duplexer
To share one antenna, the duplexer should separate the frequency spectrum into transmitting and receiving spectrum. Then, the separated bands are diverted to a transmitting network and a receiving network. To realize these functions, a duplexer which is a variant filter network should be synthesized. A simple duplexer is frequently synthesized in a parallel combination of two filters of which pass bands contain receiving frequency band and transmitting band. f
transmitting
receiving
Transfer function
Duplexer
Receiving filter
Transmitting filter
Transfer function
transmitting
receiving f

3. Example : PCS duplexer
transmitting
receiving

4. Design consideration Transmission zero
In a wireless system, the transmitting power level is very larger than the receiving power level. So care should be taken not to interfere the receiving power level. This consideration is taken into implementing the filter transfer function. Usually, each of the filter transfer functions (transmitting and receiving) should contain a transmission zero in the other frequency band. That is, the receiver transfer function should contain a transmission zero in the transmitting band and the transmitting network transfer function should contain a transmission zero in the receiving band.

5. Design assignment
To design a simple duplexer filter, implement the transmitting and the receiving filters by a cascade of low pass, high pass, band pass filter or band stop filter network that covers the specified frequency band.
Receiving or
transmitting filter
LPF, HPF, BPF
or BSF
LPF, HPF, BPF
or BSF
Reports on this design project should include the followings :
A duplexer schematic circuit.
PSpice simulation results that show the duplexer meets the design specification.
Detailed explanations on how the values of lumped elements are determined.

6. Specification 50Ω 50Ω 50Ω Receiving filter Transmitting filter
20log10|H|> -3dB
20log10|H|> -3dB
20log10|H|< -20dB
20log10|H|< -20dB
20log10|H|< -40dB
20log10|H|< -40dB
1.65GHz
1.75GHz
1.78GHz
1.84GHz
1.87GHz
1.97GHz
Receiving filter
50Ω
50Ω
Transmitting filter
50Ω

7. Example : duplexer receiving Band pass filter Band reject filter
transmitting

8. Result transmitting receiving dB(Vo1/Vi) freq, GHz dB(Vo2/Vi)
1.65
1.70
1.75
1.80
1.85
1.60
1.90
-45
-40
-35
-30
-25
-20
-15
-10 -5
-50 5
dB(Vo1/Vi)
1.65
1.70
1.75
1.80
1.85
1.60
1.90
-45
-40
-35
-30
-25
-20
-15
-10 -5
-50 5
freq, GHz
dB(Vo2/Vi)
1.65
1.70
1.75
1.80
1.85
1.60
1.90
-45
-40
-35
-30
-25
-20
-15
-10 -5
-50 5
freq, GHz
dB(Vo2/Vi)
dB(Vo1/Vi)
freq, GHz

9. Hint :
To satisfy the specifications of the duplexer, adjust Q’s, center frequencies, the order of band pass or band stop filters until they are met.
A filter with the order higher than 1 can be constructed from the low pass filter prototypes (see page 10). Relevant impedance, frequency, BPF, BSF, HPF transformation should be made.

10. Reference material N-th order low pass filter proto-types :10
N-th order low pass filter proto-types : or
N-th order Butterworth filter
Value of n g1 g2 g3 g4 g5 g6 g7 1
2.000
1.000 2
1.414 3 4
0.7654
1.848 5
0.6180
1.618
0.618 6
0.5176
1.932

11. Filter transformation11
1. Impedance transformation
Filter
Filter
2. Frequency transformation

12. LPF prototypes : ωc=1rad/s, Rs=RL=1Ω
1st order LPF
2nd order LPF
3rd order LPF