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

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

Example : PCS duplexer transmitting receiving

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.

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.

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

Example : duplexer receiving Band pass filter Band reject filter transmitting

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

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.

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

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

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