494 F. MEJRI, T. AGUILI, DESIGN OF A MICROWAVE DUPLEXER WITHOUT FERRITE AND WITHOUT MAGNET Design of a Microwave Duplexer without Ferrite and without Magnet Fethi MEJRI, Taoufik AGUILI Sys’Com Research Laboratory, National Engineering School of Tunis (ENIT), University Tunis El Manar, Tunis, Tunisia [email protected], [email protected] Submitted August 6, 2017 / Accepted November 22, 2017 Abstract. In this paper we present the design, realization and characterization of a microwave duplexer, compact, easy to realize and integrate into systems such as ground penetrating radars. It is made without the use of ferrite or magnet. This device is designed in the S band and made in micro-ribbon technology. It consists of a power divider and two RF amplifiers, low gain, using a BFR91 bipolar transistor. The latter is frequently available and inexpen- Fig. 1. Synoptic diagram of the duplexer. sive. Measurements made on a vector network analyzer have shown a low insertion loss with insulation considered The primary function of TR tubes is to direct the re- satisfactory – for low power applications – between the turn echoes to the receiver and to protect it from the trans- transmitter (Tx) and the receiver (Rx) circuits. mitted pulse [2–5]. They are usually spark gaps in a par- tially vacuum tube. An electrical arc can be generated in the tube by the ionization of the gas it contains by a suffi- cient voltage between the electrodes. Keywords The primary function of the ATR tubes is to cut the Duplexer, micro-strip, microwave, bipolar transistor, channel to the transmitter for any pulse moving in the amplifier, polarization tee, Wilkinson power divider transmission line [4], [5]. They are in fact simplified TR tubes. They are filled with an inert gas like argon because the recovery time is not critical. In addition, an exciting agent, such as a maintenance electrode, is not required. The 1. Introduction fact of not having an active gas and a maintenance elec- A duplexer (Fig. 1) is an electronic device allowing trode makes it possible to obtain a tube having a longer life the use of the same antenna for transmitting and receiving than the tubes TR. the signal [1], while respecting the optimal adaptation of Figure 1 shows the block diagram of a duplexer. Its the antenna for each input. It comprises a transmission matrix S is defined below: channel (Tx), a reception channel (Rx) and an antenna channel (Tx/Rx). The transmission and reception channels SSS11 12 13 000 (Tx and Rx) are completely isolated. It is therefore a switch . (1) S SSS21 22 23 001 that connects the transmitter to the antenna and then the SSS100 antenna to the radio receiver. It must prevent the pulses of 31 32 33 considerable power transmitted by the transmitter from The problem is that these types of devices sometimes damaging or destroying the circuits of the receiver, which exist only among sellers of radio frequency components, are calibrated for the processing of signals of very low which exist in their turns only in developed countries. Even power. Generally a duplexer is used in radar systems and in to acquire these, it takes time to deliver. Moreover, these wireless communications systems. There are different types devices do not exist for all frequency ranges. This problem of duplexers that operate on different principles, including: prompted us to design and build a duplexer based on an Hybrid circle duplexer (ferrite and magnet based). element that can exist in all sellers of electronic compo- Coaxial resonant cavity or waveguide. nents. This element is the bipolar transistor BFR91 of the NPN type. It has a cut-off frequency equal to 6 GHz. It can - with TR (Transmission-Receiving) and ATR (Anti- dissipate power up to 300 mW. It has input and output Transmission-Receiving) tubes; impedances equal to 50 . Its theoretical gain, of - or with PIN diodes. maximum power transfer, is equal to 5 dB at 2.4 GHz. The DOI: 10.13164/re.2018.0494 CIRCUITS RADIOENGINEERING, VOL. 27, NO. 2, JUNE 2018 495 Fig. 2. Synoptic diagram of a GPR radar. Fig. 4. Electrical diagram of the realized RF amplifier (VCC = +12 V, RB = 33 kΩ, RC = 220 Ω, C1 = C2 = 10.4 nF). Fig. 3. Synoptic diagram of the studied duplexer. duplexer realized with the help of this transistor presents multiple advantages such as low manufacturing cost, sim- Fig. 5. Topology and design of the RF amplifier. plicity of design and ease of reproducibility. This device [S] Parameters Module [dB] Phase [deg] will subsequently be used in a GPR RADAR [6], [7] appli- S11 –13.4 84 cation operating at 2.4 GHz (Fig. 2). S21 3.4 56 S12 –12.7 83 S22 –12.52 –90 2. Description of the Duplexer Tab. 1. Results of measurements of parameters S of the active circuit. The duplexer is realized in planar technology and its working frequency is equal to 2.4 GHz. It is composed of allows the direct current of the power supply to pass to the two RF amplifiers, based on the bipolar transistor BFR91, transistor which is polarized. The rest point of the transis- and a Wilkinson power divider. The transistors used are of tor is set by RB and RC, IC0 20 mA. The impedances of the the NPN type and are of low power. connection capacitors C1 = C2 = 10.4 nF are chosen to be This device is realized on a printed circuit that is fre- strictly less than 50 at the working frequency quently available and inexpensive. The conductor consti- f = 2.4 GHz. tuting it is copper, with a thickness T = 35 μm and a con- Stability and Gain: ductivity σ = 59.6 106 S/m. Its dielectric is Teflon glass, with a thickness H = 1.5 mm, with a relative permittivity Table 1 represents the Sij parameters of the RF ampli- εr = 4.32 and having losses modeled by tanδ = 0.02. The fier of Fig. 5. These parameters Sij are obtained by meas- synoptic diagram of this duplexer is shown in Fig. 3. urements on a vector network analyzer (VNA) at the fre- quency f = 2.4 GHz. 2.1 Characterization of the RF Amplifier These parameters S make it possible to calculate the Rollet factor K which intervenes in the stability criterion The amplifier used is shown in Fig. 4. It is based on [8] a bipolar transistor BFR91 which is mounted as a common 2 2 2 1 S S emitter. The impedances of the input and the output of this K 11 22 (2) transistor are equal to 50 . To ensure impedance match- 2 S12 S 21 ing for maximum power transfer, the input and output of the transistor have been connected to microstrip lines with with S11S 22 S 21S12 . (3) characteristic impedance ZC = 50 . The latters have a track width equal to 2.86 mm. It is calculated by the ADS Since K = 4.294 > 1 and || = 0.119 < 1, we can de- LineCalc tool. The polarization of the transistor takes place duce that the active circuit will be unconditionally stable at via a low-pass filter LC circuit which blocks the passage 2.4 GHz. The determination of the Rollet factor K is neces- of the RF signal to the supply so that it is not disturbed and sary to obtain also the maximum gain at 2.4 GHz. When 496 F. MEJRI, T. AGUILI, DESIGN OF A MICROWAVE DUPLEXER WITHOUT FERRITE AND WITHOUT MAGNET the amplifier is adapted to input and output, its maximum theoretical gain is given by: S GKK21 2 16.82dB. (4) max S12 In practice this theoretical gain is not really accessible. Input and output impedances: They are provided from the value of the reflection coefficients and by the relation [8]: e s Fig. 6. Topology and design of the insulation circuit. 1 The shape of the butterfly capacitor [9], [10] allows it z . (5) 1 to operate in a wide frequency band. It also makes it possi- ble to reduce its radiation with respect to a capacitor of Input adaptation: square or rectangular shape. Its length is chosen equal to 2 λ /4 and is obtained only by optimization under the ADS BB2 4 C g C* 11 1 (6) software since its width is increasing so its characteristic e12 2 C1 impedance and its effective permittivity vary according to its length. with: 0 c 222 . (10) g BSS111221, (7) efff eff CS S* . 11122 The butterfly capacitor is a planar line which obeys We find a coefficient of reflection: the relation (11). Its length is equal to λg/4 and it is open at its end. It returns to its input a short circuit for the RF 42 e 4.55 10 j4.1 10 signal [11]. which gives Z 49.88 j4.10 . ZZch jtan() c x IN Zx() Z Zx( ) 0. (11) c ZZ jtan() x 4 Output adaptation: cch The short circuit brought back to the input of the BB2 4 C2 * 22 2 (8) capacitor short circuits the output of a line of λg/4 length. s2C 2 C 2 This line brings back to its input an open circuit for the RF 2 signal. with: 222 Z(x ) . (12) BSS1, 22211 (9) 4 * CS22211 S. Figure 7 shows the S parameters obtained by simula- tions on ADS software and by measurements made by We find a coefficient of reflection: a VNA - of the isolation circuit of Fig. 6. Figure 7 shows 32 3.13 10 j5.22 10 that for a direct current the reflections (S11 S22) are s negligible, of the order of –70 dB, and the transmissions which gives: ZOUT 50.04 j5.25 .