The Design of Temperature-Compensated Surface Acoustic Wave Oscillator

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1 The Design of Temperature-Compensated Surface Acoustic Wave Oscillator MEI-HUI CHUNG, SHUMING T. WANG, AND JI-WEI IN Department of Electrical Engeerg I-Shou University, Taiwan, Section, Hsueh-Cheng Road, Ta-Hsu Hsiang, Kaohsiung, Taiwan 84 R.O.C. Abstract: - The design technique of a temperature-compensated SAW oscillator (TCSO) was thoroughly studied. The circuit of the TCSO contas two ma parts, one is oscillation circuit and the other is temperature compensation circuit. In this work, the oscillator circuit adopted a commercial one-port SAW resonator at MHz its resonant circuit. The reflection circuit model for oscillator design was applied. The performances of the designed TCSO are output power.6dbm, phase noise -38dBc/Hz at KHz offset from carrier frequency, and maximum frequency deviation -4ppm from -ºC to 85ºC. Key-Words: - Oscillators, Surface Acoustic Wave Resonator, Temperature-Compensated Circuit.. Introduction Surface acoustic wave (SAW) devices have the advantages of compact, superior performance, highly reproducible and reliable. Recently, owg to the improvement design and fabrication techniques, SAW devices have been widely applied on the products of communication, computer and consumer electronics, such as RF and IF filters and oscillators []. Although SAW oscillator presents better phase noise and frequency stability than C oscillator, the frequency stability with temperature changg is still a crucial issue for a high performance communication system. In this paper, a temperature-compensated SAW oscillator was designed and verified by experiment. This TCSO employed a one-port SAW resonator operated at MHz its oscillation circuit and along with a temperature-compensated circuit. 2. Method for temperature compensation The circuit of TCSO is divided to two parts, one is the voltage controlled SAW oscillator (VCSO) and the other is the temperature-compensation circuit as shown figure. The VCSO cludes an amplifier, a varactor diode and an one-port SAW resonator. As stated many circuit design textbook [2-3], the oscillation frequency can be expressed as f = 2π C () Where, and C are equivalent capacitance and ductance of the resonant tank circuit, respectively. The varactor diode is eventually a PN diode operated reverse bias. The junction capacitance of a varactor, considered as a parallel capacitor, can be calculated by C = ka d, where C is capacitance, A is the cross section area of the junction, d is the width of depletion region and k is a constant. The capacitance C is direct proportion of the area A and verse proportion of the width d. When the reverse voltage decreases, the depletion region of PN junction becomes narrower and the capacitance creases. On the contrary, the capacitance decreases as the reverse bias creases. The temperature compensation circuit senses the temperature variation and transfers to a bias voltage signal on varacator diode. By variation the junction capacitance of varacator diode, the frequency compensation can be achieved. Figure 2 depicts the prciple of temperature compensation effect. In this figure, the horizontal axis represents the variation of temperature and the vertical axis shows the relation of compensation voltage and frequency of oscillation. Curve A is the oscillation frequency without the temperature compensation, curve B is compensation voltage applied on varactor diode, and curve C depicts the oscillation frequency after compensation. Fig. Block diagram of temperature-compensated SAW oscillator.

2 Fig.2 The operation prciple of a temperature compensation oscillator. 3. Theorem of Oscillation The reflection model was used to analysis the oscillator. Figure 3 shows the equivalent circuit model of the oscillator, where Z is the impedance of active circuit and Z is the load impedance of passive circuit. a n dicates a noise source. a and b represent the amplitude of cident and reflection voltage to and from Z, and a and b represent the amplitude of cident and reflection voltage to and from Z, respectively [3-6]. where, Z is the characteristic impedance of the system. The equation (4) is valid when Z + Z =, i.e., R + R = (5) X + X = Sce the load is a passive network, the real part of load impedance R is positive. To satisfy the oscillation condition, the real part of active circuit R should be a negative value which means the active circuit should provide enough energy to support a stable oscillation. The equation of imagary part determes the frequency of oscillation that is oscillation can only occurs at the frequency where X =-X. Therefore, as illustrated by the diagram of reflection model of oscillator shown figure 4, the criteria of for itial oscillation are expressed equations (6) and (7) [5]. < Γ (6) S S = Γ (7) Fig.4 Diagram of reflection oscillated circuit. Fig.3 Equivalent circuit of oscillator usg reflection model. The reflection coefficients lookg to source and b b load are defed as Γ = and Γ =. From a a circuit analysis: Γa a = (2) ΓΓ The condition for oscillation is: Γ Γ = (3) From the relationship of load impedance and reflection coefficient, equation (3) can be derived: 2 Z Z Z ( Z + Z ) + Z Γ Γ = = (4) 2 Z Z + Z Z + Z + Z ( ) 4. Design Example Based the theory mentioned above, the designed oscillation and temperature compensation circuits are given figures 5 and 6. In figure 5, R, R 2, R c and R e are for bias resistors; C 3 and C 4 are couplg capacitors for DC block. Shunt-feedback capacitors C and C 2 are employed to grantee the active circuit operate unstable region (S >). An RF chock is placed to separate the RF signal and DC voltage. In figure 6, OP and OP2 work as voltage follower which use the nature character of operation amplifier that have a large put impedance to avoid loadg effect on output voltage. The output voltage of OP V a is the reference voltage and the output voltage of OP2 V b is the voltage transferred from the temperature sensor AD59. OP3 is a differential amplifier that takes the voltage difference between V b and V a. If R 3 =R 2 and R 4 =R 5, the ga and the R4 output voltage of OP3 are equal to K = and R3 R4 VC = ( Vb Va ). OP4 works as voltage scalar that R 3 adds a reference voltage V d to the output voltage of OP3. By adjustg K and V d, the purpose of temperature compensation can be obtaed.

The designed parameters of oscillation circuit are listed as follows: () center frequency is 433.92MHz for SAW resonator. (2) topology of the circuit: Colpitts oscillator.

The simulation results of the oscillator are shown figures 7 to, respectively. Figure 7 shows the criteria for itial oscillation where < Γ.

The power spectrum and the phase noise of the oscillator are given figures 9 and respectively. Fig.7 Simulated the criteria of oscillator, S = Γ.

3 The designed parameters of oscillation circuit are listed as follows: () center frequency is MHz for SAW resonator. (2) topology of the circuit: Colpitts oscillator. (3) active device: NEC NE85633 (4) DC bias: V ce =5V, I c =5mA (5) PCB substrate: FR4 substrate with dielectric thickness.6mm. The simulation results of the oscillator are shown figures 7 to, respectively. Figure 7 shows the criteria for itial oscillation where < Γ. Figure 8 shows S the imagary part of impedance is equal to zero at frequency MHz, and the real part of the impedance is negative. The power spectrum and the phase noise of the oscillator are given figures 9 and respectively. Fig.7 Simulated the criteria of oscillator, S = Γ. S Γ S < Γ and S Curve A Im (Z)= ohm Z Γ Curve B Re (Z)=-82 ohm Fig.5 The schematic diagram of oscillation circuit. V a Vc Fig.8 Simulated impedance performance, curve A is imagary part and curve B is real part of impedance. V b Vd Fig.6 The schematic diagram of temperature compensation circuit. Fig.9 Simulate power spectrum.

Fig. Simulated phase noise. Fig.2 Measured power spectrum. 5. Experiment and Measurement The realized TCSO hardware is shown figure, and the measurement results are represented figures 2 and 3.

4 Fig. Simulated phase noise. Fig.2 Measured power spectrum. 5. Experiment and Measurement The realized TCSO hardware is shown figure, and the measurement results are represented figures 2 and 3. The frequency versus temperature from -4ºC to 8ºC of uncompensated oscillator is displayed figure 4. The temperature characteristic of uncompensated oscillator is a parabolic curve with turnover temperature near 2 ºC. In order to compensate the frequency from -4ºC~8ºC, two compensated circuits were needs, one for below ºC and the other one for above ºC. All the measurements are done with a frequency counter an environmentally controlled chamber (temperature range and tolerance: -4 C to C and ±.2 C, humidity range and tolerance: % to 98%RH and ±2%). The oscillation frequencies are recorded on C per step. The temperature characteristics of compensated oscillator are shown figures 5 and figure 6. From the measurement results, the maximum frequency deviation above C is improved from 36ppm to 4ppm. Fig.3 Measured phase noise. Fig.4 Temperature characteristic of uncompensated VCSO Fig. Realized TCSO hardware Fig.5 Temperature characteristics of TCSO.

5 Fig.6 The frequency deviation of TCSO. 6. Conclusions In this paper, a temperature-compensated SAW oscillator was designed successfully. The reflection oscillator model was employed to design a high output power and low phase noise SAW oscillator. The oscillator frequency is MHz, output power is.6dbm and -38dBc/Hz phase noise at KHz offset from carrier frequency. The temperature compensation circuit usg temperature sensor IC AD59 improved the frequency deviation from 36ppm to 4ppm. Acknowledgements The authors would like to thank the National Science Council of the Republic of Cha for fancially supportg this research under Contract No. NSC E References: [] Chun-Chieh Chien, The Design of Surface Acoustic Wave Devices and its Application Boardband Oscillator, Master thesis, Institute of Communication National Chiao Tung University, 2. [2] R. T. Payneter, Introductory Electronic Devices ad Circuits, Prentice-Hall International, Inc., New Jersey, 989. [3] G. D. Vendel, A. M. Pavio, U.. Rohde, Microwave Circuit Design usg ear and Nonlear Techniques, John Wiley and Sons, Inc., New York, 99. [4] Yu-His Wang, Design of A VCO for WAN Application, Master thesis, Institute of Electrophysics National Chiao Tung University, 22. [5] John W. Boyes, The Oscillator as a Reflection Amplifier: An Intuitive Approach To Oscillator Design, Application Note of Hewlett-Packard. [6] R. J. Gilmore, F. J. Rosenbaum, An Analytic Approach to Optimum Oscillator Design Usg S-parameters, IEEE Transactions on Microwave Theory And Techniques, Vol. MTT-3, No. 8, pp , 983.

Understanding VCO Concepts

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