Temperature Compensated Attenuator Based On Composite Materials Of The Thermistors

In modern microwave communications, for Low Noise Amplifier(LNA) and Power Amplifier(PA) systems, the main performance of RF gain and output power and VSWR varies with temperature. In general, when the temperature becomes higher/lower, the output power and the gain of the system will be smaller/larger, then the system’s output power and the gain varies with temperature, and the performance of the system would be seriously affected. A temperature compensation element called Temperature Compensated Attenuator(TCA)is required in the RF system. The TCA uses thick film resistance materials to absorb the power then convert into heat in order to achieve attenuation. When the temperature increases/decreases, the amount of attenuation of the TCA would decrease/increase in compensation of characteristics of gain in high frequency amplifier circuits as temperature starts to increase/decrease. To achieve better temperature compensation features, we needed to design TCA by two kinds of different thermal resistances with composite properties. This paper first studied the properties of NTC resistor LSMO materials and Barium titanate series PTC resistor Y-doped BST materials, made both as thermal resistance materials and conducted a series of different sizes and proportions of different input and output ports model of TCA research work, the main following three aspects to explore TCA based on composite thermal materials was done.Materials developing aspects, first of all, the performance of NTC resistor LSMO material was studied, the thickness measured of the thick film resistor was13.5 μm, room temperature(25 ℃) resistance was 0.249 Ω ? cm and TCR was-2839.11 ppm /℃ between-20 ℃ ~+120 ℃; TCR was-3602.3 ppm/℃ between 10 ℃~+120 ℃, and the SEM testing and analysis was carried out. Then, the performance of barium titanate series PTC materials was studied, by means of BST materials doped with different proportions of Y2O3. During the temperature range of-20 ℃~+120 ℃, the resistance of Y-doped BST material increases with the increasing of temperature, showed the characteristic of PTC resistance. Under a certain doping concentration, relationship between resistance of the material and doping proportions was U shape curve. For ceramic wafer materials which were Y0.0045 doped, kept heating for 30 min, the sample had the lowest room temperature resistance, which was7.58 KΩ?cm, and the TCR was 6.34×104 ppm/℃; When temperature holding timewas 60 min, sample’s room temperature resistance was 7.58 KΩ?cm, and TCR was1.24×105 ppm/℃, and the XRD and SEM testing and analysis was carried out.Designing aspects of TCA, according to the model of pi-type attenuator and the idea of thermistors paralleled, by different input and output shape of ports of the 50 Ωresistance matching, used HFSS simulation software to design the room temperature attenuation amount which was-7 dB, and three pi-type TCA with single-stage continuous, double-stage discontinuous, triple-stage discontinuous input and output ports models.Preparation and testing aspects of TCA, the NTC and PTC resistors were printed on the alumina substrate by screen printing process, then manufactured the test instruments, and used microwave vector network analyzer and high and low temperature tool for the performance testing, operating frequency of three models were: DC-3 GHz, DC-4 GHz and DC-6 GHz, the frequency range in which VSWR was less than 2, S11 was less than-10 dB. In the temperature range of-20 ℃~+85 ℃,attenuation amount change for three models were about 1.25 dB, and attenuation amount change rate was about-0.0017 dB/dB/℃. Three models were loaded on a one hour power test, which carried the power level of 2 W, and the maximum superficial resistance temperature of the thick films were measured, lower than 100 ℃, lower than the limiting temperature of 125 ℃, met the design requirements.