Temperature-compensated lumped element tunable bandpass filter

This paper presents and compares two techniques for compensating temperature effects in varactor diode-based lumped element tunable bandpass filters (BPFs). Temperature compensation relies on first measuring the temperature and subsequently re-biasing the varactors of the filter based on pre-calculated lookup table. The lookup table is calibrated using two different methods: a) constant varactor capacitance, and b) constant filter response. Both approaches are evaluated when the filter is subjected to temperatures ranging from −40 to 100°C. In the first approach the capacitance of each of the filter's varactors is held constant at the value found to yield the desired filter response at room temperature. On the other hand, the second approach utilizes a global optimization scheme that re-biases all varactors until the filter yields the desired transfer function. The results from both methods are measured and compared for a third-order BPF tunable from 226 MHz to 333 MHz. While the first method is simpler to implement and may appear intuitive, it actually causes a worse temperature-induced drift because it ignores substrate and inductor effects. Specifically, the resulting temperature drift of the center frequency with the first method is 4.3% as compared to 3.2% when no compensation is applied. Optimal results are achieved with the second approach where the filter's center frequency remains constant to within 0.1% over the −40 to 100°C temperature range.