Design Of Complex Filter In Low Intermediate Frequency Receiver

In recent years, with the rapid development of CMOS technology, a growingnumber of single-chip radio receiver are designed to reduce power consumption andcost. Where the zero-IF and low IF architecture are two of the more popular radioreceiver structure. For zero-IF receiver, DC offset, flicker noise and other factorslimits its performance. During circuit design, we need additional circuits to eliminatethese effects, which increases the system’s complexity and power consumption. Incomparison, these factors do not affect the low-IF receiver, so this structure is widelyused. But the main problem of the low-IF receiver is image signal interference, imagesignal needs to be suppressed.This paper is to solve the image signal interference problems in low intermediatefrequency receiver, starting from the basic theory of low pass filter, we introduced theclassification and design method of low pass filter, and deeply analyzed the imagesingle suppression principle of complex filter. In the circuit design, first of all, basedon source degeneration OTA, we proposes an operational transconductance amplifiersuitable for high frequency applications. We use active inductors and resistors basedon the OTA to replace the passive inductors and resistors in low-pass filter ladderprototype and use cross-coupling transconductors to translate the frequency responseof low-pass filter to complex band-pass response. Then, a seven order Chebyshev1Gm-C complex filter is designed. Finally, an integrator and digital circuit basedfrequency tuning circuit is used to realize automatic tuning for both center frequencyand bandwidth. On the completion of tuning the circuit will automatically shut down.The ’zero power’ is particularly suitable for low power applications.The complex Gm-C filter is designed using0.18μm CMOS RF process.Simulation results show that the filter has20MHz bandwidth and is centered at46MHz, while the pass-band flatness is less than0.36dB. The image band rejection ishigher than98dB, and the Out-band rejection is higher than28dB. the tuning error isless than±1.5%. The complete filter consumes less than7.9mA with1.8V supplyvoltage.