A 20 to 560 MHz, Reduced Harmonic Selectivity and Harmonic Fold Back, 6-path Tunable Bandpass Filter

In this paper a 6-path tunable bandpass filter which is able to reduce both Harmonic Selectivity (HS) and Harmonic Fold Back (HFB) effect simultaneously is proposed. The filter contains two channels of 6-path switched capacitor filters with same circuit structure but different clocks and is designed and simulated in GSMC 180nm CMOS technology. The simulation result shows the center frequency of the designed filter s f can be tuned from 20 to 560 MHz, reduction of HS at 3 s f is from -34 to -40 dB, and reduction of H6 in HFB is -21 dB at the same time. The filter can be useful in SDR systems because of its good tunability, high Q and high linearity. Introduction Software-defined radio (SDR) is one of the most promising solutions to meet the demand of multiband wireless systems, because of its programmable circuitry which can integrate circuits with multiple purposes. To design a programmable radio, a tunable, integrated bandpass filters in analog front-end circuit, is the key and is still in challenge. Off-chip filters, like SAW filters are unable to tune. On-chip LC filters only provide limited tunability with low Q and consume much more areas. Recently, many researches have focused on the N-path switched capacitor filters due to their center frequency tunability realized by changing the frequency of clock [1-5]. Benefitted from the boost of CMOS technology, this structure has been more and more widely used in RF circuits. However, because of signal sampling, the N-path filters have two fatal drawbacks which are called Harmonic Selectivity (HS) and Harmonic Fold Back (HFB), respectively [2]. Many works have been done to reduce the HS and HFB in N-path filter. By using differential structure, all even parts of the HS can be eliminated [2]. Other HS can be suppressed when the N is decreased but the HFB will be increased at this moment [2,6]. By using the method of signal superposition, a 6-path filter can reduce the HS at 3 s f [7], and a structure which could significantly reduce HFB is proposed in [8]. But all these former proposed structures work poorly to reduce the HS and HFB at the same time. In this article, a 6-path tunable bandpass filter which is able to reduce both HS and HFB effect simultaneously is proposed. This filter contains two channels of 6-path switched capacitor filters with the same circuit structure but different clocks. With method of signal superposition, this filter can reduce HS effect at 3 s f and H6, the largest HFB effect of all, is also reduced at the same time. N-path Filter Fig. 1 gives the structure of a typical N-path bandpass filter circuit [1,2,5,7]. Each switch in this circuit works as a mixer. The input signal is down-sampled by switch networks S1 to Sn, transferred into baseband and filtered by RC lowpass filters. Since N paths are sampled by clocks with the same pulse width but different clock phases, the baseband signals are transferred to the switch frequency again. So the circuit can be taken as a bandpass filter whose center frequency can be tuned by changing the clock frequency. 2nd International Conference on Electrical, Computer Engineering and Electronics (ICECEE 2015) © 2015. The authors Published by Atlantis Press 822