Direct Downconversion of Multiband RF Signals Using Bandpass Sampling


Using Bandpass Sampling

Ching-Hsiang Tseng and Sun-Chung Chou

Abstract—Bandpass sampling can be used by radio receivers to directly digitize the radio frequency(RF)signals.Although the bandpass sampling theory for single-band RF signals is well established,its counterpart for multiband RF signals is relatively immature.In this paper,we propose a novel and efficient method tofind the ranges of valid bandpass sampling frequency for direct downconverting multiband RF signals.Simple formulas for the ranges of valid bandpass sampling frequency in terms of the frequency locations of the multiple RF bands are derived.The result can be used to design a multiband receiver for software defined radios.

Index Terms—Software defined radio,bandpass sampling, sampling methods,analog-digital conversion,signal sampling.


M OBILE communication standards emerged in an in-creasing speed in recent years.The software defined radio(SDR)[1],which was originally conceived for military applications[2],has the agility of changing its functionality through replacement of the application program and hence provides a relatively economical way to accommodate the rapid emerging mobile communication standards.The main philosophy in designing a SDR receiver front end is to push the analog-to-digital converter(ADC)as close to the antenna as possible.The ultimate goal,would be direct digitizing the radio frequency(RF)signal at the receiver antenna output[3]. To achieve this goal,one can manage to use a wideband high-speed ADC to convert the RF signal to digital.However,the required sampling rate for the ADC could often be too high to be attained if the Nyquist sampling theorem is to be satisfied [4].Alternatively,being aware of the bandpass nature of the radio signal,one can use the bandpass sampling[5]instead to directly downconvert the desired RF signal from the RF to an intermediate frequency(IF).This can significantly relax the demand for fast ADCs and digital signal processors(DSPs) in a SDR design.

In military and commercial SDRs,there is a need for a single radio to communicate simultaneously with many radios using different RF bands and different modulation schemes [2],[6],[7],[8].To attain this goal,methods for direct downconversion of multiband RF signals using a single ADC Manuscript received August11,2003;revised November13,2003;ac-cepted January11,2005.The associate editor coordinating the review of this letter and approving it for publication was T.Boucouvalas.This paper was presented in part at the IEEE International Conference on Communications (ICC’03),Anchorage,AK,May2003.

C.-H.Tseng is with the Department of Electrical Engineering,National Taiwan Ocean University,2Pei-Ning Rd.,Keelung,202,Taiwan(e-mail:

S.-C.Chou is with Wistron Corp.,21F,88,Sec.1,Hsin Tai Wu Rd., Hsichih,Teipei Hsien,221,Taiwan(e-mail:eddie chou@ ). Digital Object Identifier10.1109/TWC.2006.01012.

Direct Downconversion of Multiband RF Signals Using Bandpass Sampling

Fig.1.(a)The SDR receiver front end.(b)The spectrum of the multiband RF signal at the input of the ADC.

may be appreciated.One possible SDR receiver front end design fulfilling this purpose is shown in Fig.1(a)[9],where the input signal to the ADC would be a multiband RF signal whose spectrum is like that shown in Fig.1(b).A method to determine the bandpass sampling frequency for the ADC in Fig.1(a)has been proposed in[9].This method,however,is computationally intensive.Its computational complexity can be alleviated to certain extent by using the method presented in[10].

In this paper,a novel method is proposed to obtain the valid bandpass sampling frequency ranges for direct down-conversion of multiband RF signals.In the proposed method, wefirst observe that a valid sampling frequency must not cause aliasing in the sampled signal.To be immune from aliasing,the spectral replicas(which are produced by sam-pling)of the signals at different bands must be positioned in frequency in a particular order without overlapping in the sampled signal spectrum.By analyzing the constraints on the sampling frequency for all the possible orders of spectral replicas,we derive an efficient procedure forfinding the valid sampling frequency pared to the conventional approach,the proposed method is superior in the sense of both computational complexity and ease of implementation. Computer simulation on direct downconversion of multiband

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