INTERMEDIATE FREQUENCY(IF)SIGNAL AT 72.5MHz(=2MHz)IS ALIASED BETWEEN DC AND 5MHZ BY SAMPLING AT 10MSPS 每=100M tsa 10 000 MSPS 725:2MHE dc To 5 MHz igure 5.10 There is an advantage in choosing a sampling frequency which a sub-multiple of the lower band-edge in that there is no frequency inversion in the baseband alias as would be the case for a sampling frequency equal to a sub-multiple of the upper band-edge. (Frequency inversion can be easily dealt with in the DSP software should it occur, so the issue is not very important) Undersampling applications such as the one just described generally require sampling AD Cs which have low distortion at the high input IF input frequency. For instance, in the example just discussed, the ADC sampling rate requirement is only 10MSPS, but low distortion is required (preferably 60 to 80dB SFDR at the IF frequency of 72 5MHz A large opportunity for bandpass sampling is in digital cellular radio base stations For systems which have RF frequencies at 900MHz, 70MHz is a popular first -IF frequency. For systems using an RF frequency of 1.8GHZ, first-IF frequencies between 200 and 240MHz are often used In broadband receiver applications, one adC digitizes multiple channels in the receive path. Individual channel selection and filtering is done in the digital domain. Narrow band channel characteristics such as bandwidth, passband ripple, and adjacent channel rejection can be controlled with changes to digital parameters (i.e filter coefficients). Such flexibility is not possible when narrow band analog filters are in the receive path Figure 5.11 illustrates the kind of input spectrum an ADC must digitize in a multichannel design. The spectral lines represent narrow band signal inputs from a variety of signal sources at different received power levels. Signal"C"could represent a transmitter located relatively far away from the signal sources"A"and B". However, the receiver must recover all of the signals with equal clarity. This requires that distortion from the front-end rf and IF signal processing components,1 0 INTERMEDIATE FREQUENCY (IF) SIGNAL AT 72.5MHz ( 2MHz) IS ALIASED BETWEEN DC AND 5MHz BY SAMPLING AT 10MSPS Figure 5.10 There is an advantage in choosing a sampling frequency which a sub-multiple of the lower band-edge in that there is no frequency inversion in the baseband alias as would be the case for a sampling frequency equal to a sub-multiple of the upper band-edge. (Frequency inversion can be easily dealt with in the DSP software should it occur, so the issue is not very important). Undersampling applications such as the one just described generally require sampling ADCs which have low distortion at the high input IF input frequency. For instance, in the example just discussed, the ADC sampling rate requirement is only 10MSPS, but low distortion is required (preferably 60 to 80dB SFDR) at the IF frequency of 72.5MHz. A large opportunity for bandpass sampling is in digital cellular radio base stations. For systems which have RF frequencies at 900MHz, 70MHz is a popular first-IF frequency. For systems using an RF frequency of 1.8GHZ, first-IF frequencies between 200 and 240MHz are often used. In broadband receiver applications, one ADC digitizes multiple channels in the receive path. Individual channel selection and filtering is done in the digital domain. Narrowband channel characteristics such as bandwidth, passband ripple, and adjacent channel rejection can be controlled with changes to digital parameters (i.e. filter coefficients). Such flexibility is not possible when narrowband analog filters are in the receive path. Figure 5.11 illustrates the kind of input spectrum an ADC must digitize in a multichannel design. The spectral lines represent narrowband signal inputs from a variety of signal sources at different received power levels. Signal "C" could represent a transmitter located relatively far away from the signal sources "A" and "B". However, the receiver must recover all of the signals with equal clarity. This requires that distortion from the front-end RF and IF signal processing components