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302 Bandpass Signaling Principles and Circuits Chap. 4 |S(f)| B T s(t) f c f c f Sampler Bandpass signal input Sampled output (a) Method I—Direct sampling Clock Down converter s(t) Bandpass signal input Local oscillator 2cos v 0 t Bandpass filter Sampler Clock Sampled output (b) Method II—Down conversion and sampling Low-pass filter Sampler In-phase sampled output s(t) Bandpass signal input 2cos v c t Oscillator 2sin v c t Low-pass filter Sampler Clock Quad-phase sampled output (c) Method III—IQ (in-phase and quad-phase) sampling Figure 4–32 Three methods for sampling bandpass signals. Method II Referring to Fig. 4–32b, we see that Method II down converts the bandpass signal to an IF, so that the highest frequency that is to be sampled is drastically reduced. For maximum reduction of the highest frequency, we choose † the local oscillator frequency to be f 0 = f c - B T 2. † Low-side LO injection is used so that any asymmetry in the two sidebands of s(t) will be preserved in the same way in the down-converted signal.
- Page 602: Sec. 4-13 Detector Circuits 277 Det
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- Page 630: Sec. 4-16 Transmitters and Receiver
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- Page 642: Sec. 4-17 Software Radios 297 Zero-
- Page 646: Sec. 4-19 Study-Aid Examples 299 4-
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302<br />
Bandpass Signaling Principles and Circuits Chap. 4<br />
|S(f)|<br />
B T<br />
s(t)<br />
f c f c<br />
f<br />
Sampler<br />
Bandpass signal input<br />
Sampled output<br />
(a) Method I—Direct sampling<br />
Clock<br />
Down converter<br />
s(t)<br />
Bandpass<br />
signal input<br />
Local<br />
oscillator<br />
2cos v 0 t<br />
Bandpass<br />
filter<br />
Sampler<br />
Clock<br />
Sampled output<br />
(b) Method II—Down conversion and sampling<br />
Low-pass<br />
filter<br />
Sampler<br />
In-phase sampled output<br />
s(t)<br />
Bandpass<br />
signal input<br />
2cos v c t<br />
Oscillator<br />
2sin v c t<br />
Low-pass<br />
filter<br />
Sampler<br />
Clock<br />
Quad-phase sampled output<br />
(c) Method III—IQ (in-phase and quad-phase) sampling<br />
Figure 4–32<br />
Three methods for sampling bandpass signals.<br />
Method II Referring to Fig. 4–32b, we see that Method II down converts the bandpass signal<br />
to an IF, so that the highest frequency that is to be sampled is drastically reduced. For maximum<br />
reduction of the highest frequency, we choose † the local oscillator frequency to be f 0 = f c - B T 2.<br />
† Low-side LO injection is used so that any asymmetry in the two sidebands of s(t) will be preserved in the<br />
same way in the down-converted signal.
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