Basic Principles of Digital Modulation - Digital Video and Audio Broadcasting Technology - page 225

Very frequently, however, demodulation is performed using the fs/4
method, which requires a less complex IQ demodulator. The modulated
signal iqmod(t) is passed through an anti-aliasing lowpass filter and then
sampled by means of an A/D converter which operates at the fourfold IF of
the modulated signal iqmod(t). Therefore, if the carrier of iqmod(t) is at f IF ,
the sampling frequency is 4 • f IF . This means that a complete carrier cycle
is sampled four times (see Fig. 13.8.). Provided the A/D converter clock is
fully synchronous with the carrier clock, the rotating carrier vector is sam-
pled exactly at the instants shown in Fig. 13.17. The symbol clock is re-
covered in a carrier and clock recovery block as described above.

I

i(t)

Delay

A D

iqmod(t) +/1

q(t)

FIR
interpolation

f S =4*f IF

/+1

f S /2

Q

Fig. 13.17. IQ demodulation using f S /4 method

Q

f IF

I

Fig. 13.18. f S /4 demodulation