STA400A datasheet(14/117 Pages) STMICROELECTRONICS

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STA400A

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The second IF composite satellite signal, available at the IF2SA inputs, has the spectrum schematically shown

in Fig.7. The base band convertion of the selected satellite signal is done by programming the carrier NCO (act-

ing as local oscillator) of the demodulator. For demodulating the S1-Early Satellite signal the carrier NCO fre-

quency of the Early QPSK Demodulator must be equal to 6.095 + 0.92 = 7.015 MHz; for the S2-Late Satellite

signal the frequency at the carrier NCO output of the Late QPSK Demodulator must be 6.095 - 0.92 = 5.175

MHz.

The register map of the QPSK demodulators is described in section 2.2.

I/Q Mixer - Carrier NCO

The final downconvertion to baseband of the 2nd IF satellite signal is performed by a mixer and a local oscillator

implemented with an NCO and a Sin/Cos Look-up table (see figure 6). The signal coming from the IF Sampling

block is multiplied by the output of the quadrature NCO to produce the I and Q components of the baseband

signal. The NCO output are the sine and cosine functions obtained with a look-up table driven by a 28 bit phase

accumulator.

The IFFREQ register sets the frequency of the carrier NCO as given in the following:

Fc = FMCLK x (IFFREQ)/2

28 ,

where FMCLK is the clock frequency and IFFREQ is the 28 bit integer value loaded in the register.

For example, to program the S1-NCO to 7.015 MHz the IFFREQ register must be loaded with 04B13B14 (Hex)

equivalent to 78723860 (Dec).

The sine/cosine output frequency of the NCO is given by Fo = (Fc + Fsw + PHerr) x FMCLK/2

28 where Fc is the

nominal center frequency, Fsw is the output of the frequency sweep generator, PHerr is the filtered phase error

from the carrier loop filter output and FMCLK/2

28 is the NCO frequency resolution.

The CARFREQ read only register gives the value of the actual carrier frequency after the lock condition has

been reached. This register can be used to measure the frequency offset between the local oscillator and the

incoming carrier.

The QCHS block of the I/Q mixer changes the sign of the Quadrature component in order to adapt the demod-

ulation process to a different rotation sense of the QPSK mapping.

Figure 7. Second IF Satellite Signal Spectrum

S1A

(S1B)

F (MHz)

S2A

(S2B)

Fo + 0.92

Fo - 0.92

Fo = 6.095 MHz

RRC Symbol freq = 1.64 MHz

roll-off factor = 15 %

S2 = Late Satellite

S1 = Early satellite

A = Ensemble A; B = Ensemble B

3.726

1.886

1.64

1.84

0.92

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