STA400A datasheet(15/117 Pages) STMICROELECTRONICS

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STA400A

High-Side/Low-Side Injenction Control

In a superetherodyne tuner (using two downconversions to produce the second IF signal at the Channel Decod-

er satellite input) is possible that one LO may use an High-Side injection of the carrier and the other LO may

use a Low-Side injection.

In this case a data polarity convertion is required in the Channed Decoder. To accomodate this function two

methods are available: the first method requires a polarity change in the QCHS and CARCHS bits of the

QPSK_CTRL register, with the second method the IFFREQ register of the carrier NCO must be programmed

with the image frequency using the following formula:

FMCLK - Fc = FMCLK x (IFFREQ)/2

For example, to set the image frequency of the 7.015 MHz the value FB4EC4EC (Hex) must be loaded in the

IFFREQ register.

Matched/Interpolator Filters

The STA400A provides two matched/interpolator filters. These filters perform the Nyquist filter function

(matched with the one in the transmission side) with a Root Raised Cosine (RRC) shape and a roll-off factor of

15% and the interpolation function to evaluate the optimum sampling instant of the output symbol. The filters,

based on a poliphase structure (12 taps with 32 coefficients/ tap), receive at their inputs the separate I and Q

streams at FMCLK/FSYM samples/symbols and produce the separate I and Q output streams at one sample per

symbol. The frequency responce of one filter is given in fig.8 and fig.9.

AGC2

The AGC2 loop is designed to maintain a fixed signal level at the input of the Soft Decision Slicer. As shown in

fig.6, the AGC2 loop consists of an error detector, a loop filter and a gain multiplier. The modulus of the complex

symbols is compared to a programmable reference level (AGC2REF register) and then scaled by the

AGC2BETA coefficient and integrated. The filtered error drives two multipliers at the output of the Matched filters

to maintain constant the level at the demodulator output.

The AGC2 loop gain is given by:

βAGC2 = 2AGC2BETA. The parameter AGC2BETA can take values from 0 to 6.

When AGC2BETA="111" the loop gain is null and the AGC2 amplifier gain keeps the last value. The AGC loop

may be opened by programming "111" in the AGC2BETA parameter and writing "0x00" in the AGC2INTG reg-

ister.

The reference level set in the AGC2REF register impacts on the carrier and timing loop equations and on the

operation of the Soft Decision Slicer.

Figure 8. RRC Filter Frequency Responce

200

400

600

800

1000

1200

1400

1600

1800

2000

Frequency (kHz)

815 kHz

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