HDM8515P datasheet(21/75 Pages) HYNIX

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3.2 Variable Rate Demodulator

The block diagram illustrates the overall configuration of the variable rate QPSK demodulator.

Baseband in-phase (I) and quadrature (Q) inputs are applied to the demodulator at a fixed sampling

rate. These digital samples are produced by A/D converters which employ AC coupling to minimize

DC offset.

First

Frequency

Trans.

Dual FIR

NB AGC

Second

Frequency

Trans.

I_out

Q_out

I_in

Q_in

Carrier

Tracking

Symbol

Tracking

Lock Detect

QPSK Lock

Frequency

Sweeper

Signal

Strength

FIGURE 14 DEMODULATOR BLOCK DIAGRAM

The only significant change to this configuration over the HDM8513A is the addition of the Second

Frequency Translator. The carrier tracking block produces two outputs, one is the frequency

correction which is provided to the First Frequency Translator. This insures that the input to the

Dual FIR is always centered at zero frequency error, although there may be a phase error at this

point. The second output of the Carrier Tracking function provides the phase correction to the

Second Frequency Translator.

The carrier frequency error associated with these samples is removed digitally during tracking

operations by a complex multiplier and a digitally controlled oscillator, sometimes called a

numerically controlled oscillator (NCO). During initial acquisition, coarse frequency error is

removed by a combination of the digital AGC within the HDM8515 and external analog tuning

circuits.

A Dual filter performs the root raised cosine filtering of the frequency corrected baseband samples.

This filter, which implements the function of equation (1), is always configured to have an impulse

response duration of 8 symbols regardless of the programmed symbol rate. For low symbol rates,

a large number of samples are used, while for high symbol rates a relatively low number of samples

are processed for each filter output. The outputs of the daul filters are applied to a digital

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