PS20353 Datasheet, PDF (10/27 Pages) List of Unclassifed Manufacturers – Nordig Unified DVB-T COFDM Terrestrial Demodulator

PS20353
Figure 5- FEC Block Diagram
The FSM controller shown in Figure 4 controls both the demodulator and the FEC. It also drives the 2- wire bus to the
tuner. The controller facilitates the automated search of all parameters or any sub -set of parameters of the received
signal. It can also be used to scan any defined frequency range searching for OFDM channels. This mechanism provides
the fast channel scan and acquisition performance, whilst requiring minimal software overhead in the host driver.
The algorithms and architectures used in the PS20353 have been optimized to minimize power consumption.
2.1 Analogue-to-Digital Converter
The PS20353 has a high performance 10 -bit analogue-to-digital converter (ADC) which can sample a 6, 7 or 8
MHz bandwidth OFDM signal, with its spectrum centered at:
• 36.17 MHz IF
• 43.75 MHz IF
• 5 - 10 MHz near-zero IF
An on-chip programmable phase locked loop (PLL) is used to generate the ADC sampling clock. The PLL is highly
programmable allowing a wide choice of sampling frequencies to suit any IF frequency, and all signal bandwidths.
2.2 Automatic Gain Control
An AGC module compares the absolute value of the digitized signal with a programmable reference. The error
signal is filtered and is used to control the gain of the amplifier. A sigma-delta modulated output is provided, which
has to be RC low-pass filtered to obtain the voltage to control the amplifier.
The programmable AGC reference has been optimized. A large value for the reference leads to excessive ADC clipping
and a small value results in excessive quantization noise. Hence the optimum value has been determined assuming the
input signal amplitude to be Gaussian distributed. The latter is justified by applying the central limit theorem in statistics to
the OFDM signal, which consists of a large number of randomly modulated carriers. This reference or target value may
have to be lowered slightly for some applications. Slope control bits have been provided for the AGCs and these have to
be set correctly depending on the gain-versus-voltage slope of the gain control amplifiers.
The bandwidth of the AGC is set to a large value for quick acquisition then reduced to a small value for tracking.
The AGC is free running during OFDM channel changes and locks to the new channel while the tuner lock is being
established. This is one of the features of PS20353 used to minimize acquisition time. A robust AGC lock
mechanism is provided and the other parts of the PS20353 begin to acquire only after the AGC has locked.
2.3 IF to Baseband Conversion
Sampling a 36.17 MHz IF signal at 45 MHz results in a spectrally inverted OFDM signal centered at approximately 8.9
MHz. The first step of the demodulation process is to convert this signal to a complex (in-phase and quadrature) signal in
baseband. A correction for spectral inversion is implemented during this conversion process. Note also that the PS20353
has control mechanisms to search automatically for an unknown spectral inversion status.
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