PS20230 Datasheet, PDF (16/26 Pages) List of Unclassifed Manufacturers – COFDM demodulator with USB interface for PC-TV

PS20230
7.3 IF to Baseband Conversion
Sampling a 36.17 MHz IF signal at 45 MHz results in a spectrally inverted OFDM signal centred 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 PS20230 has control mechanisms to search automatically for
an unknown spectral inversion status.
7.4 Adjacent Channel Filtering
Adjacent channels, in particular the Nicam digital sound signal associated with analogue channels, are
filtered prior to the FFT.
7.5 Interpolation and Clock Synchronisation
PS20230 uses digital timing recovery and this eliminates the need for an external VCXO. The ADC
samples the signal at a fixed rate, for example, 45.0 MHz. Conversion of the 45.0 MHz signal to the OFDM
sample rate is achieved using the time-varying interpolator. The OFDM sample rate is 64/7 MHz for 8 MHz
and this is scaled by factors 6/8 and 7/8 for 6 and 7 MHz channel bandwidths. The nominal ratio of the
ADC to OFDM sample rate is programmed in a PS20230 register (defaults are for 45 MHz sampling and 8
MHz OFDM). The clock recovery phase locked loop in the PS20230 compensates for inaccuracies in this
ratio due to uncertainties of the frequency of the sampling clock.
7.6 Carrier Frequency Synchronisation
There can be frequency offsets in the signal at the input to OFDM, partly due to tuner step size and
partly due to broadcast frequency shifts, typically 1/6 MHz. These are tracked out digitally, up to 1 MHz
in 2 K and 8 K modes, without the need for an analogue frequency control (AFC) loop.
The default frequency capture range has been set to ±286 kHz in the 2 K and 8 K mode. However, these
values can be increased, if necessary, by programming an on- chip register (CAPT_ RANGE). It is
recommended that a larger capture range be used for channel scan in order to find channels with
broadcast frequency shifts, without having to adjust the tuner. After the OFDM module has locked (the
AFC will have been previously disabled), the frequency offset can be read from an on-chip register.
7.7 Symbol Timing Synchronisation
This module computes the optimum sample position to trigger the FFT in order to eliminate or minimize
inter-symbol interference in the presence of multi-path distortion. Furthermore, this trigger point is
continuously updated to dynamically adapt to time-variations in the transmission channel.
7.8 Fast Fourier Transform
The FFT module uses the trigger information from the timing synchronization module to set the start
point for an FFT. It then uses either a 2 K or 8 K FFT to transform the data from the time domain to the
frequency domain. An extremely hardware-efficient and highly accurate algorithm has been used for
this purpose.
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