MT352
1.4
Adjacent Channel Filtering
Data Sheet
Adjacent channels, in particular the Nicam digital sound signal associated with analogue channels, are filtered prior
to the FFT.
1.5
Interpolation and Clock Synchronisation
MT352 uses digital timing recovery and this eliminates the need for an external VCXO. The ADC samples the
signal at a fixed rate, for example, 20.48 MHz. Conversion of the 20.48 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 MT352 register (defaults are for 20.48 MHz sampling and 8 MHz OFDM). The clock recovery
phase locked loop in the MT352 compensates for inaccuracies in this ratio due to uncertainties of the frequency of
the sampling clock.
1.6
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, without the need for an analogue
frequency control (AFC) loop.
The default frequency capture range has been set to ±285 kHz in the 2 K mode and ±142 kHz in the 8 K mode.
However, these values can be doubled, if necessary, by programming an on-chip register. It is recommended that
this 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 frequency offset can be read from an on-chip register.
1.7
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.
1.8
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 8K 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.
1.9
Common Phase Error Correction
This module subtracts the common phase offset from all the carriers of the OFDM signal to minimize the effect of
the tuner phase noise on system performance.
1.10
Channel Equalisation
This consists of two parts. The first part involves estimating the channel frequency response from pilot information.
Efficient algorithms have been used to track time-varying channels with a minimum of hardware. The second part
involves applying a correction to the data carriers based on the estimated frequency response of the channel. This
module also generates dynamic channel state information (CSI) for every carrier in every symbol.
1.11
Impulse Filtering
MT352 contains several mechanisms to reduce the impact of impulse noise on system performance.
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