MT352

Data Sheet

10

Zarlink Semiconductor Inc.

1.4

Adjacent Channel Filtering

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.