A known type of DTTV receiver for receiving OFDM signals converts any selected channel for reception to a non-zero intermediate frequency. Surface acoustic wave (SAW) filters are used for filtering the intermediate frequency signal so as to pass the desired channel to, and substantially exclude other channels from, a demodulator. The intermediate frequency signal is digitised and then supplied to a digital demodulator, which extracts the in-phase (I) and quadrature (Q) channels. However, such arrangements have various disadvantages, including the need to provide off-chip discrete SAW filters.
Tuners of the zero intermediate frequency type are known for receiving digital satellite television signals. In such arrangements, the selected channel is converted directly to I and Q baseband signals, which are filtered and then digitised for subsequent demodulation. Because the intermediate frequency signal is at baseband, the SAW filters can be replaced by two low pass filters, one in each of the I and Q channels. Such filters can be integrated in an integrated circuit.
Because of a variety of factors including process variations, it is difficult or impossible to arrange for the passbands of such analog low pass filters to be exactly as required. If the filter passband is too large, it will allow high power adjacent channels to be passed and this leads to degraded performance. Conversely, if the passbands are too small, useful parts of the spectrum of the received channel will be lost and again degraded performance results. Also, the I and Q filters may have different passbands as a result of process variations and other factors.
It is known for such analog low pass filters to be adjustable so that the passbands can be adjusted and set during alignment or use of a receiver of this type. Known techniques rely on adjusting the passbands while monitoring the bit error rate from a subsequent demodulator in order to achieve an acceptable performance. However, adjustments based on bit error rate (BER) are relatively slow. In particular, after each adjustment to the filter passbands, a significant time delay has to be allowed for a valid estimate of the BER to be obtained in order to assess the result of the adjustment. Also, the BER does not by itself indicate whether the filter passbands are too wide or too narrow so that adjustment of the passbands is by undirected trial and error.