Direct-Conversion Tuner for Terrestrial, Cable and Satellite Digital TV
The goal of this project is to develop a new generation of RF tuner for front-end receiver for terrestrial, cable, and satellite for digital TV. The Direct Conversion Tuner (DCT) will significantly simplify the analog RF sections, eliminate alignment procedures, reduce the size of the RF front-end, and decrease the cost of the entire digital receiver. In spite that concept of the DCT is not new, only latest advances in digital signal processing (DSP) technology has made it possible to overcome the problems associated with development of a working DCT. New high-speed (DSP) techniques can solve the problems that could not be solved by traditional analog designs. The concept of the DCT is especially attractive for digital data communications.
Background & Objective: Advantages DCT compare with convention tuner:
* Significant reduction in the number of expensive RF components (tens versus hundred).
* No alignment of the receiver is required.
* Eliminating expensive Surface Acoustic Wave (SAW) filter.
* No image frequencies, no taboo channels.
High level of integration (implementation with, at most, two chips) easily reprogrammed for different types of modulation (VSB, QAM, BPSK, QPSK, etc.).
* Digital circuits moved closer to the antenna.
* High level of reliability.
Several leading companies such as Motorola, Philips, Maxim, Thomson are working on DCT projects. An IC DCT for satellite TV broadcasting was introduced last time. It is only a question of time when an IC DCT for terrestrial and cable broadcast will appear on the market and completely supplant conventional tuners.
Technical Discussion: A proposed block diagram of a DCT is illustrated in the figure above. The incoming RF modulated signal from antenna is bandpass filtered and amplified in the RF Processing block. The output is split into two channels and applied to mixers (I and Q) quadrature channels. These mixers downconvert the signals directly into the baseband frequency. The baseband signals in I and Q channels are digitally processed to cancel DC offset, unbalances and channel unflatness.
Some of the technical challenges for developing a DCT include:
* Amplitude and phase balance requirements between the two quadrature channels (I and Q) must be better than 0.05 dB in amplitude and 0.5( in phase. This is a very tight requirement but can be achieved by correcting the imbalances, which is mostly due to the analog circuitry imperfections, by using a DSP section in the receiver.
* Local oscillator (LO) radiation (leakage) into the antenna input. A matched impedance filter can suppress this leakage with amplifiers on the input and output of the filter.
* DC offset in the signal due to leakage from the LO, analog mixers, and analog amplifiers. This problem can be solved by careful circuit design, digital cancellation, and special modulation of the LO signal.
Contact: Johnas Cukier
Technology Area: Advanced Digital Television
Modification Date: August 1, 2001
