An optical receiver of the above kind is known from the Japanese Patent Application No. 63-33018 in which a broadband satellite receiver is disclosed. An antenna signal is fed to an optoelectric converter via a glass fibre cable. A broadband output electrical signal of the converter is fed to a down-converter. A down-converter output signal is fed to a TV-tuner. In such an optical receiver configuration, in which light from the glass fibre cable is first converted to a broadband electric signal, the optoelectric converter comprises a transimpedance amplifier. A disadvantage of such a configuration is that the whole relevant frequency range is converted while only a small band is actually used, namely the tuned channel in the TV-tuner. Due to parasitic capacitance effects, the transimpedance of the transimpedance amplifier is limited to a maximum value causing this configuration to have limited noise properties. A typical noise spectral density figure is 2 to 8 pA/.sqroot.Hz. On the one hand a very low noise figure is desired, and on the other hand a broad bandwidth is desired. Because of the fact that noise is proportional to the transimpedance and bandwidth is inversely proportional to the transimpedance, the noise and bandwidth requirements are conflicting in this configuration.
In the Article "Design and optimisation of a fully integrated GaAs tuned receiver preamplifier MMIC for optical SCM applications", P.M.R.S. Moreira et al, IEE Proceedings-J, Vol. 140, No. 6, Dec. 1993, pp. 411-415 a tuned receiver preamplifier is disclosed in which a subcarrier multiplexed light wave is inputted to an optoelectric converter which is coupled to a tuning network. The preamplifier operates in the GHz region. A broadband output signal of the tuning network is fed to a preamplifier and standard microwave techniques are used to select a channel from the broadband electrical output signal of the preamplifier. By applying an optimisation method that minimises a cost function, components of the tuning network are determined so as to globally maximise the SNR (Signal-to-Noise Ratio) over the complete tuning range. The input tuning network operates to reduce the input noise by providing a low input impedance path in parallel with a noise current source and a high impedance path in series with a noise voltage source. Said article thus discloses the use of a resonant noise shaping circuit operating over the whole tuning range, i.e. broadband noise tuning.
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