1. Technical Field
This disclosure relates to multichannel communications systems, and in particular to receivers for selecting channels.
2. Description of the Related Art
In multichannel communication systems, receivers may select a desired channel for reception from a broader input signal spectrum which includes a number of available channels. In one known implementation, a receiver having tuner capabilities may use a dual conversion reception technique, such that the entire input signal spectrum is first up-converted to a relatively high frequency. At such high frequencies, preselection of the desired channel or sub-spectrum is performed, and the remainder of the input spectrum is discarded. The desired channel is then down-converted such that it falls substantially close to the center of the operating range of a channel selection filter. The output of the channel selection filter is then input to a channel demodulator which is common to all channels.
Two critical parameters in receiver design and operation are noise figure and modulation distortion. Some receivers may minimize distortion while sacrificing good noise figure characteristics. Typically, both noise figure and modulation distortion effects may be improved using relatively expensive and/or complicated receiver implementations.
The use of a standard filter following an up-converter is known in the art for performing the basic function of filtering unwanted spectra. However, the unwanted spectrum, which may represent, for example, about 90% of the incident energy, may be reflected back into the up-converter, causing unacceptable intermodulation performance. It is also known that a buffer amplifier may be inserted between the up-converting mixer and the filter to reduce the reflected energy into the up-converter, but the use of such a buffer amplifier may cause distortion in the output stage of the buffer amplifier. Even a "unity gain" amplifier may exhibit such distortion.
Diplex filters using either high-pass or low-pass filters may be used to direct the energy of the unwanted spectrum to a termination circuit, such as a resistor, for dissipating the energy. However, such filters do not perform the filtering satisfactorily for receivers used in very broadband operation, since the unwanted spectra are below or above the desired channel for high or low frequency channels, respectively. Such deficiencies in known receiving techniques have been difficult to remedy with inexpensive implementations.