Patent EA-002515-B1 (EA)
Title: Dual-mode receiver for receiving satellite and terrestrial signals in a distal broadcast system
Inventor: EBERLEIN ERNST
IPC: H04L27/00
Filed: 1998-04-14
Abstract: 1. A receiver configured to receive broadcast signals of both a first signal type and a second signal type and to select broadcast signals of one of the signal types for output, said receiver comprising: a first oscillator and mixer circuit (140,144) for downconverting said broadcast signals of both said first signal type and said second signal type to a first intermediate frequency; a first filter (138) having a center frequency corresponding to said first intermediate frequency and frequency response selected to pass said broadcast signals of said first signal type and at least a portion of said broadcast signals of said second signal type; a second oscillator and mixer circuit (142,146) for downconverting said broadcast signals of said first signal type and said broadcast signals of said second signal type to a second intermediate frequency; a sampling and analog-to-digital conversion circuit (150,152) for converting said broadcast signals of said first signal type and said broadcast signals of said second signal type into digital signals; and a second filter (148) connected to the output of said sampling and analogto-digital conversion circuit (150,152) for filtering said digital signals, said digital signals generated from said broadcast signals of said first signal type being phase shift keying or PSK-modulated, and said digital signals generated from said broadcast signals of said second signal type being modulated in accordance with a second modulation scheme different from PSK modulation, a signal detection circuit (154) connected to said second filter (148) for determining whether a characteristic of said second signal type exceeds a predetermined threshold and for generating an output signal for controlling the operation of said first oscillator and mixer circuit (140,144), said first oscillator and mixer circuit (140,144) being configured to use one of first and second input frequencies, depending on said output signal, to generate said first intermediate frequency by mixing with a corresponding one of said first signal type and said second signal type. a first signal demodulation device (160) connected to said second filter (148) to process said first signal type,  a second signal demodulation device (156,158) connected to said second filter to process said second signal type, and a switching device (162) to select the output of one of said first signal demodulation device (160) and said second signal demodulation device (156, 158) in accordance with said output signal from said signal detection device. 2. A receiver as claimed in claim 1, wherein a third filter is used when generating said broadcast signals of said first signal type, said second filter (148) being configured to have a frequency response of a matched filter corresponding to said third filter. 3. A receiver as claimed in claim 1, said second modulation scheme is multicarrier modulation or MCM, and said second filter (148) is configured to have a frequency response to facilitate MCM and PSK demodulation. 4. A receiver as claimed in claim 3, wherein said frequency response accommodates a sampling frequency for MCM demodulation corresponding to at least one of the length of a fast Fourier transform for MCM demodulation and the length of a guard interval used during MCM modulation. 5. A receiver as claimed in claim 1, wherein said second signal type is a multicarrier modulated signal that is demodulated using downsampling and fast Fourier transform processing, said second filter (148) being configured perform in accordance with filter parameters selected to facilitate said downsampling and said fast Fourier transform processing of said second signal type and PSK demodulation of said first signal type. 6. A receiver as claimed in claim 1, wherein said second filter (148) is a root-raised cosine filter having the frequency response selected to facilitate demodulation of both said first signal type and said second signal type. 7. A receiver as claimed in claim 8, wherein said second filter (148) is a matched filter configured to substantially correspond to a third filter located at a broadcast station which produced said first signal type. 8. A receiver as claimed in claim 1, wherein said first filter (138) is a leaky filter selected from the group consisting of a surface acoustic wave filter and a ceramic filter. 9. A receiver as claimed in claim 1, wherein said second signal type is characterized by a higher signal level than said first signal type, said signal detection circuit (154) being configured to determine the difference in the signal level of said second signal type at the input and the output of said second filter (148). 10. A receiver as claimed in claim 1, further comprising a signal detection circuit (154) connected to said second filter for determining whether a characteristic of said second signal type exceeds a predetermined threshold and for generating an output signal for controlling the operation of said second oscillator and mixer circuit (142,146) to control said second oscillator and mixer circuit (142,146) to perform downconversion using said third intermediate frequency if said predetermined threshold is exceeded. 11. A method of receiving and selecting from broadcast signals transmitted in first and second frequency channels comprising the steps of: receiving signals at a carrier frequency; downconverting said received signals to a first intermediate frequency using one of said first and second input frequencies depending on weather said broadcast system in said second frequency channel exceeds a predetermined threshold; filtering said received signals to pass said broadcast signals in both said first and second frequency channels; downconverting said broadcast signals in said first frequency channel and said second frequency channel to a second intermediate frequency; sampling and converting said broadcast signals into digital signals; filtering said digital signals, wherein said digital signals generated from said first frequency channel are phase shift keying or PSK-modulated and said filtering is implemented using a root-raised-cosine filter to facilitate PSK demodulation, and said digital signals generated from said second frequency channel are modulated in accordance with a second modulation scheme different from PSK modulation and said filtering thereof is implemented using said root-raised-cosine filter; demodulating the filtered digital signals generated from the first frequency channel, and demodulating the filtered signals generated from the second frequency channel; selecting said demodulated signals generated from the first frequency channel, or said demodulated signals generated from the second frequency channel depending on weather said broadcast system in said second frequency channel exceeds a predetermined threshold. 12. A method as claimed in claim 11, wherein said filtering step comprises the step of selecting said root-raised-cosine filter to have a frequency response of a matched filter corresponding to a third filter used to generate said broadcast signals on said first frequency channel. 13. A method as claimed in claim 11, wherein said second modulation scheme is multicarrier modulation or MCM, and said providing step comprises the step of selecting said frequency response to facilitate MCM demodulation. 14. A method as claimed in claim 13, wherein said selecting step comprises the step of selecting said frequency response in accordance with a sampling frequency for MCM demodulation, said sampling frequency corresponding to at least one of the length of a fast Fourier transform used for MCM demodulation and the length of a guard interval used during MCM modulation. 15. A method as claimed in claim 11, wherein said broadcast signals in said second frequency channel having a higher signal level than said broadcast signals in said first frequency channel, and further comprising the steps of: comparing the signal level of said digital signals before and after said filtering step to determine if a characteristic of said broadcast signals in said second frequency channel exceeds a predetermined threshold; generating a detection signal indicating whether said broadcast signals in said second frequency channel have been detected; and adjusting an input frequency to a local oscillator in accordance with said detection signal to downconvert said received signals to said first intermediate frequency by mixing with said received signals in said first and second frequency channels.