Circuit for demodulating FSK signals
First Claim
1. A circuit for demodulating an FSK signal having a first frequency with a corresponding first period and a second frequency with a corresponding second period, said FSK signal having a preamble with a period corresponding to a selected one of said first period or said second period and a coded signal portion which alternates between said first and second periods, said circuit comprising:
- receiving means receiving said FSK signal;
clock means for generating a clock signal having a third frequency corresponding to a third period substantially shorter than said first period and said second period;
counter means connected to said clock means and said receiving means for generating a count corresponding to a number of third periods of said clock signal during a period of said FSK signal;
averaging means coupled to said counter means for calculating an adaptive reference count by averaging a predetermined number of counts during said preamble, said reference count representing either the high or low logic level;
comparing means for comparing said count occurring during said coded signal portion with said adaptive reference count;
output means providing a digital level output signal based whether said count is within a predetermined range from said adaptive reference count, said predetermined range corresponding to either a valid low logic or a valid high logic.
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Accused Products
Abstract
A circuit and method of demodulating an FSK signal uses digital circuitry to convert the received signal into count values. For each reception an adaptive reference count value is determined by averaging the duration of received pulses during the preamble of the transmission. The adaptive reference count corresponds to a demodulated logic "1" or logic level "0". The counts from the coded portion of the transmission are compared to the adaptive reference count. If the count is within predetermined windows from the reference count, the count value is converted to the corresponding digital signal.
172 Citations
20 Claims
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1. A circuit for demodulating an FSK signal having a first frequency with a corresponding first period and a second frequency with a corresponding second period, said FSK signal having a preamble with a period corresponding to a selected one of said first period or said second period and a coded signal portion which alternates between said first and second periods, said circuit comprising:
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receiving means receiving said FSK signal; clock means for generating a clock signal having a third frequency corresponding to a third period substantially shorter than said first period and said second period; counter means connected to said clock means and said receiving means for generating a count corresponding to a number of third periods of said clock signal during a period of said FSK signal; averaging means coupled to said counter means for calculating an adaptive reference count by averaging a predetermined number of counts during said preamble, said reference count representing either the high or low logic level; comparing means for comparing said count occurring during said coded signal portion with said adaptive reference count; output means providing a digital level output signal based whether said count is within a predetermined range from said adaptive reference count, said predetermined range corresponding to either a valid low logic or a valid high logic. - View Dependent Claims (2, 3, 4, 5, 7, 8, 9, 10)
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6. A circuit for demodulating FSK signals as recited in claim I wherein said circuit further includes a digital signal processing filter connected between said counting means and comparing means for filtering said count value.
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11. A circuit for demodulating FSK signals having a first frequency with a corresponding first period and a second frequency with a corresponding second period, said FSK signal having a preamble with a period corresponding to said first or second period and a coded signal portion with both first and second periods, said circuit comprising:
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receiving means receiving said FSK signal; clock means for generating a clock signal having a third frequency with a third period substantially shorter than said first period and said second period; counter means connected to said clock means and said receiving means for generating a count corresponding to a number of third periods occurring during a period of said FSK signal; averaging means coupled to said counter means for calculating an adaptive reference count by averaging said count from a predetermined number of periods of said preamble, said reference count representing either the high or low logic level; comparing means for comparing said count with said reference count and providing an output signal indicative of whether said count is within a first window limit from said reference or whether said count is within a second window from said reference count, said first and second windows corresponding to one of a low logic digital signal or a high logic digital signal; output means connected to said comparing means for generating a digital level output signal if said output signal indicates said signal is within said first or second windows. - View Dependent Claims (12, 13, 14, 15, 16, 17)
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18. A method for demodulating an FSK signal having a first frequency with a corresponding first period and a second frequency with a corresponding second period, said FSK signal having a preamble with a period corresponding to said first period or said second period and a coded signal portion which alternates between said first and second periods, said method comprising the steps of:
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receiving said FSK signal; generating a clock signal having a clock frequency and a clock period shorter than said first period and said second period; counting the number of clock periods in a predetermined number of periods of said preamble; obtaining a plurality of preamble counts; averaging said preamble counts to obtain a reference count; counting the number of clock periods during a period of said coded signal portion to obtain a count; comparing said count with said reference count; providing an output signal indicative of whether said count is within a first window limit from said reference or whether said count is within a second window from said reference count, said first and second windows corresponding to one of a low logic digital signal or a high logic digital signal; and generating a digital level output signal if said output signal indicates said signal is within said first or second windows; - View Dependent Claims (19, 20)
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Specification