FSK demodulator
A demodulator suitable for demodulating binary FSK signals having a small difference between carrier frequencies uses a counter-timer technique for timing a fixed number FSK cycles and comparing a count value with a threshold when a frequency change is expected. By grouping a number of FSK pulses (or cycles) together in one measurement, speed requirements on the system clock used for the counter/timer measurements can be relaxed and tolerance to noise is also improved. The demodulator is particularly suitable for wireless charging applications.
1. A frequency shift keying (FSK) demodulator for decoding a binary signal comprising data bits encoded in a received FSK signal, the demodulator comprising:
a first comparator for processing the received FSK signal to produce a FSK pulsed waveform comprising a succession of pulses wherein one bit of data comprises ‘N’ pulses, wherein N is a positive integer greater than 1;
a counter/timer, operably coupled to the first comparator, for receiving the FSK pulsed waveform and for determining measured count values of a duration of a pre-chosen number, n, where n is a positive integer and is less than N, of consecutive pulses of the FSK pulsed waveform;
a second comparator, operably coupled to the counter/timer, for receiving the measured count values and for comparing the measured count values with a pre-determined threshold value and for outputting a bit value of ‘0’ or ‘1’ depending on the comparison; and
a controller operably coupled to the counter/timer and to the second comparator for choosing the value ‘n’ and determining the threshold value.
2. The FSK demodulator of claim 1 , wherein the second comparator compares the measured count values with the predetermined threshold value only when a change of frequency of the FSK pulsed waveform is expected.
3. The FSK demodulator of claim 1 , wherein the first comparator is a zero crossing detector.
4. The FSK demodulator of claim 1 , wherein a first logical value of a data bit of the binary signal is represented by the FSK pulsed waveform running at a constant frequency during a single bit period of N pulses and a second logical value of a data bit, being of opposite polarity to the first logical value, is represented by a change of frequency of the FSK pulsed waveform in the middle of a single bit period of N pulses.
5. The FSK demodulator of claim 1 , wherein the counter/timer records a number of system clock cycles that elapse during the duration of the pre-chosen number, n, of consecutive pulses of the FSK pulsed waveform.
6. The FSK demodulator of claim 1 , wherein the counter/timer measures the duration of the pre-chosen number, n, of consecutive pulses of the FSK pulsed waveform.
7. The FSK demodulator of claim 1 , further comprising a parsing circuit operably coupled to the output of the second comparator for parsing an output bit stream received from the second comparator.
8. The FSK demodulator of claim 1 , including at least one register for storing the measured count values.
9. The FSK demodulator of claim 3 , wherein the zero crossing detector includes programmable hysteresis and a filter.
10. The FSK demodulator of claim 7 , wherein the parsing circuit generates a first interrupt signal for application to the controller after a pre-determined number of bits of data has been parsed.
11. The FSK demodulator of claim 10 , wherein the counter/timer generates a second interrupt signal for application to the controller after count values for ‘n’ consecutive pulses have been measured.
12. The FSK demodulator of claim 11 , wherein the counter/timer operates a number ‘m’ of repeating count phases, wherein m is a positive integer, whereby a first phase ends and a following phase commences after ‘n’ pulses have been received by the counter/timer, and the counter/timer further generates a third interrupt signal for application to the controller at the end of each ‘m’th phase.
13. A method for demodulating a frequency shift keying (FSK) signal, the method comprising:
processing, in a first comparator, the FSK signal to produce a FSK pulsed waveform comprising a succession of pulses wherein one bit of data comprises ‘N’ pulses, wherein N is a positive integer greater than 1;
in a counter/timer, receiving the FSK pulsed waveform and determining measured count values of a duration of a pre-chosen number, n, where n is a positive integer and is less than N, of consecutive pulses of the FSK pulsed waveform;
in a second comparator, comparing the measured count values with a pre-determined threshold value, and selecting a bit value of ‘0’ or ‘I’ depending on the comparison for outputting as a bit stream; and
in a controller, choosing the value n and determining the threshold value.
14. The method of claim 13 , wherein comparing the measured count values with the pre-determined threshold value is performed only when a change of frequency of the FSK pulsed waveform is expected.