HIGH SPEED FSK DEMODULATOR

Abstract: In an FSK demodulator, the received, modulated signal (A) is divided between two paths. In one path, the signal (A) undergoes non-linear amplification (11, 15), providing an amplitude limited output (C). In the other path, the signal (A) also undergoes non-linear amplification (10, 13), but, in addition, is subjected to a phase shift (12). The phase shift is positive at the frequency representing binary ones, and negative at the frequency representing binary zeros (or vice versa). Thus, the signal (B) from this second branch, is a phase shifted version of the signal (C) from the first. A phase detector (14), comprising a D type flip flop, provides the demodulated output.

Description: High Speed FSK Demodulator The present invention relates in general to decoding frequency-shift keying (FSK) signals, and more specifically to an FSK decoding scheme having a fast response time by avoiding integration in the decoder. Frequency-shi t keying is a well-known method tor representing binary data using a first frequency to represent a binary zero and a second frequency to represent a binary one. When the binary digit to be represented changes, the frequency of the FSK signal shifts from one frequency to the other. Although the frequency shifts abruptly, FSK systems typically employ a continuous phase, i.e., there is no phase discontinuity when the frequency shifts. Frequency-shift keying is a type of frequency modulation and can be used to transmit digital information by radio waves. FSK modulation is also used in transmitting binary information over phone lines using modems. Demodulation of FSK signals typically employs FM detection techniques which include filtering with long time constants or other integration of the FSK signal. For example, the FSK signal to be demodulated is applied to the inputs of a pair of bandpass filters having respective centre frequencies corresponding to the two frequencies of tne FSK signal. The filter outputs are coupled to the inputs of a differential amplifier which recreates the digital signal. However, the integration time constants of the filters introduce a time delay slowing down the response time of the FSK demodulator. Other demodulation tecnniques rely on the use of quadrature mixing siqnals to generate quadrature and ln- pnase FSK components tor detection, resulting in hign part count and high cost. The invention has the obηect and advantage of αecoding FSK signals at high speed with a low part count and a low cost. Specifically, the invention decodes an FSK signal which modulates between a first frequency f-^ and a second freguency f ' 2 to encode digital information. Wave-shaping means shape the FSK signal to generate substantially identical first and second wave-shaped signals. Phase- altering means receives the first wave-shaped signal for generating a phase-altered signal. The phase-altering means has a frequency response at frequency f different from its frequency response at frequency f,,. Sampling means samples one of the phase-altered signal or the second wave-shaped signal in response to a transition in the other one of the phase-altered signal or the second wave-shaped signal to generate a decoded signal. In a preferred embodiment, the phase-altering means is comprised of a resonant circuit having a resonant frequency f Q substantially halfway between f-L and f 2 • The sampling means is preferably comprised of a D-type flip-flop. The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram showing a decoder according to the present invention. Figure 2 is a frequency response curve provided by the tank circuit of Figure 1. Figures 3-6 show waveform diagrams at respective points in the circuit of Figure 1 during operation. As shown in Figure 1, an input FSK signal A obtained from an antenna, phone line, or other source (not shown) is coupled to the input of an amplifier 10 and the input of an amplifier 11. The output of amplifier 10 is connected to the input of a resonant circuit 12. The output of resonant circuit 12 is connected to the input of an amplifier 13. An output signal B from amplifier 13 is connected to the D input of a D-type flip-flop 14. The output of amplifier 11 is connected to the input of an amplifier 15 which produces a C output signal coupled to the C input of D flip-flop 14. Resonant circuit 12 includes an inductor 16 inductively coupled to an inductor 17. A capacitor 18 is connected in parallel with inductor 17. An inductor 19 is magnetically coupled to inductor 17 and provides the output of the resonant circuit 12. In this preferred embodiment, resonant circuit 12 takes the form of a tank circuit which is a parallel resonant circuit having a resonant frequency f Q at a frequency between the frequencies f-^ and f 2 used by the FSK signal to encode binary digits. Resonant circuit 12 has a high Q factor (by virtue of a low effective series resistance) in order to insure very low integration of the FSK signal. Consequently, resonant circuit 12 alters the phase of the FSK signal but introduces very little time delay, thus enabling fast decoding of the FSK signal. As shown in Figure 2, resonant circuit 12 provides a relative phase change according to the trequency of its input signal. At frequency f-^, the resonant circuit introduces a phase lag. At frequency f-,, a phase lead is introduced. Although phase shifts of about 45 provide the best accuracy of detection, much smaller relative phase shifts can be employed using the decoder of the present invention. Amplifiers 10, 11, 13, and 15 are comprised of very high gain amplifiers yielding nonlinear amplification to produce square-wave signals which indicate transitions in the respective input signals. Other forms of square-waved generators can alternatively be employed, such as zero crossing detectors. The input FSK signal A is shown in Figure 3 which shifts from frequency f^ to frequency f-, at a time t■, . FSK signal A is amplified and limited in amplifiers 10 and 11 to produce wave-shaped (i.e., square-wave) signals indicating zero crossings of FSK signal A. Prior to time t^ while FSK signal A has a frequency f- j , resonant circuit 12 introduces a phase lag resulting m output signal B relative to FSK signal A as shown in Figure . Amplifiers 11 and 15 provide an output signal C, shown in Figure 5, which is identical to the output signal produced at B except tor the phase alteration introduced by resonant circuit 12. Flip-flop 14 receives output signal C at its clock input C so that output signal B is sampled at each positive transition in output signal C. Output signal B applied to the D input of flip-flop 14 is shifted in phase relative to output signal C depending upon whether the FSK signal is transmitted at f-^ or f -, . Due to the phase shift, flip-flop 14 will provide an output signal Q according to the phase difference. As shown in Figure 6, output signal Q switches to a high output signal after a short time delay following time t j _. This delay is due to residual stored energy in the tank circuit. In the preferred embodiment of Figure 1, amplifiers- tiers 10, 11, 13 and 15 have a slew rate and a bandwidth product at least ten times greater than frequency f-,- Providing a Q factor for resonant circuit 12 of about 300, the present invention was able to detect a shift in the FSK frequency within three cycles of the FSK signal. Even faster detection can be obtained if desired using a resonant circuit with a higher Q factor. It will be apparent to those skilled in the art that other types of resonant phase-altering devices can be employed other than the tank circuit, such as a ceramic filter, a crystal or a SAW filter. In addition, nonresonant circuits can be employed to introduce the necessary phase difference, such as high-pass or low-pass RC filters. Flip-flop 14 could be replaced with other known sa ple- and-hold devices. Furthermore, the inputs to flip-flop 14 could be reversed, resulting in the inversion of the decoded output signal.

Claims: CLAIMS 1. Apparatus for decoding a frequency-shift keying (FSK) signal which modulates between a first frequency f - ^ and a second frequency f 2 to encode digital information, comprising: wave-shaping means (10,17) for shaping said FSK signal to generate substantially identical first and second wave- shaped signals; phase-altering means (12) receiving said first wave- shaped signal for generating a phase-altered signal, said phase-altering means having a frequency response at frequency f- ^ different from its frequency response at frequency f 2 ; and sampling means (14) for sampling one of said phase- altered signal or said second wave-shaped signal in response to a transition in said other one of said phase-altered signal or said second wave-shaped signal to generate a decoded signal. 2. An apparatus as claimed in claim 1 wherein said wave-shaping means includes means for generating a square- wave signal having transitions in response to zero crossings of said FSK signal. 3. An apparatus as claimed in claim 1 wherein said wave-shaping means is comprised of a nonlinear amplifier. 4. An apparatus as claimed in claim 1 wherein said phase-altering means is comprised of a resonant circuit having a resonant frequency f Q between said first frequency f^ and said second frequency f 2 . 5. An apparatus as claimed in claim 4 wherein said resonant frequency f n is substantially halfway between said first frequency f^ and said second frequency f 2 . 6. An apparatus as claimed in claim 4 wherein said resonant circuit is comprised of a tank circuit with substantially no integration. 7. An apparatus as claimed in any one of the preceding claims, wherein said sampling means is comprised of a D-type flip-flop having one of said phase-altered signal or said second wave-shaped signal coupled to its D input and having the other one of said phase-altered signal or said second wave-shaped signal coupled to its clock input. 8. A method for decoding a frequency-shift keying (FSK) signal which modulates between a first frequency f 1 and a second frequency f 2 to encode digital information, comprising the steps of: generating substantially identical first and second wave-shaped signals in response to said FSK signal; phase-altering said first wave-shaped signal to generate a phase-altered signal either leading or lagging said second wave-shaped signal depending on whether said first wave-shaped signal exhibits said frequency f, or said frequency f 2 ; and sampling one of said phase-altered signal or said second wave-shaped signal in response to a transition in said other one of said phase-altered signal or said second wave-shaped signal to generate a decoded signal.

Inventors: KENNEDY JOHN FRANCIS, PLOWDREY ROBERT DONALD, KENNEDY, JOHN, FRANCIS, PLOWDREY, ROBERT, DONALD
Applicants: FORD MOTOR CO, FORD MOTOR COMPANY
Application year: 1993
Main classification: H04L27
IPC codes: H04L27