FSK discriminator

Details FSK discriminator FSK discriminator

1989-05-03

1991-11-12

Chin, Stephen

Pulse or digital communications

Repeaters

Testing

329300, 329327, H04L 2714

Patent

active

050654098

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to discriminators for frequency shift keyed signals.
2. Description of the Prior Art
In 2-frequency fsk modulation, an on-off modulating signal is used to switch between a higher (f.sub.H) and a lower (f.sub.L) frequency, usually in a continuous phase manner. The general structure of a discriminator (FIG. 1A) is well known and comprises bandpass filters 1, 2 tuned to the two frequencies f.sub.H, f.sub.L, respectively. The modulus of the filter outputs is taken by rectifiers or (as shown) squaring circuits 3, 4, whose outputs are then subtracted at 5 to provide the demodulated output which may be further filtered at 6 to reduce high frequency ripple. The filtered signal is then converted to a square wave by a thresholding device 7.
It has also been proposed (see U.S. Pat. No. 4,437,068) to employ finite impulse response transversal filters, as illustrated in FIG. 1B. That the input signal is in the form of signal samples is illustrated by a sampler 10.
In the general case, because of the phase-sensitive nature of the FIR filter, quadrature pairs are used with vector summation of the outputs are used. Thus in FIG. 2 the higher frequency is handled by filters 12, 13 having tap coefficients which are sine and cosine functions of the tap number. The filter outputs are squared (14, 15) and summed (16).
Filters, squarers and an adder (17-21) are provided in like manner for the low frequency channel, the coefficients again being sine and cosine function of tap number but with a period corresponding to the lower frequency.
As the filter outputs have already been squared, the outputs are supplied directly to the subtractor 5.


SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided a discriminator for frequency shift keyed signals comprising first filter means for accepting a first keying frequency, second filter means for accepting a second keying frequency, and a subtractor for generating the difference between the outputs of the filter means, each filter means comprising a pair of finite impulse response filters for accepting signal components which are mutually in phase quadrature and means for forming the sum of an even function of the output of each of the two filters of the pair, characterised in that each filter response is defined by a set of coefficients and the filters of the first pair have coefficients equal to the keying frequencies, .phi..sub.H and .phi..sub.L are constants, f.sub.s is the sampling frequency at which the first and second keying frequencies are sampled, K is a normalising factor and r.sub.Hk, r.sub.Lk are sets of values, for the respective pairs, representing a modification function.
In another aspect, the invention provides a discriminator for frequency shift keyed signals comprising first filter means for accepting a first keying frequency, second filter means for accepting a second keying frequency, and a subtractor for generating the difference between the outputs of the filter means, characterised in that each filter means comprises: a pair of multipliers and means for supplying to the multipliers of the pair reference signals, mutually in phase quadrature, at the keying frequency which that filter means is to accept, each multiplier being followed by a finite impulse response filter, the two filters having the same response, representing a modification function, and means for forming the sum of an even function the output of each of the two filters of the pair.
Other optional features of the invention are defined in the sub-claims appended.


BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic illustration of the general structure of an fsk discriminator of the prior art;
FIG. 1B is a schematic illustration of the general structure of an fsk discriminator of the prior art employing finite impulse response transversal filters;
FIG. 2 is a block diagram of one form of fsk discriminator according to the invention;
FIG. 3 illustrates a typical idealised inp

REFERENCES:
patent: 3803501 (1974-04-01), Jones, Jr.
patent: 4437068 (1984-03-01), Picquendar
patent: 4475219 (1984-10-01), Puckette
patent: 4479092 (1984-10-01), Falconer
patent: 4568882 (1986-02-01), Single
patent: 4726041 (1988-02-01), Prohaska
patent: 4755761 (1988-07-01), Ray, Jr.
C. J. Waylan et al., "Detection on Fast, Noncoherent, Frequency-Hopped", see FIG. 3, IEEE Transactions on Communications, vol. 23, No. 5, May 1975, IEEE (New York, U.S.).

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