The invention relates to an arrangement for recovering a clock signal in a receiver for an angle-modulated carrier signal having a carrier frequency f.sub.c and a modulation index m=0.5 generated in response to data signals of a given clock frequency 1/T, the arrangement comprising a frequency-doubler for doubling the frequency of said modulated signal; a generating circuit for generating a signal component of clock frequency 1/T from the frequency-doubled modulated signal; a clock filter for selecting the generated clock signal component; and a pulse shaper for producing a clock pulse signal in response to the selected signal component.
Such an arrangement is described in the article by De Buda on FFSK (Fast Frequency Shift Keying) in IEEE Transactions on Communications, Vol. COM-20, No. 3, June 1972, pages 429-435 (see FIG. 4). The FFSK signal having a modulation index m=0.5 has itself a power density spectrum with only continuous information-conveying components, but the frequency doubling operation provides an FFSK signal having a modulation index m=1, which comprises both continuous information-conveying components and also discrete components at the frequencies 2f.sub.c -1/(2T) and 2f.sub.c +1/(2T). The prior art arrangement utilizes this last fact for generating a signal component of clock frequency 1/T by selecting in the generating circuit the two discrete components with the aid of narrow bandpass filters, multiplying the selected discrete components by each other in a mixing circuit and thereafter separating the mixing product at the difference frequency 1/T from the mixing product at the sum frequency 4f.sub.c with the aid of a low-pass filter. In this simple way, the clock and carrier signal references can be recovered from the received FFSK signal itself and these references can be used for an optimum detection of the data signals with the aid of orthogonal coherent demodulation and synchronous data symbol detection and regeneration. Also with a view to the required constant amplitude of these references the narrow bandpass filters in the prior art arrangement are realized in the form of a PLL (Phase-Locked Loop).
This prior art method of recovering clock and carrier signal references is particularly suitable for applications in which the data signals are transmitted continuously or at least during comparatively long time intervals. This method is, however, less suitable for application in radio communication systems in which the data signals are transmitted in time intervals of relatively short durations, such as in TDMA- or FH-systems (Time Division Multiple Access; Frequency Hopping) and the receiver must recover the references in a small initial fraction preamble of this already short time interval at the proper frequency and in the correct phase, as a reliable performance of coherent demodulation or synchronous symbol detection is not possible before the correct phase has been obtained. The reason why the known arrangement is less suitable for a fast acquisition of the reference phases resides in the fact that at the customary choice of the intermediate frequency stages of the receiver in the frequency range from several hundreds of kHz to approximately 10 MHz and the customary values for the clock frequency 1/T of the data signals of not more than some tens of kHz, the selection of the two discrete components from the FSK signal obtained by frequency doubling requires in this intermediate frequency range two very narrow bandpass filters, since their centre frequencies differ only by an amount equal to the clock frequency 1/T. Using a very narrow bandpass filter does not only require very accurate tuning but it is also accompanied by a slow acquisition of the correct reference phase and this last-mentioned problem is still further aggravated when this narrow bandpass filter is realised as a PLL, as in the prior art arrangement, since a PLL can dwell in the incorrect phase for an extended time when the initial phase error falls very close to the unstable null of the characteristic of the phase detector so that the phase detector output signal is very small. The problem of a fast acquisition of the carrier phase can be circumvented by utilizing a non-coherent demodulator, such as a frequency discriminator, but then the problem of a fast acquisition of the clock phase remains.
Although the above problems are described for FFSK signals, they hold in a much more general way and more specifically for that class of modulation methods which result in an angle-modulated carrier signal having a modulation index m=0.5 and a constant amplitude. This class of constant envelope modulation methods is particularly attractive for use in radio communication systems because of their economic use of the available bandwidth and the suitability for achieving a high power efficiency by means of circuit elements having a non-linear amplitude transfer function. Known representatives of this clas of modulation methods are TFM (Tamed Frequency Modulation), GMSK (Gaussian Minimum Shift Keying) and GTFM (Generalized TFM) which are described in IEEE Transactions on Communications, Vol. Com-26, No. 5, May 1978, pages 534-542, and Vol. COM-29, No. 7, July 1981, pages 1044-1050, and in Philips Journal of Research, Vol. 37, No. 4, 1982, pages 165-177, respectively.