Narrowband communications systems employing frequency-shift-keyed
(FSK) carrier modulation demand careful consideration of system transient
responses. Discontinuous waveforms or their derivatives, such as
produced by frequency modulation, can result in the generation of large
transient currents and voltages. These transient conditions produce
distortion which interferes with demodulation processes, and may be in
excess of the physical limitations on continuous operation of the transmitting
terminal.
It has been suggested that a phase-locked-loop (PlL), whose
reference signal is instantaneously switched between two discrete frequency
sources, be employed as the modulated source of an FSK carrier.
The characteristics of the PLL provide signal coherence and a means of
"shaping" the modulation waveform, both of which are required in
certain FSK transmissions. The investigation undertaken here is
primarily concerned with the characterization of such a PLL output
signal (exhibiting a finite frequency transition interval). Expressions
are obtained for the output signal and the resulting modulation waveform
for the case of a single frequency shift. Graphical evidence of
the dependence of the moduiation waveform on the loop parameters, gain
and bandwidth, is presented. In order to facilitate the analysis of
linear networks excited by the FS-PLL, an infinite series of damped
sinusoidal terms, representing the PLL output signal is developed. This
series is also utilized to determine the coefficients of the Fourier
series representation of a periodically keyed PLL. Using this result,
the signal bandwidth is shown to vary directly as the loop bandwidth.
Application of the FS-PLL as a voltage source is made in the case of
the equivalent circuit (a high Q series tuned circuit) of a VLf
transmitting antenna. Representative curves are provided which show
that the series tuned circuit transient response is in general the
superpositioh of a transient component, resulting from a change in
frequency, and a quasi-stationary component.
Finally, an experiment is devised which provides an additional
investigative tool. A unique method for achieving demodulation of
narrowband signals, while maintaining wideband demodulator response
characteristics, is used to experimentally substantiate the predicted
modulation waveforms. Experimental data is also obtained, and
graphically compared with the theory, to verify the frequency domain
analysis of the FS-PLL output as well as the tuned circuit time domain
analysis.
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