Pulse or digital communications – Repeaters – Testing
Patent
1986-02-24
1988-08-23
Safourek, Benedict V.
Pulse or digital communications
Repeaters
Testing
455 67, 364550, H04B 1700
Patent
active
047666005
DESCRIPTION:
BRIEF SUMMARY
This invention relates to characterisation of modulated electrical signals, particularly but not exclusively for the purposes of evaluation of performance of digital radio data signal systems or of components for use in such systems.
It is well known that radio signals generated as a modulated carrier may be received in down-graded form at the receiver of a transmission system for a variety of reasons. A particular form of signal degradation known as multipath interference occurs wherein differently directed components of the transmitted signal both reach the receiver after travelling along paths of different lengths. If the signal components arrive in-phase, constructive interference will arise, and if the signal components arrive out of phase, destructive interference will arise. For given signal paths for the two signal components, a multi-frequency signal such as the digitally modulated carrier mentioned will be affected differently at different frequencies. At some frequencies, constructive interference will occur whilst at others destructive interference will occur. In the former case an increase in signal strength will be apparent, and in the latter case, a decrease will be apparent. The resultant alternate nodes and antinodes in the plot of signal strength against signal frequency, at which constructive and destructive interference arise, may give rise to signal distortion which renders demodulation of the received signal difficult. This phenomenon arises because of existence of particular atmospheric conditions, and variation of these conditions may result in shifting of the transmitted signal nodes and antinodes back and forth along the bandwidth of the transmitted signal so that the signal, as received, is caused to vary in an unpredictable fashion, further increasing the difficulty of demodulating. Various strategies including use of various types of compensating circuitry are employed in receivers for the purpose of minimising errors in these circumstances.
In order to evaluate the performance of circuitry for reducing demodulation errors in, say, a receiver it is customary to apply to the receiver a simulated multipath interference signal. Circuitry is employed permitting generation, from a single input signal, of a pair of phase shifted component signals the relative magnitudes and phase shift and/or delay of which are variable. These component signals may be in the form of digitally modulated carriers and they are combined and fed to the receiver. The output of the receiver is monitored and the relative gains of the two component signals varied, for each of a number of phase shifts, so that various combinations of phase shift and relative gain are determined, for which combinations the error ratio in the demodulated signal just reaches some predetermined error ratio. From this data, a graph is plotted of relative signal proportion, as between the out of phase signals, against relative phase shift or against the frequency of an anti-node or "notch" in the frequency spectrum of the combined signal, since the phase shift is directly related to this notch frequency. This graph will be representative of the notch depth and notch position that will produce a particular bit error ratio in the output data signal.
A graph so obtained thus represents a characterisation of the combined test signals on the basis that notching at particular positions in the frequency bandwidth thereof will give rise to an error ratio equal to the predetermined ratio in the demodulated signal.
The preparation of these graphs is laborious, making testing slow. Furthermore, the resultant graph is obtained on the basis of static relationships between the component signals, whereon in a practical environment the relationship tends to change in a random fashion. Circuitry which performs well under static conditions in reducing demodulation errors may not be able to perform adequately under changing conditions so that the described method provides only an indirect guide to in-service performance.
An object of the invention is
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Australian Telecommunications Commission
Safourek Benedict V.
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