Telecommunications – Transmitter and receiver at separate stations – Having measuring – testing – or monitoring of system or part
Patent
1995-12-05
1999-01-19
Chin, Tommy P.
Telecommunications
Transmitter and receiver at separate stations
Having measuring, testing, or monitoring of system or part
455 676, 455304, 455305, 455306, 455 65, H04B 218
Patent
active
058624554
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to electronic fading simulators and simulation methods for use in simulating the short-term fading of radio signals due to multi-path interference phenomena. Such simulators may be used to check the ability of radio receiving equipment, such as high-capacity digital microwave receivers, to handle various types and patterns of signal fades with respect to data loss.
BACKGROUND TO THE INVENTION
The background to the present invention is set out in the specification of prior PCT patent application No. PCT/AU89/00254 filed in the name of the of the present applicant (herein called `our prior application`). Reference should be made to the specification of that application for a discussion of the technical problems faced in the design and operation of fading simulators and for an explanation of the applicant's desiderata in designing such devices.
The simulator of our prior application allowed microprocessor-controlled calibration of notch depth and notch position as independent variables thereby greatly reducing calibration time. Since the drift of component parameters normally requires frequent re-calibration of the simulator during use, the significant reduction in calibration time offered by the simulator design and method of our prior application provided a substantial advantage with respect to the art at the time it was filed. Nevertheless, calibration of that simulator could still take a few minutes and it would clearly be desirable if such calibrations were to take much less time while still allowing accurate and predictable simulation of deep signal fades which sweep rapidly across the signal spectrum.
U.S. Pat. No. 5,062,148 to Edwards discloses a multipath fading simulator in which each path includes a 180.degree. phase shifter and an attenuator which are jointly controlled so that each phase shifter contributes to the total difference in phase between the paths and so that each attenuator contributes to the required signal level difference between the paths. By closely matching the characteristics of the two phase shifters, compensation for parameter drift and errors in the phase shifters can be achieved. Similarly, by matching the attenuators, compensation for drift and errors can be achieved. However, calibration is complicated and calibration time is increased.
OBJECTIVES OF THE INVENTION
It is therefore the object of the invention to provide a fading-simulator and method which enable shorter calibration times and/or longer times between calibrations without sacrificing precision or fading performance with respect to simulators of the prior art.
OUTLINE OF INVENTION
This invention is based upon the realisation that, by using an inverting attenuator and a delay line in each path, the two paths of the simulator could be made substantially identical and the differential drift of the parameters of the components in these paths could be significantly reduced so that fewer calibration points are needed, more rapid calibration of the simulator should be possible and/or greater stability over a longer period would be obtained. In the system according to the invention, an inverting attenuator is understood to be a complex attenuator by which phase shift can be continuously controlled through 360.degree. (2.pi. radians). Such an approach is, however, counter-intuitive as it involves adding many `unnecessary` components to the signal path that is not normally deliberately phase-shifted and, therefore, increasing the complexity of the circuit and, superficially, the time required for calibration. Moreover, to be effective, the `unnecessary` components in the path which is not normally phase-shifted in a controlled manner, must now be controlled in tandem with the components in the control-path, a requirement which intuitively could be expected to result in delays and inaccuracy. Tandem control will normally be effected by using one inverting attenuator to set the nominal signal phase and amplitude difference between the two paths while simultaneously using the
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Beard David Lewis
Martin Andrew Louis
Chin Tommy P.
Martin Communications Pty Ltd
Rao Anand S.
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