This invention relates to methods and apparatus for testing frequency-dependent electrical circuits or circuit elements, particularly for example electrical filter circuits or circuit elements.
Electrical filter circuits often need to be tested, for instance for the purpose of verifying whether they are serviceable or not, i.e. to check their operational integrity, or for the purpose of assessing their electrical characteristics, or both. The usual method of testing such circuits is to apply a.c. (alternating current) input signals of predetermined voltage, e.g. of measured and/or fixed standard voltage, from a low-impedance source, over a range of frequencies, to the input terminals of a filter circuit under test, and to measure the corresponding voltage delivered at the output terminals.
Most methods for determining the frequency response of an electrical circuit rely on measurement of both magnitude and phase information at the inputs and outputs of the circuit in order to derive the frequency and phase response of the circuit. Once input and output measurements have been made it has been possible to estimate the insertion loss of the filter, i.e. ratio of output to input voltages under the test conditions, usually expressed-in decibels, and generally as a function of frequency. The insertion loss can give desired information about the integrity or otherwise, and the characteristics, of the filter under test.
However, increasingly there are situations in which it would be desirable to test frequency-dependent electrical circuits, for example filters, but in which the usual method of testing indicated above is not feasible. In modern electronic systems, especially where packaging densities are high, electrical filters are often incorporated in devices. In aircraft for example, the increasing number of pilot and navigator aids, provided by electrical equipment, means that the integration of equipment is advanced, and there is little availability for test points within equipment. Furthermore, once the circuits have been incorporated into a larger piece of equipment it may be advantageous to test the circuit for failure. Situations where it is not possible to have access both to input and to output connections, as would be required in order to carry out the traditional test methods are increasingly commonplace. Often the output side is inaccessible.
The present inventor's International Patent Application No. WO95/04935 discloses a method and apparatus for testing filters and the like in which a.c. input test signals are applied to the input of the filter under test, and measurements are made of the voltage across the input terminals and/or the current through the input terminals, and the integrity of the filter is determined by identifying a characteristic of the resultant measurements. The measurements can simply be compared with those obtained from a sample filter which is known to be good. Alternatively and preferably the measurements obtained are processed to detect expected characteristics of the measurements. This gives an indication as to whether the filter comes up to specification or not.
That system proves to be very effective, but can only identify a faulty unit. We have appreciated that there would be advantages in being able to determine why the unit has failed. In particular, it would be advantageous to determine which of the subcomponents of the filter under test are giving rise to the problem. This would make it possible to repair the unit, and/or would provide the manufacturers with information enabling them to improve the quality of their production by removing frequently appearing faults.
Thus, we have appreciated that it may be desirable to be able to determine the values of selected subcomponents of the filter under test. It may also be desirable to be able to determine the values of characteristics of the filter such as the insertion loss of the filter.