WO1995004935A1

WO1995004935A1 - Method and apparatus for testing frequency-dependent electrical circuits

Info

Publication number: WO1995004935A1
Application number: PCT/GB1994/001723
Authority: WO
Inventors: Anthony James Moore
Original assignee: Bcf Designs Limited
Priority date: 1993-08-05
Filing date: 1994-08-05
Publication date: 1995-02-16
Also published as: GB9316273D0; GB2281127A

Abstract

A method and apparatus for testing frequency-dependent electrical circuits or circuit elements, e.g. electrical filter circuits, e.g. for checking operational integrity or electrical characteristics, involves a signal generator arrangement for applying a.c. input signals over a range of frequencies of interest, to a pair of connections made to a circuit or circuit element of a selected type which is to be tested, and a measurement device for obtaining, for each frequency of a range of frequencies of interest, at least one measurement or measurement signal, related to the voltage developed across the pair of connections made to the circuit or circuit element under test and/or to the current flowing through those connections, under the test conditions, and a data-processing arrangement for identifying at least one characteristic of the measurements or measurement signals obtained from measurement device (b), or of data derived therefrom, related to the integrity or performance of the circuit or circuit element which is under test. Often the two connections used for applying the a.c. input test signals are the only signal connections made between the test apparatus and the circuit or circuit element under test.

Description

METHOD AND APPARATUS FOR TESTING FREQUENCY-DEPENDENT ELECTRICAL CIRCUITS

Field of the invention:

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.

Background of the invention:

Electrical filter circuits often need to be tested, it may be 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. input signals of determined 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. 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, can give desired information about the integrity or otherwise and the characteristics of the filter under test.

The ^"Present Invention:

The present inventor has identified situations in which it is desirable to make use of a method of testing electrical 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 or 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. Often only the input side is accessible.

Accordingly, the present invention aims to provide test methods and apparatus for testing frequency-dependent electrical circuits and circuit elements, which can be used even where only one pair of connections can be made to the circuit or circuit element which is to be tested.

The invention also aims to provide test apparatus for carrying out such test methods with useful degrees of automation.

According to the present invention there is provided a method of testing frequency-dependent electrical circuits or circuit elements, particularly for example electrical filter circuits or circuit elements, which comprises the steps of:

(a) applying a.c. input test signals over a range of frequencies of interest, to a pair of connections made to a circuit or circuit element of a selected type which is to be tested, and

(b) obtaining, for each frequency of a range of frequencies of interest, at least one measurement or measurement signal, related to the voltage across the pair of connections made to the circuit or circuit element under test, and/or to the current flowing through those connections under the test conditions.

In many cases the method also includes a further step of:

(c) identifying at least one characteristic of the the measurements or measurement signals obtained in step (b), or of data derived therefrom, related to the integrity or performance of the circuit or circuit element which is under test.

Also provided by the invention is test apparatus for testing frequency-dependent electrical circuits or circuit elements, particularly for example electrical filter circuits or circuit elements, which comprises :

(a) a signal generator arrangement for applying a.c.input signals over a range of frequencies of interest, to a pair of connections made to a circuit or circuit element of a selected type which is to be tested, and (b) a measurement device for obtaining, for each frequency of a range of frequencies of interest, at least one measurement or measurement signal, related to the voltage developed across the pair of connections made to the circuit or circuit element under test and/or to the current flowing through those connections, under the test conditions.

Embodiments of the test apparatus preferably also include:

(c) a data-processing arrangement for identifying at least one characteristic of the measurements or measurement signals obtained from measurement device (b), or of data derived therefrom, related to the integrity or performance of the circuit or circuit element which is under test.

In most cases, the pair of connections made to the circuit or circuit element which is to be used for the testing will be input connections, i.e. effectively connections made to input terminals. (But in some cases, it may possibly turn out to be necessary, or preferable, to use a pair of connections or terminals that would normally function as output connections or terminals when the circuit or circuit element is in its normal use.)

But in any given case, the test method only requires making one pair of connections to the circuit or circuit element under test, and this can be an important advantage of the method and associated apparatus, which can be used to detect and/or quantify a variety of commonly- occurring fault conditions in circuits and circuit elements of the types considered here.

The measurement or measurement signal to be obtained at (b) can be of a number of kinds, as desired.

One particularly desirable method of obtaining the needed measurement is for example to apply each of a series of a.c. input signals from the signal generator, over a predetermined frequency range, successively to the circuit or circuit element under test, from a signal source which is of standard or calibrated or otherwise determinate voltage (at open-circuit) and which also has a known and fixed source impedance (preferably a purely resistive impedance) associated with it, and then to measure the resulting voltage developed (at each of the series of frequencies) between the terminals or connections made to the circuit or circuit element under test. The source impedance should be effectively constant over the frequency range used for the testing.

For example, a suitable signal source can comprise a variable- frequency signal generator of low output impedance, of standard output voltage (or provided with means for measuring or standardising its output voltage), and a fixed impedance, e.g. a resistor, in series with its signal output. A suitable measurement device can comprise a high-impedance digital voltmeter connected across the connections made to the circuit or circuit element under test.

In practice the source impedance (usually pure resistance as far as this can be realised in practice), has a value chosen to make an appreciable difference between the resulting voltage at the connections made to the circuit or circuit element under test, on the one hand, and the open-circuit voltage of the signal source on the other hand. For example (but without limitation) a 50-ohm source resistance can be used for filters which have input impedances, at one or more of their corner frequencies or characteristic frequencies, in the range of a few ohms (e.g. 1-10 ohm) to a few hundred ohms (e.g. 100 ohm to 1 kohm).

An alternative method of obtaining a useful measurement signal at each frequency of interest is to measure the current flowing into the circuit or circuit element under test, under test conditions in which the (open-circuit) voltage of the signal source and its source impedance are also determinate. A further alternative method is to use a constant-current a.c. signal source of very high source impedance and to measure the voltage developed across the circuit or circuit element under test. (Choice of these alternatives entails corresponding change to the form of examples of the data and data- processing described below.)

The a.c. input signals are chosen so that they cover a frequency range which ranges well to each side of each corner or characteristic frequency of the selected type of circuit or circuit element which is under test, e.g. a range that includes pass and stop bands. In the case of a single parallel-capacitative filter, or other low-pass filter, the needed range extends well both below and above the corner-frequency of the configuration which corresponds to the time constant of the capacitor in conjunction with the associated source and/or load resistance.

In practice, a useful test method and apparatus catering for a wide variety of important types of filters can utilise a signal generator with a range from about 10 kHz to about 50 MHz, which can sweep the desired frequency range in suitably chosen linear/logarithmic steps to demonstrate the characteristic under test. Use of a lower frequency at the bottom of the range may be desirable if filters incorporating large capacitance values are to be tested, but the indicated range has been found suitable for most filters encountered in practice.

The data-processing arrangement (c) and identification step (c) of the method and apparatus according to the invention can be arranged or performed in any of a number of ways: for example a data-processing device can be arranged to identify the presence, in the data derived from the test, of a feature or features characteristic of test data derived from a serviceable example of the selected type of circuit or circuit element which is under test. Examples of usable characteristic features are described below.

Alternatively such a data-processing device can be arranged to compare the measurements or derived data with a set of values corresponding, under the test conditions, to those given by a serviceable example of the selected type of circuit or circuit element which is under test.

For example, several useful embodiments of the invention can be configured to detect and identify, in the case of an indicated type of of circuit or circuit element which is under test, e.g. an L-C filter, whether an example of it that is being tested is free from a number of kinds of fault that would render it unserviceable, and optionally also, if it is shown to be unserviceable, whether a given component of it is open-circuit or short-circuit, and in many cases which of its sub-components has such a defect. Such embodiments of the invention, among others, can take the form of a computer-controlled test rig comprising a digital computer and (a) a variable-frequency a.c. signal generator, of standard or calibratable output voltage, under program control by the digital computer, and arranged to feed its a.c. signal through a source resistance to a single pair of connections made to a circuit or circuit element to be tested; (b) a digital voltmeter arranged to measure the voltage developed across the single pair of connections made to the circuit or circuit element to be tested and to deliver data representing the results of such measurement as input data to the digital computer so as to cooperate with the program control; and in which the computer is arranged under program control to cause a stepwise succession of a.c. signals, of a stepwise succession of frequencies, to be applied to the circuit or circuit'element of selected type to be tested, and for the data delivered by the digital voltmeter (b) to be processed so as to provide for the user of the test rig an indication related to the integrity or performance of the circuit or circuit element which is under test, e.g. by its correspondence or otherwise with a feature or features characteristic of test data which is derived from a serviceable example of the selected type of circuit or circuit element which is under test, or by comparison between the data and a set of values corresponding, under the test conditions, to those given by a serviceable example of the selected type of circuit or circuit element which is under test.

The ^"diagnostic features of the test data show a dependence upon the type of circuit or circuit element under test, and accordingly it will often be necessary to carry_out a different form of comparison or other data-processing for each of several such types. Several representative such types of circuit or circuit element, and the characteristic forms of test data given by serviceable and unserviceable examples of each, are described below.

Further Details of the Invention, and Description of the Drawings

Embodiments of the invention are described in further detail below, but without limitation on the scope of the invention, and with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of arrangements of a computer- controlled test apparatus for testing frequency-dependent electrical circuits or circuit elements, particularly for example electrical filter circuits or circuit elements, according to an embodiment of the invention;

Figure 2 is a schematic diagram showing characteristics of the form of test data obtained when serviceable and unserviceable examples of parallel capacitative filter circuits or circuit elements are tested using the apparatus of Figure 1;

Figure 3 is a schematic diagram showing characteristics of the form of test data obtained when serviceable and unserviceable examples of low-pass T-section or L-section filter circuits or circuit elements are tested using the apparatus of Figure 1;

Figure 4 is a schematic diagram showing characteristics of the form of test data obtained when serviceable and unserviceable examples of low-pass pi-section filter circuits or circuit elements are tested using the apparatus of Figure 1.

Each of Figures 2-4 also shows a schematic partial circuit diagram of the filter type respectively concerned.

Referring to the drawings, Figure 1 shows, as a schematic diagram, arrangements of a test apparatus controlled by a computer DP. Computer DP is conveniently an industry-standard microcomputer such as a personal computer based on-a 80386 processor and equipped with hardware expansion slots and/or i/o ports such as RS-232 ports. Either by hardware expansion cards or by cable links via i/o ports, computer DP is functionally linked to control a programmable variable-frequency a.c. signal generator SG and to receive data from a programmable digital voltmeter V, under control of menu-driven software arranged to control the computer to carry out the functions described herein.

The generator SG can be industry-standard unit chosen (e.g. from the Hewlett-Packard range) to give an output level in this example of 5 v rms {+/-$%) over the range 10kHz to 50MHz, programmable so that signal frequency is also (+/-5%) relative to nominal programmed frequency. The output is sinusoidal with good purity indicated by low total harmonic distortion preferably not exceeding -40dB, and good linearity over a load impedance range from about 5 ohm to about 100 koh . The generator is of a type able to drive any impedance value from open-circuit to short-circuit, and whether resistive or reactive, without damage. The source impedance of the generator is arranged, using series resistor Rs (or internal resistance), to be effectively 50 ohm (as purely resistive as can be realised).

The programmable digital voltmeter V is an industry-standard unit chosen (e.g. from the Hewlett-Packard range) to give rms voltage detection and measurement over the frequency range 10kHz to 50MHz, over a range of levels from 20mV rms to 5V rms, with an accuracy preferably from about 1% bf measured voltage at maximum level which may gradually increase to 10% of measured voltage at minimum level. A high meter input impedance is needed, preferably at least about lOOkoh shunted by no more than lOpF capacitance over the usable frequency range. It is preferred that the measurement accuracy is maintained when about 100 different frequency settings are programmed within a period of about 10 seconds.

The arrangements described in this embodiment can conveniently be used for testing filters with C component values up to about 0.5 microF. If larger capacitance values are likely to be encountered, a lower bottom frequency may be used.

In Figure 1 the generator SG and voltmeter V are diagrammatically shown connected to a filter-F under test. Filter F may have any load impedance value ZL connected to it, or ZL may be absent. The connections of a filter F under test both to SG and to V are made via matched coaxial cables connected (via a conventional four-terminal measurement connector configuration) to the two terminals marked IN of the filter F.

It can be convenient to amke these connections switchable, e.g. by a bank of programmable relays, to any one of a number, e.g. conveniently 120 in one example, of plug/socket connectors suitable for receiving filters to be tested. Such relays can preferably be subject to operation under software control from computer DP in manner known per se.

Especially where such routing relays are used, it can be helpful to take null-signal measurements from voltmeter V when all routing relays are left open, and to apply any such null-signal values to compensate the measurements taken when a relay is closed and a circuit or circuit element is under test.

A problem of equalisation naturally tends to occur in connection with the transmission of high-frequency measurement signals along such links. Especially where an example of the system is optimised for cables filters and/or other components having characteristic impedance of e.g. 50 ohm, the measurement conditions can be controlled or compensated for inequalities in frequency response by carrying out a calibration frequency sweep of the system, (e.g. separately for all relay positions if a bank-switched system is in use), using a standard signal generator level and a pure resistance of e.g. 50 ohm in the place normally occupied by the (or each) filter to be tested, and storing calibration data or compensation data corresponding to the resultant measurements. Any departure from a 'flat' frequency response can then be applied as a factor when data-processing the measurement data obtained under test conditions, to compensate for any inequality of frequency response of the measurement system.

In use, computer DP is provided with input effectively consisting of program software P to perform the functions described herein, and a setting S set by the operator, to correspond to the type of filter to be tested, e.g. one of the types described in connection with Figures 2-4 below. Setting S then determines which of several variants of the data processing are to be carried out in connection with the test of filter F. The computer DP provides results R in any convenient from, provided by software in manner known per se.

Figures 2-4 show the forms of test data obtained when the above described apparatus is used to test each of several important types of filter in extensive use in the electronic industry.

Figure 2 shows characteristics of the form of test data obtained when serviceable and unserviceable examples of parallel capacitative filter circuits or circuit elements are tested using the apparatus of Figure 1, together with a schematic partial circuit diagram showing the configuration of this type of filter. On a logarithmic scale of voltage-decibels, the solid curve 1 continuing as la diagrammatically corresponding to the function |20*log[10](Ri/(Ri+Rs))| (ordinate) against logarithm of frequency (abscissa, high frequencies to the right in the diagram of Figure 1), where Ri is a pure capacitative frequency-dependent reactance due to parallel capacitor C, in series with the source resistance Rs. The test apparatus may be used to test capacitative and LC filters which on their output sides are either unloaded, or loaded by an effective load impedance as shown in the diagram at ZL, but several features of the curves, such as curve 1 - la, particularly their shape, are substantially unaffected over considerable ranges of frequency and values for ZL, so that it is generally unnecessary to have prior knowledge of the nature or magnitude of any such load impedance.

If, in the filter element under test here, the capacitor is faulty and open-circuit, then the falling portion of the normal characteristic, i.e. curve portion la, is replaced by curve portion 2. But if the capacitor is faulty and short-circuit, curve 1 - la is replaced by curve 3.

The data processing software, when set to examine a filter of this type, is arranged to analyse a dataset of number-pairs each representing a point on one of the curves 1, 2 or 3. These number-pairs result from applying a.c. signals of a range of frequencies, at standard voltage, to the C filter under test, and each number-pair consists of a digital numerical representation of one of the several frequency values, paired with a digital representation of the corresponding voltage measurement made by voltmeter V across the input of the filter when the signal of that frequency is applied. The data processing is arranged to discriminate between curves of the types 1, 2 and 3, and thereby to provide an output which is indicative of the state of the component under test, whether it is serviceable, open-circuit or short-circuit.

In some cases a frequency sweep may be made and controlled in response to data processing results of early data (e.g. see Appendix). Alternatively a complete frequency sweep can be made and followed up by data processing of the ensemble of results obtained.

It can be useful to arrange the computer DP so that if required it will deliver a data output of the curve diagrarnmatically shown in Figure 2, e.g. displayed as a visible plot, which provides one useful way for the operator to judge the result of the test procedure, and if the values of resistance Rs and impedance ZL are known it can also be arranged if desired to calculate the effective capacitance value of component C.

The Appendix shows (under 'Procedure for Filter Type 'C") further details of an example of useful measurement procedure and associated data processing for determining fault conditions in this type of filter.

Figure 3 shows characteristics of the form of test data obtained when serviceable and unserviceable examples of T-section and L-section low-pass L-C filter circuits or circuit elements are tested using the apparatus of Figure 1, together with a schematic partial circuit diagram showing the configuration of this type of filter. The diagram represents a T-section filter, and the L-section case corresponds to conditions resulting when the value of inductance L2 is negligible. The data given by a serviceable example of such a filter show the form of solid curve 1 (continued at la and lb) in Figure 3, with a pronounced dip in the log-log plot. This corresponds to the filter in serviceable condition, free from short-circuit and open-circuit faults. If the capacitance-is open-circuit then the dip is absent, as shown by curve portion 2a which in this case continues from curve portion la. If the front inductance LI is open circuit (i.e. in this connection of substantially zero inductance, e.g. because the ferrite bead that should provide it has split and fallen off its associated wire), the curve corresponds to curve portions 1 and la continued in curve portion 2b. If the capacitance is short circuit then the curve is replaced by curve 3. The comments about data-processing given above in connection with Figure 2 apply here as well, with necessary modifications due to the different expected forms of the data for this type of filter. The Appendix shows (under 'Procedure for Filter Type 'T" and 'Procedure for Filter Type 'L") further details of an example of useful measurement procedure and associated data processing for determining fault conditions in these types of filter.

Figure 4 shows characteristics of the form of test data obtained when serviceable and unserviceable examples of pi-section low-pass L-C filter circuits or circuit elements are tested using the apparatus of Figure 1; also shown is a schematic partial circuit diagram of the configuration of this type of filter. The data given by a serviceable example of such a filter show the form of solid curve 1 starting at la and continued at lb, lc and Id in Figure 3, with a noticeable S-shape in this log-log plot. This corresponds to an example of the filter in serviceable condition, free from short-circuit and open-circuit faults. If remote capacitance C2 is open circuit, then the data for such a faulty component correspond to a curve made up of portions la, 2a and Id. If the front capacitance Cl is open-circuit then the curve approaches the form which is normal for a L-section filter as shown in Figure 3, i.e. with a dip and a rise as shown by curve portion 2b in Figure 4. If the inductance is open circuit, then the curve corresponds to curve portions la, lb and 2c. (Curve portions Id and 2c are of the same slope and if the capacitance values Cl and C2 are equal then the vertical separation between them is 6 dB for any given frequency substantially above the turning-points of curve 1.) If either capacitance Cl or C2 is short circuit then the curve is replaced by curve 3. An inductor provided by a ferrite bead threaded around a wire does not generally develop a short-circuit fault though its inductance may largely disappear if the bead is damaged or missing. The comments about data-processing given above in connection with Figure 2 apply here as well, with necessary modifications due to the different expected forms of the data for this type of filter.

The Appendix shows (under 'Procedure for Filter Type 'Pi'') further details of an example of useful measurement procedure and associated data processing for determining fault conditions in this type of filter.

In the case of high-pass L-C filters that respectively correspond in configuration to the low-pass filters of Figures 2-4 after changing L components to C components and vice versa, the forms of the log-log characteristic curves are for many purposes substantially similar except for the reversal of the log-frequency scale, i.e. in the case of the high-pass filters the low-frequency end is to the right of each plot instead of to the left.

Several other types of filter show other characteristic data patterns and differences between serviceable and faulty examples, as may be ascertained by the reader skilled in the art by using apparatus of the present invention, e.g. that of Figure 1, to obtain data and curves corresponding to those of Figures 2 to 4 in the case of each of several filter types that may be of interest, using serviceable and faulty examples.

In addition to the use of the test system and method as described to detect fault conditions such as short circuits and open circuits, the system and method can also be used to detect other characteristics of filters or circuit elements under test. For example, also detectable are changes (e.g. by way of deterioration) with time, and/or with use or wear, of the component values in such filters or circuit elements, or changes in value of the load impedances connected to such elements.

Such changes can be detected for example by arranging to have the computer DP in Figure 1 store detailed data corresponding to good reference samples of the particular components under test and by further arranging for an indication to be given, in any desired way, of any discrepancies shown by specimens under test (e.g. beyond any allowable limit set by the operator and/or by the software, not only in the shape of the plots such as those of Figures 2-4, but also in their location, i.e. in the data values composing them.

Where alternative measurement circuit arrangements are used (e.g. current measurement or use of a high-Z constant-current source) corresponding changes generally need to be made in handling the forms of data that result, as will be appreciated by the skilled reader.

The present invention is susceptible to many other modifications and variations and the present disclosure extends to other combinations and subcombinations of the features mentioned above and illustrated in the drawings.

A P P E N D I X

Procedure for Filter Type 'C

1) Send an appropriate interface generator sub-routine to the programmable generator to set the frequency to lOKHz.

2) Without closing any of the routing relays, i.e. all relays open, use the interface detector sub-routine to read the programmable detector, and store this value as V_{GEN 0/c}.

3) Close the appropriate routing relay using the interface relay sub¬ routine as determined from the menu.

4) Use the interface detector routine to read the programmable detector and store this value as V_{fllter (10kHz)}.

Establish the filter load resistance RL using the formula

il ter Load Resistance = ^^ΓLΓ∞ .^IO^. ₌ ^R

^GENO/C -"^•L⁺-" SOURCE

the source resistance, R_SOUR_CE- being 50 ohms.

5) Send an appropriate interface generator sub-routine to set the programmable generator for a frequency of 31.62KHz.

6) Use the interface detector routine to read the programmable detector and compare with the lOKHz value.

7) If the difference in values is greater than 3dB using the equation given in Appendix I establish the cut off frequency of the filter, f_Cut off^*

8) If the difference in value is less than 3dB by sending the appropriate interface generator sub-routine, set the programmable generator to lOOKHz.

9) Compare the lOOKHz value with the lOKHz value.

10) If the difference is greater than 3dB using the equation given in Appendix I, establish the cut off frequency of the filter. ii) Repeat 8 but using a generated frequency of 316.2KHz and comparing with lOKHz.

12) Continuing the above method but at generator frequencies of 1.0MHz, 3.162MHz, 10MHz and 31.62MHz the cut off frequency of the filter will be established, with a maximum of seven measurements.

Note 1 Since the generator has a maximum frequency of

50MHz the following capacitance values are detectable with the following filter load impedances.

>50R 150pF Min (R=Resistance=Ohm) 10R 500pF Min 1 R 5.0nF Min 13) Calculate the value of R_p from the formula:

R_P _ = RL ^> RS°^∞cg where R_soaRCE = 50R

-^*\ -^SOURCE

The load resistance is calculated as in 4).

14) Calculate the value of the filter capacitance using the formula:

² - ^π * * (Cut off) ^{• R}P

Note 2 a) If when V,_{ilter (10kHz)} in 4) is measured the value is below lOOmV (RMS) or a ratio of greater than 34dB then one of three possibilities exist:

1) The filter load resistance is below 1R and is outside the specification of the instrument.

2) The capacitance value of the filter is greater than 17μF and is outside the specification of the instrument

3) The capacitance has gone short circuit

In either of the above the measurement is deemed to have failed the test.

b) If the filter capacitance has gone open circuit it will not be possible to find a frequency where the difference in loss between V_{(ilter (10kHz)} and V,_llter at any frequency up to 50MHz is more than 3dB, even at the highest frequency, or the value being measured is below the values given in note 1.

In either case the measurement is deemed to have failed the test. Procedure for Filter Type 'Pi'

1) Establish the filter load resistance using the same procedure as for the 'C type filter, steps 1) thru 4).

2) Establish the filter cut off frequency f_{cul off} using the same procedure as for the 'C type filter, steps 5) thru 12).

3) Calculate R_p as in step 13) of 'C type filter procedure.

4) Calculate the capacitance of the filter using the formula:

c ^ TOTAL

2 * ^{π •} f cutoff - R-P

Note: C_TOTAL is the sum of the Pi network input and output capacitartce.

5) Set the programmable generator to a frequency of f_cutoff and measure the value showing on the programmable detector.

Note: The value measured should be 3dB lower than the value recorded for V₍„_{ter (10kHz)}.

6) Set the programmable generator to a frequency of 3.162 x f^^ using the value established in 2).

7) Check that the programmable detector is giving a value of lOdB ±0.5dB lower than the value of V_{(ilter(10kHz)} recorded for the filter.

8) Set the programmable generator to a frequency of 10 x f_cutoff and measure the value showing on the programmable detector.

One of three conditions will apply: a) A value which is 20dB ±O.SdB lower than the value of V_filter

(10kHz) ^* b) A value which is greater than 20dB ±0.5dB lower then the value of V_{tmer (10kH2)} . c) A value which is less than 20dB ±O.SdB lower than the value of V_r„_{ler (10kHz)} .

If condition a) or b) exists, increase the frequency of the programmable generator to 31.62 x f_cutoH and measure the value showing on the programmable detector. Once again three conditions will apply:

a) A value which is 30dB ±0.5dB lower than the value of V_f fi_illter

(10kHz) ^* b) A value which is greater than 30dB ±0.5dB lower then the value of V_{filter (10kHz)} .

c) A value which is less than 30dB ±0.5dB lower than the value of V, filter (10kHz) ^•

If condition a) or b) exists, increase the frequency of the programmable generator to 100 x f_cutoff and measure the value showing on the programmable detector.

Check again for the three conditions a), b) and c) but using the value of 40dB ±0.5dB.

Continue with this procedure until condition c) is achieved.

If a generator frequency of 50MHz is reached i.e. the maximum frequency, and condition a) or b) still apply then the 'Pi' filter is faulty with the remote capacitor of the 'Pi' network being open circuit.

The above procedure is summarised as follows:

Detector measurement in dB

If condition c) is established at any time during the procedure then on of two conditions exist: a) The 'Pi' filter is good. b) The input capacitance is open circuit.

To establish which of these conditions is true the following procedure should be adopted:

Note: An open-circuit ferrite will give a similar result as an open- circuit remote capacitor, using the procedure given below, but shifted in frequency. The filter with an open-circuit remote capacitor will give a higher f_cuto(( than for a correct filter. Unless the correct f^^ has been established for the filter the procedure will not be able to distinguish between an open-circuit remote capacitor and an open-circuit ferrite but will be able to determine these faults exist in the filter.

Having established the multiple of f_cutoff at which condition c) is first encountered increase the programmable generator frequency by a factor of 3.162. i.e. if condition c) occurs first when the frequency is 10 x f_cuto(f increase the frequency of the programmable generator to 31.62 x f_cutoff and measure the value recorded for the programmable detector.

If the value is lOdB ±0.5dB below the value recorded in the previous measurement then the 'Pi' filter is good.

If however the signal measured is less than lOdB ±0.5dB below the value previously recorded and has risen by about lOdB ±0.5dB the input capacitance is open-circuit.

If either the input or remote capacitors are short circuit the note 2, 1), 2) & 3) are applicable. It is not possible for ferrites to go short-circuit. Procedure for Filter Type T'

1) Establish the filter load resistance using the same procedure as for the 'C type filter, steps l) thru 4).

2) Establish the filter cut off frequency f_{cut off} using the same procedure as for the 'C type filter, steps 5) thru 12).

Note: If f^^ can not be found below 50MHz then the capacitance has gone open circuit and the filter is defective.

3) Calculate R_p as in step 13) of 'C type filter procedure.

4) Calculate the capacitance of the filter using the formula:

c =

2 . π ^■'-cut off Rp

5) Set the programmable generator to a frequency of f^^ and measure the value showing on the programmable detector.

Note: The value measured should be 3dB ±0.5dB lower than the value recorded for V_{filter (10kHz)} at 10kHz

6) Set the programmable generator to a frequency of 3.162 x f_Cϋtoff using the value established in 2).

7) Check that the programmable detector is giving a value of lOdB ±0.5dB lower than the value of V,_ater recorded for the filter.

8) Set the programmable generator to a frequency of 10 x f^^ and measure the value showing on the programmable detector.

One of three conditions will apply:

a) A value which is 20dB ±O.SdB lower than the value of V_liller

(10kHz) - b) A value which is greater than 20dB ±O.SdB lower then the value of V_{(i]lef {10kHz)} .

c) A value which is less than 20dB ±0.5dB lower than the value of V_{ljller (10kH2)} . If condition a) or b) exists, increase the frequency of the programmable generator to 31.62 x f_cutoff and measure the value showing on the programmable detector.

Once again three conditions will apply:

a) A value which is 30dB ±0.5dB lower than the value of V_filter

(10kHz) - b) A value which is greater than 30dB ±0.5dB lower then the value of V_{(ilter (10kHz)} .

c) A value which is less than 30dB ±0.5dB lower than the value of V_{filter (10kHz)}.

If condition a) or b) exists, increase the frequency of the programmable generator to 100 x f_cutoff and measure the value showing on the 'programmable detector.

Continue with this procedure until condition c) is achieved.

If a generator frequency of 50MHz is reached i.e. the maximum frequency, and condition a) or b) still apply then the T' filter is faulty with the input inductance of the network being open- circuit

The above procedure is summarised as follows:

Detector measurement in dB Generator Frequency with respect to V^,. _(10kHz)

10kHz V - ^■ _f nn_ilii_ttt_eee_rrr

V_fater -3dB V_filter -lOdB V,_ilter -20dB V,_ilter -30dB V_fllter -40dB V_{f er} -SOdB

_fi„_e- -60dB

If condition c) is established at any time during the procedure then the T filter is good.

Note: It is not possible using this procedure to establish whether the output inductance is satisfactory.

Claims

CLAIMS :

1. A method of testing frequency-dependent electrical circuits or circuit elements, particularly for example electrical filter circuits or circuit elements, characterised in that it comprises the steps of:

(b) obtaining, for each frequency of a range of frequencies of interest, at least one measurement or measurement signal, related to the voltage across the pair of connections made to the circuit or circuit element under test and/or to the current flowing through those connections under the test conditions.

2. A method according to claim 1, characterised by a further step of:

3. A method according to claim 1 or 2, characterised in that the two connections used for applying the a.c. input test signals in step (a) are the only signal connections made between the test apparatus and the circuit or circuit element under test.

4. A method according to claim 1, 2, or 3, characterised in that the pair of connections made to the circuit or circuit element which is to be tested are input connections, i.e. effectively connections made to input terminals.

5. A method according to any of claims 1 to 4, characterised in that it further comprises the steps of applying each of a series of the a.c. input test signals from a signal generator, over a predetermined frequency range, successively to the circuit or circuit element under test, from a signal source which is of standard or calibrated or otherwise determinate voltage (at open- circuit) and which also has a known and fixed source impedance (preferably a purely resistive impedance) associated with it, and then measuring the resulting voltage developed (at each of the series of frequencies) between the terminals or connections made to the circuit or circuit element under test.

6. A method according to any of claims 1 to 5, characterised in that it is applied to testing the operational integrity of a capacitative filter which has a load resistance connected to output terminals of the filter, wherein signal generator output is connected to two input terminals only of the filter.

7. A method according to any of claims 1 to 5, characterised in that it is applied to testing the operational integrity of a pi-type filter which has a load resistance connected to output terminals of the filter, wherein signal generator output is connected to two input terminals only of the filter.

8. A method according to any of claims 1 to 5, characterised in that it is applied to testing the operational integrity of a T-type filter which has a load resistance connected to output terminals of the filter, wherein signal generator output is connected to two input terminals only of the filter.

9. Apparatus for testing frequency-dependent electrical circuits or circuit elements, particularly for example electrical filter circuits or circuit elements, characterised in that it comprises :

(a) a signal generator arrangement for applying a.c. input signals over a range of frequencies of interest, to a pair of connections made to a circuit or circuit element of a selected type which is to be tested, and

(b) a measurement device for obtaining, for each frequency of a range of frequencies of interest, at least one measurement or measurement signal, related to the voltage developed across the pair of connections made to the circuit or circuit element under test and/or to the current flowing through those connections, under the test conditions.

10. Apparatus according to claim 9, further characterised by:

(c) a data-processing arrangement for identifying at least one characteristic of the measurements or measurement signals obtained from measurement device (b), or of data derived therefrom, related r to the integrity or performance of the circuit or circuit element which is under test.

11. Apparatus according to claim 10, characterised in that it comprises a signal generator with a range from about 10 kHz to about _Q 50 MHz, which can sweep the desired frequency range in suitably chosen linear/logarithmic steps to demonstrate the characteristic under test.

12. Apparatus according to any of claims 9 to 11, characterised in that it is in the form of a computer-controlled test rig comprising a ₅ digital computer and (a) a variable-frequency a.c. signal generator, of standard or calibratable output voltage, under program control by the digital computer, and arranged to feed its a.c. signal through a source resistance to a single pair of connections made to a circuit or circuit element to be tested; (b) a digital voltmeter arranged to 0 measure the voltage developed across the single pair of connections made to the circuit or circuit element to be tested and to deliver data representing the results of such measurement as input data to the digital computer so as to cooperate with the program control; and in which the computer is arranged under program control to cause a 5 stepwise succession of a.c. signals, of a stepwise succession of frequencies, to be applied to the circuit or circuit element of selected type ^"to be tested, and for the data delivered by the digital voltmeter (b) to be processed so as to provide for the user of the test rig an indication related to the integrity or performance of the

-₀ circuit or circuit element which is under test, e.g. by its correspondence or otherwise with a feature or features characteristic of test data which is derived from a serviceable example of the selected type of circuit or circuit element which is under test, or by comparison between the data and a set of values corresponding, under

,r the test conditions, to those given by a serviceable example of the selected type of circuit or circuit element under test.