Circuit test apparatus

A fixed amplitude test waveform is concurrently applied to a reference circuit and a circuit under test that is supposed to be substantially identical to the reference circuit. The waveform comprises a composite signal whose frequency varies through a range of values during each period and is produced by a generator whose effective impedance also varies in accordance with the variation in frequency. The voltages developed at the test points of the reference and test circuits are compared and the difference between them, which is a measure of an undesired condition of the test circuit, is displayed.

The present invention relates to test apparatus for comparing a known 
circuit to an unknown circuit. Circuit test apparatus for comparing a 
known circuit to an unknown circuit are in wide use. Generally, these 
testing circuits supply a known signal in accordance with the operating 
characteristic of a given circuit to the input of that circuit. That 
circuit then processes the signal in a desired manner. That is, the output 
of the circuit in response to the supplied input signal has certain 
expected characteristics. This output signal of the circuit under test is 
compared with a signal generated synthetically or to the output of a 
standard circuit which has an identical input signal applied thereto. 
These types of testing circuits do not have universal application but must 
be established for a given circuit configuration to be tested. Such test 
apparatus can be relatively complex and costly. 
In accordance with the present invention, a circuit test apparatus compares 
a known circuit to a reference circuit by applying a test signal to any 
desired circuit location without destructive failure of any of the circuit 
components. The test apparatus is relatively easy to use and provides 
rapid fault information. The apparatus generates a test waveform 
comprising a composite signal whose frequency varies through a range of 
values during each period and is produced by a generator whose effective 
impedance varies in correspondence with the variations in frequency. The 
voltages developed at the test point of the reference and test circuits 
are compared and the difference between them, which is a measure of an 
undesirable condition of the test circuit, is developed.

In the drawing, oscillator 10 provides a square wave signal having a 
relatively high frequency f.sub.o. This signal is applied to the frequency 
divider 12 which derives from this signal a plurality of square wave 
signals f.sub.1 -f.sub.6, each at a different frequency which is a 
fraction of the frequency f.sub.o. Signals f.sub.1 -f.sub.4 are applied to 
terminals 14, 16, 18 and 20, respectively, of switch 22, these terminals 
being engagable by wiper 24. In practice, the switch 24 (and the other 
switches) may be electronic switches; however, they are shown as 
mechanical devices for ease of explanation. Wiper 24 is connected as one 
input to operational amplifier 28 through terminal 26. Wiper 24, which is 
continuously rotated, selectively connects each of terminals 14-20 to 
terminal 26 in a repeating sequence. The output of operational amplifier 
28 is applied to wiper 30 of switch 32, wiper 34 of switch 36 and wiper 38 
of switch 40. The other input of operational amplifier 28 is connected to 
switch terminals 42, 44, 46 and 48 through respective capacitances 50, 52, 
54 and 56 of different values. The wiper 38, which rotates in synchronism 
with wiper 24, contacts the terminals 42-48 in the same sequence as wiper 
42 contacts its terminal 14-20. 
The operational amplifier 28 and its feedback circuit form integrator 57. 
This integrator 57 transforms the square wave signal it receives at 
terminal 26 into an approximation of a sinusoidal signal 60. By way of 
example, signal f.sub.0 can be 82kHz, signal f.sub.1 can be 5120Hz, 
f.sub.2 can be 640Hz, f.sub.3 can be 80Hz, and f.sub.4 can be 20Hz and the 
corresponding capacitances may have values of 50 (for f.sub.1)=750 
picofarads (pF), 52 (for f.sub.2)=6000pF, 54 (for f.sub.3)=47,000pF and 56 
(for f.sub.4)=200,000pF. 
Switch control 62 operates in response to signals f.sub.5 and f.sub.6 
generated by divider 12 for switching the wipers 24 and 38 in unison. This 
switching control is represented by the dashed lines. 
The wiper 24, in the position illustrated, contacts terminal 18 so that 
signal f.sub.3 is applied to the operational amplifier 28. During the same 
time interval, wiper 38 connects the output of amplifier 28 to the other 
input of amplifier 28 via capacitance 54. 
Switch control 62 preferably is formed with micro-circuits on an integrated 
circuit chip. Signals f.sub.5 and f.sub.6 are square wave signals which 
may be at frequencies of 10Hz and 5Hz. Switch control 62 derives switch 
control signals from these two input signals for operating the wipers 24 
and 38. The wiper 24 represents an electronic device operated by a 
two-by-four encoded signal derived from the signals f.sub.5 and f.sub.6. 
That is, a binary two bit signal provides four encoded states. For example 
the derived signal may have codes 00, 01, 10 and 11. Each code represents 
a switch connection to a different one of terminals 14, 16, 18 and 20. 
Such electronic switching devices are commercially available and, for 
example, may be an RCA CD 4052A. 
Preferably, the highest of frequencies f.sub.5 and f.sub.6, e.g., 10Hz, is 
about one-half of the lowest of frequencies f.sub.1 -f.sub.4, e.g., 20Hz, 
signal f.sub.4. This permits at least one cycle of the signal 60 to be 
produced at the lowest frequency. While at least one cycle is preferred, 
signal 60 may be effective with less than one complete cycle. 
The wipers 30 and 34 operate in phase and concurrently with the wipers 38 
and 24. Wiper 30 is selectively connected to contact the terminals 64, 66, 
68 and 70 and wiper 34 is selectively connected to contact terminals 72, 
74, 76 and 78. The terminals 64-70 are each connected through 
corresponding resistances 80, 82, 84 and 86 to common terminal 88. The 
resistances 80, 82, 84 and 86 thus are sequentially connected between 
switch 32 wiper 30 and terminal 88. The terminals 72-78 are each connected 
through corresponding resistances 90, 92, 94 and 96 to common terminal 98. 
Thus, the resistances 90-96 are connected concurrently in the same 
sequence between the switch 36, wiper 34 and the terminal 98 as 
resistances 80-86 to switch 32. Only one of the resistances 80-86 is 
connected to the wiper 30 at any one time. In similar fashion, only one of 
the resistance 90-96 is connected to the wiper 34 at any one time. The 
corresponding resistors in the two banks are of substantially equal value. 
By way of example, resistance 80 and 90 may be 220k ohms. Resistances 82 
and 92 may be 22k ohms. The resistances 84 and 94 may be 2200 ohms and 
resistances 86 and 96 may be 100 ohms. Thus the relatively high frequency 
signal f.sub.1 is applied through a relatively high impedance resistances 
80 and 90 while the relatively low frequency signal f.sub.4 is applied to 
relatively low impedance resistances 86 and 96. 
Wipers 30 and 34 are operated by switch control 100, which control is 
represented by the dashed line. Switch control 100 is identical to switch 
control 62 and receives two input control signals f.sub.5 and f.sub.6. 
Switches 32 and 36 also, in the alternative, may be electronic switches 
operated by the same binary encoded signal generated by a single control. 
The code may be the same as described above in connection with the control 
62. 
Wipers 30, 24, 34 and 38 are operated by switch controls 62 and 100 in 
ganged fashion such that when the wiper 30 contacts terminal 64, wiper 34 
contacts terminal 72, wiper 38 contacts terminal 42 and wiper 24 contacts 
terminal 14. The wipers sequentially contact each of the terminals of 
their corresponding switch assemblies in synchronism continuously. In this 
embodiment the wipers are switched through all of the terminals at 5Hz or 
five cycles each second. 
Thus, the signal 60 is a composite signal whose frequency varies during 
successive time intervals of each period from f.sub.1 to f.sub.2 to 
f.sub.3 to f.sub.4 in correspondence with the rotation of the wipers 
through the four switch positions. The signal 60 is continuous since there 
is substantially negligible delay time between the connection of each of 
the switch terminals to the wipers. The identical signal 60 appears on 
each of the wipers 30 and 34. 
Common terminal 88 is connected to circuit probe 106 and common terminal 98 
is connected to circuit probe 108. The standard circuit 102 and the unit 
under test (UUT) 104 are each represented by the parallel combination of a 
resistance and capacitance connected between the test point and ground. 
These circuits may also include inductive components to ground (not 
shown). Terminals 88 and 98 are connected as separate inputs to 
differential amplifier 110. The output of amplifier 110 is supplied as an 
input to the vertical drive of oscilloscope 112. The output of operational 
amplifier 28, signal 60, is applied to the horizontal drive of 
oscilloscope 112. The differential amplifier 110 may be included in the 
internal circuitry of the oscilloscope 112 and is available in present 
commercial oscilloscopes. 
The resistances 80-86 and 90-96 are of significance in the present system. 
At any particular time, a resistance from bank 80-86 and a resistance from 
bank 90-96 together form two arms of a bridge, and the impedances between 
the test point and ground of the standard unit and the unit under test 
(UUT), respectively, form the other two arms of the bridge. In the 
position illustrated of the circuit, resistors 84 and 94 form the two 
known bridge arms, the known parallel combination of resistors 103 and 
capacitor 107 form the third bridge arm, and the parallel combination of 
resistor 105 and capacitor 109 form the fourth bridge arm. The 
differential amplifier 110 is connected between the nodes 88 and 98 of the 
bridge. Elements 84 and 94 are of the same value and if the impedance of 
the standard unit (STD) between node 88 and ground is equal to that of the 
UUT between node 98 and ground, the bridge is in balance and the output of 
amplifier 110 is zero. 
It is important in the operation of the bridge that there be available, for 
any impedance in the UUT it is desired to test, a value of impedance in 
roughly the same range for the reference arms (the resistors in banks 
80-84 and 90-96) of the bridge. This is to obtain optimum sensitivity from 
the bridge (comparable to the use of different ranges in an ohmmeter). 
Thus, if say the impedance in the UUT which is being measured is purely 
resistive and is say in the range of 100k ohms, then the present test set 
will provide a good "reading" of this impedance when the reference arms of 
the bridge consist of resistors 80 and 90, each having a value of 200k 
ohms. That is, when these two resistors are in the circuit, a relatively 
small difference in value between the impedance in the UUT and that in the 
STD will unbalance the bridge and result in an easily detectable reading 
on the test oscilloscope 112. At other positions of the switches where the 
reference arms differ very greatly in value from the impedances 
(resistances only in this example) being measured, the bridge is much less 
sensitive. For example if in the UUT the resistance being measured is 100k 
ohms and the reference arms are 86 and 96 (100 ohms each), the bridge will 
be insensitive to small differences between the values of the resistances 
in the STD and the UUT. The values of resistance given by way of example 
have been found to provide good performance over a large range of 
impedances in the UUT it is desired to test for. The electronic switches 
forming controls 62 and 100, add in practice, about 100 ohms to the 
circuit. This does not adversely affect the performance of the circuit. 
In the discussion above, it is assumed that only resistance is being tested 
for, however, in practice the STD and UUT bridge arms are more complex. 
For example, the impedance of each arm may be in the form of a resistive 
and a reactive component, where the latter may have a value X.sub.c = 
(1/2.pi.FC), where C is the value of the capacitance, such as 109, and F 
the value of the applied frequency. It is for this reason that in the 
present system it is not only the impedance of the reference arms of the 
bridge that is varied through a range of values but also the frequency is 
varied. As the impedance X.sub.c increases with a decrease in capacitance, 
the optimum bridge configuration is such that small values of the 
capacitance are best tested with reference bridge arms which have large 
values of resistance and with the testing being carried out at higher 
frequency (the higher frequency causes the capacitive reactance X.sub.c to 
be smaller). For example, a capacitance of say a nominal value 100 
picofarads in the UUT would be sensed best in the present circuit with 
resistors 80 and 82 (200k ohms each) in the circuit and with frequency 
f.sub.1 = 5120Hz. Assuming resistors 103 and 105 to be present and to be 
say 1 or 2 Megohms or more, with the numbers given the reference arm 
impedances, 200k ohms each, is in the same general range as the STD and 
UUT bridge arm impedance and small differences between these latter two 
impedances will easily be detected by the circuit. Similarly, with small 
values of mainly reactive capacitive reactance in STD and UUT, the bridge 
works optimally with the reference arms at lower values of resistance and 
with the frequency also lower. 
The value of frequencies f.sub.1 and f.sub.4 at the high and low end of the 
range were chosen empirically. It is found, in practice, that if f.sub.1 
and resistance 80 are made too high, then the circuit is too sensitive and 
slight, relatively insignificant differences in capacitances, produced 
even by differences in lead lengths, produce error signals. The lowest 
frequency, on the other hand, must result in a readable fault signal. If 
the frequency is made significantly lower than 20Hz, then flutter is 
introduced on the oscilloscope 112 display, interfering with the reading. 
If other indicators are used instead, and if flutter is not a problem with 
such indicators, then lower frequencies can be used. 
The value of the frequencies of signals f.sub.2 and f.sub.3 is arbitrarily 
chosen within the range of the extreme frequencies of signals f.sub.1 and 
f.sub.4. The value of resistances 82, 84 and 92 and 94 are chosen to 
correspond to these frequency values. While four frequencies are employed 
in the present system and provide good performance, more or fewer than 
four frequencies could be used. 
In operation, switch controls 62 and 100 automatically switch wipers 24, 
38, 30 and 34 in a given sequence to each of the switch contact terminals, 
repeating the switching action continuously. This action produces a 
continuous sinusoidal signal 60 at the frequencies of signals f.sub.1, 
f.sub.2, f.sub.3 and f.sub.4. This signal is concurrently applied through 
the corresponding resistances 80 and 90, 82 and 92, 84 and 94 and 86 and 
96 in sequence. Thus, identical signals are applied by the probes 106 and 
108 to the assumed identical standard and circuit under test 102 and 104, 
respectively. The signals are applied anywhere to the circuits at 
identical corresponding locations. 
Due to the impedance to ground produced by the various components in the 
circuits under test and the standard circuits, the voltages appearing at 
common terminals 88 and 98 should be identical if the components are, in 
fact, identical. If this is the case then differential amplifier 110 will 
produce a signal at zero volts resulting in a horizontal line on the 
oscilloscope display. Should one of the components in the unit under test 
have an incorrect value or be a short circuit, or the component be placed 
incorrectly in the circuit, then the voltages appearing between the 
terminal 88 and ground and terminal 98 and ground will differ. This 
difference is produced as an output from amplifier 110 which produces a 
signal on the display of oscilloscope 112 other than a horizontal straight 
line. 
Since the signals f.sub.1 -f.sub.4 are derived from the same square wave 
signal source, these signals each have about the same amplitude. There may 
be slight variations in amplitude, but integrator 57 smooths these 
variations. Thus, no high frequency switching transients are produced. If 
the amplitudes were different, then at the times of switching between the 
f.sub.1 -f.sub.4 signals, the difference in amplitudes would result in the 
generation of a high frequency spike. This spike would be passed on to the 
circuits under test. The capacitive components 107 and 109 would generate 
a difference voltage due to their sensitivity at these high frequencies. 
This difference voltage would appear as an erroneous fault condition on 
the display. Thus, by making the signals f.sub.1 -f.sub.4 of the same 
fixed amplitude, erroneous transient signals are avoided. 
Further, in accordance with a feature of the present invention, the voltage 
applied by the probes 106 and 108 has the value within a range which is 
safe to use with most integrated circuits, FET, transistors, and other 
semiconductor devices. This voltage is preferably 1.8 volts peak-to-peak, 
i.e., 0.9 volts positive and negative with respect to ground. This voltage 
generally can be applied to any point in most circuits, including 
integrated types, without destructive failure of components in that 
circuit. 
The following are examples of the visual defects which can be observed. A 
reversed diode or a transistor installed incorrectly with the wrong 
polarity will appear as a horizontal line with several diagonally spaced 
tails at the ends of the horizontal line. An incorrect capacitance appears 
as a tilted ellipse, a solder short across a reactive component appears as 
a tilted loop, an incorrect resistance value appears as a tilted line, and 
an incorrect inductance appears as loops centered about a horizontal line. 
Other components or a combination thereof will have various combinations 
of these and other easily identifiable display curves. In the case of 
resistances, by merely removing the differential amplifier and applying 
the voltage at terminal 98 to the vertical input of oscilloscope 112, the 
incorrect resistance is displayed as a tilted line. The angle of the tilt 
of the straight line can be calibrated to provide an accurate reading of 
the resistance value. 
Additional features can be provided (not shown) for increasing the 
versatility of the apparatus. For example, the input to switch control 62 
and 100 can be connected to a signal source which cause switch controls 62 
and 100 to produce only one of the four encoded binary codes that will 
place the wipers 24, 28, 34 and 30 in a given switch position. Further, 
the common terminals 88 and 98 may be connected through switching devices 
(not shown) to the probes 106 and 108 so that one of the probes is 
grounded and the other is active. This results in the oscilloscope 112 
displaying the signal appearing only on the one circuit. 
There has thus been described a test circuit for dynamically testing and 
comparing an unknown circuit with a standard circuit. The unkown circuit 
can be tested almost at any location within the circuit. Sealed integrated 
circuit chip devices provide suitable test points via the extending leads. 
Tests can be performed readily quickly by unskilled technicians and 
provide accurate rapid checks of complex circuits within a minimum of 
set-up time. The composite sinusoidal signal formed of contiguous signals 
of different frequencies in a continuous wave are applied through suitable 
resistances to standard and unknown circuits for generating two signals 
representing the condition of the two circuits at the point of test. By 
simply comparing the two signals and displaying the compared results for 
visual observation on a suitable display, circuits can be quickly 
analyzed. Further, set-up time is minimized in applying the test signal to 
a circuit under test.