Compensation indicator for attenuation probe

A circuit including a peak detector detects the distortion of the leading corner of a square-wave reference signal in the over-compensated condition of an attenuator probe and turns on an indicator light. The light turns off as the probe is adjusted to the properly compensated condition because the distortion is no longer detected.

BACKGROUND OF THE INVENTION 
This invention relates to impedance-matching devices in general, and in 
particular to devices for indicating the proper adjustment of a 
frequency-compensated attenuation probe. 
Attenuation probes are generally associated with oscilloscopes; however, 
they may be used as a means for coupling an input signal to any electronic 
test and measurement instrument. A typical method of compensating the 
probe is to connect it to an oscilloscope and apply a precise square-wave 
reference signal thereto while viewing the resultanting signal on the 
oscilloscope screen. Distortions in the square-wave signal produced by the 
impedance mismatch are indicated on the wave form by either peaked or a 
rolled-off leading edge of the square-wave. The probe may be adjusted 
while watching the display to adjust the square-wave to a square leading 
corner. 
Other electronic instruments having wide-band capability, such as events 
counters and the like, have come into wide usage, and such instruments 
lend themselves to usage of an attenuation probe therewith to reduce the 
amplitude of an input signal to a usable level. However, these instruments 
do not have a display device for viewing an input waveform and it is 
difficult to properly compensate the probe without the use of an 
oscilloscope. One solution to this problem has been described in U.S. Pat. 
No. 4,070,615, which patent is assigned to the assignee of the present 
invention. This patent teaches the application of a square-wave signal to 
both inputs of a differential amplifier, one input receiving the signal 
directly and the other input receiving the signal via an attenuation probe 
and input circuitry of an electronic instrument. Any impedance mismatch 
causes an output from the differential amplifier which is rectified and 
filtered to produce a control voltage to be applied to an indicator light. 
A drawback to this system is that the indicating circuitry has the 
undesirable effect of loading down the input. Furthermore, proper 
compensation of the probe is indicated when the light is its brightest, 
and thus accuracy is dependent upon the judgment of the person 
compensating the probe. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a compensation indicator for an 
attentuation probe is provided in which proper compensation of the probe 
is indicated when a light turns off during the compensation adjustment. A 
square-wave signal is applied via an attenuation probe and input circuitry 
of an electronic instrument to a peak detector. The peak detector 
iteratively detects the peak voltage of the square-wave signal. When a 
detected peak voltage is greater than that previously stored, an indicator 
light is turned on. When a detected peak voltage is less than or equal to 
a previously stored voltage, the indicator light is turned off and the new 
peak value stored in place of old one. Thus, an attenuator probe may be 
adjusted from an over compensated condition while watching the indicator 
light, which turns off when the properly compensated condition is reached. 
It is therefore one object of the present invention to provide indication 
of proper compensation during the adjustment of attenuation probe. 
It is another object of the present invention to provide a compensation 
indicator for an attenuation probe which does not add substantial 
additional circuit components to the input circuit of an electronic 
instrument. 
It is a further object of the present invention to provide a compensation 
indicator for an attenuation probe which does not add additional impedance 
loading to the input circuit of an electronic instrument. 
It is an additional object of the present invention to provide a method for 
compensating an attenuation probe without the use of an oscilloscope. 
Further objects, features, and advantages will be apparent from 
consideration of the following description taken in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a preferred embodiment of the present invention, and may be 
described in connection with the waveforms of FIGS. 2A-2C. A reference 
signal having a constant amplitude and frequency is generated by a 
square-wave generator 10. The square-wave reference signal, the duty cycle 
of which is not critical and may be any value, is applied via an 
attenuation probe 12 to an input buffer amplifier stage 14 of an 
electronic instrument. The probe 12 includes a probe tip 16 connected to a 
jack 18, and further includes at least one internal resistor 20 shunted by 
a capacitor 22, and an adjustable capacitor 24. The probe 12 is connected 
to the input circuit 14 via input connectors 26 and 28. Input connector 26 
passes the square-wave reference signal, while input terminal 28 provides 
a ground connection for the probe. The input buffer amplifier 14 may 
suitably include a first field-effect transistor 30 and a second 
field-effect transistor 32 serially connected between suitable sources of 
positive and negative voltage. Field-effect transistor 30 provides a high 
input impedance to the signal, and behaves as a source follower. 
Field-effect transistor 32 provides a current source for operation of 
field-effect transistor 30. The input impedance of input buffer amplifier 
14 is established by the parallel combination of a resistor 34 and a 
capacitor 36. Resistor 34 may suitably be a one-megohm resistor, and the 
value of capacitor 36 may be chosen to provide a predetermined frequency 
response. Probe 12 may suitably be an attenuation probe having any 
predetermined attenuation ratio, for example, 5:1, 10:1, 100:1, etc. The 
value of resistor 20 inside the probe is chosen to provide the proper 
voltage divider ratio in conjunction with resistor 34. Capacitors 22 and 
24 may be chosen to provide the appropriate capacitance divider ratio in 
conjunction with capacitor 36 to provide proper frequency compensation for 
the probe. That is, the impedance of probe 12 should be matched to the 
impedance of input buffer amplifier 14 to provide best frequency response 
so that an input signal is not distorted because of impedance mismatch. 
Capacitor 24 is typically adjustable through a 360.degree. range to permit 
such probe to be operated with a number of electronic instruments. An 
operator connects the probe and adjusts the capacitor 24 for best 
compensation. Such probe 12 and input buffer amplifier 14 have been 
utilized for many years and are well known to those skilled in the art. 
The square-wave reference signal at the output of input amplifier 14 on 
line 40 may appear as one of the wave-forms shown in FIGS. 2A-2C. The wave 
form of FIG. 2A is an over-compensated square-wave from an improperly 
compensated probe. Here, the leading edge overshoots the square-wave 
amplitude and thus has a peaked leading edge FIG. 2B is a properly 
compensated square-wave signal from a properly compensated probe. That is, 
the square-wave signal is not distorted at all. The waveform of FIG. 2C is 
an under-compensated square-wave in which the leading edge rolls off to 
the desired amplitude. As the capacitor 24 of the probe is rotated through 
its range, all of the waveforms of FIGS. 2A-2C can be obtained; however, 
the probe is properly compensated when the waveform is undistorted as 
shown in FIG. 2B. 
The output of input amplifier 14 is applied to a peak detector circuit 44. 
The peak detector 44 in this embodiment comprises a comparator 50, a 
flipflop 52, and a digital-to-analog converter 54. The signal from 
amplifier 14 is applied via line 40 to the inverting input of comparator 
50, where such signal is compared with a triggering level voltage applied 
from the digital-to-analog converter 54 to the non-inverting input 
thereof. When the signal voltage exceeds the triggering level voltage, the 
output of comparator 50 switches, producing a negative-going clock edge 
which is applied to flipflop 52. The flipflop 52 switches states each time 
a peak signal is detected. A control circuit 60 monitors the output of 
flipflop 52, and generates a digital signal which is applied to the 
digital-to-analog converter 54 to change the triggering threshold by some 
predetermined increment. Thus, it can be discerned that if it is desired 
to detect peak voltage, a repetitive signal is applied to the inverting 
input of comparator 50, and the comparator switches each time the signal 
voltage exceeds the trigger threshold from the digital-to-analog converter 
54. The control circuit 60 then causes the output of converter 54 to 
ratchet incrementally toward the peak voltage until the comparator ceases 
to switch on each cycle. At that point, the trigger threshold voltage is 
equal to the peak amplitude value of the input signal. The control circuit 
60 may suitably include a memory device which stores the value of each 
previous trigger threshold, and this information may be made available as 
needed within the electronic instrument. Also connected to control circuit 
60 is an indicator lamp circuit which may comprise a light-emitting diode 
diode 70, a resistor 72, and a transistor switch 74 serially connected 
between a suitable source of positive voltage and ground. A control signal 
is applied from the control circuit 60 through a resistor 76 to the base 
of transistor 74 to cause the switching thereof. 
The peak detector circuit 44 and the control circuit 60 may be a portion of 
signal conditioning circuits within an electronic instrument, for example, 
such as circuits which might be found in an events counter or the like. 
Thus, a peak detector other than the one shown may be suitable, depending 
upon its intended function within the instrument. Control circuit 60 may 
suitably be an arrangement of logic gates coupled to a memory, or in a 
more sophisticated form, it may be a microprocessor or the like. The 
light-emitting diode 70 may be energized to indicate a triggered 
condition. The control circuit 60 stores the value of the trigger 
threshold, and as long as signal peaks are being detected as indicated by 
the switching of flipflop 52, the control circuit 60 applies a positive 
voltage to the base of transistor 74, holding the transistor on and 
energizing the indicator light. Similarily, when the detected voltage is 
equal to or less than the stored threshold value, as indicated by the lack 
of switching of flipflop 52, the indicator light 70 is switched off. 
For probe compensation, the circuit operates as follows: the triggering 
level voltage at the non-inverting input of comparator 50 is initially set 
to some high voltage level V.sub.0, which may be the upper voltage limit 
of the triggering window. Under control of the control circuit 60, the 
triggering threshold is lowered until a peak voltage of the reference 
signal is detected. No matter how the probe 12 is adjusted, the detected 
peak voltage will be either V.sub.1 or V.sub.2. The person compensating 
the probe will know that this condition has been reached because the 
indicator light 70 will come on momentarily and then turn off as the 
threshold level is adjusted toward the detected peak value. The person 
compensating the probe now makes the compensation adjustment while 
watching the indicator light. For example, if the probe is initially 
under-compensated, the light will remain off until the leading edge of the 
square-wave signal is raised above the detected and stored peak level 
V.sub.2, indicating an over-compensated condition. Of course, at this 
point the indicator light will come on, and the probe can be adjusted 
until the light turns off, at which point the waveform will be as shown in 
FIG. 2B, indicating a properly compensated probe. If the probe is 
initially over-compensated, detecting and storing V.sub.1 as shown in FIG. 
2A, as the compensation adjustment is made, the probe will actually be 
adjusted to the under-compensated condition, and the voltage level V.sub.2 
will be detected and since V.sub.2 &lt;V.sub.1, V.sub.2 will be stored, 
replacing the V.sub.1 value previously stored. This will be indicated by 
the light 70 turning on momentarily and then turning off as before. Proper 
compensation is then adjusted from the under-compensated condition to the 
over-compensated condition and back as previously described. 
It will be obvious to those having ordinary skill in the art that many 
changes and modifications may be made in the details of the 
above-described embodiment of the present invention. For example, a 
different type of buffer amplifier or peak detector could be employed 
without changing the basic operating principles of the system hereinabove 
described. Therefore, the scope of the present invention should be 
determined only by the following claims.