Waveform observing method and apparatus

A waveform observing method and apparatus incorporating signal sampling and digital technologies is disclosed. An input signal to be observed is compared with the output from a digital-to-analog converter and the comparison output is used for controlling display means such as a cathode-ray tube. A wideband input signal can be measured with much higher accuracy than the conventional apparatus.

BACKGROUND OF THE INVENTION 
The present invention relates to a waveform observing apparatus for 
sequentially scanning recording means such as an electron beam or a 
recording pen of a cathode-ray tube, X-Y recorder or the similar planar 
display apparatus for a plurality of times in a predetermined manner and 
modulating the recording means in accordance with an input signal to be 
observed for reproducing the waveform thereof. 
Examples of such waveform observing apparatus are oscilloscopes and 
oscillographs which are being used in various fields. In these 
conventional apparatus, the electron beam of a cathode-ray tube 
(hereinafter referred to as a CRT) or the recording pen moving on a 
recording paper is deflected or moved in orthogonal two directions (X- and 
Y-axes); one (for example X-axis) represents the time axis to which a 
sweep signal varying linearly with time is applied; and the other (Y-axis) 
is controlled by an input signal voltage to be observed. The electron beam 
or the pen position is deflected or moved in accordance with the input 
voltage as a function of time and displaying or recording the trace of the 
input signal waveform on the screen of the CRT or on the recording paper. 
The conventional waveform observing apparatus, however, has disadvantages 
that the use of analog signal processing means limits the measurement 
accuracy to approximately a few percent which is much lower than 0.1 
percent or so required in many high precision applications and that the 
measurements are not in such a form that is convenient for storage or 
further arithmetic processing. 
SUMMARY OF THE INVENTION 
According to the present invention, the use of sampling and digital 
technologies permits the wideband signal measurement of the conventional 
sampling oscilloscope with improved accuracy as high as 0.1 percent or 
better of the conventional digital-to-analog converters (hereinafter 
referred to as a DAC). The displayed waveform consists of a plurality of 
parallel lines densely positioned and modulated in accordance with the 
input signal, so that the modulated boundaries of the parallel lines 
represent the input signal waveform. Digital outputs representing both of 
the signal and timing information are also obtained for digital storage 
for a controllable finite time or for further arithmetic processing. 
The object of the present invention is to overcome the disadvantages of the 
conventional waveform observing apparatus and to provide a unique waveform 
observing method and apparatus of displaying a fast input signal waveform 
with high precision as well as obtaining a digital output, if necessary, 
for recording, calculating, or other processing. 
This invention is pointed out with particularity in the appended claims. A 
more thorough understanding of the above and further objects and 
advantages of this invention may be obtained by referring to the following 
description of a preferred embodiment taken in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
In FIG. 1, an input signal to be observed is applied to the non-inverting 
input terminal of a comparator 12 through an input terminal 10 and, if 
necessary, an input circuit (not shown) including an attenuator and a 
buffer amplifier. An external trigger signal is applied to a terminal 11. 
A reference output voltage from a DAC 14 is applied to the inverting input 
terminal of the comparator 12 as a reference input. The output from the 
comparator 12 is supplied to a Z-axis amplifier 16 for intensity control 
or modulation. The Z-axis amplifier 16 provides its output to an 
appropriate intensity control electrode, for example, the control grid, 
cathode, or blanking deflection plates of an electron gun in a CRT 18 
after amplification to a desired amplitude and polarity. The output of the 
DAC 14 is also applied to the vertical deflection plates of the CRT 18 
after converting it into a push-pull output by a vertical amplifier 20. 
Referring to a timebase circuit 22 in FIG. 1, a push-pull slow ramp or 
staircase signal for driving the horizontal deflection plates of the CRT 
18 is compared with a fast ramp signal generated in synchronism with the 
input signal to be observed and a narrow strobe pulse is generated 
whenever they coincide for driving the comparator 12. A digital processing 
circuit 24 is added to store and arithmetically process the digital 
outputs from the DAC 14 and the comparator 12. 
Now, the operation of the apparatus in FIG. 1 is described by reference to 
FIGS. 2 through 4. Shown in FIG. 2 is the outline of the operation for the 
apparatus in FIG. 1, especially the timebase circuit 22. FIG. 3 shows the 
detailed operation of the apparatus, especially the comparator 12. FIGS. 
4A and B show respectively partial and entire signal waveforms displayed 
on the CRT screen as an example. 
Shown in FIG. 2A is the slow ramp (or sweep) signal from the timebase 
circuit 22 to be applied to one of the horizontal deflection plates of the 
CRT 18. The slow ramp signal is generated at a controllable constant 
period. Shown in FIG. 2B is a reset pulse for the slow ramp signal of FIG. 
2A. Shown in FIG. 2C is a fast ramp signal to be generated in synchronism 
with the input signal to be observed. The frequency of the fast ramp 
signal is several hundreds to several thousands times higher than that of 
the slow ramp signal. Shown in FIG. 2D is a narrow strobe pulse train 
which is generated whenever the fast ramp signal in FIG. 2C coincides in 
amplitude with the slow ramp signal in FIG. 2A. Shown in FIG. 2E is a 
reference voltage output from the DAC 14. The reference voltage output 
increases (or decreases) by one bit unit at every generation of the reset 
pulse (see FIG. 2B) for the slow ramp signal in FIG. 2A. The reference 
voltage waveform in FIG. 2E is also applied to the vertical deflection 
plates of the CRT 18, so that the electron beam of the CRT 18 is 
sequentially scanned across the entire screen from the bottom to the top 
(in a similar manner to a television raster). If no output is applied to 
the Z-axis amplifier 16 from the comparator 12, each scanning line remains 
constant brightness. The entire screen may be either light or dark. 
Actually, a digital voltage, which is either "H" (high) or "L" (low), is 
applied from the comparator 12 to the Z-axis amplifier 16. Each scanning 
line is, therefore, modulated in two tones-either bright or dark-depending 
upon the amplitude relationship between the input signal to be observed 
and the reference voltage from the DAC 14. This will be described in 
detail by reference to FIG. 3. 
FIG. 3A shows one example of the input signal to be observed, which is 
represented by the voltage amplitude (vertical axis) against time 
(horizontal axis). The levels V.sub.1, V.sub.2 and V.sub.3 superimposed 
with the input signal waveform in FIG. 3A are the reference output from 
the DAC 14 in FIG. 2E at different timing. Shown in FIG. 3B is the strobe 
pulse train of FIG. 2D to be applied to the comparator 12. Shown FIGS. 3C, 
D and E are comparison outputs from the comparator 12 at the specific time 
when the reference voltages V.sub.1, V.sub.2 and V.sub.3 are applied from 
the DAC 14 to the comparator 12. That is, the comparator 12 compares the 
input signal to be observed and the reference voltage whenever each strobe 
pulse is applied and latches the comparison output. 
When the reference voltage from the DAC 14 is V.sub.1, the output of the 
comparator 12 remains L during the peroid (t.sub.0 -t.sub.1) and changes 
to H at time t.sub.1 ; thereafter remaining H until time t.sub.n. The 
waveform in FIG. 3C will result at the output of the comparator 12. 
Similarly, when the reference voltage is V.sub.2, the output of the 
comparator 12 is L during the period (t.sub.0 -t.sub.2) and H during the 
subsequent period (t.sub.2 -t.sub.n) as shown in FIG. 3D. When the 
reference voltage is V.sub.3, the transition in the comparison output from 
L to H level takes place at time t.sub.3. As understood from FIG. 3, 
varying the reference voltages from the DAC 14 to V.sub.1, V.sub.2 and 
V.sub.3 will result in shifting the transition timing of the comparison 
output to t.sub.1, t.sub.2 and t.sub.3, respectively. The resulting 
display on the CRT screen will be divided into bright and dark portions as 
shown by solid lines a, b and c in FIG. 4A. A very high resolution can be 
realized because as many as 1024 different reference voltages are 
available if, for example, a ten-bit DAC is used for the DAC 14. FIG. 4B 
shows the display on the CRT screen in such an instance and the borderline 
of the bright and dark areas of the plurality of scanning lines or raster 
will represent the waveform of the input signal to be observed as shown in 
FIG. 3A. 
The timebase of the waveform represented by the borderline of different 
brightness in FIG. 4B is determined by the slope of the fast ramp signal 
in FIG. 2C and not affected by the slow ramp signal in FIG. 2A. This 
allows the apparatus to observe fast and ultra high frequency signals as 
is possible in the conventional sampling oscilloscope. 
Although the above description was made on one example using an 
electrostatic deflection type CRT as the display apparatus, those skilled 
in the art can understand easily that any other planar display apparatus 
can be used. The alternative planar display apparatus includes a storage 
type CRT, and electrodynamic deflection type CRT, and X-Y pen recorder, or 
a display panel consisting of LED (light emitting diode) matrix, liquid 
crystal or plasma display. In addition, the comparison outputs such as 
FIGS. 3C, D and E are applied directly to the Z-axis of the CRT 18 in the 
above mentioned embodiment, but they can be applied through a 
differentiation circuit. The electron beam is then modulated only at one 
portion per every scanning line when the transition occurs in comparison 
output, or at time t.sub.1, t.sub.2, t.sub.3 . . . . The resulting 
waveform will be similar to that in the conventional oscilloscope or 
oscillogram. A monostable multivibrator being triggered at time t.sub.1, 
t.sub.2, t.sub.3 . . . can be used to provide the output to the Z-axis 
circuit. 
It can be understood that the Z-axis signal corresponds to a pen up-down 
control signal if an X-Y pen recorder is used as a display or recording 
apparatus. 
The digital processing circuit 24 in FIG. 1 may be either semiconductor IC 
or magnetic memory circuit. The memory circuit stores the digital signal 
from the DAC 14 (for example the output of a counter for counting the 
number of the slow ramp signals from the timebase circuit 22) and the 
digital ouput from the comparator 12 either directly or by digitizing the 
sweep voltage level at the transition of the comparison output of the 
comparator 12. The stored data can be utilized at any desired time for 
reproducing the signal waveform on an display apparatus. 
If the digital code of the DAC 14 is stored at the instance of the 
transition of the comparison output, the apparatus will serve as an 
analog-to-digital converter for converting instantaneous amplitudes of the 
input signal into digital codes. 
The comparator 12 in FIG. 1 may be the strobed voltage comparator 
commerically available from, for example, Advanced Micro-devices Corp. 
FIGS. 5A and B are examples of the comparator 12. Emitter coupled 
transistor pair 26a, 26b constitutes a differential comparator and 
compares the signal to be observed and the reference voltage from the DAC 
14 which are applied respectively to the base input terminals 28a, 28b. A 
latch transistor pair 30a, 30b is cross connected to the collectors of the 
transistor 26a, 26b. The emitters of the latch transistor pair 30a, 30b 
are coupled together and output terminals 38a, 38b are connected to the 
collectors of these transistors 30a, 30b. The common emitters of the 
transistor pairs 26a-26b and 30a-30b are connected to a current switch 
consisting of transistors 32a, 32b. The current switch is connected at the 
common emitter electrodes to a current source 34 and receives at base 
input terminals 36a, 36b the strobe pulse generated in the timebase 
circuit 22 in FIG. 1 directly and through an inverter 36c, respectively. 
In operation, under the normal condition before the strobe pulse is applied 
to the terminal 36, the transistors 32a, 32b of the current switch are 
respectively off and on. The latch transistor pair 30a, 30b holds the 
state when the previous strobe pulse was applied. On applying the strobe 
pulse, the current switch is rapidly inverted with the transistor 32a and 
the transistor 32b off. The comparison in amplitude is made at this 
instance between the signal to be observed and the reference signal 
applied to the terminals 28a 28b, respectively. The resulting voltage 
difference appearing across collectors of the comparison transistors 26a, 
26b is amplified by the latching transistors 30a, 30b constituting a 
positive feedback amplifier when enablied at the termination of the strobe 
pulse. This latches (holds) the voltage at the output terminals 30a, 30b 
to either H or L state. The similar operation is repeated hereinafter. 
Since the comparator circuit in FIG. 5B is essentially identical in 
construction and operation to the circuit in FIG. 5A, its detailed 
description will be omitted. Primary differences therebetween are that the 
common emitters of the transistors 30a, 30b are not only connected to the 
collector of the transistor 32b constituting the current switch in 
combination with the transistor 32a but also prebiased through a resistor 
39 and an emitter follower stage 40a, 40b is inserted at the input side of 
the latching transistor pair 30a, 30b. Such differences provide remarkable 
improvement in the response time and permit a fast signal to be observed 
because the comparison can be made by a narrow strobe pulse. 
Although the foregoing description was made on one example for observing a 
fast, ultra high frequency signal by equivalent time signal sampling 
techniques, this invention is not intended to be limited to such 
embodiment only but can be applied to a real time display apparatus for 
observing a low frequency signal. 
In such a case, the slow sweep signal from the timebase circuit 22 as shown 
in FIG. 2A is generated in synchronism with the signal to be observed and 
the binary output (H or L) from the comparator 12 is used to control the 
Z-axis of the recording apparatus. Since the vertical deflection signal 
int this instance is low frequencies as shown in FIG. 2E. Thus, any 
problems such as drift, distortion, and nonlinear frequency response can 
be solved because the input signal to be observed cannot be amplified to a 
large amplitude up to thirty to fifty volts with high fidelity unlike a 
conventional oscilloscope. 
Since any CRTs generally have 50 MHz or higher bandwidth, they can be used 
for observing considerably high frequency repetitive signals. The use of 
digital circuits makes it possible to store the measurements and 
reproduces then at any desired time. That is, the reference voltage of the 
DAC 14 can be extracted easily as a digital signal thus the timing 
information when the comparator output changes from H to L or L to H can 
also be stored in a digital form by simply digitizing the instantaneous 
values of the sweep signal. 
As understood from the description hereinbefore, the waveform observing 
apparatus according to this invention is completely different in principle 
from any conventional signal display/observing apparatus which process the 
input signal to be observed by analog means such as amplification or 
attenuation for moving a recording means (such as electron beam or 
recording pen) on recording media (such as phosphor screen or recording 
paper) in response to the signal and forming a trace of such signal. That 
is, in the present apparatus, the voltage axis movement of the recording 
means is a low frequency reference signal from a DAC which is closely 
related to the time axis movement. The input signal to be observed is 
compared in amplitude with the reference voltage to provide the digital 
output for controlling the recording means (blanking/unblanking of an 
electron beam, or up/down of a pen) to display the signal waveform by the 
boundaries. This invention ensures highly accurate measurements of 0.1% or 
better determined by the accuracy of the DAC and the comparator and 
eliminates such undesirable problems as waveform distortion and drift due 
to frequency response and nonlinearity of the amplifiers in the 
conventional apparatus. Also, the digital output corresponding to the 
input signal to be observed is available for being stored in a memory 
circuit for a controllable time, so that reproduction of the input signal 
or any other digital processing can be performed at any desired time. In 
addition, processing such as differentiation of the output from the 
comparator will display a substantially continuous trace representing the 
input signal waveform in the similar form in appearance to the 
conventional waveform display apparatus. 
The waveform display apparatus according to this invention may be used for 
various applications for observing repetitive signals but is particularly 
suited for measuring the settling time of operational amplifiers and DACs, 
and operation characteristics of devices or elements including switching 
(or sampling) operations such as, for example, sample-and-hold circuits 
(hereinafter referred to as S/H circuits) in which very high precision 
measurements are required. Such an application is described briefly 
hereinafter. 
Assume that a S/H circuit is operated to sample and hold an input signal 
having a frequency close to or higher than its maximum sampling frequency. 
Then, the input signal varies significantly during two neighbouring 
sampling operations or sampling strobe pulses. The S/H circuit is unable 
to reproduce the input signal accurately on the real time basis. However, 
the apparatus in FIG. 1 can solve this problem. If the sampling operation 
of the S/H circuit is performed by using the strobe pulse as shown in FIG. 
2D, the actual signal sampling takes place once every one or a plurality 
of cycles of the input signal on sequentially delayed point by .DELTA.t 
with respect to the trigger point of the input signal. Thus, the variation 
of the input signal between the neighbouring sampling times is very small. 
The S/H circuit can accurately reproduce the input signal in a lower 
frequency as long as the signal is repetitive. Although not shown in the 
drawings, the circuit arrangement required for this application is to 
connect the S/H circuit to be tested between the input terminal 10 and the 
comparator 12 in FIG. 1. This permits the S/H circuit to be operated at 
around its maximum frequencies for evaluation purposes.