Triggering for waveform recording

A drop out trigger captures waveform data when a normally continuous input signal is disrupted or interrupted. In the preferred embodiment, a conventional trigger detects a parameter periodically satisfied by the continuous signal and resets a timer whenever a trigger event occurs. The alarm count of the timer is set for an interval slightly larger than the periodic interval. If the timer reaches the alarm count, a measurement signal is generated.

BACKGROUND AND SUMMARY OF THE INVENTION 
The invention relates to waveform recorders and in particular to a means 
and method for triggering a waveform recorder upon the disruption or 
interruption of a normally continuous signal. 
Waveform recorders are general purpose, time domain measuring instruments 
that take samples of analog waveforms and store the sample values as 
digital data suitable for analysis by a computer. Waveform recorders are 
generally suited to measurement of both transient or single-shot waveforms 
and continuous or repetitive waveforms. Waveform recorders typically 
acquire a large amount of data over a short period of time to capture a 
representative sample of a repetitive waveform or to record the occurrence 
of a single-shot event. 
Since the waveform is captured in a digital memory, all digital waveform 
recorders have some type of pretrigger. Typically, the memory records data 
continuously in a circular configuration, recording new data over existing 
data until the trigger stops the recording. An adjustable delay may be 
used after the trigger to change the time frame of the data captured. For 
example, with zero delay, the recorder will capture waveform data that 
occurred before the trigger event, so the cause of the trigger event can 
be investigated. With a delay equal to the duration that can be stored in 
the memory, the captured data will start at the trigger event. With longer 
delays, data at some time after the trigger event can be captured. 
One of the most important aspects in designing a digital waveform recorder 
is to trigger accurately and properly. Converting the data to digital form 
has made setting the trigger level both accurate and repeatable. The 
gintroduction of hysteresis trigger controls, which vary the width of the 
band that the signal must go through before a trigger can occur, has made 
it possible to trigger reliably on small signals or on large signals with 
large amounts of noise. However, all triggers have required the occurrence 
of an event in which the signal crosses the preset trigger level. Thus it 
has been difficult to record waveforms that are characterized by the 
non-occurrence of an expected event, where a normally active signal 
becomes inactive or altered, because there has been no way to 
automatically trigger on such a non-event. 
The area of computer disc testing has made clear the limitations and the 
disadvantages of conventional triggering techniques. The electromechanical 
analysis of the head positioning servo system, the read/write electronics 
and the magnetic material on the disc requires a search for and isolation 
of infrequent errors and imperfections. An error is characterized by a 
"drop out", a missing pulse in an otherwise continuous string. In the 
past, these dropouts were located by testing a checkword at the end of 
each sector of the disc. An error condition in the checkword was a 
pretrigger for recording the signal from the defective sector--256 bytes. 
Then the recorded data was sent to a computer and a pattern recognition 
routine run to locate the error. 
In many other measurement situations a "drop out" trigger would make 
waveform recording possible or at least greatly simplify the procedure. In 
industrial power distribution, a device with high in-rush current can 
cause a temporary drop in line voltage, adversely affecting other devices 
on the same main, especially computers. It has been difficult or 
impossible to capture such a disruption of a continuous signal with 
conventional triggers. However, a drop out trigger could trigger on the 
drop in line voltage for one cycle, allowing the events surrounding the 
line voltage drop to be captured and studied. Carrier loss or distortion 
in communication systems, and sync loss or distortion in video 
transmission systems could similarly be captured using a waveform recorder 
equipped with a drop out trigger. 
The invention provides means and a method for achieving a "drop out" 
trigger function. According to a preferred embodiment in accordance to the 
invention, the drop out trigger is realized by adding a timer circuit to a 
conventional trigger detection circuit. The timer monitors the elapsed 
time between trigger events. The conventional trigger detects a parameter 
periodically satisfied by the continuous signal and resets a timer 
whenever a trigger event occurs. The alarm count of the timer is set for 
an interval slightly larger than the periodic interval. Whenever a trigger 
event occurs, the timer gets reset. If the timer reaches the alarm count, 
a measurement signal is generated which can be used to stop the 
measurement and view the waveform or to begin another countdown timer 
interval.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a schematic diagram of the elements of a waveform recorder 10 
which incorporates the trigger circuit of the invention. Input signals for 
recorder 10 are applied to signal conditioning circuitry 12, which can 
include multiple sensitivity ranges and high impedance circuits. The input 
signals then pass to an analog to digital converter (ADC) 14 which 
transforms the analog input signals into digital form for storage in 
memory 18. Memory 18 records the digital signal data continuously in a 
circular configuration, so that when the memory is filled, the entering 
data is written over the data already stored. Memory 18 is generally 
rather small and fast, on the order of 16K-256K words, thus depending on 
the sampling rate, only data for a short time interval can be captured. 
In order to capture the data for a particular event of interest, trigger 22 
is programmed to produce a trigger signal on the occurrence of some aspect 
of the input waveform that is characteristic of the event to be captured. 
In response to the signal from trigger 22, memory controller 24 stops the 
flow of data into memory 18, so the memory holds the last iteration of 
data written into memory, which is the data from the event of interest. 
Depending on the delay from the trigger signal to the stop writing 
command, data prior to, subsequent to, or surrounding the trigger event 
will be captured. 
Oscillator 16 generates a clock signal for controlling the sampling 
interval of ADC 14 and for use by timebase 20 to produce clock signals for 
trigger 22 and for memory controller 24. 
Triggering 
The waveform recorder 10 has a variety of conventional triggering modes, 
based on the amplitude or the slope of the input waveform. For example, by 
using hysteresis bands, the trigger can be set to occur when the input 
signal crosses a threshold voltage in a positive direction, a negative 
direction, or either direction. Alternatively, by not using hysteresis 
bands, the trigger can be set to occur when the slope of the input signal 
goes above a certain programmable threshold value or below a programmable 
threshold certain value. The hysteresis bands reduce sensitivity to noise. 
The drop out trigger of the invention can be used in conjunction with any 
of these conventional trigger modes in the implementation of FIG. 1. 
In its most basic form, the dropout trigger is comprised of a conventional 
trigger circuit 201, a reset timer 203, and a control circuit 205, 
configured as shown in FIG. 2. The conventional trigger 201 produces an 
output pulse on line 209 each time the input signal satisfies the selected 
trigger condition. These trigger output pulses are applied to the reset 
terminal 211 of timer 203, so that the timer is reset every time the input 
signal satisfies the trigger condition. Thus, the timer 203 monitors the 
time between trigger events. Control circuit 205 sets the timer alarm 
count, i.e., the length of time until the timer count will overflow. If 
the timer reaches the predetermined alarm count set by control 205, the 
drop out trigger generates a "trigger ocurred" signal that is sent to the 
memory controller. The memory controller then stops the measurement after 
the appropriate post-trigger delay period. 
The operation of the dropout trigger shown in FIG. 2 is illustrated in FIG. 
3. The drop out trigger is configured to detect and capture any disruption 
of normally continuous signal 31. The threshold level 33 of trigger 201 is 
set just below the expected peak amplitude of signal 31 and trigger 201 is 
set to produce an output pulse when signal 33 goes through threshold 31 in 
the positive direction. The alarm count of the timer 203 is set to an 
interval t slightly longer than the period T of signal 31. 
As long as the amplitude and the frequency of signal 31 remain constant, 
trigger 201 will reset timer 203 before the alarm count is reached, and 
the drop out trigger will not produce a measurement signal. However, if 
the amplitude of signal 31 decreases and the signal does not cross the 
threshold level for one or more cycles, as between trigger points 35 and 
37, timer 203 reaches the alarm count after time t and produces a "trigger 
occurred" signal, as at point 39. 
FIG. 4 shows an alternative embodiment for the drop out trigger adapted for 
analog input signals. In this embodiment, the drop out trigger is 
configured to produce an output signal when the input signal crosses 
either of two programmable threshold in either direction. Like the first 
embodiment, if the programmed trigger event does not occur for an interval 
that is longer than the alarm count interval programmed into the timer, 
the drop-out trigger will produce a "trigger occurred" signal. 
The analog input signal is applied to the inverting input of comparator 407 
and to the noninverting input of comparator 409. A programmable low 
threshold voltage is applied to the noninverting input of comparator 407. 
A programmable high threshold voltage is applied to the inverting input of 
comparator 409. The output of comparator 407 is applied to the D input of 
D flip-flop 411 and to one input of exclusive or gate 415. The Q output of 
flip-flop 411 is applied to the other input of exclusive or gate 415. 
Similarly, the output of comparator 409 is applied to the D input of D 
flip-flop 413 and to one input of or gate 417. The Q output of flip-flop 
413 is applied to the other input of or gate 417. The outputs of both or 
gates 415 and 417 are connected to the reset terminal of timer 421. The 
sample rate clock signal from timebase 20 is applied to the clock inputs 
of flip-flops 411 and 413, and to timer 421. 
The operation of the drop-out trigger of FIG. 4 is illustrated in FIG. 5. 
The trigger is configured to capture data if the input signal 51 does not 
cross either the high threshold 53 or the low threshold 55 for a 
predetermined interval. 
Each time the input signal 51 passes through the low threshold 55 the 
output of comparator 407 will change sign. This makes the input to the 
exclusive or gate on line 423 different from the one on line 425, and the 
exclusive or gate 415 turns on producing a reset pulse for timer 421. On 
the next clock pulse, flip-flop 411 switches its Q output to the same 
state as its D input, and exclusive or gate 415 turns off. Similarly, each 
time the input signal passes through the high threshold 53, the output of 
comparator 409 changes state, and exclusive or gate 417 produces a reset 
pulse for timer 421. 
If the alarm count of timer 421 is set slightly longer than the longest 
expected interval between threshold crossings, e.g., t' as shown in FIG. 
5, the timer will be reset as long as the input signal behaves normally, 
and the drop out trigger will not produce a measurement signal. 
However, if input signal 51 remains above the high threshold, as at 54, 
below the low threshold, as at 56, or between the thresholds, as at 58, 
longer than the alarm count interval programmed into the counter 421, the 
drop-out trigger will produce a measurement signal. 
This triggering mode is particularly useful in finding out how an 
oscillator or clock signal is gated off, or in capturing instances of 
random missing pulses.