by DWIGHT E. PATRICK
If fate tosses a glitch you can't see, stretch it and let your scope catch it!
THE SIMPLE CIRCUIT OF THE SCOPE GLITCH STRETCHER uses only one 74LS221 dual one-shot, one 74LS86 quad exclusive OR (EXOR), and one 74LS32 OR to stretch TTL-level glitches and narrow pulses, that might otherwise be invisible on a scope. The stretcher is especially useful in applications where a narrow pulse has such a low repetition rate that it either fails to trigger a standard scope or triggers the scope and still cannot be observed, a common occurrence. But, unlike many stand-alone glitch-catching probes, which respond to valid as well as invalid signals (making interpretation impossible when invalid signals are mixed in with valid signals), this circuit simply extends the duration of all signals fed to it by a predictable and adjustable amount. However, the duration of time added to an input pulse can be increased (stretched) to any arbitrary and useful value, thus making the Scope Glitch Stretcher usable for driving LED indicators.
The two channels of the Scope Glitch Stretcher allows use with a dual trace scope. In the case of using the device with a single trace scope, one channel may be deleted or used to light a LED readout, while the other channel provides a scope input. Pulses 40-ns wide and wider pulses may be stretched to any desired length, usually the 1 to 10 µs range is about right for most scopes, and the 40-ns figure can be improved by selecting high-speed IC devices which are more expensive.
How it works
The selected positive or negative edge of a signal or signals at inputs Y1 or Y2 trigger one shots, whose outputs are summed with the signal triggering them.
See Fig. 1. Thus, an input signal's duration can be increased. Where the one-shot's duration arbitrarily can be anything we choose, a long duration could be used to light an LED: And, a short duration pulse to added to that of a glitch or short-duration pulse to make it just visible on a scope, where it might otherwise be invisible. S1 and S2 select the triggering edge-positive or negative.
Semiconductors IC1-a, IC2-a, IC3-a and IC 1-b are configured in the same way as IC1-c, IC2-b, IC3-b and IC1-d; therefore, the circuit explanation for one will be the same as for the other.
IC1-a, an Exclusive OR gate acts as a programmable inverter. It senses the negative- or positive-going edge of a square wave input at input Y1, by its output going high when the edge of the input signal goes in the opposite direction, set by S1. That is, when Sus in the NEGATIVE EDGE position, IC1-a's input, pin 2 held high, IC1-a's output pin 3 will go high on the negative edge of input at pin 1. When Si is in the POSITIVE EDGE position, IC1-a's input pin 2 held low, I C1's output pin 3 will go high on the positive edge of input Y1 at pin 1.
FIG. 1. THE SCHEMATIC DIAGRAM FOR THE SCOPE GLITCH Stretcher is segmented
into three parts, two of which are identical signal channels, and the
power-supply circuit. If you have no need for a second channel to drive
a scope input, use it to Might an LED indicator for snappy troubleshooting
tests and checks.
The 5-VDC supply should be regulated. Use a 7805 chip.
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PARTS LIST
RESISTORS
1/4 watt, 5%, unless otherwise noted
R1, R3-1500 ohms
R2, R4-100,000 ohms, linear-taper potentiometers
R5, R6-4700 ohms
CAPACITORS
C1-500 pf, Mylar (see text)
C2-10 MF, electrolytic (see text)
C3-1 µF, 10 volts, tantalum
C4, C5-0.1 uF Mylar
SEMICONDUCTORS
IC1-74LS86 quad exclusive OR (see text)
IC2-74LS221 dual one-shot
IC3-74LS32 quad OR IC4-7805 5-volt regulator
S1, S2, S3-1 SPST toggle switch
J1-J4-BNC or UHF jacks
BATT1-9-volt transistor battery
MISCELLANEOUS
RV Cement, PC boards, Hardware, Battery holder, Battery Connector Clip, Aluminum or Plastic Chassis box (2 1/2 x 5 x 1 1/2 approx.), wire, hardware, etc.
The following are available from: ETC Company, P.O. Box 29278, Denver, CO 80229.
(1) A complete set of parts for Scope Glitch Stretcher, excluding optional one-shot capacitor switches, including PC board, IC's case, etc.-specify types of I/O connectors BNC or UHF @ $25.00, order #2248-90 (2) A complete set of parts for Scope Glitch Stretcher, including one-shot capacitor switches and capacitors, with all the above-specify types of I/O connectors to BNC or UHF @ $30.00, order #2248-91 (3) A complete set of parts for Scope Glitch Stretcher and Variable Pulse Generator, excluding capacitor switches for Pulse Generator-specify types of I/O connectors BNC or UHF @ $40.00, order #2248-92 (4) A complete set of parts for Scope Glitch Stretcher and Variable Pulse Generator-specify types of I/O connectors BNC or UHF @ $45.00, order #2248-93 Include $3.50 to cover postage and handling. Only checks or money orders will be accepted.
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On the positive-going output of IC1-a's pin 3 into the B input of IC2-a, pin 2, the one-shot will fire. IC2-a's Q output pin 13 will go high for a duration set by its timing resistors and capacitor, RI , R2, and C1, such that time t = C1 x (R1 + R2) x 1n2 = 0.7C x R. The t values (actually pulse widths) are arbitrary and may be set to any value at the user's discretion, where the minimum value of (R1 + R2) is 1200 ohms and the maximum value is 100,000 ohms, and the maximal value of C1 is 1000 F. For the sample values shown for IC2-a, t is approximately 500 ns to 35µs and for the sample values shown for IC2-b, R3, R4, and C2, t is approximately 10 ms to 700 ms. The values in Fig. 1 are selected for discussion purposes only. Note: Should either or both capacitor C1 and C2 be electrolytic (polarized types) connect the positive terminal to pins 6 and/or 14.
IC2-a's pin 13 Q output is summed with IC 1-a's pin 3 output via OR gate IC3-a, IC3-a's pin 3 output is the glitch or pulse time and IC2-a's time, which feeds the programmable inverter IC 1-b, pin 4. IC 1-b is controlled by S1. as was IC1-a, to give the same polarity signals at input Y1. Therefore, the output of IC1-b, pin 6 will be a signal with the same polarity as the input, whose duration has been increased or stretched in time. Also, the similar circuit configuration of IC 1-c, IC2-b, IC3-b, and I C1-d, provides a similar output at I C1-d's pin 8 output.
Application and construction hints The pulse widths of the 74LS221 (IC2) can be any value from nanoseconds to seconds, where the capacitor values are switched, as in Fig. 2. And junk box capacitors will do just fine, since there's nothing critical about capacitor or resistor values.
The minimum pulse width for the 74LS221 is specified at 40 ns, with propagation delays also in the 40-ns range; however, you can help that along by selecting devices that will actually do a lot better than 40 ns.
But, don't get too inventive and substitute two 74121's in place of the 74LS221, without taking into consideration the fact that 74121's are specified to have a minimum pulse width at 50 ns. Also, the LSTTL one-shot has generally better specs over all, compared to the TTL one-shot, while the propagation delays through LTTL devices rules out their use.
On the other hand, when comparing TTL, LSTTL, and STTL 7486 and 7432 devices, you'll find that the propagation delays for the TTL and LSTTL devices are about the same, while STTL devices have smaller propagation delays. 74LS32 and 74LS86 devices have typical delays around 14 ns, while TTL devices have delays around 10 ns and STTL devices have delays around 7 ns. Therefore, your best bet is to go with the LSTTL stuff for the lowest power consumption, which you can select if you want better propagation-delay specs to go with a selected one-shot. Also, if you end up with a real dog of a one-shot, with a long propagation delay through it, you can use one of the extra OR gates in IC3-c or 1C3-d (Fig. 1) to delay the signals input to the summing OR gates IC3-a, pin 2 and IC3-b, pin 5.
If you intend to use optional LED outputs, the LED's must be the low-current variety, if driven directly from LSTTL, albeit you could use the extra OR gates IC3-c and IC3-d (Fig. 1) to increase drive capability.
And the whole circuit can be built on a small board, which can be placed in a metal mini-box about the size of a pack of cigarettes if you wish. Amphenol 31-102/ UG657U BNC or 83-878 UHF connectors should be used for input and output, with the circuit plugged in between scope and suitable X1 probes.
---------- HERE'S AN INSIDE VIEW OF AUTHOR'S PROTOTYPE Scope Glitch
Stretcher with visible parts identified. PC board is cemented to bottom
of plastic case-no shorting problem here.
FIG. 2--IF YOUR NEEDS DEMAND MANY DIFFERENT PULSE widths, consider adding
a capacitor switch. A miniature rotary switch works fine. Information
in parentheses are for channel-2 circuit point in the schematic diagram.
FIG. 3--TECHNIQUE FOR CONNECTING TWO ONE-SHOT IC'S to form an astable test circuit. The IC's may be any combination of 74LS221, 74121, 74LS121, 74122, 74LS122, 74123, 74LS123, et al, as discussed in text.
Notes and optional circuits
If you don't have a good fast-rise-time pulse generator to test the circuit, you can use two interconnected one-shots as in Fig. 3. The one-shots form a self-starting multi-vibrator whose frequency and pulse width are adjustable, via their timing networks. Pulse width is controlled as described earlier. Also, several one-shot IC's may be used, which include 74121, 74LS121, and 74LS221, with t = 0.7CR and 74122, 74LS122, 74123, 74LS123, et al, with t = K RC (1 + 0.7/R), where K = 0.32 for the 74122 and 0.37 for the 74123.
The switch in Fig. 2 may be added for wide range operation, and independent pulse width and frequency control can be added with a third one-shot IC. Also, for minimum t, the one-shots can be selected, as outlined earlier.