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Here's a sweet-acting single-sweep, blanking circuit packaged to cope with the limitations of your old, but still serviceable oscilloscope!
By D.E PATRICK
MANY SINGLE-SWEEP, BLANKING CIRCUITS HAVE BEEN proposed as add-on options to oscilloscopes like older Dumonts and the Philips PM3210, which have external sweep-ramp or gate outputs and Z-axis inputs, (Fig. 1). However, none match the performance of the built-in circuits they are attempting to mimic, based on disabling sweep or trigger circuits after one sweep in Fig. 2.
Most add-on designs un-blank the scope's CRT for one-sweep cycle via its Z-axis or intensity-modulation input after a reset or new sweep command, then blank the CRT again by triggering off the trailing edge of a sweep-ramp or gate output (Fig. 1). But, most add-on designs have partial display, asynchronous reset, and lock-out problems, Further, little effort has been made to apply those circuits to scopes like the Tektronix T921/T922 or Heathkit 10-4510; the former has a Z-axis input, but no external-ramp or gate output, and the latter has neither external-sweep output nor Z-axis input.
Taking a closer look
With the typical marginal, sweep-blanking circuit in Fig. 1, the oscilloscope's CRT is held blanked via its Z-axis by the flip-flop's Q or outputs. Depressing the RESET switch sets the flip-flop and un-blanks the scope's CRT. After a one-sweep cycle, the negative trailing edge of the sweep-ramp or sweep-gate fires the one-shot, which resets the flip-flop via its clock input. And the flip-flop holds the scope's CRT blanked once again until the next manual reset. However, that (and similar circuits which have appeared in the literature) works when the scope's ramp generator is being triggered only occasionally; otherwise, the asynchronous manual reset causes partial display and lock-out problems. That design, and similar ones, cannot be used with free-running synced scopes; it is sensitive to ramp or gate rates, and generally isn't worth the time to build.
On the other hand, single-sweep disable circuits built into a scope, as in Fig. 2, operate on the principle of disabling trigger or sweep circuits, as opposed to blanking the CRT. They also cannot be used with free-running, un-triggered scopes; and since the trigger and sweep circuits of every oscilloscope are quite different, trying to build a general-purpose circuit would be next to impossible. Further, on some scopes, where major modifications to existing circuits would be necessary, the implementation of such a circuit would be too advanced a job for a neophyte.
In any case, in Fig. 2, when the momentary manual RESET switch SI is pushed and released, ICI-a sets and resets, providing a debounced pulse output at Q (pin 6) and Q (pin 5), where either may be used as the clock input to IC1-b at pin 11. The former case assures that the manual RESET switch S1 must be pressed and released for the action to take place. In the latter case, IC1-b gets set as soon as the manual RESET switch is pressed, which is similar to the configuration used in the Tektronix T922R. (The T922R is the rack-mount version of the T922, but unlike the T922, it has external ramp, gate, and a single-sweep option.) Now, in Fig. 3 we can see that IC1-b sets on the positive edge of IC1-a's Q output, at time t1. At time t2 the oscilloscope's trigger circuit produces a pulse, which in turn causes the sweep gate to go low. (The bar symbol indicates an active low signal.) Sweep gate going low resets IC1-b, and the scope's sweep cycle begins. However, when IC1-b gets reset, the scope's trigger circuits are disabled. Therefore, after the completion of one sweep cycle, there won't be another sweep displayed until the manual RESET or sweep command switch S1 is depressed again, because the trigger circuit itself cannot retrigger the sweep circuit.
Obviously, the difference between Fig. 2 and IB is the fact that the former is inherently "synced-up" and asynchronous reset is an impossibility. However, the partial, single-sweep disable circuit's (Fig. 2) deceptive simplicity belies the fact that it cannot be generally applied.
One that really works
In order tor a single-sweep, blanking circuit to be as effective as a single-sweep, disable circuit, the former must sync-up the un-blanking of the scope's CRT with the start of a sweep cycle. Only one complete sweep cycle, with each RESET or new sweep command request must be allowed.
Partial displays and lock-out conditions must be eliminated. And, the circuit must be relatively insensitive to ramp, or gate, repetition rates.
Fig. 4 is the circuit of the basic Single Sweeper One. It accomplishes all the above by debouncing the reset or sweep-command signal from ICI-a. It stores the command in ICI-b when the RESET switch S 1 is released or an external pulsed reset command is applied to IC 1-b's set input. IC2-b syncs up the reset command and the leading edge of sweep gate or similarly derived pulse, indicating the start of a scope's sweep cycle, unblanking the scope's CRT. IC2-a resets the single-sweep, blanking-control flip-flop (IC2-b), blanking the scope's CRT again on the trailing edge of sweep gate, which indicates that one sweep cycle has been completed.
When IC2-b unblanks the scope's CRT, it simultaneously resets ICI-b, in which the reset command was stored, and when IC2-b begins to blank the CRT again, it simultaneously resets IC2-a, the flip-flop that reset it.
Therefore, at the completion of the above cycle of events, the scope's CRT will remain blanked until either the RESET button S I is depressed and released, or a pulsed reset-command signal initiates another cycle.
PARTS LIST FOR SINGLE SWEEPER ONE
IC1. IC2-7474 dual D edge- triggered flip-flop integrated circuit
IC3-7404 hex inverter integrated circuit
IC4--7805 5-volt regulator integrated circuit
LED1-LED3--Light emitting diode with red diffused lens
-- Q1--2N2222 or 2N3904 NPN transistor
+ Q2--2N2907 or 2N3406 PNP transistor
All resistors are fixed composition, 1/4-watt, 5% components
R1, R2. R3--4700-ohm
R4--100,000-ohm (this value is suitable for sweep gate or ramp voltage levels of 30-volt approximately.
Use 1- Megohm value for higher values.)
R5-1000-ohm R6-470-ohm R7-R9- 470-ohm
C1--100-u.F, electrolytic 10-W VDC
ADDITIONAL PARTS AND MATERIALS
S1-DPDT momentary pushbutton switch (RESET)
S2-SPST toggle switch (ON-OFF)
J1, J2-BNC connector to mate with oscilloscope's patch cable
J3-open- circuit miniature jack (power connection)
Plastic cabinet 4 1/4 x 2 1/2 x 1 1/4-in. printed-circuit board materials, wire, hardware, solder. etc.
"Optional on-board regulated 5-volt DC power supply used with 9-12-volt DC battery eliminator Optional circuit for high-level sweep gates and ramps-may be deleted with TTL-level design and applications.
The following is available from E.T.C., P.O. Box 29278, Denver, CO 80229.
Bare circuit board 510.00. A complete set of parts for the Single Sweeper One per Fig. 4- 540.00. Optional power supply, which includes filter and bypass capacitors, regulator, and battery eliminator which plugs directly into the wall, supplying 9 to 12-volts of unregulated DC to unit-515.00.
Now, that was a little quick; let's do it again, by the numbers this time. See Fig. 4. When the momentary RESET switch Si is pushed and released, IC1-a sets and resets, providing a debounced pulse output at Q (pin 6). On the positive-going edge at pin 6, which coincides with the release of S 1, IC1-b will be set by its clock input (pin 11) going high, with its D input (pin 12) tied high. ICI-b stores or holds the manual-reset or new-sweep command signal, and may also be used to store or hold an external-pulsed reset command applied at IC1-b (pin 10). But, whether by pulse or manual command, IC1-b's Q output (pin 9) going high enables IC2-b D input (pin 12). IC2-b is the single-sweep blanking control flip-flop which will set on the low-to-high leading edge of the sweep gate or similarly derived pulse, applied to its clock input (pin 11). When IC2-b gets set, it unblanks the scope's CRT via its Q (pin 9) or Q (pin 8) outputs. Also, IC2-b's Q (pin 8) going low resets IC1-b via its reset input (pin13). At the end of one scope-sweep cycle, the sweep gate will go low, which is inverted high by IC3-a and applied to the clock input of IC2-a (pin 3). IC2 Q output (pin 6) will go low resetting IC2-b via its reset input (pin 13), blanking the scope's CRT again. Also, the IC2-b Q output (pin 9) going low resets IC2-a via its reset input (pin 1). The scope will remain blanked until a new sweep command is received by manual operation of S 1 or a pulse is applied at IC1-b's set input (pin 10). See the typical timing diagram for a triggered scope using the single- sweep, blanking circuit in Fig. 5.
Operational status LED indicators (see Fig. 6) may be added and driven by the unused outputs of the flip-flops, or unused inverters which are used as buffer drivers. LED1 driven by ICI-a' s outputs would indicate a manual reset.
LED2 driven by IC1-b's outputs would indicate when the circuit was ready for a new command. And LED3 driven by IC2-b's outputs would indicate blanking status.
Where high-level sweep gate or ramp inputs were used, the input signals might be pre- conditioned by a current-limiting resistor followed by a two-transistor scheme. See Fig. 7. However, the pre- conditioning circuit could be used to amplify a signal; but, the use of low-level sweep ramp signals is not advised. Also, the user should avoid loading scope circuits or signals.
Where a scope has buffered ramp and/or gate outputs and Z-axis or intensity modulation inputs, like some Philips, Tektronix, Dumont, et al, oscilloscopes, adding the single-sweep blanking circuit of the Single Sweeper One (Fig. 4) is a simple matter. A "single sweeper" could be built into a mini box and plugged or switched in circuit, as in Fig. 1.
In the case of scopes like the Tektronix T 921/T922, with a TTL active-high and active-low sweep gate (IC- U2234D, pin 11 and IC-U2234A, pin 3, respectively) the blanking circuit in Fig. 4 could be switched in when needed, requiring no input pre-conditioning. However, in other scopes, using TTL logic on the sweep, time base, and /or horizontal amplifier boards, those signals can generally be located by refer- ring to a schematic diagram or user manual. Also, a 5V signal into the Z-axis input on the back of this particular scope will cause a noticeable decrease in intensity, where the maximum input is 50 volts.
Therefore, a low level input into the Z-axis, with front-panel intensity adjustment might be used, or a voltage translation circuit with output-level adjustment could be used. The circuit could be plugged in when needed, if the sweep gate was made an external output or added internally, with suitable switching. Further, you might give some thought to buffering the sweep ramp and gate, bringing them to the back panel as outputs.
In the case of scopes like Heathkit's I0-4510, with TTL active-high and active-low sweep gate (IC-404, pin 9 and IC404, pin 8, respectively) the blanking circuit (in Fig. 4 for the Single Sweeper One) again requires no input pre-conditioning. However, there's no Z-axis input you can tie into, so you'll have to use the blanking-control circuits. In that, and similar applications, where TTL level signals control the blanking amplifier on the deflection circuit board, additional AND or NAND gates may be used to take control of the blanking amplifier.
Now, virtually any scope, where you can pick off the sweep-time-base and which has Z-axis control, intensity modulation, and/or blanking can be modified for single-sweep blanking control with the Single Sweeper One. And we've only covered a few specific cases here; however, with a working knowledge of how your particular oscilloscope operates, the circuit in Fig. 4 can generally be modified to work.
You could either outboard a 5-volt DC supply that is regulated, or pump in approximately 12-volt DC from a plug-in battery eliminator and install a regulated 5-volt DC circuit within the unit. Additionally, you could use a 9 to 12-volt DC battery pack with the regulator circuit. Refer to Fig. 8 for the schematic diagram. The parts for that circuit addition have foil leads and holes provided on the printed-circuit board illustrated in Fig. 9. The location of parts and jumpers are illustrated in Fig. 10.
The Single Sweeper One is a designer's dream project. If you use your oscilloscope a great deal, this project is one that may be what the Z-axis ordered.
Adapted from: Radio-Electronics--Special Projects (Summer 1983, #7)
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Radio-Electronics--Special Projects (USA print magazine)
Also see: Supersensitive Simple Voltmeter
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