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Triggered Sweep / Timebase Generator

http://www.williamson-labs.com/scope-main.htm

Oscilloscopes have several sync modes: internal sync; external sync; line sync and delayed sync. The selection of any of these sync sources sends the signal to the "sync detector." For example: if the internal sync is selected, it sends the input signal, picked-off from the vertical input amplifier, to the "sync detector" which is a fast analog comparator that gets its other comparison input from a front panel mounted control, called the "LEVEL" control. This allows the user to preset the exact voltage level and polarity, at which the scope "triggers" the sweep generator to sweep the beam across the screen once. Every time the signal goes above this preset voltage (except during an active sweep time), a new sweep is started: this is called "Triggered Sweep."

"Delayed Sweep" Another feature of most good scopes is "Delayed Sweep." Delayed Sweep, allows the user (that could be you, if you behave and eat your vegetables) to trigger on an event and observe the signal after some predetermined time interval. For example: if you wanted to observe one single scan line--out of 525--in a television signal, you would trigger off of the beginning of each television field time (16.67 msec) but holdoff displaying that particular scan line (HD = 63.5 usec) until the correct amount of time has passed. In essence the Delayed Sweep is just a fancy "One-Shot" multivibrator with a ten-turn pot, triggered from the scope's normal sweep circuit; and holds off the sweep across the screen until some time interval--determined by our old friends Ms. R & Mr. C--has passed, after which time the scope sweeps for the interval of one TV scan line.

Numbers Don't Lie... People Do: Let's say I wanted to eyeball the 188th horizontal scan line, and I had dialled in both the "Delay" and the "Sweep" intervals: as soon as the field sync pulse (VD) occurs the delay sweep starts to time out for 11,885 usec (187 x 63.5), at which point it triggers the scope's sweep generator. The sweep runs for ~ 65 usec--displaying one complete horizontal scan line (the 188th). Other features of the oscilloscope that are available, are left for the user to discover in the user's manual: you know, that thing you read when "all-else-fails!"

DELAYED_SWEEP_ADAPTER

The schematic for the delayed sweep adapter is shown in the figure. Power for the circuit is pro-vided by a dual-polarity, 5-V supply. An LM360 high-speed comparator (IC1) with complementary TTL outputs is at the heart of the circuit. A trigger-level control, potentiometer R2, allows the adapter to trigger on any part of the waveform being displayed. With the values shown, the circuit functions well with a Hitachi V-212 oscilloscope, whose channel 1 output is about 25 mV per vertical division of the signal display. Assuming a normal display of about six divisions, the level control pro-vides a range of ±150 mV. You can change that range to suit your scope output by adjusting the ratio of R3 (15,000 Ω) to R4 (470 Ω); however, maintain the ratio of R5 (100,000 Ω) to R4 (470 Ω) for proper hysteresis. Resistor R1 (51 Ω) terminates the 50-Ω cable from the channel 1 output jack. IC2, a 7473 TTL dual J/K flip-flop, is configured in its toggle mode to divide the input frequency by 2. That ensures that IC3, a 74121 TTL monostable multivibrator, will function accurately over the in-put waveform's full time period. The pulse output from IC3 is coupled to the trigger-input jack of your oscilloscope, and the slope is selected via S2 to match the slope selected on your oscilloscope. Po-tentiometer R9 allows fine positioning of the trace and potentiometer R7 allows the output pulse width selected to be multiplied by a factor from x1 to over x2.5. Switch S1 lets you select the de-sired pulse width by switching in C8 through C13, in a 1-2-5 sequence. The adapter circuit was de-signed to operate on an external ±5-V, 100-mA power supply to avoid 60-Hz pickup in the unit.