By Carl Babcoke, CET
--------- Model LBO-515 dual-trace 25-MHz scope by Leader has a second
horizontal-sweep time base for expanding small areas of a waveform.
The trace is very bright and sharp because of a high accelerating voltage at the CRT, and an internal illuminated graticule eliminates parallax.
Each report about an item of electronic test equipment is based on examination and operation of the device in the ELECTRONIC SERVICING laboratory. Personal observations about the performance, and details of new and useful features are spotlighted, along with tips about using the equipment for best results.
Leader Model LBO-515 Scope
Scopes with additional functions and improved performances are becoming necessary for testing digit al circuits, micro-processers, and other sophisticated devices. The Leader LBO-515 has those extra capabilities.
General Features
Leader model LBO-515 has all usual functions of a modern dual trace triggered-sweep scope. The waveform of either channel 1 or channel 2 can be displayed. Also, pushbuttons provide a manual choice of either "chop" or "alternate" display of both waveforms. However, the unique features are in the horizontal-sweep section, where the conventional (or "A") sweep has a "hold-off' control, and a second "B" sweep is provided to enlarge specific areas of the wave form. This is called "delayed sweep", and the enlargement is additional to the X10 magnification. Such wide horizontal expansions are required for analyzing narrow pulses.
Vertical Amplifiers
There are two identical vertical amplifiers. The bandwidth is rated at -3 dB from 2 Hz to 25 MHz for AC coupling, and from DC to 25 MHz for DC coupling. The rise time is 14 nanoseconds. No ringing or overshoot was noticed in any of the complex waveforms tested.
Lever switches select channel inputs of DC coupling, AC coup ling, or ground. The grounded input mode is very helpful for moving the zero-voltage line to any desired point. The lever switches were faster and easier to operate than are some other types.
Maximum sensitivity of both channels is 5 millivolts (0.005 volts) per calibration division on the CRT screen. Ten ranges are provided, in a 1-2-5 sequence, up to 5 volts per division. A variable control allows adjustments of waveform heights between the ranges.
When the variable control is being used, the calibration is not known, and an "uncal" LED lights to remind you. For ±3% calibration accuracy, turn the variable control clockwise until a switch clicks and the "uncal" LED goes dark. Maximum input voltage is 600 volts peak.
Each channel has a delay line, which permits the horizontal to be triggered by the un-delayed signal, and thus display the leading edge of the waveform (usually eliminated by the time required for the sweep to begin). Centering controls are provided for both vertical channels.
Pushbutton switches select: channel 1 waveform; channel 2 wave form; a composite of both wave forms; alternate waveforms; or chopped waveforms (where the two waveforms are switched at 250 KHz). The polarity of the channel 2 waveform can be inverted by an other pushbutton, while the trigger button allows either channel 1 or channel 2 waveform to sync the horizontal sweep.
CRT and Controls
Size of the rectangular CRT was not listed, but the screen has 8 vertical and 10 horizontal divisions.
Each division is 0.8 centimeter.
Parallax error is eliminated by the internal graticule lines. Variable graticule illumination can change the lines from black to white.
Waveform sharpness and brilliance are very good because of the 6-KV accelerating voltage and the screwdriver-adjustable astigmatism control. A similar control eliminates any waveform tilting.
The LED on-indicator, the on/off switch, and a terminal with a 0.5 VPP squarewave (for probe adjustments) are located to the left of the CRT controls.
Horizontal Sweep
Two time-base horizontal-sweep circuits are included. Time base "A" has 20 steps from 0.5 second per-division to 0.2 microsecond per division, plus a variable control (with an LED to show uncalibrated operation). The other time base will be explained later.
Sync for triggering the horizontal sweep can come from either vertical channel or from an external source (through the "source" pushbutton). Four other pushbuttons modify the sync signal. The "AC" button does not allow locking below 20 Hz, and the "HF Rej" (high-frequency rejection) button limits the high frequency locking to 10 KHz. The "TV" button supplies vertical or horizontal sync from any composite video, and a "DC -ext only" button extends the external-sync locking down to DC. In addition, a "slope" button allows locking either to positive-going or negative-going signals.
The "level" control adjusts the amplitude level that starts the sweep, and also selects a positive or negative point. This action is conventional. For simple waveforms, the control can be turned fully CC W to the "preset" position where locking occurs automatically.
When sync is triggering the horizontal sweep correctly, the "trig'd" LED lights (a help in making locking adjustments). Associated with this operation is the "A holdoff" control that removes an adjustable amount of sweep near the end of the trace; an action that's helpful with some digital waveforms.
Delayed "B" sweep
Eight pushbuttons are arranged vertically down the right edge of the front panel. Most of these are used for the delayed sweep that has the effect of widening selected areas of the regular waveform.
Eighteen positions from 0.1 second-per-division to 0.2 micro second-per-division are provided for time base "B," which is completely separate from the conventional "A" time base.
First, the normal "A" time base controls are adjusted to obtain a stable waveform that includes the area where the trace expansion is needed. For this step, the "A" button under "horiz display" is depressed.
Next, the "A inten by B" (A display intensified by the B display) button is pressed, and the area (which will become the expanded trace) has more brightness. The "delay time" control moves the brighter area horizontally, and the setting of the "B time/div" switch determines the width of this area; therefore, these two controls are adjusted so only the portion of the waveform that's to be examined is in the brighter area.
When the "B dly'd" (B time base) button is pressed, only the area formerly brightened will cover the screen (same height, and full width). If needed, the "delay time" knob can be used to move the waveform across the screen for pin pointing any specific area.
Some types of waveforms will have less jitter if the "B trg'd" (B time base triggered) button is used.
Evidently, the sweep is started by the first edge of the expanded area.
Refer to the instruction book for more details.
Other buttons allow single traces of infrequent signals. Also, the "X-Y" button uses channel 1 for the X input and channel 2 for the Y input of a vectorscope. The advantage is that both channels have identical phase and frequency response.
----------- Stable VITS and VIR waveforms were displayed on the LBO-515.
------- Multi: frequency waveform from a video generator was used to
show the operation of time-base B. (A) Five bursts of different frequencies
are part of this video waveform. The normal "horizontal display" button
A was used. (B) When the "A inten by B" button was pressed,
only part of the waveform was brightened (the scope intensity was reduced
slightly to avoid overexposing the brighter area). The area brightened
included the 3-MHz and 3.58-MHz bursts. (C) Pressing the "B dely'd" button
expanded the two bursts to fill the entire screen. More or less expansion
is possible with other adjustments. This feature is highly recommended
for digital waveforms.
Comments
Two x1/x10 probes with insulated hook tips are included, and they functioned very well. Accuracy of the various calibrations seemed to be excellent, and the internal CRT graticule allowed the same accuracy of peak-to-peak readings regardless of the angle of view. No drift of the two traces was observed (very rare with dual-trace scopes). Operation of the delayed "B" sweep enabled me to obtain several waveforms not possible with conventional triggered-sweep scopes.
This function is essential for proper display of certain digital wave forms.
(adapted from: Electronic Servicing magazine, Feb. 1978)
Also see: Test Equipment REPORT (Aug. 1978)