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THE CRO CONTROLS
(The screen is shown below)
The first thing you
need to know is the function of each control.
To make sure you can identify Channel 1
probe, it is marked with "white-out" on the end
and also where it is connected to the CRO.
Some CRO's do not have all the features we will cover and some have different
names.
The On/Off Switch may be a press-button, toggle switch or rotary switch.
It is generally clearly marked - this doesn't need any more coverage.
The Focus control adjusts the
clarity of the trace on the display.
The Intensity control adjusts the
brightness of the trace on the display.
The Volts/Div is a multi-position rotary switch with settings: 5mV/div to
20v/Div
When viewing two waveforms, the Volts/Div may be different for each
waveform, even though the two graphs are equal on the screen.
The Variable Volts/Div control is located in the centre of the
Volts/Div control. It must be turned clockwise and "clicked" so the settings
on the Volts/Div control are accurate. This is called the "cal"
(calibrated) state.
The Variable Volts/Div control will provide intermediate volts/div values
but the exact values are unknown and this control is rarely used.
The Vertical Position knob will move the trace up or down the screen
until it is in the most suitable place.
The Horizontal Position is turned
clockwise or anticlockwise to shift the trace right or left.
This will also shift two traces if the CRO is in DUAL Trace mode.
This does not affect the point at which the trace is triggered.
If the Horizontal Position knob
is pulled, the Sweep is multiplied by a
factor of 5. It's a quick way to observe the details of a waveform. It's effect is
this: If 5 cycles are displayed on the screen, only one cycle will be displayed
when the Horizontal Position knob
is pulled.
Sweep is also known as Time/Div or Time-base. It is the speed at which the dot
is moved across the display. The rotary switch is marked: 0.2s,
0.1s, 50mS, 20mS etc.
The VARIABLE Timebase control is located in the centre of the Time/Div
control. It can be turned to make a fine adjustment to the
speed, but must be left at the position "CAL" (calibrated) or turned fully until
it is clicked off so the values on the
Sweep control are accurate.
The TRIGGER controls are used to maintain a steady trace on the screen. If
they are set incorrectly, the trace will drift, flicker or produce no trace at all! The trigger maintains a steady
trace by starting the dot on the screen when the input signal
reaches the same point in each cycle.
Trigger Level sets the CRO to begin a trace when the input
voltage reaches a certain value. The (+) or (–) control (called SLOPE) lets you
choose whether you want the CRO to trigger on a positive or negative
voltage.
It is best to leave the Trigger Level set to AUTO. (Pull the
Trigger Level to set to Auto). If you have difficulty obtaining a steady trace, try adjusting the Trigger Level
to set the level manually.
AC-GND-DC Selector.
The AC position only allows "alternating" or "varying" signals to appear
on the screen. The signal passes through a large capacitor that blocks any DC
component. This allows signal such as ripple on a DC voltage to be viewed on the
screen.
When the selector is moved to GND, the signal from the probe is
disconnected from the CRO. (The signal is not "grounded" or "shorted"). This
allows you to check the position of the trace for a 0v signal.
The DC position sends the signal (detected by the probe) to the screen (the
amplitude on the screen will depend on the setting of the Volts/Div
control). If you select the DC position, the waveform may be "off-screen" as shown below:
A typical example to show the effect of selecting DC on the
AC-GND-DC Selector is shown above. The CRO is connected to
a 10v DC voltage from a power supply with 1v ripple.
The Volts/Div control is set is 1v/div.
In diagram "A," the waveform cannot be seen as it is above the screen. This is
due to the CRO being set to "DC" on the
AC-GND-DC Selector.
The mistake we are making is this: The CRO is set to 1v/div as we expect to see
the 1v ripple on the screen, but the 10v is pushing the waveform above the
screen. We need to remove the effect of the 10v. This is done by selecting "AC"
on the
AC-GND-DC Selector.
This is shown in diagram "B."
Diagram "C" shows the volts/div control turned to 0.2v/div to get the
waveform to fill the screen.
Mode Selector Switch has 4 functions:
Channel A operation
Channel B operation
Dual operation
Add Operation
When two waveforms are being detected, they can be ADDED by moving the
Mode Selector to Add. The screen will display a single waveform.
Subtraction can be achieved by inverting channel 2 by pulling out the
vertical position control for channel B.
SINGLE INPUT
When the CRO is used with a single input, the probe is connected to channel 1
(channel A). 
It is very important that Channel 1 probe is clearly identified, especially when two
probes are in use.
DUAL INPUTS
A Dual Trace Oscilloscope will produce two traces on the screen at
the same time. It actually draws one at a time, but the image persists on the
screen long enough so your eye cannot see any flicker, and it appears as if
there are two steady traces.
When two signals are to be viewed, the waveform for channel
A
is generally displayed at the top and channel
B is displayed at the bottom. This makes their locations easy to remember.
The two waveforms can be moved to any part of the screen and positioned on top of
each other to detect any slight differences.
This is the advantage of a dual-trace CRO. It makes comparisons very quick and
easy.
The two waveforms will have the same "time-base" (the same sweep time) as only
one sweep-time can be selected at any given time.
Triggering is normally applied to channel A (Ch 1) by the CRO ( when the
Trigger Source control is set to INT) and this means the waveform being
detected by channel B may not be steady.
You may have to swap the points on the circuit where the probes are taking a
reading, to see if the CRO will produce better triggering on
waveform B. If the CRO has Trigger Source Ch2, the CRO will trigger on channel 2
and the probes need not be swapped.
(The earth clips have been omitted from the diagram above for clarity)