PART 1 - BASIC DESCRIPTION, CALIBRATION AND CONTROLS
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NOTE: This is a text version of an article appearing in the Summer
1997 issue of "QRPp." The article contains numerous illustrations and
photos of oscilloscopes displays, which unfortunately can not be
included in a text file.
GENERAL O-SCOPE DESCRIPTION.
THE VERTICAL INPUT is applied to the vertical input amplifier, which
is quite sensitive, designed for a 25-50mV input. For larger inputs,
the signal is routed through attenuators comprised of simple voltage
dividers. These attenuator dividers is what forms the VERTICAL
SENSITIVITY, calibrated in mV/division or V/div. An INPUT COUPLING
switch selects DC or AC coupling, and sometimes a GROUND position.
The output of the vertical input amplifiers is a differential signal,
amplified up to high voltages and applied to the CRT (cathode ray
tube) vertical plates for deflecting the beam in the vertical axis.
THE HORIZONTAL AMPLIFIERS are driven by an internal sweep generator,
amplified to a high voltage and applied to the CRT horizontal plates
for deflecting the beam in the horizontal axis ... that is, the sweep
that moves the beam from left-to-right.
Thus, for a proper oscope display, such as displaying a sinewave, it
is a combination of moving the trace from left to right to show TIME,
and up-and-down to show MAGNITUDE.
THE SWEEP GENERATOR is a constant current source charging a capacitor
to make a linear sawtooth waveform. The value of the capacitor will
determine the time is takes to eventually move the beam across the
screen, and is selected by the HORIZONTAL SWEEP control, calibrated in
seconds, mS, uS (or nS) per division. The faster the beam moves
across the screen, the higher the frequency that can be displayed.
An important task of a scope is to display a stable waveform, which is
done by starting the sawtooth sweep at exactly the same time in
respect to the input signal. A switch labeled TRIGGER SOURCE
determines what initiates the sweep. In the INTERNAL position, a
sample of the input signal from the vertical amplifiers is used,
and when it reaches a certain level, WOOSH, the sweep occurs. In the
AUTO mode, the sweep is free running and not necessarily synchronized
with the input signal. In LINE position, the sweep is triggered off
of 60-Hz from the power supply (useful for synchronizing to TV/VCR
signals), and EXTERNAL the sweep is triggered from an external input
applied to a BNC (on the front or the back of the scope).
OTHER FEATURES your scope may have are:
* Two vertical input channels for dual-trace operation
* Two separate time bases for delayed sweep operation
* Various modes to display the input signals (alternate, chopped,
A+B added, invert B, A intensified by B, etc.)
* Built in calibrators
CALIBRATING YOUR SCOPE.
Likely, you obtained your scope from a hamfest, the company junk
bin, etc. The first thing you should do upon acquiring a scope is
to check its calibration.
THE VERTICAL AMPLIFIERS can be checked with a known voltage source,
such as a 9v battery. Measure the battery output with an accurate
voltmeter. Let's say it's exactly 9v. Set the input coupling to the
GND position (0v) and move the trace to the bottom division. Switch
the input coupling to DC and set the attenuators to 2v or 5v/div. to
give a nearly full scale deflection. For example, if your scope has
four vertical divisions, setting the attenuators to 2v/div. would be
8V full scale deflection, and at 5v/div., full-scale would be 20V.
With the 9v battery applied, the DC deflection should be 1.8 divisions
at 5v/div. Switching to 10v/div., the deflection should be just a bit
less than one division. Internal to the scope (or perhaps accessible
from the outside) are adjustments for the VERTICAL AMPLIFIER GAIN.
Adjust this pot for the proper deflection described above. The procedure
can be repeated with a 1.5v battery for the lower sensitivity ranges
(which you'll be using more of the time anyway).
Also note that when you adjust the Vertical Amplifier Gain adjustment,
the 0v (ground) reference on the bottom division may also shift. So
after each adjustment, reposition the trace on the bottom division for
0V input, then recheck the trace position for the 9V or other test
voltage you are using to calibrate against. It takes a few times
going back and forth to get it right.
THE HORIZONTAL AMPLIFIERS should be checked/calibrated using a signal
generator. For example, a 1 MHz signal has a period of 1uS per cycle.
Setting the SWEEP RATE to 1.0uS/div., a 1 MHz signal should take
EXACTLY one division per cycle. Ensure the horizontal WIDTH control
is set so the beam starts at the first division and ends on the last
one, and the HOR SWEEP VERNIER (fine adjustment) is in the OFF or CAL
position. If the sweep rate appears incorrect, an internal SWEEP GAIN
ADJustment can be set for proper display of the test signal. This
should be repeated at different frequencies, and some scopes will have
a separate adjustment for each time base setting. Once the Sweep Gain
has been set as above for 1MHz = 1 cycle per division, go to the next
faster sweep speed, which should
usually be 0.5uS/div. In this 1 2 3 4 5
case, the 1MHz sinewave should |--**--|------|--**--|------|
take TWO divisions to display | * * | | * * | |
a complete cycle, as shown in the |* *| |* *| |
quasi-illustration to the right. *----- * -----*------*------*
Trigger the scope for a stable | |* *| |* *|
display so that the zero crossings | | * * | | * * |
or the peaks are on the vertical |------|--**--|------|--**--|
graticle lines. The illustration | |
shows the positive going "zero- |<---1.0uS--->| 1MHz sinewave
crossings" occuring on the vertical at 0.5uS/div.
graticles labeled "1" - "3" - "5".
For proper zero-crossings, the waveform should be centered between
two divisions, also as shown in the illustration.
If you can't find the adjustment to tweak the horizontal gain, you
can shrink or stretch out the test signal to the desired divisions
using the HORIZONTAL WIDTH control, usually a front panel control.
Then you can mark on the front panel where the HOR WIDTH control must
be at each SWEEP setting for proper calibration.
Without checking and calibrating the accuracy of your time base sweep,
time and frequency measurements performed on your scope may contain
significant errors.
If you don't have a signal generator, you might use the audio from a
receiver tuned to WWV. The various tones transmitted throughout the
minute and hour are listed in various references. And of course,
there's always 60-cycles floating around the ham shack somewhere!
MAIN OPERATOR CONTROLS.
INTENSITY - controls the brightness of the beam. Adjust for a clear
trace, but not too bright. A very bright trace can cause
permanent damage to the CRT, particularly on a well-used scope.
FOCUS - adjusts the beam for the thinnest and sharpest display.
VERT & HOR POSITION controls the vertical and horizontal position
of the trace.
VERT VOLTS/DIV - controls the vertical sensitivity of the display,
i.e., how may volts or mV per division.
VERT VERNIER - adjust the vertical sensitivity in fine steps. Should
be off (or CAL position) for calibrated measurements.
TIME BASE/HOR SWEEP SPEED - sets the horizontal sensitivity, i.e.,
how many second, mS or uS per division.
HOR VERNIER - ajusts the horizontal sensitivity, or sweep speed, in
fine steps. Should be in OFF or CAL for calibrated measurements.
OTHER ADJUSTMENTS YOU MAY FIND:
ASTIGMATISM - With the scope INTENSITY and FOCUS properly set, this
adjustment compensates for the curvature of the CRT tube by making
it in-focus across the entire sweep. If your trace is out-of-focus
in certain areas, but in-focus elsewhere, the ASTIGMATISM needs to be
adjusted.
TRACE ROTATION - is a small coil around the CRT that skews the trace
to ensure it is perfectly horizontal. Set the scope to GND, free-run
the sweep and adjust the vertical position so the beam is along a graticle
(division) line. Adjust the TRACE ROTATION until the beam
is perfectly parallel to the horizontal graticles. On scopes without
this adjustment, leveling the trace is performed by loosening the
CRT mounting brackets and physically rotating the CRT tube for a level
trace, then re-tightening the CRT brackets.
!!! WARNING!!! HIGH VOLTAGE ARE PRESENT AROUND THE CRT TUBE. USE
EXTREME CAUTION WHEN PERFORMING THE ABOVE PROCEDURE.
DC BALANCE - is a DC offset in the vertical amplifiers that causes a
shift in the trace baseline when changing vertical scales. It is
most obvious when displaying AC signals. For example, you are
displaying a 10Vpp sine wave, centered on the center graticle at
2v/div. Changing to 5v/div, the sine wave shifts away from the
center graticle, up or down ... that is, it assumes a DC bias error
in the verticle amplifiers. The DC BAL is adjusted until no shift
occurs when changing vertical scales. Admittedly, setting the scope
for perfect dc balance on all scales is an exercise in patience!
DC BAL is often an internal adjustment, or on the rear panel. On
dual trace scopes, there will be one for each channel.
HV ADJUST - is the high voltage that controls the intensity of the
trace. Turn up the INTENSITY control to its brightest position,
then adjust the HV ADJ for a trace slightly brighter than normal
intensity. Return the INTENSITY for normal brightness. The INTENSITY
control now has the proper range. On some scopes, it takes a little
piddling around to properly set the HV ADJ, intensity and focus for proper
operation. The HV ADJ is often an internal adjustment.
!!! If you adjust the HV ADJUST, you may also have to recalibrate
the VERTICAL and HORIZONTAL GAINS as described above for proper
calibration (V/division and time/division accuracy).
An oscilloscope is an amazing instrument for making voltage, time and
frequency measurements ... however, all of these measurements are
worthless unless you ensure the vertical and horizontal stages of
your scope are reasonably calibrated. The time to calibrate your
scope will be worth the ease and reliability of subsequent measure-
ments you will make.
END OF PART 1
72, Paul Harden, NA5N
NA5N@Rt66.com
To part 2
Many thanks to Paul for allowing me to use his work
Frank G3YCC