'&21752/6
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.
*(1(5$/26&23('(6&5,37,21
THE VERTICAL INPUT is applied to the vertical input amplifier, which is quite sensitive,
designed for a 25-50mV input. For larger inputs, the signal is routed through attenuators
comprised of simple voltage dividers. These attenuator dividers is what forms the VERTICAL
SENSITIVITY, calibrated in mV/division or V/div. An INPUT COUPLING switch selects DC or
AC coupling, and sometimes a GROUND position. The output of the vertical input amplifiers
is a differential signal, amplified up to high voltages and applied to the CRT (cathode ray
tube) vertical plates for deflecting the beam in the vertical axis.
7+(+25,=217$/$03/,),(56
7+(+25,=217$/$03/,),(56 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.
7+(6:((3*(1(5$725
7+(6:((3*(1(5$725 is a constant current source charging a capacitor to make a
linear sawtooth waveform. The value of the capacitor will determine the time is takes to
eventually move the beam across the screen, and is selected by the HORIZONTAL SWEEP
control, calibrated in seconds, mS, uS (or nS) per division. The faster the beam moves
across the screen, the higher the frequency that can be displayed. An important task of a
scope is to display a stable waveform, which is done by starting the sawtooth sweep at
exactly the same time in respect to the input signal. A switch labelled TRIGGER SOURCE
determines what initiates the sweep. In the INTERNAL position, a sample of the input signal
from the vertical amplifiers is used, and when it reaches a certain level, WOOSH, the sweep
occurs. In the AUTO mode, the sweep is free running and not necessarily synchronized with
the input signal. In LINE position, the sweep is triggered off of 60-Hz from the power supply
(useful for synchronizing to TV/VCR signals), and EXTERNAL the sweep is triggered from an
external input applied to a BNC (on the front or the back of the scope).
OTHER FEATURES your scope may have are:
* Two vertical input channels for dual-trace operation
* Two separate time bases for delayed sweep operation
* Various modes to display the input signals (alternate, chopped, A+B added, invert B, A
intensified by B, etc.)
* Built in calibrators
&$/,%5$7,1* GQRP Club
Datasheet
OSCILLOSCOPES - BASIC USE AND
MEASUREMENTS by Paul Harden, NA5N
Page 2 of 12
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 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-
crossings" occurring on
the vertical at graticles
labelled "1" - "3" - "5". For
proper zero-crossings, the
waveform should be
centred 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 GQRP Club
Datasheet
OSCILLOSCOPES - BASIC USE AND
MEASUREMENTS by Paul Harden, NA5N
Page 3 of 12
somewhere!
0$,123(5$725&21752/6
,17(16,7<
,17(16,7< - 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.
)2&86
)2&86 - adjusts the beam for the thinnest and sharpest display.
9(57 +25326,7,21
9(57 +25326,7,21 controls the vertical and horizontal position of the trace.
9(5792/76',9
9(5792/76',9 - controls the vertical sensitivity of the display, i.e., how may volts or
mV per division.
9(579(51,(5
9(57 9(51,(5 - adjust the vertical sensitivity in fine steps. Should be off (or CAL
position) for calibrated measurements.
7,0(%$6(+256:((363(('
7,0(%$6(+256:((363((' - sets the horizontal sensitivity, i.e., how many second,
mS or uS per division.
+259(51,(5
+259(51,(5 - adjusts the horizontal sensitivity, or sweep speed, in fine steps. Should be
in
2)) or &$/ for calibrated measurements.
OTHER ADJUSTMENTS YOU MAY FIND:
$67,*0$7,60
$67,*0$7,60 - With the scope INTENSITY and FOCUS properly set, this adjustment
compensates for the curvature of the CRT tube by making it in-focus across the entire
sweep. If your trace is out-of-focus in certain areas, but in-focus elsewhere, the
ASTIGMATISM needs to be adjusted.
75$&(527$7,21
75$&(527$7,21 - is a small coil around the CRT that skews the trace to ensure it is
perfectly horizontal. Set the scope to GND, free-run the sweep and adjust the vertical
position so the beam is along a graticle (division) line. Adjust the TRACE ROTATION until
the beam is perfectly parallel to the horizontal graticles. On scopes without this adjustment,
loosening the CRT mounting brackets and physically rotating the CRT tube for a level trace,
then re-tightening the CRT brackets perform levelling the trace.
!!! WARNING!!! HIGH VOLTAGE ARE PRESENT AROUND THE CRT TUBE. USE
EXTREME CAUTION WHEN PERFORMING THE ABOVE PROCEDURE.
'&%$/$1&(
'&%$/$1&( - is a DC offset in the vertical amplifiers that causes a shift in the trace
baseline when changing vertical scales. It is most obvious when displaying AC signals. For
example, you are displaying a 10Vpp sine wave, centered on the center graticle at 2v/div.
Changing to 5v/div, the sine wave shifts away from the center graticle, up or down ... that
is, it assumes a DC bias error in the vertical amplifiers. The DC BAL is adjusted until no shift
occurs when changing vertical scales. Admittedly, setting the scope for perfect dc balance
on all scales is an exercise in patience! DC BAL is often an internal adjustment, or on the
rear panel. On dual trace scopes, there will be one for each channel. GQRP Club
Datasheet
OSCILLOSCOPES - BASIC USE AND
MEASUREMENTS by Paul Harden, NA5N
Page 4 of 12
+9$'-867
+9 $'-867 - 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 calibr