Use and Compensation of Probes
To
display an undistorted waveform on an oscilloscope, the
probe must be matched to the individual input impedance
of the vertical amplifier.
The
built-in calibration generator provides a
squarewave signal with a very low
and
switch-selectable frequencies of approx.
1
and 1
MHz
at two output sockets below the CRT
screen. One output
provides
%
for
10: 1
probes, and
% are
present at the other, for 100: 1 probes.
When the attenuator switches are set to
vertical
deflection coefficient, these calibration voltages corre-
spond to a screen amplitude of
The output sockets have an internal diameter of
to
accommodate the internationally accepted shielding tube
diameter of modern
and
F-series
probes. Only this type of construction ensures the
extremely short ground connections which are essential for
an undistorted waveform reproduction of non-sinusoidal
high frequency signals.
Adjustment at 1
The C-trimmer adjustment compensates the capacitive
loading on the oscilloscope input (approx.
with the
HM604).
By this adjustment, the capacitive division
assumes the same division ratio as the ohmic voltage
divider to ensure an equal division ratio for high and low fre-
quencies, as for DC. (For
1: 1
probes or switchable probes
set to 1: 1, this adjustment is neither required nor possible).
A baseline exactly parallel to the horizontal graticule lines is
a major condition for accurate probe adjustments. (See also
‘Trace Rotation
TR’,
page M7.)
Connect the probes (Types HZ51, 52, 53,
54,
or HZ37) to
CH.1
input. All pushbuttons should be released (in the ‘out’
position), and all push-pull knobs pushed ‘in’. Set the input
coupling switch to
DC,
the
attenuator switch to
and the
switch to
and all variable
controls to
CAL.
position. Plug the probe tip into the appro-
priate calibrator output socket, i.e.
probes into the
socket, 100: 1 probes into the
socket.
1
incorrect
correct
incorrect
Approximately 2 complete waveform periods are displayed
on the CRT screen. Now the compensation trimmer has to
be adjusted. Normally, this trimmer is located in the probe
head. On the 100: 1 probe HZ53,
however, it is located in
the connecting box at the other end of the cable. Using a
small insulated non-metallic screwdriver or trimming tool,
the trimmer has to be adjusted slowly until the tops of the
squarewave signal are exactly parallel to the horizontal
graticule lines. (See Fig. above for
1
The signal
amplitude shown should be 4cm
1.2
mm
3
During
this adjustment, the signal edges will remain invisible.
Adjustment at 1 MHz
Probes
and 54 will also allow for HF-adjustments.
They incorporate resonance deemphasizing networks
trimmer in conjunction with inductances and capacitors)
which permit for the first time probe compensation in
the range of the upper frequency limit of the vertical oscillo-
scope amplifier.
Only this compensative adjustment
ensures optimum utilisation of the full bandwidth, together
with constant group delay at the high frequency
end,
thereby reducing characteristic transient-distortion near the
leading signal edge (e.g. overshoot, rounding, ringing, holes
or bumps) to an absolute minimum.
Using the probes
52,
and 54, the full bandwidth of
the HM 604 can be utilized without risk of unwanted wave-
form distortion.
Prerequisite for this HF-adjustment is a squarewave
generator with fast
(typical
and low output
impedance (approx.
providing
and 2V at a fre-
quency of approx.
1
MHz. The calibrator output of the
HM604 meets these requirements when the pushbutton
1 MHz is depressed.
Connect the probe
or 54) to CH.1 input. Depress
the calibrator pushbutton
All other pushbuttons
should be released (‘out’ position). Set the input coupling
switch to DC, attenuator switch to
and TIME/
DIV. switch to 0.1
Set all variable controls to CAL.
position.
Insert the probe tip into the output socket marked
A
waveform will be displayed on the CRT screen, with leading
and trailing edges clearly visible. For the HF-adjustment
now to be performed, it will be necessary to observe the ris-
ing edge as well as the upper left corner of the pulse top. To
gain access to the
trimmer, the plastic
cover of the probe connecting box has to be slid off after
unscrewing the probe cable. The connecting boxes of the
HZ51 and HZ54 contain one R-trimmer screw, each, while
that of the HZ52 provides three. These R-trimmers have to
be adjusted in
such a manner that the beginning of the pulse
top is as straight as possible. Overshoot or excessive round-
ing are unacceptable. This is relatively easy on the HZ51
and HZ54, but slightly more difficult on the HZ52. The rising
edge should be as steep as possible, with the pulse top
remaining as straight and horizontal as possible.
On the HZ52, each of the three trimmers has a clearly
M8 604
Subject to change without notice
