Subject to change without notice
22
First Time Operation
For this purpose a square wave signal with a very fast rise
time and minimum overshoot should be used, as the sinusoidal
contents cover a wide frequency range.
The built-in calibration generator provides a square wave
signal with a very fast risetime (<4ns), and switch-selectable
frequencies of approx. 1kHz and 1MHz from the output
socket below the CRT screen. As the squarewave signals are
used for probe compensation adjustments, neither the fre-
quency accuracy nor the pulse duty factor are of importance
and therefore not specified.
The output provides 0.2Vpp ±1% (tr <4ns) for 10:1 probes.
When the Y deflection coefficient is set to 5mV/div, the
calibration voltage corresponds to a vertical display of 4
divisions (10:1 probe).
The output socket has an internal diameter of 4.9mm to
accommodate the internationally accepted shielding tube
diameter of modern Probes and F-series slimline 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 1kHz
The C-trimmer adjustment (low frequency) compensates the
capacitive loading on the oscilloscope input. By this adjust-
ment, the capacitive division assumes the same ratio as the
ohmic voltage divider to ensure the same division ratio for
high and low frequencies, as for DC. (For 1:1 probes or
switchable probes set to 1:1, this adjustment is neither
required nor possible). A baseline parallel to the horizontal
graticule lines is essential for accurate probe adjustments.
(
See also “Trace rotation TR”
).
Connect the probes (Types
HZ51
,
52
,
54
, or
HZ36
) to the CH
I input. Set the deflection coefficient to 5mV/div and the input
coupling to DC. The time deflection coefficient should be set
to 0.2ms/div. All deflection coefficients should be calibrated.
Plug the probe tip into the calibrator output socket.
Approximately 2 complete waveform periods are displayed
on the CRT screen. The compensation trimmer should be
adjusted. The location of the low frequency compensation
trimmer can be found in the probe information sheet. Adjust
the trimmer with the insulated screwdriver provided, until the
tops of the square wave signal are exactly parallel to the
horizontal graticule lines (see 1kHz diagram). The signal height
should then be 4div ± 0.16div (= 4% (oscilloscope 3% and
probe 1%). During this adjustment, the signal edges will
remain invisible.
Adjustment at 1MHz
Probes HZ51, 52 and 54 can also be HF-compensated. They
incorporate resonance de-emphasing networks (R- trimmer
in conjunction with inductances and capacitors) which permit
probe compensation in the range of the upper frequency limit
of the vertical oscilloscope amplifier. Only this compensative
adjustment ensures optimum utilization of the full bandwidth,
together with constant group delay at the high frequency end,
thereby reducing characteristic transient distortion near the
leading edge (e.g. overshoot, rounding, ringing, holes or
bumps) to an absolute minimum.
Using the probes
HZ51
,
52
and
54
, the full bandwidth of the
oscilloscope can be utilized without risk of unwanted wave-
form distortion.
Prerequisite for this HF compensation is a square wave
generator with fast risetime (typically 4ns), and low output
impedance (approx. 50
Ω
), providing 0.2V at a frequency of
approx. 1MHz. The calibrator output of this instrument meets
these requirements when the CAL. pushbutton is depressed.
Connect the probe to
CH I
input. Depress the CAL. pushbutton
for 1MHz. Operate the oscilloscope as described under 1kHz
but select for 0.2µs/div time deflection coefficient setting.
Insert the probe tip into the output socket. 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 rising edge as
well as the upper left corner of the pulse top. The location of
the high frequency compensation trimmer(s) can also be
found in the probe information sheet. These R-trimmer(s)
have to be adjusted such that the beginning of the pulse is as
straight as possible. Overshoot or excessive rounding are
unacceptable. The adjustment is relatively easy if only one
adjusting point is present. In case of several adjusting points
the adjustment is slightly more difficult, but gives a better
result. The rising edge should be as steep as possible, with a
pulse top remaining as straight and horizontal as possible.
After completion of the HF-adjustment, the signal amplitude
displayed on the CRT screen should have the same value as
during the 1kHz adjustment.
Probes other than those mentioned above, normally have a
larger tip diameter and may not fit into the calibrator output.
Whilst it is not difficult for an experienced operator to build a
suitable adapter, it should be pointed out that most of these
probes have a slower risetime with the effect that the total
bandwidth of scope together with probe may fall far below
that of the oscilloscope. Furthermore, the HF-adjustment
feature is nearly always missing so that waveform distortion
can not be entirely excluded. The adjustment sequence must
be followed in the order described, i.e. first at 1kHz, then at
1MHz. The calibrator frequencies should not be used for
timebase calibration. The pulse duty cycle deviates from 1:1
ratio.
Prerequisites for precise and easy probe adjustments, as well
as checks of deflection coefficients, are straight horizontal
pulse tops, calibrated pulse amplitude, and zero-potential at
the pulse base. Frequency and duty cycle are relatively
uncritical. For interpretation of transient response, fast pulse
risetimes and low-impedance generator outputs are of par-
ticular importance. Providing these essential features, as well
as switch-selectable output-frequencies, the calibrator of the
instrument can, under certain conditions, replace expensive
squarewave generators when testing or compensating
wideband-attenuators or -amplifiers. In such a case, the input
of an appropriate circuit will be connected to the CAL.-output
via a suitable probe.
