Hameg HM504-2, Manual

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25
Subject to change without notice
Prerequisite for this HF compensation is a square wave generator
with fast risetime (typically 4 ns), and low output impedance
(approx. 50 Ohm), providing 0.2 V at a frequency of approx. 1MHz.
The calibrator output of this instrument meets these requirements.
Connect the probe to the input previously used when 1 kHz
adjustment was made. Select 1 MHz output frequency. Operate
the oscilloscope as described under 1 kHz but select for 0.2 µs/
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 is 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
causes 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 1 kHz 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 rise time 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 1 kHz, then at 1 MHz.
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 rise times and low
impedance generator outputs are of particular importance.
Providing these essential features, as well as selectable output
frequencies, the calibrator of the instrument can, under certain
conditions, replace expensive square wave generators when
testing or compensating wideband attenuators or amplifiers. In
such a case, the input to an appropriate circuit will be connected
to the
calibrator output via a suitable probe.
The voltage provided by the probe to a high impedance input
(1 MOhm II 15 - 30 pF) will correspond to the division ratio of the
probe used (10:1 = 20 mV
pp
output). Suitable probes are HZ51 ,
52 and 54 .
Operating modes
of the Y amplifiers in Yt mode
The most important controls regarding the operating modes of
the Y amplifiers are the pushbuttons:
CH I [15] , DUAL [16] and
CH II [19] . Their functions are described in the section ”Controls
and Readout”.
In most cases oscilloscopes are used to display signals in Yt
mode. Then the signal amplitude deflects the beam in vertical
direction while the time base causes an X deflection (from left to
right) at the same time. Thereafter the beam becomes blanked
and fly back occurs.
The following Yt operation modes are available:
1
st Single channel operation of channel I (Mono CH I).
2
nd
Single channel operation of channel II (Mono CH II).
3 rd Two channel operation of channel I and channel II (DUAL).
4
th
Two channel operation of channel I and channel II displaying
the algebraic result as the sum or difference (“add”).
The way the channel switching is determined in DUAL mode
depends on the time base setting and is described in the section
”Controls and Readout”.
In ADD mode the signals of both channels are algebraically added
and displayed as one signal. Whether the resulting display shows
the sum or difference is dependent on the phase relationship or
the polarity of the signals and on the invert function.
In ADD mode the following combinations are possible for
In phase input voltages:
Channel II invert function inactive = sum.
Channel II invert function active = difference.
Antiphase input voltages:
Channel II invert function inactive = difference.
Channel II invert function active = sum.
In the ADD mode the vertical display position is dependent upon
the Y position setting of both channels. The same Y deflection
coefficient is normally used for both channels with algebraic
addition.
Please note, that the Y-position settings are also added but
are not affected by the invert function.
Differential measurement techniques allow direct measurement
of the voltage drop across floating components (both ends above
ground). Two identical probes should be used for both Y inputs.
In order to avoid ground loops, use a separate ground connection
and do not use the probe ground leads or cable shields.
X-Y Operation
The important control for this mode is the pushbutton labelled
DUAL and MENU [16] .
In XY mode the time base is deactivated. The signal applied to the
input of channel I front panel marking INPUT CHI (X) causes the
X deflection. The input related controls (
AC/DC/GND pushbutton
and the VOLTS/DIV knob) consequently affect the X deflection.
For X position alteration, the X-POS.- control knob must be used,
as the Y-POS./CURS.I control is automatically deactivated. The
input deflection coefficient ranges are the same for both channels,
because the
X x10 magnifier is inactive in XY mode.
Operating modes oft the Y amplifiers in Yt mode