Operating Modes Of The Vertical Amplifi Er; Xy Mode; Measurement Of Phase Differences In Dual Channel Operation; Triggering And Time Base - Hameg HM400 Handbuch

O p e r a t i n g m o d e s o f t h e v e r t i c a l a m p l i f i e r
Operating modes of the vertical amplifi er
The most important controls determining the operating modes of the
vertical amplifer are the mode buttons CH 1, CH 2, DUAL, ADD and
XY 32 .
Changing the modes is described in the chapter Controls. Yt is by far
the mode most used: the input signal defl ects the trace vertically while
a time base moves the trace from left to right. The Y amplifi er offers
these modes:
1. Single channel operation of CH 1.
2. Single channel operation of CH 2.
3. DUAL trace two channel operation.
4. Algebraic addition of CH 1 + CH 2 and subtraction of CH 1 – CH 2.
In DUAL trace mode both channels are operating, the time base de-
termines the exact mode of representation, see the chapter Controls.
Switching of the channels may either happen alternately after the com-
pletion of each time base cycle, or the switching occurs at a high rate
during the course of a time base cycle (chopped). The alternating mode
is unsuited at slow time bases because the alternation becomes visible
with disturbing fl icker, here, the chopped mode will yield a fl icker-free
display. At high sweep speeds the chopped mode is unsuited because
the switching transients are disturbing.
In the ADD mode the signals of both channels are algebraically added
(CH 1 + CH 2) or subtracted (CH 1 – CH 2) if CH 2 is inverted. If the si-
gnals of CH 1 and CH 2 happen to be of opposite phase they may fully
or partly cancel, of course.
It is important to bear in mind that the two inputs of the scope are not
to be mistaken as the inputs of a true difference amplifi er! When using
this feature to measure the difference signal between two measuring
points, restrictions must be observed: both input attenuators must be
switched to the same setting, the common mode rejection is very mo-
derate, and the common mode range is limited to the normal operating
range of the input amplifi ers. This means in practice that, before the
ADD mode is entered, it has to be checked whether each input signal can
be displayed, i.e. that is in within the normal operating range; if that is
the case for both signals, switch to ADD. Please note further that both
POSITION controls affect the vertical position of the added signals. If
probes are used, their tolerances will also diminish the common mode
rejection; this can be checked by connecting both probes to the same
measuring signal, the resulting display should be zero. It is preferable
to use the probe adjust or a pulse generator for this test.

XY mode

For this mode use the button XY as described in the chapter Controls
32
under .
In this mode the time base is disabled. The CH1 input signal will defl ect
the trace horizontally, the CH 2 input signal vertically. The horizontal
position is controlled as usual with the X-POSITION control
position control is disabled. The magnifi er is also disabled. When using
this mode, the low bandwidth of the X amplifi er (see the specifi cations)
has to be observed, the phase difference between the wide band vertical
amplifi er and the X amplifi er increases with frequency.
The Y signal may be inverted by pressing the button INV
CH 2.
Using Lissajous patterns it is possible
STOP
– to compare two signals of different frequency and to adjust one to
the frequency of the other until both are synchronized. This applies
also to multiples or fractions of one of the frequencies
– to measure the phase difference between two signals of the same
frequency.
34 Subject to change without notice
Measurement of phase differences in dual channel
operation
A much more precise and convenient method of measuring phase
differences which is also applicable up to high frequencies is the
measurement of the time difference in dual channel operation. Please
note: It is mandatory that the trigger signal is taken from only one
signal.The phase difference can be easily calculated as the frequency
is known. Another advantage of this me-
thod is the fact that the time difference
is still measurable even if the signals
are corrupted by hum, ripple or noise.
Also, there are no ambiguities. Alterna-
tively Lissajous patterns can be used for
measurement of phase differences.
The pertinent controls are located to the right of the VOLTS/DIV knobs,
see the chapter Controls.
In Yt operation the signal defl ects the trace vertically while it is defl ected
horizontally with constant selectable velocity from left to right. The time
base is started by a socalled trigger signal which is derived from any
of the available sources. The time base performs one cycle and rests
waiting for the next trigger. It is hence immaterial when the next signal
arrives, the signal needs only to be repetitive, it need not be periodic!
The time between triggers may be any, at low repetition frequencies the
display becomes darker, at very low ones the trace will not be visible
any more. In order to achieve a stable display, the trigger must always
be derived from the very same portion of the signal. The slope and the
level of the triggering signal can be chosen.
Note: Various trigger sources are available: the two input channels, an
external input, a signal taken from the mains, a TV trigger. Of course, the
triggering signal must be synchronous to the signal to be displayed. The
minimum amplitude for stable triggering is called the trigger threshold.
With internal triggering the trigger signal is taken off in the two input
amplifi ers, the minimum amplitude here is given in mm vertical defl ec-
tion, independent of the positions of the VOLTS/DIV switches.
With external triggering the minimum amplitude is given in Vpp at the
external trigger input connector. The trigger amplitude may be much
larger than the threshold, but it is advisable not to exceed 20 times
the threshold.
The oscilloscope features two trigger operation modes to be descri-
bed.

Automatic peak-to-peak triggering

13
, the CH 1
Please refer to the chapter Controls for specifi c information about the
controls SLOPE
When using AUTOSET, this trigger mode will be automatically selected.
If DC coupling is selected, the peak-to-peak detection will be disabled,
while the function of the auto trigger will remain active. With auto trigger
selected, there will be always a trace visible, because the time base will
restart periodically if no trigger signal is present or if only a DC voltage
is applied. The auto trigger function implies that the user is only required
to operate the VOLTS/DIV and TIME/DIV controls.
The TRIGGER-LEVEL knob is active with auto peak-to-peak triggering,
its range is automatically adjusted to the peak-to-peak level measured,
sin ϕ = —
cos ϕ =
ϕ = arc sin —

Triggering and time base

9
, TRIGGER-LEVEL 10 , and TRIGGER
a
b
a
( ) 2
1 – —
b
a
b
16 to 23 .