Hameg HM1008-2 Manual Download Page 14

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Subject to change without notice

may result as the input range of one or both ampliﬁ ers may
be exceeded.
Another precondition for obtaining true displays is the use of
two identical probes at both inputs. But note that normal probe
tolerances (percent) will cause the CM rejection to be expected
to be rather moderate. In order to obtain the best possible re-
sults proceed as follows: First adjust both probes as carefully
as possible, then in Add mode select the same sensitivity at
both inputs and connect both probes to the output of a pulse
generator with sufﬁcient amplitude to yield a good display. Re-
adjust one (!) of the probe adjustment capacitors for a minimum
of over or undershoot. As there is no adjustment provided with
which the resistors can be matched a residual pulse signal will
be unavoidable. When making difference measurements it is
good practice to ﬁrst connect the ground cables of the probes
to the object prior to connecting the probe tips. There may be
high potentials between the object and the scope. If a probe tip
is connected ﬁrst there is danger of overstressing the probe or/
and the scope inputs! Never perform difference measurements
without both probe ground cables connected.

XY operation

This mode is accessed by VERT/XY

XY. In analog mode the

timebase will be turned off. The channel 1 signal will deﬂ ect in X
direction (X INP. = horizontal input), hence the input attenuators,
the variable and the POSITION 1 control will be operative. The
HORIZONTAL control will also remain functional.

Channel 2 will deﬂ ect in Y direction.

The x10 magniﬁ er will be inoperative in XY mode. Please note the
differences in the Y and X bandwidths, the X ampliﬁ er has a lower
–3 dB frequency than the Y ampliﬁ er. Consequently the phase
difference between X and Y will increase with frequency.
In XY mode the X signal (CH1 = X INP). cannot be inverted.

The XY mode may generate Lissajous ﬁ gures which simplify
some measuring tasks and make others possible:

– Comparison of two signals of different frequency or adjust-

ment of one frequency until it is equal to the other and
becomes synchronized.

– This is also possible for multiples or fractions of one of the

frequencies.

Phase measurements with Lissajous ﬁ gures

The following pictures show two sine waves of equal amplitude
and frequency but differing phase.

Calculation of the phase angle between the X and Y signals (after
reading a and b off the screen) is possible using the following
formulas and a pocket calculator with trigonometric functions.
This calculation is independent of the signal amplitudes:

Please note:

– As the trigonometric functions are periodic, limit the cal-

culation to angles

90 degrees. This is where this function

is most useful.

– Do not use too high frequencies,

because, as explained above, the
two amplifiers are not identical,
their phase difference increases
with frequency. The spec gives the
frequency at which the phase diffe-
rence will stay

3 degrees.

– The display will not show which of the two frequencies does

lead or lag. Use a CR combination in front of the input of the
frequency tested. As the input has a 1 MΩ resistor it will be
sufﬁ cient to insert a suitable capacitor in series. If the ellipse
increases with the C compared to the C short circuited the
test signal will lead and vice versa. This is only valid

degrees. Hence C should be large and just create a barely
visible change.

If in XY mode, one or both signals may disappear, showing only
a line or a point, mostly very bright. In case of only a point there
is danger of phosphor burn, so turn the intensity down imme-
diately; if only a line is shown the danger of burn will increase
the shorter the line is. Phosphor burn is permanent.

Measurement of phase differences in dual channel
Yt mode

Please note:

Do not use ”alternate trigger“ because the time

differences shown are arbitrary and depend only on the respec-
tive signal shapes! Make it a rule to use alternate trigger only
in rare special cases.
The best method of measuring time or phase differences is using
the dual channel Yt mode. Of course, only times may be read off
the screen, the phase must then be calculated as the frequency
is known. This is a much more accurate and convenient method
as the full bandwidth of the scope is used, and both ampliﬁ ers
are almost identical. Trigger the time base from the signal
which will be the reference. It is necessary to position both
traces without signal exactly on the graticule center (POSITION
1 and 2). The variables and trigger level controls may be used,
this will not inﬂ uence the time difference measurement. For
best accuracy display only one period at high amplitude and
observe the zero crossings. One period equals 360 degrees.
It may be advantageous to use ac coupling if there is an offset
in the signals.

In this example t = 3 cm and T = 10 cm, the phase difference in
degrees will result from:

° = —

360° = —

360° = 108°

or in angular units:

t 3

arc

° = —

= —

= 1,885 rad

Very small phase differences with moderately high frequencies
may yield better results with Lissajous ﬁ gures.

However, in order to get higher precision it is possible to switch
to higher sensitivities after accurately positioning at graticule