Hameg HM504-2, Manual

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Subject to change without notice
26
The bandwidth of the X amplifier, is lower than the Y amplifier
and the phase angle which increases with higher frequencies,
must be taken into account (please note data sheet).
The Y signal applied at INPUT CHII can be inverted.
Lissajous figures can be displayed in the X-Y mode for certain
measuring tasks:
I
Comparing two signals of different frequency or bringing
one frequency up to the frequency of the other signal.
This also applies for whole number multiples or fractions
of the one signal frequency.
I
Phase comparison between two signals of the same
frequency.
Phase comparison with Lissajous figures
The following diagrams show two sine signals of the same
frequency and amplitude with different phase angles.
Calculation of the phase angle or the phase shift between the X
and Y input voltages (after measuring the distances a and b on the
screen) is quite simple with the following formula, and a pocket
calculator with trigonometric functions. Apart from the reading
accuracy, the signal height has no influence on the result.
The following must be noted here:
I
Because of the periodic nature of the trigonometric functions,
the calculation should be limited to angles
≤
90° However
here is the advantage of the method.
I
Due to phase shift, do not use too high a test frequency.
I
It cannot be seen as a matter of course from the screen display
if the test voltage leads or lags the reference voltage. A CR
network before the test voltage input of the oscilloscope can
help here. The 1MOhm input resistance can equally serve as
R here, so that only a suitable capacitor C needs to be
connected in series. If the aperture width of the ellipse is
increased (compared with C short-circuited), then the test
voltage leads the reference voltage and vice versa. This applies
only in the region up to 90° phase shift. Therefore C should
be sufficiently large and produce only a relatively small, just
observable phase shift.
Should both input voltages be missing or fail in the XY mode, a
very bright light dot is displayed on the screen. This dot can burn
into the phosphor at too high a brightness setting (INTENS.
setting) which causes either a lasting loss of brightness, or in the
extreme case, complete destruction of the phosphor at this point.
Phase difference measurement
in DUAL mode (Yt)
Phase differences between two input signals of the same
frequency and shape can be measured very simply on the screen
in Dual mode. The time base should be triggered by the reference
signal (phase position 0). The other signal can then have a leading
or lagging phase angle. In alternate triggering condition, phase
difference measurement is not possible.
For greatest accuracy, adjust the time base for slightly over one
period and set approximately the same height of both signals on
the screen. The Y-deflection coefficients, the time base coefficient
and the trigger level setting can be used for this adjustment,
without influence on the result. Both base lines are set onto the
horizontal graticule center line using the Y-POS.-knobs before the
measurement. With sinusoidal signals, use the zero (crossover
point) transitions; the sine peaks are less accurate. If a sine signal
is noticeably distorted by even harmonics, or if a DC voltage is
present, AC coupling is recommended for both channels. If it is
a question of pulses of the same shape, read off at steep edges.
It must be noted that the phase difference cannot be determined
if alternate triggering is selected.
Phase difference measurement in DUAL mode
t = horizontal spacing of the zero transitions in div
T = horizontal spacing for one period in div
In the example illustrated, t = 3 div and T = 10 div, the phase
difference in degrees is calculated from
or expressed in radians
Relatively small phase angles at not too high frequencies can be
measured more accurately in the X-Y mode with Lissajous
figures.
Measurement of an amplitude modulation
The momentary amplitude u at time t of a HF carrier voltage,
which is amplitude modulated without distortion by a sinusoidal
AF voltage, is in accordance with the equation
where: UT = unmodulated carrier amplitude
Ω
= 2 pF = angular carrier frequency
ω
= 2 pf = modulation angular frequency
m = modulation factor.
As well as the carrier frequency F, a lower side frequency F-f and
upper side frequency F+f arise because of the modulation.
Operating modes oft the Y amplifiers in Yt mode