User manual

28
Subject to change without notice
5 Extended operating modes
5.1 Available modulation
types (MOD)
A modulated signal consists of
a carrier signal and a superim-
posed modulation signal. The
HMF2525 / HMF2550 offers the
following types of modulation:
AM (amplitude modulation),
FM (frequency modulation), PM
(phase modulation), PWM (pulse width modulation), and FSK
(frequency shift keying). The type of modulation is selected by
pushing the MOD key and choosing it in the soft key menu TYPE.
Only one type of modulation may be active at any time.
With AM the amplitude of the carrier signal will be changed by
the amplitude of the modulating signal. After selecting AM in the
soft key menu TYPE the modulation depth can be set from 0 to
100 % in 0.1 % increments (AM DEPTH). Internal or external mo-
dulation can be chosen. When external modulation was selected,
the carrier will be modulated with this external signal.
Fig. 5.2: Sine wave with amplitude modulation
External modulation signals are connected to the rear panel
connector MODULATION INPUT.
With FM the frequency of the carrier signal will be varied ac-
cording to the instantaneous value of the modulating signal,
the amplitude remains unaffected.
With phase modulation the phase of the carrier signal will be shifted
according to the instantaneous value of the modulating signal.
The socalled pulse width modulation (PWM) is only available
with the pulse waveform; it will be automatically chosen when
the waveform „pulse“ is selected.
The soft key menu item SHAPE offers the following modulation
waveforms, available for the modulation types AM, FM, PM,
and PWM:
– Sine Function
– Square Function
– Positive / Negative Ramp Function
– Triangle Function
– Noise Function
– Arbitrary function
Apart from the limitations given by the specifi cations it should
be kept in mind that waveforms freely defi ned and digitally ge-
nerated may contain high frequency harmonics far above the
signal frequency. When using arbitrarily defi ned signals their
possible effects on the circuits tested should be evaluated.
The parameters frequency, amplitude, and offset can be set
as described above, by using the SELECT functions waveforms
can also be recalled from RAM (random access memory), ROM
(read-only memory) or via USB. The menu item ROM offers
these signal waveforms:
– Sine Function
– Square Function
– Positive Ramp Function
– Negative Ramp Function
– Triangle Function
– Noise Function
– Cardinal Sine Function
– Exponential Rise Function
– Exponential Fall Function
The arbitrary waveforms for the HMF2525/2550 can only be defi -
ned via the interface (USB/RS-232, GPIB, LAN). Once a waveform
has been defi ned it can be stored in an EEPROM (nonvolatile
memory) and used like any predefi ned one. Additionally, stored
waveforms will be available in the listing of predefi ned wave-
forms. Please refer to the section „Remote Operation“.
The HMF2550 / HMF2525 offers a memory of 1 Mpoints resp.
512 kpoints for arbitrary signals. The Y axis represents the
amplitude and the X axis the time or phase values.
When defi ning a new waveform, it is not always necessary to
erase old data fully. The soft key menu EDIT offers the pos-
sibility of modifying existing waveforms or to use a portion of
an old waveform. (The internal EDIT function nwill work from
fi rmware 1.2)
Operation of the HMF2525 / HMF2550
Fig. 4.6: Arbitrary signal
Fig. 5.1:
Key’s for additional
functions