User`s guide

8
When the program is running,
the top trace of the display
shows the first order sideband
pair, and the bottom trace shows
the baseband signal, but this
time the signals are centered on
the screen (figure 7).
With the zoom FFT mode, both
amplitude and phase versus
frequency can be measured,
with no additional measurement
time.
Marker Options
(zoom FFT on)
When the zoom FFT mode is
selected, the group delay
program defaults to placing the
marker on signals using the
highest peak function. This
allows the measurement to be
done even if the LO in the DUT
drifts somewhat, causing the
sidebands to move around, or if
the modulation frequency is not
known exactly (this is often the
case when using an external
signal generator to generate the
modulation signal).
If there are contaminating
signals present, it is possible
that the marker will find an
incorrect signal which may be
bigger than a desired signal.
This can happen, for example,
when the DUT produces
significant carrier harmonics
that are not filtered away. To
avoid this, an option is available
to use direct marker placement
and the local peak function. In
this mode, the marker is set to
where the signal is expected,
and a local peak is done to
ensure that the peak of the
signal is found. However, if the
LO frequency of the DUT has
drifted enough, or has
significant amounts of residual
FM, or the modulation
frequency cannot be set
accurately enough, the marker
may be placed too far from the
signal for local peak to work
properly. This will result in a
bad measurement. Direct
marker placement with local
peak should only be used in an
environment with very stable
and accurately known signal
frequencies.
When using a manual
modulation source, it may be
necessary to fine-tune the
modulation frequency to match
the value used in the program,
to ensure correct marker
placement. If it appears that
neither marker mode is finding
the correct signals, then the
direct demodulation mode
should be used. The user can
choose between highest or local
peak, by using the markers
softkey under the Options
menu.
Figure 7: Screen display using zoom FFT mode. The upper trace shows the
first sideband pair of the modulated RF carrier. The lower trace shows the
modulation signal. Both signals are in the frequency domain.