User`s guide

15
The circuit ON/OFF
softkey is used to display or
disable the block diagram
drawing. Although the
drawing shows a connection
of the 10 MHz reference
between the sources and the
HP 71500A, it is usually not
necessary to do so.
4. Choose the options (Options
menu): the sglshot ON/OFF
softkey determines whether
the measurement is performed
in the single-shot or repetitive
mode of the HP 71500A. This
product note describes the
operation when the single-
shot mode is used, where the
modulation frequency must
be less than one half of the
single-shot sample rate,
which is usually between 10
and 20 MHz. The repetitive
mode can be used for higher
modulation frequencies, but
all of the frequencies involved
(including the LO in the DUT)
must be known much more
accurately than in the single-
shot case. In general, it is
recommended that everything
be locked together for the
repetitive mode. The zoom
FFT mode is not available
when sglshot is set to OFF.
The zoom ON/OFF softkey
determines whether the zoom
FFT mode (zoom ON) or the
direct demodulation mode
(zoom OFF) is used. When
using FFT zoom, the ZOOM
FACTOR softkey is used to
enter the amount of zoom
desired (see discussion of
Measuring in the
Frequency Domain on
page 6). This zoom factor is
only used during the actual
measurement, and not when
VIEW SPECTRM is pressed.
The markers softkey is used
to select between the two
marker modes, LOCAL or
HIGHEST (see discussion of
Marker options on page 8).
The phase ON/OFF softkey
determines whether a phase
trace is displayed below the
group delay trace. When
using the zoom FFT mode, an
amplitude trace is also
displayed when phase is set
to ON.
When set to ON, the invert
ON/OFF softkey adds 180°of
phase shift to each
measurement. This is
to account for some
modulators that perform an
inversion of the modulation
signal. This has the effect of
adding .5/fmod worth of delay
to all of the group delay values.
5. Set the sweep parameters
(Sweep menu): enter the
start, stop and step
frequencies of the RF source
with the top three softkeys.
If the source is to be swept
down in frequency, make the
start frequency higher than
the stop frequency, and enter
a negative value for the
frequency step.
Ideally, the frequency step
should be equal to twice the
modulation frequency for
uniform sampling across the
measurement band. This
provides full group delay
characterization with the
minimum number of data
points. Setting the frequency
step smaller than this fills in
the trace with more points,
but doesn’t provide any
additional group delay
resolution (over-sampled
case). If the frequency step is
larger than twice the
modulation frequency, a
warning message is displayed
on the screen to alert the
user that group delay
variations may be missed
(under-sampled case). Care
should be taken to make the
measurement aperture and
frequency step small enough
to resolve all of the group
delay variations which occur
across the measurement
band for the particular DUT
being tested.
The AVERAGE softkey is used
to enter the number of
averages used during the
measurement.
Before doing an actual
measurement, it is a good
idea to confirm that
everything is set up correctly.
One way to do this is to press
the VIEW SPECTRM softkey.
An FFT of the modulated
carrier is displayed, centered
at the DUT output frequency.
If no signal appears, check
that everything is correctly
set in the Config menu. This
spectrum display is very
useful for setting the right
amount of modulation when
a manual modulation
generator is used.
If the frequency span of the
spectrum is too wide to
clearly see the modulation
sidebands, then the zoom
transform can be used from
the standard HP 71500A
menus to expand the display
(increase the frequency
resolution). To access the
zoom transform, press MENU,
then Traces, then trnsfrm
control. Set zoom to ON
after setting the correct
CENTER and SPAN values.