User`s guide

6
Figure 4: Screen display using direct demodulation mode (zoom FFT off).
The upper trace shows the demodulated RF carrier. The lower trace shows
the modulation signal. Both signals are in the frequency domain.
Measuring in the
Frequency Domain
This program makes extensive
use of the Fast Fourier
Transform (FFT) to perform the
group delay measurement.
FFTs are used to demodulate
the carrier and compute the
phase of the baseband signal.
Zoom FFTs are also used as a
noise reduction technique.
A zoom FFT takes a larger time
record to get better frequency
resolution, and displays only a
portion of the frequency
domain data.
The program operates in two
basic measurement modes. The
recommended practice is to use
the zoom FFT mode. This
mode yields the most accurate
measurements. The user can
select the trade-off between
accuracy and measurement
speed. The greater the amount
of zoom used, the more accurately
the carrier and modulation
frequency must be known as well.
The direct demodulation mode
does not use zoom FFTs and
is generally faster than the zoom
FFT mode, but not as accurate.
It is less sensitive to frequency
inaccuracy, and should be
used when the carrier and
modulation frequencies are not
controlled well enough to use
the zoom FFT mode.
Trace averaging can be used
with either mode to further
improve measurement
accuracy. For a given level of
performance, the zoom FFT
mode is faster than using the
direct demodulation mode with
averaging.
Direct Demodulation Mode
(Zoom FFT Off)
This mode uses the built-in FM
demodulation routine of the HP
71500A. Trace 3, which shows
the demodulated carrier in the
frequency domain, is defined as
FM(CH1), and trace 4, which
shows the modulation signal in
the frequency domain, is
defined as CH2. Both traces are
transformed into the frequency
domain using the FFT. The
displayed signals appear at the
far left-hand portion of the
screen (figure 4). The markers
take one reading on each trace,
using the local peak function
to ensure the signal peak is
measured.
The RF signal is sampled such
that the carrier is centered in
the IF, and the modulation
sidebands are unaliased (not
folded about DC or the Nyquist
frequency). If they were
displayed, they would appear as
they would on a spectrum
analyzer (figure 5).
When using the direct
demodulation mode (zoom FFT
off), phase versus frequency
data is available, however,
amplitude versus frequency
information of the DUT is not
measured. Amplitude
information is available when
using the zoom FFT mode.