User`s guide

7
Figure 5: Screen display showing an FM modulated RF signal as it would
appear in the IF using direct demodulation mode, but without the FM
demodulation function applied.
Zoom FFT Mode
With this mode, the user
specifies the amount of zoom
used, from 1 to 128. Higher
zoom values result in better
accuracy, but slower
measurement times. Larger
zoom values are analogous to
smaller resolution bandwidths
in a spectrum analyzer.
A typical value which
represents a good trade-off of
accuracy versus measurement
time is 16. Each factor of two
increase in the zoom FFT value
will slow the measurement time
down by two as well. In this
mode, trace 3 (the carrier) is
defined as CH1, and trace 4 (the
modulation signal) is defined
as CH2.
In the zoom FFT mode, the RF
signal is sampled so that the
carrier gets mixed to the IF
near DC, and half of the
modulation sidebands are folded
around DC such that sideband
pairs of the same order end up
very close together (for AM,
there is only one pair of
sidebands). If this result was
displayed without the zoom
FFT, it would look like figure 6.
The zoom FFT is used to expand
the first order sidebands. For
FM, all sideband pairs higher
than first order are ignored.
Markers are used to measure
the amplitude and phase of the
sidebands, and the program
uses this information to perform
the necessary demodulation.
Normally, if a modulation
frequency of 1 MHz is used, the
first order sidebands will be
separated by 2 MHz. With the
zoom FFT mode, the program
sets the sampling frequency of
the HP 71500A such that the
sideband separation is reduced
by about twice the zoom factor.
Carrier
1st order
sideband pair
2nd order
sideband pair
3rd order
sideband pair
4th order
sideband pair
Area expanded by zoom FFT
Figure 6: Screen display of an FM modulated RF signal as it would appear
in the IF using zoom FFT mode, but without the zoom FFT applied.