HP 8901B Operation And Calibration Manual page 124

5-8 Modulation and Audio
Modulation flatness
The HP 8901Bs Ratio Mode makes it easy to make relative measurements
such as modulation flatness.
1 . Connect your modulated source to the HP 89018's
INPUT.
2.
Select the appropriate modulation measurement on the HP 8901B,
3. Key
in a reference modulation and press the
RATIO key. (For instance,
(AM,
FM,
or
@W.
press
3
and then the RATIO key to establish a reference of 3% AM depth,
or 3 kHz FM deviation.) Or, simply press the
RATIO key by itself to
establish the signal present at the HP 8901Bs INPUT as the reference.
4.
Press the LOG/LIN key if you wish to toggle between
O h
and dB units.
5.
Vary
the modulation rate of your source across the audio
spectrum. The HP 8901B
will
display modulation flatness as
the change in measured modulation relative to the reference.
SA Down-Conversion The measurement range of the
HP
8901B can be extended above
1300 MHz for modulation and audio frequency measurements by
using a spectrum analyzer as
a
fixed-tuned receiver. (This is one
of three techniques available for extending the RF frequency range
of the HP 8901B. The other two are also shown in this chapter.)
SIGNAL SOURCE HP 89018
1. Connect the
signal
source, spectrum analyzer and
HP
89018 as
shown above.
2.
Tune the spectrum analyzer to the output frequency of the signal source.
Set the spectrum analyzer in its zero span mode with linear detection, and
select a resolution bandwidth that is wide enough to pass the input signal
(e.g. 3 MHz).
3.
Adjust the spectrum analyzer fine frequency control to peak the signal
on the spectrum analyzer display.
4.
Tune the
HP
8901B to the
IF
output frequency of the spectrum
analyzer and select the desired modulation or audio measurement.
(Note that residual FM measurements
will
generally be limited by the
phase noise characteristics of the spectrum analyzer's local oscillator.)