3-6
Extending
Operation
Driffy Carrier
The HP
8901B
is
able to track drifting or noisy RF signals without sacrificing
measurement bandwidth or level sensitivity.
A
drifting input signal can
cause the instrument to continuously display four dashes (Auto Tuning
mode) or display Error
01
(Manual Tuning mode). Press the
TRACK MODE
key to select the IF-bandwidth track mode function. Track Mode can be
selected with either the Auto or Manual tuning modes. Note that the
Track Mode function reduces the low frequency response of modulation
measurements and increases
the
level of residual modulation. (For details on
the capabilities and limitations of Track Mode operation, refer to
Chapter
2.)
Frequency Overrange
There are three techniques for extending the HP
8901Bs
frequency
range above
1300
MHz: internal harmonic mixing, external local
oscillator and mixer down-conversion, and down-conversion with a
spectrum analyzer.
All
three techniques extend the
AM,
FM, and
@M
measurement capabilities of the
HP
89018
to microwave frequencies. Refer
to
Chapter
5
for measurement examples using these tuning techniques.
Frequency Underrange
The specified low frequency limit of the HP
8901B
is
150
kHz.
However, you can extend this measurement range to below
50
kHz.
The
procedure is simple. First, manually tune the HP
8901B
to
100
MHz
and override Error
01
by keying
in
8.1
SPCL.
This enables the input
signal to pass directly through the input mixer without frequency
conversion. Next, suppress IF feedthrough by selecting the
3
kHz
or
15
kHz
low-pass filter. The analyzer
is
now ready to make modulation
measurements. If you wish to display the carrier frequency, key in
34.0
SPCL
(press the
AM
key to return to the AM measurement mode).
Unstable Carrier
If you are measuring modulation on a carrier signal whose
RF
level does
not remain above
-20
to
-25
dBm (for example, a carrier that is being
pulsed on and
off),
select the Manual Tuning mode by keying in the
frequency of your carrier signal and then pressing the
MHz
key. In Manual
Tuning mode, the instrument
will
remain locked at the frequency of
the carrier after the carrier
is
turned
off.
(For details on the instrument’s
operation when in the Manual Tuning mode, refer to
Chapter
2.)
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