HP 8901B Operation And Calibration Manual page 104

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.
A l l
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 o f f . (For details on the instrument's
operation when in the Manual Tuning mode, refer to Chapter 2.)