HP 8901B Operation And Calibration Manual page 90

2-4 Tuning Modes
Manual Tuning The Manual Tuning mode allows you to set the
HP
8901B's tuning
at a desired frequency. This is done by keying in the desired
frequency and then pressing the
MHz
key. The manual tuning
mode can also be entered by pressing the
MHz
key by itself.
This
locks the instrument at the frequency it
is
currently tuned to.
In Manual Tuning mode, the HP 8901Bs local oscillator (LO) is locked
to the HP 8901B's internal crystal oscillator whether an input signal is
present or not. Once the LO is locked to the crystal oscillator in Manual
Tuning mode, tuning does not change unless a new frequency
is
keyed
in and the
MHr
key is pressed, or tuning
is
incremented up or down
using one of the increment keys,
tkHz
or
UkHz.
(Set an increment by
keying in the desired increment value and pressing one of the increment
keys. The instrument
will
increment by the keyed-in value each time
one of the increment keys
is
pressed.) Press the
AUTO TUNING
key or
the
AUTOMATIC OPERATION
key to exit the Manual Tuning mode.
Manual Tuning is a good choice when measuring an input signal
whose amplitude is less than -25 dBm or unstable, or where
multiple RF signals are present. If the input signal is
d r i f t y ,
however, it may be necessary to select the Track Mode function.
Track Mode The Track Mode function enables the
HP
8901B to track drifting input
signals from 10 MHz to 1300 MHz. Track mode can be selected with
either Auto Tuning or Manual Tuning by pressing the TRACK MODE
key.(A drifting signal will cause the
HP
8901B to place four dashes
in its display when Auto Tuning is selected or Error 01 when the
instrument is manually tuned.)
In Track Mode the Local Oscillator (LO)
is
locked to the input signal.
The instrument remains locked to the signal throughout each of the
LO'S tuning octaves. When an input frequency change causes the LO
to cross into a new octave, lock may be broken. If the instrument is
in Auto Tuning Track Mode, the HP 8901B will relock to the input
signal. If the instrument
is in
Manual Tuning Track Mode, the LO
will
return to the manually tuned setting after lock
is
broken. The octave
boundaries occur at 2 . 5 , 5, 10, 20, 40, 80, 160, 320, 640, and 1280 MHz.
Track Mode is the right choice when measuring drifty input signals.
However, certain operating limitations are present when using Track Mode.
These limitations are listed below:
0
Not allowed below 10 MHz
0
Not allowed with 455 kHz IF
0
Raises residual FM level
0
Attenuates low-rate FM on input signal
(use
for FM rates
>1
kHz only)