HP 8901B

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 drifty,
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)