HP 8901B

2-2 Tuning Modes
If You Need to Know:
Refer to:
Tuning Modes
0 How the Auto Tuning Mode operates
0 How the Manual Tuning Mode
Auto Tuning (2-2)
operates . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0 How Track Mode operates.. . . . . . . . . .
Manual Tuning (2-4)
Track
Mode (2-4)
Auto Tuning
Auto tuning allows the HP 8901B to automatically find and tune to the
input signal. It is automatically selected on Power-Up, or when the
AUTOMATIC OPERATION or INSTR PRESET key is pressed. Auto Tuning
mode is also selected by pressing the
Blue Shift key and then the AUTO
TUN
I NO key.
When the HP 8901B enters the Auto Tuning mode, it first checks for an
input signal below
2.5 h4Hz. If no signal is detected below 2.5 MHz, the
instrument then begins sweeping its local oscillator
(LO) from 1300 MHz
down to
2.5 MHz. The LO continues sweeping until a difference frequency,
created
by the mixing of the LO and the RF input signal, appears within
the IF passband. If no signal
is detected after multiple sweeps, the
instrument places
two dashes in its display while it continues to sweep.
When a signal does appear within the IF passband, the instrument performs
a series of tests to determine whether the signal it has found
is the
appropriate signal to tune to.
This testing enables the HP 89018 to Auto
Tune to the fundamental in harmonically-rich specbums or to a carrier
with high-depth
AM. Once the instrument determines that it is tuned to
the correct frequency, the
HP 89018’s LO is locked to an internal crystal
oscillator to ensure high-stability, low-noise measurements. If the input
signal changes after the
LO has been locked to the crystal oscillator in Auto
Tuning mode, the
LO automatically begins the sweep search again. Auto
Tuning mode
is a good choice when measuring a single, stable input signal.
Keep in mind that certain input signal conditions can make it difficult
for the HP
8901B to Auto Tune. The Manual Tuning mode is a good
choice when these conditions exist. For a
listing of these conditions,
refer to
Table 2-1. Input Conditions that May Require Manual Tuning.