HP 8901B Operation And Calibration Manual page 226

Model
8901B
Operation
RF Frequency Tuning
FUNCTIONS
TRACK MODE, AUTO TUNING, DISPLAY
FREQ
INCR,
kHz
$
and kHz
4
keys
DESCRIPTION
The Modulation Analyzer is considered tuned to an input signal when the frequency of the Local
Oscillator (LO) produces an
IF
signal centered in the
IF
passband. Normally the
LO
frequency is tuned
either 1 . 5 MHz or 455
kHz
above the input signal, depending on which
IF
is selected. When the input
signal is less than 2.5 MHz, the
LO
is tuned to 101.5 MHz and the input passes directly into the
IF'.
The Modulation Analyzer can be automatically or manually tuned
to
the input
signal:
In automatic tuning, the instrument searches for the presence of a signal over the entire input
spectrum. Once found, the
LO is tuned so that the signal is centered properly in the
IF' .
In manual tuning, the desired input frequency is entered via the numeric keyboard or
W z
fi and
kHz
keys, and the LO is tuned so that a signal of that frequency would be centered properly in
the
IF.
For both of these tuning modes, the
LO can be configured to lock itself to an internal crystal oscillator
(for low-noise measurements) or to track the input signal as it
drifts
(keeping the down-converted signal
centered in the
IF).
The four tuning modes are listed below:
0
Automatic
h i n g
Low-noise
LO
0
Automatic Thing; Track Mode,
0
Manual n n i n g ; Low-noise
LO,
and
0
Manual h i n g ; Track Mode.
Automatic Tuning-Low-Noise LO
If not already tuned, the Modulation Analyzer searches throughout its frequency range for an input
signal. If two input signals have similar power levels, the higher frequency signal is usually selected.
The
RF input signal's second and third harmonic levels must be at least
10
dB
below the level of
the fundamental, and
all
other
signals
at the input must be at least 30
dB
below the level of the
desired signal. Once tuned, the
LO
is locked to an internal, voltage-controlled, crystal oscillator for
highly-stable, low-noise measurements. T h i n g is maintained as long as a detectable signal is present.
If the input signal disappears (drops below -20 to
-25
a m ) , the Modulation Analyzer initiates a
signal search. This tuning mode is selected whenever Automatic Operation is pressed and when the
instrument powers-up.
Automatic Tuning-Track Mode
The instrument searches for the input signal in the same way as described in the previous paragraph;
however, it does not lock to the internal reference oscillator. Instead, the LO is locked
to
the input signal
itself and tracks it as it drifts. This function is useful for checking modulation or level as a function
of carrier frequency.
If
the input signal disappears (for example, when the
LO
switches
bands)
the
Modulation Analyzer searches for, and re-acquires the input signal. Track Mode tuning is not allowed
with the 455 kHz
IF
or with input signals below 10 MHz. Track Mode tuning somewhat attenuates
low-rate FM on the input signal; thus, FM measurements should only be made in track mode when
modulation rates exceed 1 kHz.
Also,
Track Mode tuning is not recommended where optimum noise
performance is required.
RF
Frequency %ing
3-115