HP 8901B Operation And Calibration Manual page 216

Operation Model 8901B
COMMENTS
Frequency Modes Possible During the Selective Power Measurement
During the selective power measurement, the instrument must be properly tuned to the signal and must
be in one of the following RF Frequency measurement modes:
0
RF Frequency (the FREQ key, HP-IB Code: M5),
0
RF Frequency Error (the S, FREQ ERROR keys, HP-IB Code:
S5),
0
IF Frequency (Special Function 34, HP-IB Code: 34.0SP),
or
0
LO Frequency (Special Function 33, HP-IB Code: 33.0SP).
When To Use an External LO
A low-noise, external LO
is recommended for RF input frequencies above 300 MHz. Also, an external
LO
can be selected for lower phase noise and finer resolution in its frequency steps.
When an external LO is used, the internal LO should be manually tuned to an arbitrary frequency (such
as
100 MHz) to defeat automatic signal search. Refer to
RF Frequency 7Lning for tuning information.
(Note that the 455 kHz IF will be used.)
Frequency Offsetting
The IF frequency can be offset by one of the following three methods:
0
offsetting the RF input signal.
0
offsetting the external LO, or
0
offsetting the internal LO using the
fi
kHz or
4 )
kHz keys.
An increase in the RF input signal moves the
IF signal down the lower skirt of the IF bandpass filter.
An
increase in the LO frequency (either external or internal) moves the IF signal down the upper skirt.
Refer to Measurement Technique for an illustration of this.
The measurement specification may require that the frequency offset be made from the 6
dB
corner
frequency of the IF bandpass filter.
Finding the IF Peak
For maximum accuracy, the frequency offset should be relative to the frequency at which the IF signal
is peaked in the IF bandpass filter.
(IF filter ripple can be as high as
2 B.) Automatic tuning brings
the IF close to the peak, but the instrument controller does not specifically check for an IF peak.
Finding the
IF
peak is most easily accomplished by tuning the RF input signal (if this is possible) or
by tuning the external LO while Special Function 24.1, 24.3, or 24.5 is active. The RF input signal or
external
LO
is tuned until the displayed
IF voltage is at its peak. Finding the IF peak by incrementing
the internal LO is similar, but the active special function must be re-entered after each frequency
increment. Tuning requeires
a
range of only a few kHz.
In all the measurements, you can tune the input signal instead of the LO. This is the easiest way if
the source
is
tuneable. However, you must tune the source opposite to the direction you would tune
the
LO.
For example, if you tune the LO down, you would tune the source up in frequency.
3-102
Selective Power Measurements (Option 030)