HP 8901B

Operation Model 8901B
WIOE 455kHZ
INPUT EP OR 2.5MHZ
ATTENUATION ,.,-.,. LP FILTER
AM
y$p~ OEMOOULATOR 7 VOLTMETER 7
run 1F IYC
(OPTION SERIE5 030 ONLY1
Selective Power Measurement Simplified Block Diagram
Initializing
The Modulation Analyzer must be set up for the first part of the measurement which establishes the
IF reference level. This is most easily done by keying in Special Function
24.0 which
0 holds the RF input attenuation;
0 sets the IF to 455 kHz, retunes the LO to accommodate the 455 kHz IF, and sets the LO to the
manual-tune, low-noise mode;
0 inserts the RF Input High-Pass Filter (which limits the lowest RF input frequency to 10 MHz
but filters
any low-frequency RF input signals from interfering with the measurement); and
0 disables Error Messages 01 through 04.
The Measurement Sequence
The first step in the measurement sequence is to tune the LO to the RF input signal to center the IF
in the IF filter passband. Refer to (a.) in the figure "Signal Tuning Illustrating Frequency Offsetting".
After the instrument
is properly tuned the IF (in-band) level is measured with the IF RMS Detector.
The in-band IF level is defined
as a ratio reference.
Next the frequency is offset either by tuning the RF input signal (if this is possible) or by tuning the
LO as illustrated in (b.) and (c.) of the same figure. Note that tuning is in opposite directions. If it is
desired to offset the IF to the upper skirt of the IF filter, the RF frequency must be decreased or the
LO frequency increased.
After the signal is offset, a second (out-of-band) IF level measurement is made with the same IF RMS
Detector. Since the IF level has decreased considerably, the IF gain
is adjusted to bring the IF level
within the optimum range of the IF RMS Detector. The ratio of the out-of-band
IF level to the in-band
level is computed and displayed. Out-of-band
IF signal appears in the IF passband because of
0 sideband noise on the RF input signal,
0 modulation sidebands on the RF input signal (if the signal is modulated),
0 sideband noise on the LO, and
0 the limited signal rejection in the stopband of the IF bandpass filter.
Since it is desired to measure only the first two contributions (that is, the contributions of only the
RF input signal), care must be taken to choose the correct frequency offset, the proper
LO, and the
proper IF filter.
3-100
Selective Power Measurements (Option 030)