3
It can make more than just a
single form of measurement; it
combines the capabilities of
three separate instruments. The
HP 8901A Modulation Analyzer
has the capability of a frequency
counter for measuring carrier
frequency. It can measure RF
peak power with typical measure-
ment accuracy of ±1.5 dB. It can
also accurately measure modula-
tion and recover the modulating
signal. This allows the user to
make those measurements most
commonly needed to totally char-
acterize a signal. The modulation
analyzer can measure a signal’s
frequency, frequency drift, peak
power level, AM, FM, or
φ
M, and
AM and FM noise components.
It recovers the modulating signal
with very low added distortion for
audio analysis.
Besides combining several mea-
surements in one instrument the
HP 8901A Modulation Analyzer
makes a second contribution to
signal analysis—extremely pre-
cise modulation measurements.
Its ability to make highly precise
depth and deviation measure-
ments coupled with very low in-
ternal noise enables the analyzer
to characterize very accurate
signal sources. Modulation depth
or deviation accuracy is generally
<1 percent of reading. Residual
noise in a 50 Hz to 3 kHz band-
width is 0.02 percent for AM and
<8 Hz for FM at 1300 MHz carrier
frequencies, decreasing to <1 Hz
below 100 MHz.
The modulation analyzer is fully
automatic and all major measure-
ments can be made by pushing a
single key. The modulation
analyzer’s large digital display
shows measurement results with
excellent resolution and is easy
to read. All measurements can
be easily controlled remotely
and data transferred via the
Hewlett-Packard Interface
Bus (HP-IB).*
Frequency Measurements
In automatic operation the
HP 8901A Modulation Analyzer
has the performance of a high
quality 150 kHz to 1300 MHz
frequency counter. Resolution
is 10 Hz below 1000 MHz and
100 Hz above 1000 MHz. Sensi-
tivity is
−
25 dBm (12 mVrms)
below 650 MHz and
−
20 dBm
(22 mVrms) above 650 MHz.
Besides normal frequency
measurement capabilities the
analyzer’s counter has several
additional capabilities. Like most
frequency counters it will mea-
sure signals over a wide dynamic
range, >50 dB (22 mVrms to
7 Vrms), and is protected from
damage for signals up to 35 Vrms.
Unlike many frequency counters,
however, it automatically adjusts
itself as the input level changes.
There is no need to manually set
or adjust any input attenuator.
Because the modulation analyzer
is usually used to measure modu-
lated signals, its frequency
counter accurately measures
signals with significant levels
of AM modulation.
The modulation analyzer uses an
indirect technique for measuring
RF frequencies. Instead of count-
ing directly, the input signal is
down-converted to an interme
diate frequency (IF) using a mixer
and a local oscillator (LO). By
counting the frequency of both
the IF and LO and calculating
their difference, the modulation
analyzer can determine the
frequency of the input signal.
In automatic operation the ana-
lyzer automatically tunes to the
largest input signal and measures
its frequency.
In manual operation the user de-
termines the frequency to which
the modulation analyzer tunes.
When more than one signal is
present it is thus normally pos-
sible to select which signal is
counted. Entering the approxi-
mate frequency on the keyboard
causes the IF filter to eliminate
all but very close interfering sig-
nals. This allows the modulation
analyzer to selectively count sig-
nals other than the largest. Also,
because of the large IF gain of the
modulation analyzer, it is possible
to measure very low-level signals.
In manual operation the modula-
tion analyzer has sensitivity of
0.22 mV
rms
, and dynamic range
of >90 dB (0.22 mV
rms
to 7 V
rms
).
* HP-IB is Hewlett-Packard's implementation
of IEEE standard 488.
The HP 8901A Modulation Analyzer
is a complete measurement system for
accurately characterizing signals in the
150 kHz to 1300 MHz frequency range.