4
The ability to measure low
residual FM is one of the key
contributions of the modulation
analyzer. A low noise local
oscillator in combination with a
low noise discriminator allows
residual FM measurements of
<8 Hz at 1300 MHz and <1 Hz
below 100 MHz. This is low
enough to allow the direct
measurement of residual FM
of such low noise sources as
crystal oscillators.
For all AM depth and FM deviation
measurements the user can select
from three detectors. Both posi-
tive and negative peak (trough for
AM) can be measured. The modu-
lation analyzer also has an aver-
age-responding detector which is
RMS sine wave calibrated. This
type of detector is useful for
determining the residual noise
on a signal where the RMS value
and not the peak is generally the
desired measurement.
The modulation analyzer also has
a Peak Hold function that is used
with either the positive or nega-
tive peak detectors. This function
captures and displays the maxi-
mum peak modulation of a signal
and is ideal for making measure-
ments such as modulation limiting
on mobile radios. Peak Hold can
capture even very short tran-
sients and display their peak
value indefinitely. Measurements
can be made for any length of
time and either the largest posi-
tive or negative peak that occurs
will be measured. Pushing the
Peak Hold key resets the
display and initiates a new
measurement cycle.
RF Power Measurements
The HP 8901A Modulation Ana-
lyzer uses a diode detection cir-
cuit to measure RF input power.
This technique measures peak
voltage and is calibrated from
1 mW to 1 W for sine wave inputs.
In the case of amplitude modu-
lated signals, the modulation ana-
lyzer measures the peak envelope
power with ±1.5 dB accuracy,
thus eliminating the need for a
power meter in most applications.
The modulation analyzer is
equipped with input power protec-
tion to prevent damage from the
accidental application of exces-
sive power. This is a common
cause of damage in equipment,
such as the modulation analyzer,
used to measure transmitters.
The modulation analyzer is tested
for inputs up to 25 watts. Protec-
tion is provided by limiting diodes
and an RF relay. When excessive
power is applied the relay opens
and protects sensitive compo-
nents, and the analyzer displays
an error message. The circuit
automatically resets whenever a
key is depressed. This technique
is superior to fuses which in many
cases are too slow for adequate
protection and require replacement
each time an overload occurs.
In addition to normal RF level
measurements made directly on
the input signal, the modulation
analyzer can measure the signal
level in the constant-gain IF filter
pass band. This is the Tuned RF
Level function. In this mode the
analyzer accuracy is degraded
from normal RF measurements,
but relative power measurements
at a single frequency can be made
with increased resolution. Because
the IF filter allows some selectiv-
ity, one signal can be measured
even when others are present.
Modulation Measurements
In AM, high accuracy and low
noise are coupled with resolution
of 0.01 percent below 40 percent
depth and 0.1 percent resolution
to over 100 percent. AM signals at
rates up to 100 kHz can be mea-
sured and the modulation accu-
rately recovered. AM signals with
significant levels of FM can be
measured because of excellent
FM rejection.
Most AM depth measurements can
be made with accuracies better
than 1 percent of reading. This is
made possible by very linear am-
plifiers and detectors. Because
these amplifiers and detectors
are also low noise, residual AM
in a 50 Hz to 3 kHz bandwidth is
<0.02 percent rms.
FM deviation can be measured
with accuracy of 1 percent and
displayed with resolution ranging
from 1 Hz for deviations below
4 kHz, to 100 Hz for deviations
greater than 40 kHz. Modulation
is recovered with less than 0.1
percent distortion, and most
AM is rejected.