Modulation Calibrators
One of the most difficult problems
involved in making very accurate
measurements of AM depth or FM
deviation is generating a pre-
cisely modulated signal to use as
a calibration standard. By order-
ing Option 010 a precise AM and
FM modulation standard is in-
cluded in the HP 8901A modula-
tion analyzer.
The AM standard is generated by
summing two identical 10 MHz
signals. When one of the signals
is switched on and off at a 10 kHz
rate, the result is 33.33 percent
AM depth. By internally measur-
ing any slight difference in the
levels of the 10 MHz signals the
analyzer is able to determine the
actual depth to ±0.1 percent accu-
racy. To further improve the
modulation envelope the rise and
fall transitions are smoothed to
eliminate ringing that might
otherwise occur when this signal
is measured.
The FM standard is generated by
square-wave modulating a VCO
with a nominal 33 kHz peak devia-
tion. By using the internal counter
to measure the upper and lower
frequency of this signal, the ac-
tual peak deviation is determined
to ±0.1 percent accuracy. To
prevent ringing, the square
wave is modified to a round
edge trapezoid.
Because the modulation stan-
dards are internal to the analyzer,
there is little need for metrology
laboratories to purchase separate
calibration standards. Also, be-
cause of the technique used, it is
easy to verify that the calibrators
are operating properly.
Post Detection Audio
Filters
The modulation analyzer has two
high-pass and three low-pass post-
detection audio filters for filtering
the recovered modulation. These
filters can be selected individu-
ally or in combination. Their cut-
off frequencies have been chosen
to match those needed for appli-
cations such as transmitter or
signal generator testing. The
>20 kHz filter is a Bessel filter.
It minimizes overshoot for
square-wave modulation so that
this type of modulating waveform
can also be accurately measured.
The modulation analyzer contains
four de-emphasis networks that
can be used in addition to the
audio filters. These are the ones
commonly used in FM communi-
cations—25, 50, 75, and 750
µ
s.
When selected, the de-emphasis
networks always affect the de-
modulated output. The user can
select whether or not the de-em-
phasis network affects the devia-
tion measured and indicated by
the display. The ability to select
either the actual or “de-empha-
sized deviation” increases the
usefulness of the modulation
analyzer in many applications.
5
Operation
Often instruments with state-of-
the-art accuracy require tedious
setup or highly skilled operators
in order to be used. This is not the
case with the HP 8901A Modula-
tion Analyzer. It provides excel-
lent accuracy while remaining
easy to use. The front panel is
simple, uncluttered, and easy
to understand.
The user need only select the
measurement to be made. There
is no need to tune, adjust levels,
or select the appropriate range;
the internal microprocessor does
all this quickly. Because the mi-
croprocessor determines the best
instrument settings, most mea-
surements require only a single
keystroke.
For those applications requiring
tuning to a specific frequency,
automatic tuning may be overrid-
den. This feature allows a single
signal to be selected in the pres-
ence of others but retains the
speed and convenience of the
rest of the automatic functions.
The user can also make measure-
ments relative to either a mea-
sured value or one entered from
the keyboard by using the ratio
keys. Relative measurements can
be expressed in either dB or per-
cent. This means that when test-
ing FM mobile transmitters a user
could enter 3 (kHz), depress the
dB key, and make measurements
in dB relative to 3 kHz deviation.
Similarly, in broadcast FM appli-
cations, deviation could be dis-
played in percent relative to
75 kHz deviation where 75 kHz is
defined as 100 percent. The user
can also enter a measurement
limit on the keyboard which will
cause the modulation analyzer to
signal whenever the measured
value exceeds the value entered
as a limit.