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HEWLETT' PACKARD
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; f T E C H N I C A L I N F O R M
JOURNAL
A T I O N F R O M T H E - d p - L A B O R A T O R I E S
VOL. 1 No. 12
PUBLISHED BY THE HEWLETT-PACKARD COMPANY, 395 PAGE MILL ROAD, PALO ALTO, CALIFORNIA AUGUST, 1950
A New Monitor for
FM Communications Services
IN recent years the use of radio communica single FM (or PM) transmission, while the
tions has grown enormously in fields such Model 337B monitors up to four transmis
as land transportation, public safety, and sions. The latter instrument is similar to the
public utilities. The consequent crowding of Model 337A except for a panel switch that
available spectrum has resulted in FCC connects any one of the four carrier frequen
adoption of tighter carrier frequency toler cies to the monitoring circuits. All coil
ances for these radio services. At present changes are performed automatically by the
such services are, in general, required to panel switch and no plug-in coils are used.
maintain their carrier frequencies accurate The heart of these new monitors is the
within 0.01% when below 50 me and within pulse-counter type discriminator which was
0.005% when above 50 me. In addition, developed for the popular -hp- Model 335B
periodic measurements must be made to de FM broadcast service monitor1. This dis
termine that carrier frequencies are within criminator is extremely stable; in fact, no
FCC tolerances. zeroing control is necessary or provided for
The new -hp- Model 337 FM monitor the discriminator in the Model 337. The
shown in Figure 1 directly measures and con stability depends only on two small capaci
tinuously indicates both the carrier frequen tors and four resistors, for which precision
cy and modulation deviation of an FM trans parts are used. The reliability of the circuit
mitter operating in the range from 30 to 175 and the stability of its measurements have
me. Two models of the instrument are avail been proved thoroughly in the Model 335B
able. The Model 337A monitors the carrier and 337A-B monitors now in the field.
frequency and modulation deviation of a
Besides their primary function of indicat
ing carrier frequency and modulation devia
tion, the Model 337 monitors are valuable to
the station for making audio measurements
on the transmitter. With the use of a distor
tion meter such as the -hp- Model 330B, the
audio frequency characteristics of the trans
mitter are easily measured. In addition, a
monitoring amplifier can be driven from the
monitors for aural monitoring purposes.
In production work the Model 337 moni-
Figure 1. -hp- Model 337B FM Monitor can be used '. B. Schrock and D. Packard. A Pulse Counter Type FM Station
with four channels in the 50-/75 me range. Monitur, Proceedings of the National Electronics Conference, Vol.3. 1947.
P R I N T E D I N U . S . A . C O P Y R I G H T 1 9 5 0 H E W L E T T - P A C K A R D C O .