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Page Title: Shunt Method of Calibration Using Internal Signal Source
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antenna is used with tuning unit 2, 3, or 4
the measurement of broadband radio inter-
(20- to 1,000-mc frequency range).
ference (noise). Broadband interference is
p r o d u c e d by switching transients in elec-
t r o n i c and electrical equipment, such as
3. Shunt Method of Calibration Using
t h a t caused by the throwing of switches,
Internal Signal Source
vibrating electrical contacts of relays, ro-
t a t i o n of commutators, electrical arcing
a. General.  The signal path for cali-
o r sparking, and other sudden changes in
brating the test set by the shunt method of
e l e c t r i c or magnetic potentials. This type
calibration is shown in figure 1. Basically,
of interference normally consists of short-
a self-contained inpulse generator is used
d u r a t i o n pulses that have either a com-
a s a calibrating signal to produce a full-
scale meter indication. The setting of six-
p a r a t i v e l y regular or a random repetition
r a t e and amplitude. For measuring pur-
s t e p signal attenuator AT702 control de-
poses, broadband interference is ex-
termines the amount of rf and intermediate
pressed in terms of microvolt per unit of
frequency (if.) attenuation inserted into the
b a n d w i d t h , such as microvolt per kilo-
s i g n a l path. After the meter is calibrated
c y c l e (uv/kc) of bandwidth or microvolt
f o r a full-scale meter indication, the im-
p u 1 s e generator is deenergized. The
p e r megacycle (uv/mc) of bandwidth. In
these measurements, the peak value of the
magnitude of the rf signal under measure-
i n t e r f e r i n g signal is used.
ment (fig. 2) is then directly read from the
(1) Assume that a signal of 1 uv/mc of
meter scale.. taking into account the setting
bandwidth is applied to the input of
o f the six-step signal attenuator control.
a radio receiver with an equivalent
T h e following subparagraphs, based on
noise bandwidth of 1 mc.
placing the main unit calibration switch in
(2) The definition of a 1 uv/mc of band-
the SHUNT CAL position, give a brief func-
width signal is as follows: it is the
tional description of the calibrating signal
peak value of that broadband signal
w h i c h produces a specific output
b. Impulse Generator Circuit. With IM-
f r o m this receiver. If a cw signal
of 1 microvolt is then applied to the
ON-OFF switch S705 in the ON position,
i n p u t of this receiver, the output
t h e signal path is produced as follows:
will be equivalent to that produced
(1) R e p e t i t i o n rate generator and
by the broadband signal.
d r i v e r V703. Repetition rate gen-
d. A fourth application of this test set is
erator V703A is a resistance-
t h e measurement of the field intensity of
c a p a c i t a n c e type oscillator which
b r o a d b a n d r a d i o interference (noise).
produces a fixed output of approxi-
These measurements, expressed in mi-
mately 70 cycles per second (cps).
c r o v o l t per megacycle per meter (uv/mc/
This output, increased in amplitude
m ) , are used when the test set is located
b y impulse generator driver stage
in the far field of a radiating antenna. In
V703B, is fed to impulse generator
this application, the peak amplitude of the
n o i s e signal under measurement is indi-
(2) I m p u l s e generator G701. The im-
c a t e d on the meter. An antenna factor,
p u l s e generator produces abroad-
which accounts for the effective height of
band signal by the transient
t h e antenna (a above), and a correction
d i s c h a r g e of a direct-current (de)
f a c t o r for losses in the rf cable must be
voltage on a short length of coaxial
a d d e d to the meter indication. Typical
l i n e , working into a 50-ohm im-
graphs of antenna factors are provided in
p e d a n c e load. A mercury switch,
T M 11-6625-351-12. One graph is appli-
cable when the vertical or loop antenna is
operated by impulse generator
driver stage V703B, opens and
used with tuning unit 1 (150 kc to 30 mc
f r e q u e n c y range); a second graph is ap-
closes the coaxial line pulse-form-
plicable when the appropriate dipole
i n g circuit.

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