Shunt Method of Calibration Using Internal Signal Source

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

path.

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

PULSE GENERATOR DB ABOVE lUV/MC

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

G701.

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.