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Section Ш
Paragraphs 3-50 to 3-51
3-51,
Figure3-11illustrates a typical value of Noise
Figure that would be encountered
in a Model 415Е.
The
following
example
of Model
415E
noise figure
measurement is presented to illustrate the particular
considerations that must be made to calculate instru-
ment noise figure.
а. Calculate the meter
indication when a 5000 ohm
resistoris connected as a source, assuming the Model
415E is noiseless (0 db noise figure).
b. Any excess indication of 415E meter is then one-
half of its noise figure.
T
T
T
4]
|
Ap 415Е
9
INPUT AT HIGH
S3
в
IMPEDANCE POSITION
7
ul
5
86
u
° 5
°
24
3
2
І
Les
1250
2500
5000
IOK
50K
(OHMS —=)
SOURCE
IMPEDANCE (Rg)
setas
19
зс
„ттл
зо
9
hp 4\5Е
INPUT AT LOW
7]
З
IMPEDANCE POSITION
Rs
т
шб
a
©
5-5
ш
Фа
о
2
ЗЕ.
2
Th
4
|
25
50
100
200
409
(OHMS — )
SOURCE
IMPEDANCE (Rs)
mnm
Figure 3-11.
415E Noise Figure Curves
3-10
Model 415E
с. Calculation example:
(1) Assume 415E with controls setfor the following
conditions: EXTAL IMPED HIGH (input imped-
ance 200K), 1 uv RMS sine wave at input causes
full scale deflection (0 оп Oto 10 DB scale), 130
cps bandwidth at 3 db points (1.5 db points on
Model
415Е
meter
which
is calibrated
for
square-law.
Noise equivalent bandwidth is 7/2
times 3 db bandwidth).
(2) The open-circuit noise voltage across а 5000
ohm resistor at 295^ K (22°C) in a bandwidth of
(130 times 7/2) cps is as follows:
Y з
Z2IKTBR
v= Жі,38к10:29) (295130
x 7/2) (5000)
(3) The 0.129 цу open circuit voltage is reduced to
0.126
uv by the 200K ohm
input resistance of
the 415E which is assumed to be noiseless.
(4) 0.126 цу is 18.0 db below 1 uv but square-law
calibrated
meter
of the 415E,
set as
above,
would indicate 1/2 of 18 db or 9.0 db below full
Scale.
Also, since the 415E meter is average-
reading and calibrated to read RMS value of a
sine wave, it reads 1.05 db below the RMS value
of Gaussian noise.
Therefore,
the 415E reads
1/2 of 1.05 db or 0.525 db less than 9.0 and the
415E
would
read
9.525 db with а 5000
ohm
resistor
connected
to the input as described.
Hence, a 7.525 db
(9.525 - 2) meter reading
indicates a 4 db noise figure.
MEASUREMENT
ERROR
(DB)
2
4
6
8
(9)
12
SIGNAL
PLUS NOISE MINUS
NOISE ALONE (DB)
везла
Figure 3-12.
Meter Noise Correction Curve
02152-3
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