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Section HI
Paragraphs 3-50 to 3-51
Model 415E
3-50. MODEL 4!5i MOiSi FIGURE .
3-51,
Figure 3-11 illustrates a typical value of Noise
Figure that would be encountered in a Model 415E.
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.
a. Calculate the meter indication when a 5000 ohm
resistor is connected as a source, assuming the Model
415E is noiseless (0 db noise figure).
b. Any excess indication of415E meter is then one-
half of its noise figure.
SOURCE
IMPEDANCE(RS)
SOURCE
IMPEDANCE (Rs)
Figure 3-11. 415E Noise Figure Curves
c. Calculation example:
(1) Assume 415E with controls set for the following
conditions: EXTAL IMPED HIGH (input imped¬
ance 200K), 1 pv RMS sine wave at input causes
full scale deflection (Oon Oto 10 DB scale), 130
cps bandwidth at 3 db points (1.5 db points on
Model 415E meter which
is calibrated for
square-law. Noise equivalent bandwidth is n/2
times 3 db bandwidth).
(2) The open-circuit noise voltage across a 5000
ohm resistor at 295° K (22°C) in a bandwidth of
(130 times n/2) cps is as follows:
Vn = 2HKTBR
Vn = 2]|(1.38 x 10"23) (295/130xtt/
2
) (5000)
Vn = 0.129 x 10" 6 volts = 0.129 pv
(3) The 0.129pv open circuit voltage is reduced to
0.126 pv by the 200K ohm input resistance of
the 415E which is assumed to be noiseless.
(4) 0.126 pv is 18.0 db below 1 pv 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 a 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.
2
4
6
8
10
12
SIGNAL PLUS NOISE MINUS NOISE ALONE {DB)
Figure 3-12. Meter Noise Correction Curve
3-10
02152-3
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