Model 350C/D Section V
Paragraphs 4-1 to 4-8
SECTION IV
PRINCIPLES OF OPERATION
4-1. GENERAL. The basic rules for exponents of logarithms then
allow equation (4) to be written as:
4-2. The Model 350C/D is shown in simplifiedsche-
matic form in figure 4-1. In the complete schematic
(figure 5-6) note that eachattenuator section, 10 db and (5)
100 db, is composed of four segments, each basically no. of db = 20 loglo (%)
the same configuration as shown in figure 4-1. The
attenuator circuit ground is isolated from the cabinet 4-6. The values of attenuation factor given in table
ground by capacitor C1, to allow a floating input, Le. 3-1 are based on a voltage ratio assuming the resist-
an ac signal at a dc level. ance at the input and output is the same. Values for
Af are computed using equation (5) where Vi = Vin
4-3. ATTENUATION EXPRESSED IN DECIBELS. and V2 = Vout:
4-4. POWER AND VOLTAGE RATIOS.
'in 1
4-5. The basic equation for computing attenuation in Vout = VinAf or - =- Af (6)
decibels is based on a power ratio where P = power, Vout
V = voltage, and R = resistance:
Then substituting equation (6) in equation (5) gives
no. of decibels = 10 loglo 7
no. of db = 20 loglo (7)
(*)
V2
Since power is expressed as: P = - (2) Solving for Af gives
R
Equation (1) may be rewritten as:
1
(8) A= no. of db
v1 antiloglO 20
An example will check the value for Af given in table
no. of db = 10 loglo k) (3) 3-1 to 24 db.
1 1
- -
- (9)
antiloglO ( 20 24) antiloglO (1.2)
and if R1 = R2 then,
From a log table, the antiloglo of 1.2 is 15.85 and
no. of db = 10 loglo (4)
A = ---= 0.0631 (10)
f 15.85
4-7. REFERENCE FOR DB.
4-8. The db levels given in figure 3-1 are referenced
to a milliwatt of power, hence the term dbm. This
indicates that the logarithm is taken of a power ratio
where 1 milliwatt is the reference. For the 37 dbm
shown in figure 3-1E, equations (1) and (2) show that:
I1
- 1 1 1 / 502\
100 Q0 IO DB so-Y-116
\*I
ATTENUATOR ATTENUATOR
dbm = lo loglo
1 milliwatt
dbm = 10 loglo (5000) = 10 (3.7) = 37
Figure 4-1. Model 350C/D Simplified Circuit
01307-1
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