Canon fax basic 2000 서비스 매뉴얼

A–35

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8. RATIO, dB, dBm & dBV

(1) Ratio and dB

There are two ways of comparing values: by ratio and by dB indication

(logarithm indication).

Fig. A-36   Input and Output

When the power is compared on a circuit such as the above (Fig. A-36),

the ratio becomes

Take the common logarithm of this ratio and multiple it by 10 times:

This is called dB (decibel). Transforming the above equation results in the

following:

If the input and output impedance are equal, then the above equation

changes as follows:

The same applies to the current ratio

When these ratios of power, voltage and current are greater than 1, the dB

value is a positive value. When they are smaller, the dB value is a negative

value. So, a positive dB value expresses gain while a nagative dB value

expresses attenuation.

• Practical dB values

The following advantages are obtained from the logarithmic characteris-

tics.

Po

[times]

Pi

10 log

10

Po

[dB]

Pi

10log

10

Eo

2

/Zo

[dB]

Ei

2

/Zi

10 log

10

Eo

2

[dB]

=

20 log

10

Eo

[dB]

Ei

Ei

20 log

10

Io

[dB]

Ii

Zi

Ei

Pi

Ii

Io

Zo

Eo

Po

Circuit

network

where

: Input power
: Input voltage
: Input current
: Input impedance

Pi
Ei

li

Zi

: Output power
: Output voltage
: Output current
: Output impedance

Po

Eo

Io

Zo