Heathkit IG-18 User Manual

dB MEASUREMENT
The red scale on the panel meter is used for
decibel (dB) comparisons. The decibel is a ratio,
or comparison, of power levels. It is defined by
the equation:
dB
10 log
PI
P
2
Since P = EI and I = E
R
equation may be restated as:
dB 10 log
E12
Rl
E ,2
_2
R
z
the above
While
the decibel is basically a relative ex-
pression between two power levels, it can be
used as a quantitative expression if one of the
levels is defined as a standard level.
After various levels in several industries were
partly accepted as "standard levels", the audio
industry settled on a standard level of 1 milliwatt
Page 49
of power into a 600 ohm load. This standard
level may be used in 600 ohm circuits only. In
these circuits Rl and R2 in the above equation are
equal and cancel out, simplifying the expression
to:
dB
10
log
E
J
2
==
- 2' or
E
z
dB
10
log (~): or
dB 20 log
E
J
E2
Accordingly, with the standard 600 Q load across
the signal output, you can read relative power
on a voltmeter, such as the front panel meter
on the Generator or any other voltmeter with the
appropriate calibration (dB scale).
As zero dB is defined as 1 milliwatt in a 600 Q
load, and P = E2 ,then.OOl = ~~ , orE = '\16-
R 600 • ,
or E = .775 V. It is for this reason that the zero
dB mark is in line with the 7.75 mark on the 0-10
meter scale.

Heathkit IG-18 Specifications

General

Brand	Heathkit
Model	IG-18
Category	Test Equipment
Language	English