Lesson l/Learning Event 12
(5) DBm. This term refers to power level with respect to milliwatt,
whatever the source.
Thus dBm can represent either signal level, noise
level, or a combination of both.
The power level of 1 milliwatt is
indicated by the term 0 dm.
NOTE:
In American practice, unweighted measurement is normally
understood, applicable to a certain bandwidth which must be
stated or implied.
In European practice, psophometric
weighting may be implied, as indicated by content;
equivalent to dBm0p, which is preferred.
Only American
practice is discussed in this text.
(6) DBm0.
In noise power measurement, noise power in dBm can be
referred to the normal signal level at a point in the circuit.
This
measurement is designated dBm0.
(7) DBrn. Reference noise (rn) signifies a level 90 dB below 0 dBm,
or -90 dBm (1 picowatt). 0 dBrn is therefore equivalent to -90 dBm of noise
power, while 0dBm is equivalent to +90 dBrn.
When C-message weighting is
used to measure noise across the VF band, noise level equivalent of 0 dBm (1
milliwatt) at 1,000 Hz is +88 dBrn.
(8) DBrn (C-message)..
Weighting should always be indicated in
parenthesis.
DBrn (C-message) weighting is level of noise measured on a
line-measuring set with that weighting. Noise power is usually abbreviated
dBrnc when so weighted, and parentheses thereby become unnecessary.
(9) DBrnc0.
When noise power is measured on a set with C-message
weighting and referred to the normal signal level passing the point of
measure, the noise level is quoted as dBrnc0.
(10) DBrn (f1-f2). This expression indicates the measured noise level
compared with the reference of -90 dBm (0 dBrn) when measured across the
voiceband shown within the parentheses.
c. Calculations.
Noise levels in dBa and darn are measured by test
sets. Noise levels in dBa0 and dBrnc0 are not measured; they are calculated
by taking into account meter readings, circuit weightings, and relative
signal levels.
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