User guide

MS-DMT Guide v1.04
Released 05-30-2014
133
The signal to noise ratio (SNR) is defined as the ratio between the signal and noise levels, and is
usually expressed in decibels (dB). 0 dB means the ratio is 1, the signal and noise power levels
are the same. a 10 dB SNR means the signal power is 10 times the noise power, 20 dB means the
signal is 100 times (it is a log based scale). These are for power values, for voltage ratios the SNR
is twice the power value. A SNR of 0 dB would just be barely detectable, in practice you need a
few dBs for even a weak signal, and a SNR of 30 or 40 dB is considered an excellent quality
signal.
For a correlation to MIL-STD-188-110B 5.3.2.5 Performance requirements. TABLE XX. In terms of
S meter reading only, the International Amateur Radio Union (IARU) Region 1 agreed on a
technical recommendation for S Meter calibration for HF and VHF/UHF transceivers in 1981.
IARU Region 1 Technical Recommendation R.1 defines S9 for the HF bands to be a receiver input
power of -73 dBm. This is a level of 50 microvolts at the receiver's antenna input assuming the
input impedance of the receiver is 50 ohms.
S-reading
HF
Signal Generator emf
μV (50Ω)
dBm
dB above 1uV
S9+10dB
160.0
-63
44
S9
50.2
-73
34
S8
25.1
-79
28
S7
12.6
-85
22
S6
6.3
-91
16
S5
3.2
-97
10
S4
1.6
-103
4
S3
0.8
-109
-2
S2
0.4
-115
-8
S1
0.2
-121
-14