Signal-To-Noise Ratio Measurements; Note - AMERITECH AM5 Instruction Manual

AM5(e) Classic Instruction Manual
6.6.1 Signal-to-Noise Ratio Measurements: The range of the S/N Ratio
measurements is 10 to 50 dB with accuracy as follows:
• 10 to 40 dB at ±1 dB
• 40 to 45 dB at ±2 dB
• 45 to 50 dB at ±3 dB
The measurement technique for S/N Ratio incorporates the following steps
which are automatically repeated while the measurement is being made:
1. The average detector measures the amplitude of a received holding tone
(1004 Hz) and keeps this value in memory. This is the "signal" reading.
The incoming signal range is -40 to +10 dB.
2. The 1004 Hz signal is notched out, and the RMS (or Q-PEAK) detector
measures the signal again. This is the "noise" reading.
NOTE: The "noise" reading, could be 0 to 40 dBrn (–90 to -50 dBm) for a –40
dBm holding tone, up to 40 to 80 dBrn (–50 to 0 dBm) for a +10 dBm holding
tone. If the "noise" reading is less than 20 dBrn (–70 dBm), the accuracy of the
computed S/N ratio is reduced to ±2 dBm unless CMSG or PSHO filter is used.
3. A computation occurs which determines the difference, in dB between the
"signal" (step 1) and the "noise" (step 2). The result is the signal-to-noise
ratio.
4. This value is displayed in the left display window as a range of 10 to 50
dB. The holding tone frequency is displayed in the right display window.
For example, a measured S/N ratio of 30 dB and 1004 Hz hold tone
would be displayed as follows:
SEND
MEAS
DISPLAY
For a line that is so quiet that the S/N ratio exceeds 50 dB, the left
window will show "oVEr" as illustrated below:
SEND
MEAS
DISPLAY
A very noisy line (worse than 10 dB), would show "undr" and the right
window would be blank. The HOLD TONE indicator will be lighted but
kHz is not. Tones outside the notch band are treated as noise and may be
filtered.
November 19, 1997
(18-0047) MEASURE FUNCTIONS
dB
dBm
dBrn
mSEC
HOLD TONE
dB
dBm
dBrn
mSEC
HOLD TONE
Page 6-7
KHz
SEC
MIN
CNT
KHz
SEC
MIN
CNT