ETL600 1KHW001489-EN ABB Switzerland Ltd
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]
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dBuLdBAdBmL
tsuts |
.
(18)
Example:
For the test socket on the RF hybrid, we have A
ts
= 50 dB and the input impedance of
the measuring set is specified to 50 Ohms. A measured voltage level L
u|ts
of -10 dBu
indicates a power level L of 50 - 10 = 40 dBm equivalent to 10 Watts at the RF
connector, which is the measuring point. If the measurement was made with a test
tone of level L
0
= 0 dBm0, the signal at the RF connector is shown to have a relative
level of L
rel = 40 dBr. This follows from equation (17).
5.2.10. Signal weights
A multi-purpose PLC equipment transmits different kinds of individual
signals such as speech, teleprotection and various modem and pilot
signals. Each of these signals occupies an associated frequency band.
All signals are summed up, the resulting sum signal is transposed to
RF and is amplified by the power amplifier, which is peak power
limited, the maximum power being specified by the PEP of the
amplifier. The sum of the PEPs of the individual signals is limited by
the maximum PEP of the amplifier, otherwise the amplifier will
saturate. It is therefore mandatory to keep track of the PEPs of the
individual signals that have to be transmitted.
A simple method is used to set the PEPs of the individual signals in
the transmitter before superposition. A dimensionless number called
“weight” is defined for each individual signal. The scaling is done such
that the PEP of each individual scaled signal is proportional to its
weight. There are two exceptions to this simple scaling method: The
weights of the MOD600 signal and of NSD600 guard signal are not
fixed but depend on the weights of the other signals.
A number of factors are taken into account when determining the
weights, such as
- the bandwidth occupied by the signal,
- the SNR for adequate performance,
- the PEP to average ratio of the signal,
- the importance of the service provided by the transmission of the
signal.
Example:
Given that the weight of a speech channel 300 to 2000 Hz is 1.41, what should be the
weight of a 100 Bd VFT channel?
Contribution 100 Bd VFT
channel
Speech channel
300-2000Hz
Difference (expressed
in dB)
Noise bandwidth 150 Hz 1700 Hz 10*log(1700/150) =
10.5 dB
SNR for adequate
performance
16.5 dB 20 dB 3.5 dB
PEP to average 2 dB 3 dB 1.0 dB
Total 15.0 dB
It turns out that the 100 Bd VFT channel is superior to the speech channel mainly in
two aspects: It has a smaller noise bandwidth and requires a lower SNR. The
differences of all contributions add up to 15 dB, so that for the 100 Bd VFT channel, a
weight of 0.25 should be chosen: 20*log(1.41/0.25)= 15 dB.
Configuration and Set-up November 2005 5-7
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