LTC487
6
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PPLICATI
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I FOR ATIO
Losses in a transmission line are a complex combination
of DC conductor loss, AC losses (skin effect), leakage, and
AC losses in the dielectric. In good polyethylene cables
such as the Belden 9841, the conductor losses and dielec-
tric losses are of the same order of magnitude, leading to
relatively low overall loss (Figure 7).
FREQUENCY (MHz)
0.1
0.1
LOSS PER 100 FT (dB)
1.0
10
1.0
10
100
LTC487 • TA08
Figure 7. Attenuation vs Frequency for Belden 9841
When using low loss cables, Figure 8 can be used as a
guideline for choosing the maximum line length for a given
data rate. With lower quality PVC cables, the dielectric loss
factor can be 1000 times worse. PVC twisted pairs have
terrible losses at high data rates (> 100kbs) and greatly
reduce the maximum cable length. At low data rates
however, they are acceptable and much more economical.
DATA RATE (bps)
10k
10
CABLE LENGTH (FT)
100
1k
10k
100k
1M
10M
LTC487 • TA09
2.5M
Figure 8. Cable Length vs Data Rate
Cable Termination
The proper termination of the cable is very important. If the
cable is not terminated with its characteristic impedance,
distorted waveforms will result. In severe cases, distorted
(false) data and nulls will occur. A quick look at the output
of the driver will tell how well the cable is terminated. It is
best to look at a driver connected to the end of the cable,
since this eliminates the possibility of getting reflections
from two directions. Simply look at the driver output while
transmitting square wave data. If the cable is terminated
properly, the waveform will look like a square wave
(Figure 9).
Rt
DRIVER
DX
RECEIVER
RX
Rt = 120
Ω
Rt = 47
Ω
Rt = 470
Ω
LTC487 • TA10
PROBE HERE
Figure 9. Termination Effects
If the cable is loaded excessively (47
Ω
), the signal initially
sees the surge impedance of the cable and jumps to an
initial amplitude. The signal travels down the cable and is
reflected back out of phase because of the mistermination.
When the reflected signal returns to the driver, the ampli-
tude will be lowered. The width of the pedestal is equal to
twice the electrical length of the cable (about 1.5ns/foot).
If the cable is lightly loaded (470
Ω
), the signal reflects in
phase and increases the amplitude at the driver output. An
input frequency of 30kHz is adequate for tests out to
4000 feet of cable.