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FIGURE 6.
Using Serial And Parallel Termination
Many applications, such as video, use a series resistance
between the driver and the transmission line (see Figure 1).
In this case the transmission line is terminated with the char-
acteristic impedance at both ends of the line. See Figure 6
trace B. The voltage traveling through the transmission line is
half the voltage seen at the output of the buffer, because the
series resistor in combination with Z0 forms a two-to-one volt-
age divider. The result is a loss of 6dB. For video applications,
amplifier gain is set to 2 in order to realize an overall gain of
1. Many operational amplifiers have a relatively flat frequency
response when set to a gain of two compared to unity gain.
In trace B it is seen that, if the voltage reaches the end of the
transmission line, the line is perfectly matched and no reflec-
tions will occur. The end point voltage stays at half the output
voltage of the opamp or buffer.
Driving More Than One Input
Another transmission line possibility is to route the trace via
several points along a transmission line (Figure 2) This is only
possible if care is taken to observe certain restrictions. Failure
to do so will result in impedance discontinuities that will cause
distortion of the signal. In the configuration of Figure 2 there
is a transmission line connected to the buffer output and the
end of the line is terminated with Z0. We have seen in the
section 'Connecting a load using a transmission line' that for
the condition above, the signal throughout the entire trans-
mission line has the same value, that the value is the nominal
value initiated by the opamp output, and no reflections occur
at the end point. Because of the lack of reflections no inter-
ferences will occur. Consequently the signal has every where
on the line the same amplitude. This allows the possibility of
feeding this signal to the input port of any device which has
high ohmic impedance and low input capacitance. In doing so
keep in mind that the transient arrives at different times at the
connected points in the transmission line. The speed of light
in vacuum, which is about 3 * 108 m/sec, reduces through a
transmission line or a cable down to a value of about 2 * 108
m/sec. The distance the signal will travel in 1ns is calculated
by solving the following formula:
S = V*t
Where
S = distance
V = speed in the cable
t = time
This calculation gives the following result: s = 2*108 * 1*10−9
= 0.2m
That is for each nanosecond the wave front shifts 20cm over
the length of the transmission line. Keep in mind that in a dis-
tance of just 2cm the time displacement is already 100ps.
Using Serial Termination To More Than One
Transmission Line
Another way to reach several points via a transmission line is
to start several lines from one buffer output (see Figure 3).
This is possible only if the output can deliver the needed cur-
rent into the sum of all transmission lines. As can be seen in
this figure there is a series termination used at the beginning
of the transmission line and the end of the line has no termi-
nation. This means that only the signal at the endpoint is
usable because at all other points the reflected signal will
cause distortion over the line. Only at the endpoint will the
measured signal be the same as at the startpoint. Referring
to Figure 6 trace C, the signal at the beginning of the line has
a value of V/2 and at T = 0 this voltage starts traveling towards
the end of the transmission line. Once at the endpoint the line
has no termination and 100% reflection will occur. At T = 10
the reflection causes the signal to jump to 2V and to start
traveling back along the line to the buffer (see Figure 6 trace
D). Once the wavefront reaches the series termination resis-
tor, provided the termination value is Z0, the wavefront un-
dergoes total absorption by the termination. This is only true
if the output impedance of the buffer/driver is low in compar-
ison to the characteristic impedance Z0. At this moment the
voltage in the whole transmission line has the nominal value
of 2V (see Figure 6 trace E). If the three transmission lines
each have a different length the particular point in time at
which the voltage at the series termination resistor jumps to
2V is different for each case. However, this transient is not
transferred to the other lines because the output of the buffer
is low and this transient is highly attenuated by the combina-
tion of the termination resistor and the output impedance of
the buffer. A simple calculation illustrates the point. Assume
that the output impedance is 5Ω. For the frequency of interest
the attenuation is VB/VA = 55/5 = 11, where A and B are the
points in Figure 3. In this case the voltage caused by the re-
flection is 2/11 = 0.18V. This voltage is transferred to the
remaining transmission lines in sequence and following the
same rules as before this voltage is seen at the end points of
those lines. The lower the output resistance the higher the
decoupling between the different lines. Furthermore one can
see that at the endpoint of these transmission lines there is a
normal transient equal to the original transient at the begin-
ning point. However at all other points of the transmission line
there is a step voltage at different distances from the startpoint
depending at what point this is measured (see trace D).
Measuring The Length Of A Transmission Line
An open transmission line can be used to measure the length
of a particular transmission line. As can be seen in Figure 7
the line of interest has a certain length. A transient is applied
at T = 0 and at that point in time the wavefront starts traveling
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