Coax Cables; Impedance Matching - Tektronix 1S1 Instruction Manual

Operating Instructions – Type 1S1
istics of a 50-ohm transmission line. This permits the use of
50-ohm coax cables for applying the input signals with mini-
mum signal loss or distortion.
When connecting the signal from the source to the Type
1S1, many factors must be taken into consideration including
loading of the source, signal losses in the cables, time delay,
coupling and attenuating the signal, and matching imped-
ances. This portion of the manual discusses these factor with
respect to the vertical input signal. Connections for applying
an external triggering signal are discussed later under "Trig-
gering the Display".

Coax Cables

Signal cables that connect the vertical signal from the
source to the Type 1S1 SIGNAL IN connector should have a
characteristic impedance of 50 ohms. Impedances other
than 50 ohms will cause reflections that will make it difficult
to interpret the display. High-quality low-loss coaxial cables
should be used to insure that all the information obtained at
the source will be delivered to the Type 1S1 vertical input.
If it is necessary to use cables with characteristic impedances
other than 50 ohms, suitable impedance-matching devices will
aid in the transition.
The characteristic impedance, velocity of propagation and
nature of signal losses in a coaxial cable are determined by
the physical and electrical characteristics of the cable. Losses
caused by energy dissipation in the dielectric are proportional
to the signal frequency. Therefore much of the high-fre-
quency information in a fast-rise pulse can be lost in a very
few feet of interconnecting cable.
Fig. 2-6. shows the relative increase in output signal rise-
time when a step input signal is passed through several types
of commonly used 50-ohm coax cables. This increase in out-
put risetime must be taken into consideration when making
risetime determinations. For example, a length of cable with
a 225-psec output risetime will degrade a 500-psec input rise-
time by about 10%. (This may be determined by the "root
of the sum of the squares" formula.) As can be seen from
Fig. 2-6. Output signal risetime in response to a step input, given as
a function of cable length for some common coaxial cables
2-10
the graph, it takes only 6 feet (9 nsec) of RG-58A/U cable to
cause this 10% change. However, it will take about 15 feet
(22.5nsec) of RG-8A/U or 80 feet (95nsec) of Spir-o-line to
cause the same amount of change. If signal delay greater
than 60nsec is required, the use of a Tektronix Type 113
Delay Cable is recommended.
It is important to note that the risetime of the transmitted
signal deteriorates approximately in proportion to the square
of the length of the cable. As an illustration, a 500-psec rise-
time would be increased to 1000psec (100% increase) by
a length of cable with a risetime of about 867 psec. From
Fig. 2-6 it is seen that approximately 10 feet (15 nsec) of RG-
58A/U, 25 feet (37.5 nsec) of RG-8A/ U or 115 feet (137 nsec)
of Spir-o-line would cause this amount of risetime change.
Comparing this result with the previous determination, it
is seen that a 67% increase in cable length would produce
a 900% increase in risetime deterioration for the 500-psec
pulse risetime.
Due to the high-frequency losses in coax cables, the
0-50% half-amplitude risetime ("T nought", T
instead of the 10-90% measurement that is used with ampli-
fiers. T is approximately equal to 1/30th the 10-90% rise-
0
time of the coax cable output.
Occasionally, it may be desirable to use long 50-ohm
cables to move reflections out of the "time window" of
interest (delayed by double the transit time of the cable).
Keep in mind, however, the degrading effect that long
lengths of delay cable have on the pulse risetime.
Coupling
If there is a dc voltage greater than ±2 volts associated
with the signal source, a coupling capacitor must be used to
block the dc voltage. If a capacitor is not used, the signal
will be offset and will not be positionable on the screen.
In addition, voltage in excess of ±5 volts may damage the input
circuitry.
Attenuation
Maximum signal amplitude that should be applied to the
Type 1S1 SIGNAL IN connector is ±2 volts, combined dc
and peak ac. If the signal amplitude at the source is too
great, use an attenuator probe and/or externally-connected
T-attenuators. The attenuators that are used must have a
bandpass to about 2 Gc to avoid reducing the performance
of the system. High-quality 50-ohm attenuators are available
with attenuation factors of 10X, 5X and 2X. When the
attenuators are stacked, their attenuation factors multiply.
Thus, two 10X attenuators produce a 100X attenuation
factor.

Impedance Matching

To provide a smooth transition between devices of different
characteristic impedance, each device must encounter a
total impedance equal to its own characteristic impedance.
Thus, when a signal is applied to the Type 1S1 SIGNAL IN
connector, if the source impedance of the signal is not 50
ohms, a suitable impedance-matching device must be pro-
vided. If the impedances are not matched, reflections and
standing waves in the cables will result in distortion of the
.
displayed waveform.
) is often used
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