Electrical Impedance Matching and Termination
When computer systems were first introduced decades ago, they
were large, slow-working devices that were incompatible with each
other. Today, national and international networking standards
have established electronic control protocols that enable different
systems to "talk" to each other. The Electronics Industries
Associations (EIA) and the Institute of Electrical and Electronics
Engineers (IEEE) developed standards that established common terminology
and interface requirements, such as EIA RS-232 and IEEE 802.3.
If a system designer builds equipment to comply with these standards,
the equipment will interface with other systems. But what about
analog signals that are used in ultrasonics?
Data Signals: Input versus Output
Consider the signal going to and from ultrasonic transducers.
When you transmit data through a cable, the requirement usually
simplifies into comparing what goes in one end with what comes
out the other. High frequency pulses degrade or deteriorate when
they are passed through any cable. Both the height of the pulse
(magnitude) and the shape of the pulse (wave form) change dramatically,
and the amount of change depends on the data rate, transmission
distance and the cable's electrical characteristics. Sometimes a marginal
electrical cable may perform adequately if used in only short
lengths, but the same cable with the same data in long lengths
will fail. This is why system designers and industry standards
specify precise cable criteria.
manufacturer's recommended practices for cable impedance, cable
length, impedance matching, and any requirements for termination
in characteristic impedance.
possible, use the same cables and cable dressing for all inspections.
Cable Electrical Characteristics
The most important characteristics in an electronic cable are
In this page, we can only review these characteristics very
generally, however, we will discuss capacitance in more detail.
Impedance (Ohms) represents the total resistance that
the cable presents to the electrical current passing through it.
At low frequencies the impedance is largely a function of the
conductor size, but at high frequencies conductor size, insulation
material, and insulation thickness all affect the cable's impedance.
Matching impedance is very important. If the system is designed
to be 100 Ohms, then the cable should match that impedance, otherwise
error-producing reflections are created.
Attenuation is measured in decibels per unit length (dB/m),
and provides an indication of the signal loss as it travels through
the cable. Attenuation is very dependent on signal frequency.
A cable that works very well with low frequency data may do very
poorly at higher data rates. Cables with lower attenuation are
Shielding is normally specified as a cable construction
detail. For example, the cable may be unshielded, contain shielded
pairs, have an overall aluminum/mylar tape and drain wire, or
have a double shield. Cable shields usually have two functions:
to act as a barrier to keep external signals from getting in and
internal signals from getting out, and to be a part of the electrical
circuit. Shielding effectiveness is very complex to measure and
depends on the data frequency within the cable and the precise
shield design. A shield may be very effective in one frequency
range, but a different frequency may require a completely different
design. System designers often test complete cable assemblies
or connected systems for shielding effectiveness.
Capacitance in a cable is usually measured as picofarads
per foot (pf/m). It indicates how much charge the cable can store
within itself. If a voltage signal is being transmitted by a twisted
pair, the insulation of the individual wires becomes charged by
the voltage within the circuit. Since it takes a certain amount
of time for the cable to reach its charged level, this slows down
and interferes with the signal being transmitted. Digital data
pulses are a string of voltage variations that are represented
by square waves. A cable with a high capacitance slows down these
signals so that they come out of the cable looking more like "saw-teeth,"
rather than square waves. The lower the capacitance of the cable,
the better it performs with high speed data.