Greetings dacflyer,
dacflyer wrote:i never understood 50ohm - 75 ohm cables..
i never could detect a resistance.. whys it called that ?
The cables in question are transmission lines used
to convey RF energy (or high speed digital data).
There are two important characteristics of a transmission
line.
One is that the source impedance and load impedance
are the same. This is not "resistance" as
AC circuits have complex notation with both real
and imaginary components. The process is called
matching and allows the most efficient transfer
of energy over the transmission line. Long distance
high voltage powerlines have a charateristic impedance
to match the generator and sub-station transformers
at 60Hz (or 50Hz elsewhere).
The second one is that the transmission line is
not reactive at the desired operating frequency,
as energy will be reflected or attenuated at the
boundaries of the different impedance sections.
Measuring any cable with an ohmmeter will only
reveal it's DCR (DC resistance) which increases
with length. To measure the impedance of the
cable would require a TDR (Time Domain Reflectometer)
or NA (Network Analyzer) or similar RF test
equipment to measure complex impedances.
A transmission line can be an open feeder for
a broadcast transmitter tower for example, or a
cable that is designed to have a known, or
"Charactersistic", impedance that can be used
in a circuit with the same impedance.
Cables come in different styles and sizes, the
popular ones are coax and twisted pairs, and
they may also have shields or be bundled with
other conductors. Coax operates as a shielded
single-ended line, as the shield is usually also
system ground. Twisted pairs are not grounded
and each line has an equal and opposite signal
that cancels (balances) out. Twisted pairs are
used by the phone company and for professional
microphones. Other uses are for high speed
digital data.
The most abundant is coax for TV antennae and
cable TV feeds in the home, and these
are 75 ohms impedance. Coax replaced
twin-lead open feeders of 300 ohms impedance,
and so 4:1 balun transformers are often found
in adapters to go between the two cable types.
Professional video systems use a standard 75
ohms impedance cable, and require long runs
and high performance in broadcast plants that
in turn require qood quality connectors and cables.
Coax in 50ohm impedance was used for radio
applications originally on Navy ships as the
shorter antenna wire required lower feeder
impedances.
Modern instruments have 50 ohms connectors
for RF, TV and consumer appliances use 75
ohms, and high speed digital data cables (CAT5,
etc.) are 120 ohms characteristic impedance.
Cable impedance is only critical in instrumentation
to prevent measurement errors, and in both
low power and high power RF to prevent signal
loss. The GPS signals are low power and
high frequency as are cellphone receivers so in
both cases the circuits require good matching
and low-loss (gold or silver plated) connectors.
In the digital world the cable impedance only
shows up on long runs and is measured as BER
(bit error rate) rather than amplitude (dB) scales
used for analog signals.
Comments Welcome!
Any tips, ideas, experience with this? Thanks! Dave