# characteristic impedance

K

#### Krish

Jan 1, 1970
0
characteristic impedance of a line is square root of L/C. It does not
depend on the length. But the losses occuring in resistance, no matter
how small, is dependent on the length. Any literature to read on how
the length affects the charactristic impedance.
When doing circuit analysis, will this charactreritic impedance be
added to the series connecting element?
I have a device that says input impedance 50ohm, i am not using it.
But to use other part of the device i have terminate it with 50ohm.
How can i do it? can i just connect a 50ohm characterisitc impedance
cable and leave it?

A

#### Active8

Jan 1, 1970
0
characteristic impedance of a line is square root of L/C.

Something like that.
It does not
depend on the length. But the losses occuring in resistance, no matter
how small, is dependent on the length. Any literature to read on how
the length affects the charactristic impedance.

Most texts leave out the resistance as negligible, but I've seen it
figured in - just don't remember where.
When doing circuit analysis, will this charactreritic impedance be
added to the series connecting element?

Read up on VSWR, reflection coefficient, return loss, ... If a 50
ohm source is connected to a 50 ohm line terminated with 50 ohms,
you'd ignore the line impedance, but the line loss is another story.
There's different kinds of analysis, so exactly how you do it is
another story.
I have a device that says input impedance 50ohm, i am not using it.
But to use other part of the device i have terminate it with 50ohm.
How can i do it? can i just connect a 50ohm characterisitc impedance
cable and leave it?

No. You'd want a 50 ohm resistor. Whether or not you use the line is
up to you, but it would need to be terminated. You can buy
terminators that mate to different connectors. Good connections like
that keep water out and RF leakage down. You should probably
terminate the input whether or not you use it.

And your secret device isn't worth keeping secret, so if this

P

#### PeteS

Jan 1, 1970
0
Zo is sqr(L/C), to a close approximation, provided the following is
true:

1. The frequencies of interest are well above the skin effect knee
frequency. Above this frequency, R(s) is fairly constant (but not
completely).

2. Rs is negligible (per unit length) relative to Zo.

As noted, you should read up on transmission line theory and filter
theory (which is where that equation comes from). For most circuits,
provided the above are true, you can simply terminate to a resistor,
although many circuits designed for high frequency have terminators
internally. Some do, some don't.

This is something you can spend years on and still not understand
completely (there are many things in this field which have been found
empirically).

Note that to get a decent 'feel' for characteristic impedance (and
transmission line theory in general), you should be comfortable
analysing in the frequency domain and dealing with S-parameters

Cheers

PeteS

J

#### John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that Krish <[email protected]>
characteristic impedance of a line is square root of L/C. It does not
depend on the length. But the losses occuring in resistance, no matter
how small, is dependent on the length. Any literature to read on how
the length affects the charactristic impedance.

The full expression for characteristic impedance is (use Courier font):
________
/R + jwL
Zo = /---------
\/ G + jwC

where R = series resistance per unit length
L = inductance per unit length
G = shunt conductance (dielectric loss) per unit length
C = capacitance per unit length
When doing circuit analysis, will this charactreritic impedance be
added to the series connecting element? I have a device that says input
impedance 50ohm, i am not using it. But to use other part of the device
i have terminate it with 50ohm. How can i do it? can i just connect a
50ohm characterisitc impedance
cable and leave it?

No. Use a 50 ohm resistor.

M

#### Mac

Jan 1, 1970
0
characteristic impedance of a line is square root of L/C. It does not
depend on the length. But the losses occuring in resistance, no matter
how small, is dependent on the length. Any literature to read on how
the length affects the charactristic impedance.
When doing circuit analysis, will this charactreritic impedance be
added to the series connecting element?
I have a device that says input impedance 50ohm, i am not using it.
But to use other part of the device i have terminate it with 50ohm.
How can i do it? can i just connect a 50ohm characterisitc impedance
cable and leave it?

In real coaxial cable, the effect of resistance can be specified as a
certain amount of dB power-loss per foot of cable. For example, if the
cable has 10 dB of loss per foot, then 5 feet of cable would
absorb 50 dB of the total power.

Another effect to keep in mind is that as the frequency goes up, the dB
losses per foot usually go up. So if a particular cable has 10 dB of
loss per foot at 500 MHz, it may have 50 dB of loss per foot at 1.5
GHz. Note that I'm just making these numbers up.

So, to answer your question, this power loss doesn't really effect the
characteristic impedance of the cable. To a first approximation, the loss
can be modeled separately from the characteristic impedance.

Note that the above discussion assumes you have a resistive load at the
end of the transmission line whose impedance is the same as the
transmission line's impedance. For example, if it is a 50 Ohm transmission
line, you should have a 50 Ohm resistor (or 49.9) at the end as a load.

And to answer your other question, the best way to terminate a device at
frequencies up to UHF is to use a small surface-mount resistor (49.9 Ohms
in this case) as close as possible to the device. If you can't get close
to the device, for some reason, use a 50 Ohm cable with a 49.9 Ohm
resistor at the end of it. You can also buy 50 Ohm terminators for cables
and connectors. If the device has some kind of an RF connector on it, it
may be easier to use a terminator for it.

If the frequencies are down around 100 MHz or less, then you don't really
have to be all that close to the device or do anything fancy. Just put any
type of 50 Ohm resistor across the terminals somehow.

--Mac

M

#### Mark

Jan 1, 1970
0
in theory yes, a VERY long cable will look like 50 Ohms due to it's
losses ...

in practice, what you want is a termination plug

just by a 50 Ohm termination plug to connect to your source.

mark

D

#### Don Pearce

Jan 1, 1970
0
in theory yes, a VERY long cable will look like 50 Ohms due to it's
losses .

It will indeed look like 50 ohms - but this is nothing to do with its
losses. It is simply that power is propagated rather than dissipated.
Provided none is reflected back while you are measuring, the cable
will appear to be a 50 ohm resistor.
in practice, what you want is a termination plug

just by a 50 Ohm termination plug to connect to your source.

mark

d

Pearce Consulting
http://www.pearce.uk.com

R

#### Rene Tschaggelar

Jan 1, 1970
0
Krish said:
characteristic impedance of a line is square root of L/C. It does not
depend on the length. But the losses occuring in resistance, no matter
how small, is dependent on the length. Any literature to read on how
the length affects the charactristic impedance.
When doing circuit analysis, will this charactreritic impedance be
added to the series connecting element?
I have a device that says input impedance 50ohm, i am not using it.
But to use other part of the device i have terminate it with 50ohm.
How can i do it? can i just connect a 50ohm characterisitc impedance
cable and leave it?

A bit of thinking is required.
Apart what has been said, there is DC termination and
there is AC termination. The AC termination being an
additional cap in series with the 50 ohm.
It all depends whether a DC is required or not.

Rene

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