Hi,
In its simplest form Zo = sqrt ( L / C ) where L and C are per unit length.
The above does not take into account resistive / conductive losses,
consequently it is of little value at audio frequencies / long cables.
Telecoms uses ~ 600 ohms for audio frequencies whilst the actual
characteristic impedance at high frequencies will be nearer 100.
As far as I know it is far easier to model / calculate / estimate than
actually measure. There are lots of data standards for twisted pair
interfaces that yield values in the range 50 ohm to 150 ohm.
Spaced twin antenna lead (think cheap FM aerials) takes it up to 300 ohm.
FWIW most phono plugs mismatch the cable they are used to connect,
yeilding the idea of characteristic impedance a rather moot point.
/sreten.
In its simplest form Zo = sqrt ( L / C ) where L and C are per unit length.
The above does not take into account resistive / conductive losses,
consequently it is of little value at audio frequencies / long cables.
Telecoms uses ~ 600 ohms for audio frequencies whilst the actual
characteristic impedance at high frequencies will be nearer 100.
As far as I know it is far easier to model / calculate / estimate than
actually measure. There are lots of data standards for twisted pair
interfaces that yield values in the range 50 ohm to 150 ohm.
Spaced twin antenna lead (think cheap FM aerials) takes it up to 300 ohm.
FWIW most phono plugs mismatch the cable they are used to connect,
yeilding the idea of characteristic impedance a rather moot point.
/sreten.
http://www.sigcon.com/lib/htm/TWIST.htm
The simplest way of determining the impedance of a cable is with a network analyzer, effectively by measuring the reflected wave at the discontinuity with a known cable. You might be able to terminate the cable with a known resistor and use a ham radio SWR meter.
Cablle impedances are irrelevant where the cable is electrically 'short', i.e. less than a tenth of a wavelength.
w
The simplest way of determining the impedance of a cable is with a network analyzer, effectively by measuring the reflected wave at the discontinuity with a known cable. You might be able to terminate the cable with a known resistor and use a ham radio SWR meter.
Cablle impedances are irrelevant where the cable is electrically 'short', i.e. less than a tenth of a wavelength.
w
sreten said:[snip]FWIW most phono plugs mismatch the cable they are used to connect,
yeilding the idea of characteristic impedance a rather moot point.
Sreten,
What would be a good XLR or RCA plug for digital audio interface, like AES/EBU or S/PDIF? Are there any connectors that preserve the characteristic impedance?
Jan Didden
You can use a square-wave generator, a 'scope, a resistor and and a 200 Ohm variable-resistor to make a simple time-domain-reflectometer.
Simple drive a length of 15 metres or so of the cable through a 100 Ohm (or there-abouts) resistor from the square-wave generator. Use the 'scope to monitor across the line after the series resistor. If you do this with a 1MHz square wave, you should see, at about 150nsec after the driving edge, a small reflection from the far end. If you now try it with the far end of the cable shorted, you should see the reflection alter in polarity.
Use the variable resistor to terminate the far end of the cable, and as you vary the resistance, there will be a value where the amplitude of the reflection is at a minimum. This resistance value is close to the characteristic impedance of the cable.
Simple drive a length of 15 metres or so of the cable through a 100 Ohm (or there-abouts) resistor from the square-wave generator. Use the 'scope to monitor across the line after the series resistor. If you do this with a 1MHz square wave, you should see, at about 150nsec after the driving edge, a small reflection from the far end. If you now try it with the far end of the cable shorted, you should see the reflection alter in polarity.
Use the variable resistor to terminate the far end of the cable, and as you vary the resistance, there will be a value where the amplitude of the reflection is at a minimum. This resistance value is close to the characteristic impedance of the cable.
this I can manage.Ouroboros said:
Could an error here be causing the test signal I input into my amps to appear peaked rather than square/squarish?
If I'm sending a mixture of analoque and digital signals around the house over twisted pair varying between 10m and 100m long, do I need to know my characteristic impedance?
Does this impedance change with the varying twist found in CAT5?
janneman said:
Hi,
BNC's (bayonet connectors) aka oscilloscope inputs match
the cable impedance they are designed for (50 or 75 ohm).
XLR's AFAIK are hopeless.
Video phono's with circular crimping (like BNC's) are better than the usual.
(But what about the phono inputs - same issues apply as plugs .....)
For a DAI there are possible tweaks using matching resistors etc
at either end to improve matters but are no fixed rules AFAIAA.
/sreten.
AndrewT said:
Like I said, this is a good idea in theory, but hard to implement effectively.
It is impossible to say whether you require specialist cables and knowledge on the basis of 'a mixture of analog and digital signals'.
Signal at radio frequencies ~100kHz (3,000metre wavelength)up require increasing attention. The general rule is that cables under a tenth of a wavelength are 'short', which is a technical term in this context.
The speed of propagation is 300 million metres/sec, in a vacuum, and may fall to 200,000,000 in a cable, so at audio frequencies you could probably tolerate a kilometer.
w
so a 20kHz square wave into a 1m length of test lead cannot cause reflection problems.
But can improper termination cause peaking of the HF frequencies in the square wave?
Test signals pass through:-
50ohm source into 100/110ohm cable into 600ohm attenuator.
Then from 600ohm attenuator with an output load of 600ohm (Rs=300r) into 75ohm coax into RCA plug to DUT.
But can improper termination cause peaking of the HF frequencies in the square wave?
Test signals pass through:-
50ohm source into 100/110ohm cable into 600ohm attenuator.
Then from 600ohm attenuator with an output load of 600ohm (Rs=300r) into 75ohm coax into RCA plug to DUT.
In fact, it is the edge rate that is important for reflections, not the frequency. The frequency is relevant only for sinusoidal signals.
TDR technique can be used wery well using, say, 1 Hz square wave, if the edges are steep enough, rise/fall times below 1/10 of the round-trip time. For example, 100 ps rise/fall time pulse to 1 meter cable sure exhibits transmission line effects, regardless of the frequency.
Regards,
Janne
TDR technique can be used wery well using, say, 1 Hz square wave, if the edges are steep enough, rise/fall times below 1/10 of the round-trip time. For example, 100 ps rise/fall time pulse to 1 meter cable sure exhibits transmission line effects, regardless of the frequency.
Regards,
Janne
The key word is 20kHz *square* wave. If the rise and fall are very fast, you'll see reflections and ringing with no problem, even in a foot of cable, if the driving and terminating impedances don't match. IMO, feeding audio equipment with fast square waves, usually with an unknown rise and fall, is foolish. That signal qualifies as an RF signal, and can cause all sorts of problems that may not be relevant in actual use. In the audio regime, cable impedance is rarely even a valid concept. Now, it may be reasonable to make a conscious decision to feed some RF into your inputs, in the form of a square wave or other, to find out how the equipment reacts, but you want to do it with a known signal. When I do this, I use a 50 ohm generator, 50 ohm cable, and terminate the cable with 50 ohms right at the input to the equipment. Thus, the equipment is presented with a near perfect signal. IMO again, audio inputs (and sometimes outputs) should be filtered to prevent entry of RF signals.
Member
Joined 2003
not to completely upset the apple cart here, but "characteristic impedance" really doesn't apply here. an audio cable doesnt act as a transmission line. thats an RF phenomenon. you won't have any significant "reflection" happening in an audio cable less than about 1875 meters long (1/8 wavelength at 20khz). i don't know where the "audio cables as transmission lines idea came from, but it's incorrect. unless youre Ma Bell, you don't have any audio signal paths long enough to act as a transmission line.
DcibeL said:
Sure, but I would like to keep the original type connector in my equipment ;-)
I asked the local Neutrik rep, and they have a special XLR connector that they recommend for digital audio interfaces. It is NOT specified for 110 ohms, and according to him, there is no XLR 110 ohms connector in existence on this planet. However, it is designed with additional HF screening. Type numbers are NC3FXCC and NC3MXCC.
Jan Didden
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