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Old 20th November 2012, 12:22 AM   #1
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Default i2s cable/wire type

I have two pcbs - one is a usb/i2s receiver and the other is a dac with i2s input - there are six pcb holes for the i2s connections on each. Is it ok to use any copper cable type to connect these pcbs, or is a digital wire (eg 75R coax) required?
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Old 20th November 2012, 02:53 AM   #2
grufti is offline grufti  United States
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50R coax and/or ribbon cable for a short connection. Each signal needs to be close to its common/ground companion.
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Old 20th November 2012, 03:13 AM   #3
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Quote:
Originally Posted by grufti View Post
50R coax and/or ribbon cable for a short connection. Each signal needs to be close to its common/ground companion.
Thanks - any idea where to get 50R coax? I can't find any on Partsconnexion. The connection is barely 10cm, so ribbon cable is an option, if I can find some with 6 wires. I assume you mean the ribbon cable where the casing is all connected?

Why does the signal need to be close to the ground companion?
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Old 20th November 2012, 06:46 AM   #4
qusp is offline qusp  Australia
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if it doesnt have a nearby ground/return for every signal, it will affect the signal integrity, but also spray RF around your case. i2s signals, particularly MCLK are quite high frequency, they are up in the radio frequency bands. high frequencies/radio frequencies travel quite well through the air, as you know, because the air presents a low impedance at high frequencies, so if there is no lower impedance path nearby, it will just find its own way through the air to the nearest ground or signal trace or component in a fairly unpredictable way. return signals even on a ground plane, will tend to travel as close to the signal trace on the other side because naturally it prefers to be as close as possible.

i'll have a look if I have some spare, or alternatively you can buy some at element14. I wouldnt use ribbon cable really, mini coax is preferred

Last edited by qusp; 20th November 2012 at 06:49 AM.
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Old 20th November 2012, 07:15 AM   #5
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Just reaching here, maybe look for an application that uses it? For mini 75 Ohm coax I've cut S-video cables up. I've seen 50 ohm test leads. Maybe something like that would be easy and inexpensive to find?
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Old 20th November 2012, 08:12 AM   #6
grufti is offline grufti  United States
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u.fl cable assembies are inexpensive even by diy standards.
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Old 20th November 2012, 08:40 AM   #7
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qusp is generally correct. Though I do have to make a correction, the current flow is not via the air in this case. That would mean that the space between the signal and return conductor/pcb are ionised. Which I am reasonably sure isn't going to happen with the signals we're dealing with in I2S. I am certain he understands the issue (I've talked with him in person about it a few times already) most likely that he was in a hurry when he typed that out.

I felt across the forum this question comes up often enough that its worth ensuring it is written out somewhere.

Here is my take on behaviour of I2S signals and how the diy-er should treat them:

Consider that between the signal and return conductors there exists a magnetic field as well as electric field. An increased area between them decreases their efficiency and more of the magnetic and electric field radiates rather than being canceled out by the opposing field generated in the return conductor. This is to say that the signal behaves less like an electric transmission line; it starts to behave more as an RF transmission antenna.

With high frequency signals, a poorly terminated electrical transmission line will radiate out the 'open' end and have some portion of the signal reflected back down the transmission line. If the 'loop area' is large there will be radiation generated along the transmission line.

This comes down to two things:
  1. Each signal must have independent return path, so that each signal acts as an independent transmission path.
  2. The return path should be as close as practical to the signal.


This is to ensure both the signal sent and received correlate closely AND to minimise the radiated noise that will contribute to the noise in components nearby.

The distance at which the conductor separation becomes significant depends on the frequency of the signal (ie the combined harmonics needed to produce the 'square' wave). Refer here - Red Flags -I read that to mean that the distance between signal and its return conductor should be less than 1/20th of the highest harmonic needed to form the 'square' wave. Note that the 'squarer' the analogue waveform, the higher the number of harmonics required to produce that wave.


This is why the signal transmission is considered an analogue system and termination resistors help slow the rise time to reduce the band of frequencies required to produce the signal.


In the end, twisted pairs of copper may work quite well if the pcb has proper termination resistors for each signal AND the sending and receiving PCB have a nearby ground termination point at both ends.

For absolute best signal termination a signal will travel a pcb trace with a characteristic impedance matched to a termination resistor, leave the pcb and travel an impedance matched coax cable, land and the next pcb and have a pcb with similarly well designed pcb trace and termination resistor. For best performance of the termination of the cable the signal landing pad will have a circular return path termination around it. As the size of this is reduced, the performance increases. This is because it reduces the 'opening' at the end of the transmission line.


And this boys and girls, is why the smart guys on these pages use u.fl and w.fl terminations on all of their high speed signals. These coaxial cables tick all of the boxes, all you need to do is preach the good word to the designers for the devices at either end of the cable to keep up their end of the bargain; or speak with your wallet and support the guys who are 'doing it right'.


Chris
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Old 20th November 2012, 08:59 AM   #8
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Default Reflections

I think I missed some info on signal reflections.

For these I imagine my high school science light experiments. As light travels from air->glass, at the interface between these two some light is reflected back and some continues. This is exactly the same in this case. Think of electrical signal as your light and characteristic impedance as your optical density.


For this reason I believe that there is a disadvantage in soldered wire terminations where the impedance at the interface is not controlled.
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Old 20th November 2012, 12:14 PM   #9
marce is offline marce  United Kingdom
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As signal frequency increases the return current will follow more closly the path of least inductance, on a PCB it will track under the trace on the ground or power plane(that is hopefully adjacent to the tracking layers)on cables use either a co-ax or wo layer flexi cables with a contigous grouns plane.
The signals travel as a psuedo TEM wave or TEM wave dependant on the cable configuration or whether the trace is routed as microstrip (top or bottom layer, adjacent ground) or stripline(between two grounds). Any seperation between a signal and its return path will cause signalintegriy problems and increase the risk of EMC problems such as radiating noise (which is what Qusp was reffering to, not ionisation.) For ultimate signal integrity a signal should be tracked next to an adjacent ground plane so the e and h field terminate at this ground plane. The frequency spectrum of a square wave that we are concerned about is determined from:
Fknee=0.5/Tr, where Tr is the 10-90% risetime of the signal, this also determines whether a design is high speed or not.
The best sources of info for thie are Henry Ott for the EMC implications:
home page
And either of these two for signalintegrity:
beTheSignal.com
Signal Consulting, Inc. - Dr. Howard Johnson

A couple of links that help illustrate the points.
http://www.ultracad.com/articles/slots.pdf
Printed Circuit Design & Fab Magazine Online
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Old 20th November 2012, 12:18 PM   #10
marce is offline marce  United Kingdom
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Signal reflections are caused by impedance mismatches, and are treated by:
Reducing the signal Tr, thus reducing the current drive of the ransmitter, or by either a combination of or one off series and parallel termination. Effectively a reflection is due to to much energy being injected into the line and the reciever being unable to sink all the current.
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