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About ribbon cables at fairly high frequencies.

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Spatterlight:
Hi, I just tried to google the answer to this, but it seems to be ungoogleable.

From a practical point of view, how high frequencies can be transferred via a ribbon cable?  Let's say the common 1.27 mm type. I made some calculations on my SPICE software on transmission lines, but I'm not sure whether a ribbon cable can be regarded as a transmission line.

Really, the frequency isn't that important, it's how the transients will propagate that limits the length, I think.

In my particular case, I need to transfer bitrates at a maximum of 12Mhz. Can someone perhaps estimate the maximum length of the cable?  I'm using it with a ground wire inserted between the signal wires.

Also, I intend to insert resistors of some 22 ohms at the TX point.  And the receiver is a MCU that has some 5 or 10 pF on the inputs.

For me, 50cm ( 20 inch ) will be perfectly alright. Actually, it's a I2S interface, but it's insensitive to jitter. The only importance is that there are no bit errors. Possible ringings must have subsided after a maximum of 30ns.

Thanks

helius:
Unshielded ribbon cable may have common-mode noise immunity issues, which the twisted-pair type cable (AMP Spectrastrip) was designed to address. It's possible that the twisted pairs self-inductance limits the risetime, I'm not sure. Some ribbon cables used to connect computer busses were also sandwiched between aluminum shields.

T3sl4co1l:
I'd seriously reconsider it as a medium by a few GHz.  Full shielding would probably be needed below that point.

SATA cables are basically full-shielded ribbon cables made of better-performing materials.

As for range vs. frequency, given reasonable drivers and receivers -- PVC insulation isn't fantastic, so don't expect any miracles.  At 12MHz, and average logic type transceivers, it's probably 10s of meters, maybe a bit over 100.  Further than you would ever want to run a ribbon cable (for a number of reasons)... ;D

12MHz, that's comparable to the bandwidth of old school hard drives -- cool fact, the ST506 interface used single-ended (TTL level) control signals, chained between one or two drives and terminated with a resistor pack in the control board and in the last drive; and differential (RS-422 level) data signals (from/to the head amplifier, point-to-point -- no split cable allowed), terminated with 120 ohms or so.  It was high tech in 1980. :D

I would be more concerned about the EMI implications of the arrangement -- if you're doing a one-off and don't have any particularly sensitive equipment nearby, you probably won't mind the emissions, but the susceptibility may still be an issue.  At the very least, assign every other wire to ground, and connect them to ground at both ends.  This gives a typical about 100 ohm impedance for the each signal line inbetween, so yeah a little source termination resistance is desirable (most logic devices have a 50-100 ohm source resistance, so a little extra to make up the difference is about right, or more can be used to slow the risetime further).

50cm is short.  Short enough that inter-integrated circuit (I2C, I2S, SPI, etc.) signals are very possible, even in a full commercial setting, without a shielded enclosure, testing to EMI standards.  In addition to interleaved grounds, it might also need some ferrite beads, and cautious filtering at the transmitter and/or receiver.  RS-422 signalling would be wise, but may not be required.  (I would strongly recommend RS-422 and/or shielding for anything much longer.)

Tim

jbb:
You can probably do it with LVDS. It’s differential which will improve radiation and noise rejection.

langwadt:

--- Quote from: T3sl4co1l on July 23, 2020, 01:00:45 am ---12MHz, that's comparable to the bandwidth of old school hard drives --

--- End quote ---

afair parallel-ata went up to 16MHz DDR before it went to the 80 conductor cable with the extra grounds

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