Frequency for ribbon cable

I’m using a FPGA to drive a matrix keyboard and I try to connect it with a ribbon cable. I have problems and I don’t know if it’s because of the cable. The signals are 20 ns pulses.

Some simple math says that the fundamental frequency for a 20 ns pulse is 50 MHz (T = 1/f). If you assume that a square wave needs at least 5 harmonics to look "square", then you need a cable that can support up to 250 MHz. A ribbon cable driven in a single-ended fashion is highly unlikely to work well up at those frequencies.

If you need to operate that fast, then look into using a high speed logic like PECL, LVPECL or LVDS to route the signal, then use translators at both ends to convert it back to TTL/CMOS/etc. Also, be sure to understand the characteristic impedance of the ribbon cable for a balanced, two-wire transmission line so you don't induce unintended reflections at your source and load. You may need to use small coax cables to route your high-speed signals.

Or, slow your data waaaay down. You might grab an oscilloscope and look at the output signal at the display. Have the FPGA send a running clock (101010101010...) and see how it looks. If it's distorted, then you need to reduce your speed to something that the ribbon cable can support, or improve your transmission medium (the cable) to something that supports higher frequencies.

I could use a lot more information pertaining to this subject (for ex: what is the characteristic impedance of ribbon cable, when to use coax vs twisted pair vs other cable types and at what frequencies). Does any information exist on this? I've looked but it seems hard to find.

Naive question: what the hell do you need 20 ns pulses for with a matrix keyboard?

How about slowing things down by oh lets say a factor of 1000? That should still give you plenty of time for a keyboard readout, AND you can suddenly use ribbon cable. Life is simple.

Agreed 20ns is ridiculously fast, your signals don't need to be anywhere near that.

BlackHelicopter said:

I could use a lot more information pertaining to this subject (for ex: what is the characteristic impedance of ribbon cable, when to use coax vs twisted pair vs other cable types and at what frequencies). Does any information exist on this? I've looked but it seems hard to find.

Click to expand...

A lot of the choice for transmission line type is determined by the application. If you running differential signals over long distances (and probably using a connector on the box), then you'd go with twisted pair. For unbalanced RF signals, you typically use coax so that you retain the ground reference at both ends of the cable (i.e. not differential signaling).

The frequency, signal power level and signal type (system impedance) will often drive the type of transmission medium. Most RF systems are 50 or 75 ohm and are well-suited to drive coaxial lines. High speed digital drivers may specify a 300 ohm transmission line, which is readily supported by twisted pair or other differential style transmission mediums.

The answers are mostly given by experience and looking at implementations of similar systems, or (hint hint) vendor recommended implementations.

enjunear said:

A lot of the choice for transmission line type is determined by the application. If you running differential signals over long distances (and probably using a connector on the box), then you'd go with twisted pair. For unbalanced RF signals, you typically use coax so that you retain the ground reference at both ends of the cable (i.e. not differential signaling).

The frequency, signal power level and signal type (system impedance) will often drive the type of transmission medium. Most RF systems are 50 or 75 ohm and are well-suited to drive coaxial lines. High speed digital drivers may specify a 300 ohm transmission line, which is readily supported by twisted pair or other differential style transmission mediums.

The answers are mostly given by experience and looking at implementations of similar systems, or (hint hint) vendor recommended implementations.

Click to expand...

Thanks for the advice, much appreciated!

In applications like IDE cable the ribbon is used in a signal-gnd-signal-gnd-signal-gnd scheme I think

Maybe you'll find this interesting:
http://www.signalintegrity.com/Pubs/news/3_10.htm

Thanks, I slowed the data speed and now it's working well.