ribbon cable TDR test

I'm going to have one board in the front of a rackmount box, the controller, with an FPGA. In the back of the box will be an output board with an ADUM7703 isolated delta-sigma converter measuring current. The boards will be connected by an 18" ribbon cable. The signals are a 20 MHz clock to the ADUM and 20 mpbs delta-sigma data coming back to the FPGA. Both sigs are source terminated.

I was worried about signal integrity and didn't find much useful stuff online, so I tested a ribbon cable .

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The waveform and edge rate look fine, clean 390 ps rise at the end. The FPGA will have to deal with the prop delay.

I was going to derive a lossy-line Spice model of the cable based on these measurements (still might some day) but it looks like this cable will be plenty good enough.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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John Larkin
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Ribbon cable with every second wire grounded is pretty good. The next stage up is a balanced pair of signals on adjacent wires with grounded wires on either side of the pair.

The dielectric/insulator isn't chosen to give good very high frequency characteristics, but eighteen inches (half a metre) isn't far.

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Bill Sloman, Sydney
Reply to
Bill Sloman

fredag den 25. oktober 2019 kl. 00.57.24 UTC+2 skrev John Larkin:

IDE Harddrives managed 33MHz or something like that on ribbon cables, thought at the end they used the special cable with twice the conductors every other one ground

if two pairs and three grounds will do you could use SATA cables

Reply to
Lasse Langwadt Christensen

Any one care to comment on this related problem.

8 digital outputs (3.3V CMOS from FPGA (guess)) and other things on 37 way D connector, one ground pin for the digital outputs, at one end of the connector. I have to connect this, via about 30cm of cable, to my box and buffer the signals. The goal is to get the best result (for speed and decent pulses) that I can.

Plan A is to try with IDC D plugs on the cable and use ribbon cable with T networks of 0603 parts to be optimised in my box at the other end.

Plan B is to use tiny resistors in the cable D connector at the source end and T networks of 0603 parts to be optimised in my box at the other end. Cable to be whatever it takes.

Plan C is to have an active board at the source plug.

Plan D (doing it right with LVDS drivers in the source box) is too late because the source box has been bought and paid for already.

A good result would be decent 20ns pulses at 25MHz rate.

MK

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Reply to
Michael Kellett

l=0

s

an.

It's a nice short cable - about 1.5nsec of propagation delay - so it should n't be too difficult.

You should have enough connections to let you put a ground wire between eac h of the eight signal wires. This gives each line a characteristic impedanc e of between 110R and 130R (depending on the ribbon - measure it if you hav e to, but the manufacturer should be able to tell you if it isn't in the da ta sheet).

If you put a big enough damping resistor on each driver to boost the output impedance to this level, the signal will travel along the cable as half vo ltage step, but if your receiver has a fairly high input impedance, the ref lection at the receiving end will mean that you will see a full step at the receiver, and the should be very little reflection when this reflected ste p gets back to the driver.

Eight resistors at the driving end should be all that you need.

They only dissipated heat whole the cable capacitance is charging up (1.5 n sec in each 40nsec for a 25MHz signal rate) so 0603 should be fine (but che ck).

Okay. So what I am saying is that plan B should be fine, and the T network shouldn't be necessary.

If you need a high current drive to cope with getting your voltage swing in to a 110R t0 130R transient load (for 1.5 nsec in 40nec whenever you are se nding data at 25MHz, this might be necessary.

LVDS sounds like a bit of an overkill for a 25MHz data rate and a 30cm cabl e.

At 25MHz you should see steps no more frequently than once every 40nsec.

You want them to be reliably high or low when the receiving circuit looks a t them, which means getting a 25MHz clock - high for 20nsec, low for 20nsec - down the ribbon cable, and using balance clock signal minimises the amou nt of noise (cross-talk) injected into adjacent wires.

Not doing that certainly screwed up one system that I worked on, which had been fine when originally designed for a 5MHz data rate over a few metres o f ribbon cable, but got very cranky when the data rate got pushed up to 10M Hz and the cable stretched to 18 metres, when the receiving circuits starte d getting moved into clean areas.

Throwing in opto-isolators made life even more complicated. It wasn't hard to fix, but getting it right took a bit of care.

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Bill Sloman, Sydney
Reply to
Bill Sloman

Schmitt triggers are always good. You could leave the transmit end alone and lowpass+Schmitt each line on your end.

First guess, 100 ohms and 50 pF then a Schmitt, 5 ns time constant and a partial termination to damp any ringing.

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John Larkin         Highland Technology, Inc 

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Reply to
jlarkin

dl=0 You've basically reinvented IBM tri-lead, which they devised for the IBM

370 series. They used smaller wires, and got a 91 Ohm single-ended single-signal cable.

Jon

Reply to
Jon Elson

It's standard 3M ribbon cable. Wiki says "The ribbon cable was invented in 1956 by Cicoil Corporation."

I just couldn't find any data on pulse behavior.

I should test a much longer chunk, to see some serious drool, then hack a Spice lossy transmission line model to match.

I was happy with the rise time over a 2' length.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

can.

This is remarkably bad advice. The problem with fast signals on ribbon cabl es (and every other kind of transmission line) is reflections.

If you drive a 1.5nec long transmission line with a device that has a lower output impedance than the transmission line (about 120R in this instance) the amplitude of the signal is doubled when it hits far end (potentially da maging the receiver, or at least putting it into a state where it can behav e very strangely). Using a Schmitt trigger receiver won't help.

If the driver is fast enough to produce a 390psec rise-time, you will see t his problem.

At the driven end, the reflection will come back as the same double height signal after 3nsec, and wont do the driver any good either.

A 100R resistor as source terminator (at the driving end) should do all tha t is necessary.

Futzing around at the receiver end is the sort mindless twiddling that igno rant amateurs go in for.

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Bill Sloman, Sydney
Reply to
Bill Sloman

Some data sheets list the characteristic impedance. The trick of using a grounded wire between each signal wire makes the signal wires look like tolerable transmission lines.

You should think about what's actually going on - reflections and some attenuation of the higher frequency components in the current that charges up the transmission line and gets reflected at discontinuities in the transmission line - the ends, mostly.

Even Howard Johnson could do better than that.

When you should have been worrying about the reflections, if the rise-time were shorter than the roughly 3nsec propagation delay that you'd expect with two feet (61cm) of ribbon cable.

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Bill Sloman, Sydney
Reply to
Bill Sloman

On a sunny day (Fri, 25 Oct 2019 14:44:14 -0700) it happened John Larkin wrote in :

There are many qualities of ribbon cable, the cheap stuff I got from China has less copper[1], weaker plastic than the flat cables to my old harddisks. Never measured those.

[1] it is not copper I think...

It all depends, I have cheap ethernet cable from China that does not even have twisted pairs.

Reply to
Jan Panteltje

Thanks (JL and BS) for all the suggestions.

I'll try " Futzing around at the receiver end" first - not because it's good, but because it would be so much nicer to be able to use a ribbon cable with IDC connectors plugged straight into the source connector.

One of my (many) concerns re. the source box is that it has just one ground pin for many fast digital outputs. Unless I do lots of fancy work inside the connector most of my signals are adjacent to two other independent signals.

I'm hoping that the source box has some kind of series termination between connector and drivers but I don't know yet.

If I get the job I'll let you know how it goes.

MK

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Reply to
Michael Kellett

The traditional techniques are source termination or receiver termination, but not both.

Source termination inserts a series resistor R at the driver so that the driver's output resistance plus R equals the transmission line impedance, Z. Driver supplies V/2Z current.

Receiver termination has several options, each with benefits: 1 single resistor to ground Z=R 2 resistor R1 to Vcc, R2 to gnd, such that R1//R2=Z and the potential divider voltage is the receiver's threshold voltage 3 AC only version 1 inserts a capacitor in series

All those require the driver can supply double the current, V/Z.

1 asymmetrically loads the driver. 2 has a constant current through R1+R2. 3 is suboptimum except in special circumstances.

A schmitt trigger does no harm in properly terminated transmission lines. Your situation (1 gnd in a 37-way cable) is not going to have a well-defined impedance, and there will be more crosstalk, so a schmitt /might/ make the difference between it working or failing.

You might also like to consider the effects of static hitting either the driver or the receiver. That can result in catastrophic damage or more subtle parametric shifts.

Reply to
Tom Gardner

37

of

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t I can.

with

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her

too late

cables (and every other kind of transmission line) is reflections.

lower

ce) the amplitude of the signal is doubled when it hits far end (potentiall y damaging the receiver, or at least putting it into a state where it can b ehave very strangely). Using a Schmitt trigger receiver won't help.

see this problem.

ight

d

l that is necessary.

good, but because it would be so much nicer to be able to use a ribbon ca ble with IDC connectors plugged straight into the source connector.

It's not nice to run the risk of blowing up the receiver.

It's not wise to risk feeding current into the catching diodes on the recei ver - the current going into them can have unexpected effects. Even if they works as advertised, not blowing up the receiver is not the same as having it work properly while the inputs go outside the supply rails.

A T-network can stop that happening, but it messes up the waveform.

Source termination is a lot neater, and only needs one damping resistor per lead.

ound pin for many fast digital outputs. Unless I do lots of fancy work insi de the connector most of my signals are adjacent to two other independent signals.

The paths inside the connectors are pretty short, and there are going to be lots of grounded pins in the immediate vicinity too.

een

The source box should have a source termination resistor between each drive r pin and the corresponding pin on the connector.

The question is whether it's going to have the right value for your particu lar ribbon cable - it should be pretty much okay since most ribbon cables are pretty similar but you really need to at the driver and the cable befor e you can be sure.

You can do both. It halves your signal level, but really cuts down reflecti on at both ends of the cable.

But only while the cable is getting charged up (or down).

All the time.

ECL is designed to drive precisely such terminating resistors - signal swin g is from -0.6V (one) to -1.2V (zero), with the terminating resistor return ed to -2V

But it dissipates less heat.

If you link every second cable in the ribbon to that single ground you kill almost all the cross-talk.

30 cm of cable is likely to be well enough grounded at both ends to make th is an unlikely problem.

The electron microscopes at Cambridge Instruments had a 30kV supply for the electron gun, and when that flashed over there were unfortunate side effec ts.

Most of them went away when the gun got a proper 30kV coaxial connector. Th e flash-over currents stayed inside the coax cable, and didn't show up as b rief hundred ampere currents in inconvenient places.

--
Bill Sloman, Sydney
Reply to
Bill Sloman

Source termination would be great. It reduces the drive currents, which may be a benefit with your grounding. That would involve adding, say, 100 ohms in series with each line at the source, and maybe adding a cap to ground and a Schmitt at the receive end.

But you don't know if the source connector is actually close to the source! A series termination midway in a transmission line can get messy.

You should probably experiment with the real source and an oscilloscope and some parts. Or find out what's in the source box.

LT Spice transmission line models work pretty well.

One shared ground is scary. Can you use a cable shield as another ground?

Ask! It's a very reasonable question.

If it's a big job, adding a little driver board inside a connector shell wouldn't be a big deal.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
jlarkin

In most cases you "throwing away" voltage swing is suboptimum.

In special, well-designed, cases it might be beneficial, but clearly the OP's case doesn't fall into that category.

Yes, but that's long enough for problems w.r.t. ground bounce and crosstalk.

Of course.

ECL and "derivatives" are nice to work with, but I doubt they are relevant in this case.

That's about the only benefit.

Yup, but that isn't the case here :(

There's a minor industry devoted to just that problem!

If the cable is entirely within a cabinet, and basic anti-static precautions are taken when the cable isn't connected, then I wouldn't be too concerned. I don't know whether that is the case here.

Yes.

Reply to
Tom Gardner

ion,

ection at both ends of the cable.

Obviously, but there are situations where minimising reflections is essenti al, and trumps every other consideration.

Whoever said it did? You were talking about "traditional techniques", not t he OP's situation, and termination at both ends of cable is a traditional t echnique in certain particularly demanding situations. You may not have acc ess to a complete set of traditional techniques ...

he

ce,

But can reduce power supply load and heat dissipation. Coping with V/Z all the time isn't usually necessary.

//R2=Z

3 AC

swing

rned

Probably not, but they do show up frequently in the kinds of high-speed cir cuits that need this sort of attention.

But it can be a useful - sometimes vital - benefit.

es.

kill

Michael Kellett just said that he had a 37-way ribbon cable. That should be enough ways to let him devote 15 ways to burying each of the eight data li nes between grounded wires.

He'd be mad not to.

he

e this an unlikely problem.

Cleaning up after people who didn't think about it in advance.

It ought to be. Somebody who thinks hard enough to post questions here can probably be relied on to have done their homework.

the

. The flash-over currents stayed inside the coax cable, and didn't show up as brief hundred ampere currents in inconvenient places.

The odd part about the story was that the proper 30kV coaxial connector got bought and fitted as a sort of make-the-product-look-neater exercise.

The improvement in flashover performance came as a surprise.

The people surprised were remarkably competent, which is why I mention it h ere from time to time. As patent lawyers say, everything is obvious to the Supreme Court, but lesser mortals have blind spots.

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Bill Sloman, Sydney
Reply to
Bill Sloman

Well yes. I presume there aren't sufficient spare connections, or that the pinout is pre-ordained.

If the latter, I'd try to find a way of mutating the pinout.

Or where cables will be external and inserted/removed by the ignorant in uncontrolled conditions. USB is the obvious example, but there are many others.

Indeed, but there may be other "users" later on.

Well, everyone except me. I'm perfect.

My daughter believed that for a while; it was a useful "life lesson" for her.

Reply to
Tom Gardner

Thanks for further suggestions. Seems I didn't make the initial situation quite clear, the source box is a standard thing already bought by my customer. The 37 way D connector has all 37 pins committed. There are 3 gnd pins, all at one end of the D connector. The pins are obviously driven by octal bus drivers (spec says max 35mA per pin and max 70mA per group of 8.) So I have to take the signals from where they tell me (opposite end of D conn from the gnd pins !).

MK

Reply to
Michael Kellett

Oops. You may have to find out if you can make it work, and if not, reconcile your customer to a little board to swap the connections around, possibly onto a wider cable.

It takes a while to get naive customers to appreciate how expensive it can be to spend money before they know exactly what they need.

Something that can't be made to do what they need done is entirely worthless, and the time spent working why it can't be made to work isn't cheap either.

--
Bill Sloman, Sydney
Reply to
Bill Sloman

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