PECL and Transmission line length

Hello all, I am looking to transmit a 66MHz clock signal over a 50ft length of 50ohm coax. I am considering using a Texas Instruments TB5D1, which is capable of driving 50 ohm loads, but I am unsure as to what the max switching frequency vs length of cable is for PECL. I would appreciate any comments or suggestions as well. Thanks in advance.

Reply to
bart
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Just use source termination.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

The TB5D1 is a differential driver. Do you plan to use two pieces of coax, or a twinax or something?

What receiver chip did you have in mind? Do you need to just send a clock, or is there an associated data stream?

John

Reply to
John Larkin
66 MHz isn't a particularly high frequency, so you probably don't have to worry about any form of signal attenuation other than that due to the resistance of the inner and outer conductors. Some 50R cable can be quite resistive, so it might be worth checking out the specification of the cable you have in mind.

50 feet of cable is roughly 75nsec of propagation delay, or six wavelengths of your 66MHz clock, so it would be a very good idea to terminate the cable at one end or the other. Terminating at both ends does absorb any residual reflections twice as fast, but for a simple

66MHz clock this isn't really necessary - all you need to do is make sure that any reflections aren't big enough to cancel out your clock signal at the receiving end.

PECL-style outputs are emitter-followers, so they need pull-down resistors to sink enough current to keep the output transistors turned on and linear over the full signal voltage excursion, which is easier to arrange with the terminating resistors at the receiving end.

Hope this helps.

----------- Bill Sloman, Nijmegen

Reply to
bill.sloman

Hello Jim,

That's one option but would lead to one reflection at the far end. More EMI. What about termination at the end?

If there is a remote chance that the end of the coax might occasionally be left dangling then source termination is the better option, of course.

Regards, Joerg

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Reply to
Joerg

PECL style GND and cable shield are -5.2V down -making that terminating resistor quite a load.

Reply to
Fred Bloggs

Hello Fred,

Probably has to be some kind of AC coupled deal. Else something might rub through the outer jacket, shield touches chassis and... "Bzzzt".

Regards, Joerg

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Reply to
Joerg

PECL is ECL run with the negative rail at ground and the positive rail at +5V or +.3.3V (if you've got the right parts).

ECL terminating/pull-down resistors are normally returned to an auxiliary rail set 2V lower than the the positive rail. If you don't want to add a third power supply, you can put a second resistor in series with the 50R terminating resistor to add enough DC resistance to get the right current in the output transistor (around 5mA IIRR) and by-pass it to ground with a capacitor to restore the terminating impedance.

It is all spelled out in the Motorola ECL application notes.

--------- Bill Sloman, Nijmegen

Reply to
bill.sloman

It's also spelled out in the TI datasheet for this part, which , if believed, indicates that a single ended drive from this part may not be compatible with a DC coupled single ended PECL receiver, whereas a differential output will be. Looks like it was designed with UTP in mind, with the ultra-low skew "and all." As usual, the original standalone question is pointless- what is the environment, what is the receiver and coupling, CW or not etc...

Reply to
Fred Bloggs

You count on the reflection to make the signal whole. The termination is at the source. It's a very effective low-power termination but is only a good choice in point-to-point nets because the intermediate points on the line see V/2. I don't believe EMI is any worse.

It's a better choice for most point-to-point nets.

--
  Keith
Reply to
keith

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