Twisted-pair & PECL


with twisted

PECL is just ECL run between +5V and 0V rather than 0V and -4.5V. Because ECL works with current sources, the package runs only 11% hotter, so you don't lose much of your temperature range.

ECL outputs are designed to drive terminating resistors - as low as 50R

- returned to a -2V rail, from the logic high of -0.8V and low of -1.6V and can source the necessary 24mA.

PECL lines thus need to be terminated at the receiver to +3V with resistors that match the characteristic impedance of your twisted pair. Typical telecoms twisted pairs have an impedance of between 100R and

120R. Your will be driving the pair differentially, so you'd need two resistors that sum to the characteristic impedance - say 56R. You can set the 3V with a zener diode, or just join both 56R resistors to a 68R resistor which is returned to 0V - this third resistor would be dissipating close to 150mW, so don't use a tiny SMD part.

The bad thing about PECL is that the logic outputs can be instantly destroyed by shorting them to 0V (not a problem when the same parts are used with regular ECL power supplies) so make sure that the outputs are insulated against ground clips and other grounded metal-work - one of HPs interferometers used a bit of PECL and more than half the boards returned for repair had lost their PECL outputs.

------- Bill Sloman, Nijmegen

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DIFFERENTIAL line driver and receiver ??

...Jim Thompson

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

Hi All!

I am going to connect FastEthernet LAN adapter via twisted-pair to my device with PECL logics. Does anybody know, how can I negotiate PECL input with twisted pair TX and PECL output with twisted pair RX ?

Reply to
Roman D. Sinjuk

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