[time-nuts] LVPECL logic for dummies (that would be Moi)
lists at rtty.us
Sun Jan 23 19:25:07 UTC 2011
One other reason for lightly source terminating ECL (or PECL) in an oscillator - there will be an output signal at the pin when there's no load. Helps in troubleshooting things.
On Jan 23, 2011, at 2:14 PM, Bruce Griffiths wrote:
> Michael Baker wrote:
>> It has been shown that that ECL outputs are open emitters. Without
>> pull-down resistors, the outputs are turned off and therefore, there is
>> no output voltage. Even if the output has an internal pull-down
>> it may still not be possible to measure the true output signal either,
>> unless the measurement device is impedance-matched to the ECL
>> output structure. The reason for this problem is that the internal
>> connection between the output ECL device pin and the output connector
>> is most likely a "long line," and neither the scope probe nor the high
>> impedance scope input represents an impedance match to the ECL
>> output structure.
> Utter nonsense!!
> The output impedance of an standard ECL emitter follower output is 10 ohms or less depending on the emitter current.
> Using a 10 ohm load would result in an emitter current that exceeds the design limits and significantly reduce the life of the part.
> ECL outputs are intended to drive transmission lines terminated in their characteristic impedance albeit to a terminating voltage other than ground.
> Sometimes (rarely) the transmission lines are source terminated by adding series resistors at the emitters to match the output impedance to that of the transmission line being driven.
> A pulldown resistor (or current source) is then required from the emitter to the negative supply (for NECL) or to ground (for PECL).
>> If one was to connect the ECL output directly to a 50 Ohm oscilloscope
>> input, there would no output either, because the output emitter will be
>> turned off by the ground-referenced 50 Ohm input, even if the output
>> a 200 Ohm pull-down resistor. However, AC coupling an ECL output with
>> an internal 200 Ohm pull-down resistor to a 50 Ohm input instrument is
>> So much for not being able to measure an ECL signal, now we shall
>> show how it can be measured using an ECL Terminator.
> Its very easy to measure correctly terminated ECL outputs with a low capacitance probe.
>> ECL/PECL output circuits are designed to drive 50 Ohm loads
>> terminated into a terminating voltage V[TT]= V[CC]-2 V.
>> For ECL, V[CC] = 0 V, and V[TT] = -2 V. For PECL, V[TT] = +3 V.
>> If the input of a measurement instrument is made to look just like a
>> 50 Ohm/V[TT] termination, then all should be well. In fact, that is
>> what an ECL or PECL Terminator is.
>> An ECL Terminator is basically a biased 50 Ohm microwave attenuator.
>> The input has an equivalent 50 Ohm/-2 V termination, and the output is
>> suitable for driving a ground referenced 50 Ohm input instrument.
>> the input of a PECL Terminator has an equivalent 50 Ohm/3 V
>> In order to protect sensitive instruments, however, a properly
>> ECL/PECL terminator should have a near ground level output
> The usual way of ensuring this for test purposes during development is to shift the ECL supplies so that the -2V termination voltage is shifted to ground.
> For 5V ECL this requires Vcc = +2V and Vee = -3.2V.
>> For measuring a differential ECL output either an instrument with a
>> differential input and the proper termination or a differential to
>> single-ended converter is required.
>> Caution! Do not connect the output of a PECL device to an ECL
>> or to a ground-referenced 50 Ohm input instrument. This could spell
>> disaster for the PECL device or the instrument Although connecting an
>> ECL output to a PECL Terminator may not destroy the ECL device, it
>> could cause gradual degradation of the output emitter follower, due to
>> possible excessive reverse bias voltage developed across the base
>> to emitter junction.
>> It is also shown that the collectors of the ECL output emitter
>> followers are
>> connected to V[CC]. When V[CC] is ground, shorting the emitter to
>> merely turns off the emitter follower and no damage will occur.
>> This is not the case when V[CC] is = +5 V. The transistor output
>> is limited only by b times its base current, which is supplied by R
>> R connected to V[CC]. In most cases, the device is destroyed
>> In fact, connecting a PECL output device to a ground-referenced 50 Ohm
>> load often destroys the device instantly as well.
>> Now-- back to the breadboard to see if I can get this ornery LVPECL
>> oscillator to show me some output... (next time, I am going to make
>> such chips I use are CMOS !!)
> CMOS is not a panacea, the faster CMOS families are prone to generating lots of Vcc and ground bounce as well as supply noise.
> Used properly ECL produces relatively little supply or ground noise compared to CMOS of equivalent performance.
> The close in phase noise of ECL is superior to that of CMOS.
> Low close in phase noise can be important when measuring ADEV for Tau > 1s or so.
>> Mike Baker
>> Micanopy, FL USA
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