[time-nuts] LVPECL logic for dummies (that would be Moi)

Bob Camp 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
>>    resistor,
>>    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
>>    has
>>    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
>>    OK
>>    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
>>    exactly
>>    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.
>>    Similarly,
>>     the input of a PECL Terminator has an equivalent 50 Ohm/3 V
>>    termination.
>>     In order to protect sensitive instruments, however, a properly
>>    designed
>>    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
>>    terminator
>>    or to a ground-referenced 50 Ohm input instrument. This could spell
>>    instant
>>    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
>>    ground
>>    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
>>    current
>>    is limited only by b times its base current, which is supplied by R[1]
>>    or
>>    R[2] connected to V[CC]. In most cases, the device is destroyed
>>    instantly.
>>    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
>>    sure
>>    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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> Bruce
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