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

ehydra ehydra at arcor.de
Sun Jan 23 18:06:59 UTC 2011

I think you have chosen the worst output possible.
CMOS is good but better is CML. With CML you generate constant-current 
load at the VCC supply. So lower back-induced noise! And suppression of 
the symmetrical noise to the next stage. This next input is a 
differential transistor stage. CML to CMOS translator chips are 
available too.
CML is also more immun to shorts. Good for breadboarding :-)

- Henry


Michael Baker schrieb:
>    Hello, TimeNutters--
>    Part deux of the continuing saga of the SiLabs
>    oscillator chip that has no output....
>    I did not realize that LVPECL chips were such a
>    hassle...  Next time, I am going insure that such
>    devices I use are CMOS and not LVPECL. This
>    particular SiLabs Si595 chip has complementary
>    LVPECL output.
>    As you may recall, I wired up dead-bug legs
>    on the 110MHz Si595 VCXO and hooked the
>    circuit up on a breadboard and got no output.
>    Sadly, the chip behaved as if it were dead.
>    (Sniff....)
>    Poking around (in-between bouts of sniffling)
>    I came across the following in an app note
>    by another manufacturer of LVPECL logic chips:
>    Why can't I get any output from an ECL Output Connector,
>    and how should I measure it?
>     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.
>    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.
>    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
>    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 !!)
>    Mike Baker
>    Micanopy, FL   USA
> _______________________________________________

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