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

[Michael Baker]

   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