[time-nuts] LVPECL logic for dummies (that would be Moi)
mpb45 at clanbaker.org
Sun Jan 23 12:38:35 UTC 2011
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
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.
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
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
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.
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
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
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 !!)
Micanopy, FL USA
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