Hi 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.
Bob 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 >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> > Bruce > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
