[time-nuts] Noise of LT3042 vs ECL noise (was: Generating a solid PPS from 10Mhz source)

Attila Kinali attila at kinali.ch
Tue Jan 26 22:15:43 UTC 2016


On Sun, 17 Jan 2016 17:32:58 +0100
Gerhard Hoffmann <dk4xp at arcor.de> wrote:

> BTW the extra quiet reference of the LT3042 is a current source, so 
> there is no real justification for the repeated claims here that ECL
> must be noisy because of its integrated current source.

I had a closer look at the datasheet of the LT3042 and disagree with you
on this conclusion. Yes, the LT3042 uses a current source into a resistor
as reference. But the resistor is bypassed by a HUGE ceramic capacitor.
The resulting low-pass filter has a cut-off frequency in the low hertz
region, if not sub-hertz. Or to cite the datasheet:

One problem that conventional linear regulators face is
that the resistor divider setting the output voltage gains up
the reference noise. In contrast, the LT3042's unity-gain
follower architecture presents no gain from the SET pin
to the output. Therefore, if a capacitor bypasses the SET
pin resistor, then the output noise is independent of the
programmed output voltage. The resultant output noise
is then set just by the error amplifier's noise -- typically
2nV/sqrt(Hz) from 10kHz to 1MHz and 0.8µVRMS in a 10Hz
to 100kHz bandwidth using a 4.7µF SET pin capacitor.

In contrast to that, the current sources in ECL circuits do not have a
bypass capacitor. Or rather, you cannot bypass the current source itself
as it is used as an infinitely large resistor for the differential "pairs"
in the ECL circuit.

If you decrease the value of the by-pass capacitor such that you get
a low pass cut-off frequency in the, let's say, 100Hz range, then you
should see the effect clearly in your measurements.

			Attila Kinali
Reading can seriously damage your ignorance.
		-- unknown

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