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January 26, 2012


david pacholok

You come up with the most arcane BUT USEFUL circuits and great analyses thereof!
Back at Motorola Comm div, about 1978 I remember a similiar circuit used to isolate the VCO in an 800 MHz PLL from +12 VDC supply noise corruption... it worked great!

Jerry Meyerhoff

John & David (Hello, yes we're still alive !),
At MOTO many called the VCO application a "super filter" and cloaked it in mystery & reverence. I'm trying to recall its initiator. Could have been Bob Loving who had a knack for making truly effective circuits. Others might have called it a "Beta Multiplier" because crudely the effective capacitor value was roughly multiplied by the transistor Beta or current gain. At one point I think the circuit was integrated into a "wrap-around" ASIC customized to do the PLL frequency phase detector and/or some quasi-analog functions of a typical PLL frequency synthesizer ? But I think it's useful bandwidth was mostly in the base station AC power supply hum region. There was still 1-over-f and flicker noise to contend with from multiple sources within the VCO itself. Ah, strolling down memory lane... jdm 2/1/2012

John Dunn

Hi, Jerry.

The origin of this thing seemed to be shrounded in mystery when I first came across it back around 1994. Nobody could remember who created it, but it's arrangement because quite widely known and widely used. It didn't behave as a capacitive multiplier though. Rather, it's a Sallen-Key filter element turned sideways to be used as a series impedance versus the shunt impedance of the capacitor going to ground. The two impedances form a current divider.

This thing can be extended to three poles and more too, at least according to theory. I ran some promising looking simulations, but I never tried to build the hardware for that.

Dave Peter

Thanks for posting this stuff, it's interesting and makes me think. Is there a reason this would be preferable to something like a LDO regulator with decent bandwidth?

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