[SI-LIST] Re: varactor diode in clock delays

  • From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Mon, 24 Apr 2006 09:12:56 -0400

> Can someone please tell me the use and the benefit of a varactor diode in
> having an automatically programmable clock delay. And why exactly varactor
> diodes and not other diode?

Do you know what the varactor diode is doing?  The purpose of the diode is
to provide an electrically adjustable delay, by way of the diode's voltage-
dependent reverse-bias capacitance.

In fact many people would call it a varactor diode even if it wasn't one
designed specifically for that purpose.  i.e., if you used some other type
of diode there, it would be called a "varactor diode" anyway, because of how
it functions in that circuit.  (Just as a "clamp diode" doesn't need to be
one that was designed as a clamp diode.)

Varactor diodes that are designed as such are optimized for use as an
electrically controlled capacitance.  Many have a controlled doping profile
which gives them a particular C(V) characteristic that may be desired,
and/or a wider max/min capacitance range (wider tuning range when used in
tuned circuits).  Yes, you could use some other type of diode instead, like
a small signal switching diode (and such diodes have been used as varactor
diodes) but it might not work as well as a diode that was designed to be a
varactor diode.

Regards,
Andy



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