In answer to your first question regarding how a capacitor blocks DC,
think of this.
 A capacitor is basically a pair of METAL plates seperated by an
INSULATOR.
 We know that the definition of an insulator is, in it's simplest form,
a NON CONDUCTOR.
 So, if we have two conductors, seperated by a NON CONDUCTOR, it would
seem ratheer obvious=20
 that current can't flow through the NON CONDUCTOR.  Therefore, if we
have no current flow,
 we say that current flow is blocked bythe capacitor.  More precisely,
the current flow is=20
 blocked by the insulator (dielectric) of the capacitor.

 A capacitor stores energy in an electric field between the plates when
a voltage is applied.=20
 Even in DC circuits, with a fully discharged capacitor, the current
that flows the instant=20
 the voltage is applied is high, and is limited only by the series
resistance. =20

 An AC signal constantly changes polarity, therefore the plolarity of
the stored charge, in the=20
 form of an electric field, changes too.  This allows current to flow at
the ame frequency as the
 voltage source.  The higher the frequency, the faster this field
changes, and theefore, the effective
 reactance goes down.


  Regards,
=20
  Jim

> -------- Original Message --------
> Subject: [EE] Understanding filters and resonator circuits
> From: V G <x.solarwind.x@gmail.com>
> Date: Tue, June 05, 2012 12:54 pm
> To: PICLIST <piclist@mit.edu>
>=20
>=20
> Capacitors:
>=20
> I'm trying to (fully) understand the principle of capacitor reactance and
> high pass filters. I'm having trouble forming a mental picture of how a
> capacitor in series with a signal blocks DC and has the lowest impedance
> for the highest frequency signals on a voltage and charge-on-the-plate
> level. I can understand how a capacitor tied to ground acts as a buffer a=
nd
> smoothes out high frequencies by averaging the voltage. But I don't reall=
y
> get how the reverse is true for a capacitor in series with a signal. I ca=
n
> see how it blocks DC after it is fully charged.
>=20
> 1. How does a capacitor in series work to block a DC signal, but pass a
> high frequency signal?
> 2. If you apply a voltage to a capacitor in series (keeping in mind that =
it
> takes time for the voltage from the power source to rise), I understand
> that the current before the capacitor is NON-zero. But what about the
> current AFTER (on the other side) of the capacitor? Is it zero or non-zer=
o
> (considering that a capacitor is just two parallel plates)?
>=20
>=20
> LC circuits:
>=20
> 1. Consider a parallel LC circuit where the bottom is tied to ground and
> the top is tied to a mixed-frequency signal source. I understand that the
> parallel LC circuit will shunt all frequencies to ground other than those
> around its resonant frequency. After the signal source is removed, the
> circuit will eventually lose its energy. How do I make an oscillator out =
of
> such an LC circuit that self-starts (that is, I can simulate in LTSPICE
> which doesn't take into account external noise coming into an oscillator
> circuit, which is required for some oscillator designs)? I've looked at L=
C
> oscillator circuit types like the Hartley oscillator, but I don't see how
> this can self start in theory under ideal conditions.
> --=20
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