Instruction manual

FMLP1 • 6
If you think for a moment about the fundamental operation of a
capacitor and an inductor when an RF (radio frequency) signal is applied, the
reasoning for the layout seen in the schematic is obvious. Thinking in terms of
extremes and going back to the basics of reactive elements, let’s examine each
one separately.
At very high frequencies a capacitor looks like a short circuit meaning
signals pass through it like it was just a piece of wire. On the opposite extreme,
at very low frequencies a capacitor looks like an open circuit blocking the signal
from passing through. Granted there are many factors that determine just how
much signal can pass through the capacitor at different frequencies due to X
C
( X
C
= 1 / (2 x π x F x C)
,
the capacitive reactance in ohms. The math is getting
too specific at this time for our proposes!
At very high frequencies an inductor looks like an open circuit blocking
the signal from passing through. On the opposite extreme, at very low
frequencies an inductor looks like a short circuit passing signals through it like it
was just a piece of wire (Hey that’s what it is!). Keep in mind again there are
many factors that determine just how much signal can pass through the
inductor at different frequencies due to X
L
( X
L
= 2 x π x F x L), the inductive
reactance in ohms.
Armed with these simple concepts, look at the schematic again. We
stated earlier that a lowpass filter allows signals up to a certain cut-off
frequency (f
c
) to pass through while blocking any signals that are higher. Take
the following two opposite extremes:
1) f
c
= a constant DC Voltage, the lowest possible frequency input.
Look back at the schematic and remember the characteristics of a capacitor
and an inductor at low frequencies. Will the f
c
signal (DC for this example) pass
through a capacitor? Will the f
c
signal pass through an inductor? The lowpass
orientation of the components is easily apparent when you ask these questions.
A DC input, the lowest
possible frequency, cannot pass through a capacitor so
it will not be shorted to ground by the shunt caps (C1, C2, C4, C5, C6, C7, C9,
& C10). The inductors however (L1, L2, & L3) will pass the DC input signal from
input J1 to the output J2 without any attenuation. Therefore low frequency
signals (f
c
= a DC voltage) pass through the circuit, hence Lowpass Filter!
2) f
c
= an infinitely high frequency, the highest possible frequency input.
Look back at the schematic again and remember the characteristics of a
capacitor and an inductor at high frequencies. Will the f
c
signal (very high for
this example) pass through a capacitor? Will the f
c
signal pass through an
inductor? At a very high frequency, the input will pass right through a capacitor
so it will be shorted to ground by the shunt caps. The inductors however will not
pass the high frequency input signal. Therefore any high frequency is blocked
from passing through the filter! Again, Lowpass Filter!