Why Ferrite Beads Aggravates ACLR

1. 1

2. Ferrite
Bead
Introduction
 A ferrite bead is a passive device that filters high
frequency noise energy over a broad frequency range.
It becomes resistive over its intended frequency range
and dissipates the noise energy in the form of heat.
 The ferrite bead is connected in series with the power
supply rail and is often combined with capacitors to
ground on either side of the bead.
This forms a low-pass filter network,
further reducing the high frequency
power supply noise[1].
2

3. Introduction
 Ferrite beads are categorized by three response regions:
inductive, resistive, and capacitive. To reduce high
frequency noise, the bead must be in the resistive
region, and it acts like a resistor[1].
 The inductive region occurs
below the bead crossover
frequency (X = R),
where the bead becomes
inductive and acts like
a inductor[1].
3

4. Unwanted resonance
 Nevertheless, improper combining ferrite beads with a
decoupling capacitor for low-pass filtering can lead to
unwanted resonance below the bead crossover
frequency (X = R).
 Unwanted resonance may amplify ripple and noise in a
given system instead of attenuating it[1].
V+
V-
Ferrite bead, but
acts like a
inductor
Series Resonance
Decoupling
Capacitor
 Unwanted resonance frequencies for typical bead filters
are generally in the 0.1 MHz to 10 MHz range.
For typical switching frequencies
in the 300 kHz to 5 MHz range[1].
4

5. Unwanted resonance
 For example, the ferrite bead used is a TDK
MPZ1608S101A (100 Ω, 3 A, 0603) and the decoupling
capacitor used is a Murata GRM188R71H103KA01 low
ESR ceramic capacitor (10 nF, X7R, 0603).
5

6. Unwanted resonance
 Apparently, unwanted resonance occurs at ~2.5 MHz
due to (bead-C) combination, which forms series
resonance mechanism.
 As mentioned earlier,
this unwanted resonance
may amplify
ripple and noise.
6

7. Unwanted resonance and IR Drop
 Thus, (bead+C) combination makes PA power supply
unstable and ACLR aggravates.
 In addition, the ferrite bead’s large DCR may lead to
large IR drop, which aggravates ACLR as well,
especially at maximum RF output power because
current is large.
PA
Bead
IR Drop = I*DCR
7

8. How to solve this problem ?
 In terms of DCR, we ought to choose a ferrite bead with
small DCR.
 In terms of unwanted resonance, using an additional
RC decoupling filter can reduce the unwanted
resonance[1].
8

9. Reference
[1] Ferrite Beads Demystified, Analog Devices
9