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Phase-Locked Loop Design Fundamentals
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The fundamental design concepts for phase-locked loops implemented with integrated circuits are outlined. The necessary equations required to evaluate the basic loop performance are given in conjunction with a brief design example
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© Freescale Semiconductor, Inc., 1994, 2006. All rights reserved.
Freescale Semiconductor
Application Note
Document Number: AN535
Rev. 1.0, 02/2006
Abstract
The fundamental design concepts for phase-locked loops
implemented with integrated circuits are outlined. The
necessary equations required to evaluate the basic loop
performance are given in conjunction with a brief design
example.
NOTE
This document contains
references to obsolete part
numbers and is offered for
technical information
only.
1 Introduction
The purpose of this application note is to provide the
electronic system designer with the necessary tools to
design and evaluate Phase-Locked Loops (PLL)
configured with integrated circuits. The majority of all
PLL design problems can be approached using the
Laplace Transform technique. Therefore, a brief review
of Laplace is included to establish a common reference
Phase-Locked Loop Design Fundamentals
by: Garth Nash
Applications Engineering
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 Parameter Definition . . . . . . . . . . . . . . . . . . 2
3 Type - Order . . . . . . . . . . . . . . . . . . . . . . . . . 3
4 Error Constants . . . . . . . . . . . . . . . . . . . . . . 4
5 Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6 Bandwidth . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7 Phase-Locked Loop Design Example . . . 11
8 Experimental Results . . . . . . . . . . . . . . . . . 18
9 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
10 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . 21
Parameter Definition
Phase-Locked Loop Design Fundamentals Application Note, Rev. 1.0
2 Freescale Semiconductor
with the reader. Since the scope of this article is practical in nature all theoretical derivations have been
omitted, hoping to simplify and clarify the content. A bibliography is included for those who desire to
pursue the theoretical aspect.
2 Parameter Definition
The Laplace Transform permits the representation of the time response f(t) of a system in the complex
domain F(s). This response is twofold in nature in that it contains both transient and steady state solutions.
Thus, all operating conditions are considered and evaluated. The Laplace transform is valid only for
positive real time linear parameters; thus, its use must be justified for the PLL which includes both linear
and nonlinear functions. This justification is presented in Chapter Three of Phase Lock Techniques by
Gardner
1
.
The parameters in Figure 1 are defined and will be used throughout the text.
Figure 1. Feedback System
Using servo theory, the following relationships can be obtained
2
.
Eqn. 1
Eqn. 2
These parameters relate to the functions of a PLL as shown in Figure 2.
Figure 2. Phase Locked Loop
θ
e
s()
1
1Gs()Hs()+
---------------------------------
= θ
i
s()=
θ
o
s()
Gs()
1Gs()Hs()+
---------------------------------
= θ
i
s()=
Type - Order
Phase-Locked Loop Design Fundamentals Application Note, Rev. 1.0
Freescale Semiconductor 3
The phase detector produces a voltage proportional to the phase difference between the signals θ
i
and θ
o
/N.
This voltage upon filtering is used as the control signal for the VCO/VCM (VCM – Voltage Controlled
Multivibrator).
Since the VCO/VCM produces a frequency proportional to its input voltage, any time variant signal
appearing on the control signal will frequency modulate the VCO/VCM. The output frequency is
Eqn. 3
during phase lock. The phase detector, filter, and VCO/VCM compose the feed forward path with the
feedback path containing the programmable divider. Removal of the programmable counter produces
unity gain in the feedback path (N = 1). As a result, the output frequency is then equal to that of the input.
Various types and orders of loops can be constructed depending upon the configuration of the overall loop
transfer function. Identification and examples of these loops are contained in the following two sections.
3 Type - Order
These two terms are used somewhat indiscriminately in published literature, and to date there has not been
an established standard. However, the most common usage will be identified and used in this article.
The type of a system refers to the number of poles of the loop transfer function G(s) H(s) located at the
origin. For example:
Let
Eqn. 4
This is a type one system since there is only one pole at the origin.
The order of a system refers to the highest degree of the polynomial expression
Eqn. 5
which is termed the Characteristic Equation (C.E.). The roots of the characteristic equation become the
closed loop poles of the overall transfer function. For example:
Eqn. 6
Then
Eqn. 7
Therefore
Eqn. 8
Eqn. 9
which is a second order polynomial. Thus, for the given G(s) H(s), we obtain a type 1 second order system.
f
o
Nfi=
Gs()Hs()
10
ss 10+()
----------------------
=
1Gs()Hs()+0∆C. E. =
Gs()Hs()
10
ss 10+()
----------------------
=
1Gs()Hs()+1
10
ss 10+()
----------------------
+0==
C. E. ss 10+()10+=
C. E. s
2
10s 10++=
Error Constants
Phase-Locked Loop Design Fundamentals Application Note, Rev. 1.0
4 Freescale Semiconductor
4 Error Constants
Various inputs can be applied to a system. Typically, these include step position, velocity, and acceleration.
The response of type 1, 2, and 3 systems will be examined with the various inputs.
θ
e
(s) represents the phase error that exists in the phase detector between the incoming reference signal θ
i
(s)
and the feedback θ
o
(s)/N. In evaluating a system, θ
e
(s) must be examined in order to determine if the
steady state and transient characteristics are optimum and/or satisfactory. The transient response is a
function of loop stability and is covered in the next section. The steady state evaluation can be simplified
with the use of the final value theorem associated with Laplace. This theorem permits finding the steady
state system error θ
e
(s) resulting from the input θ
i
(s) without transforming back to the time domain
3
.
Simply stated
Eqn. 10
Where
Eqn. 11
The input signal θ
i
(s) is characterized as follows:
Step position:
Eqn. 12
or, in Laplace notation:
Eqn. 13
where C
p
is the magnitude of the phase step in radians. This corresponds to shifting the phase of the
incoming reference signal by C
p
radians:
Step velocity:
Eqn. 14
or, in Laplace notation:
Eqn. 15
where C
v
is the magnitude of the rate of change of phase in radians per second. This corresponds to
inputting a frequency that is different than the feedback portion of the VCO frequency. Thus, C
v
is the
frequency difference in radians per second seen at the phase detector.
Lim θ t()[]Lim sθ
e
s()[]=
t ∞→ so→
θ
e
s()
1
1Gs()Hs()+
---------------------------------
θ
i
s()=
θ
i
t() C
p
=t0≥
θ
i
s()
C
p
s
------
=
θ
i
t() C
v
t=t0≥
θ
i
s()
C
v
s
2
------
=
Error Constants
Phase-Locked Loop Design Fundamentals Application Note, Rev. 1.0
Freescale Semiconductor 5
Step acceleration:
Eqn. 16
or, in Laplace notation:
Eqn. 17
C
a
is the magnitude of the frequency rate of change in radians per second per second. This is characterized
by a time variant frequency input.
Typical loop G(s) H(s) transfer functions for types 1, 2, and 3 are:
Type 1
Eqn. 18
Type 2
Eqn. 19
Type 3
Eqn. 20
The final value of the phase error for a type 1 system with a step phase input is found by using Equation 11
and Equation 13.
Eqn. 21
Eqn. 22
Thus, the final value of the phase error is zero when a step position (phase) is applied.
Similarly, applying the three inputs into type 1, 2, and 3 systems and utilizing the final value theorem, the
following table can be constructed showing the respective steady state phase errors.
Table 1. Steady State Phase Errors for Various System Types
Type 1 Type 2 Type 3
Step Position Zero Zero Zero
Step Velocity Constant Zero Zero
Step Acceleration Continually increasing Constant Zero
θ
i
t() C
a
t
2
=t0≥
θ
i
s()
2C
a
s
3
---------
=
Gs()Hs()
K
ss a+()
------------------
=
Gs()Hs()
Ks a+()
s
2
--------------------
=
Gs()Hs()
Ks a+()sb+()
s
3
-------------------------------------
=
θ
e
s()
1
1
K
ss a+()
------------------
+
----------------------------
⎝⎠
⎜⎟
⎜⎟
⎛⎞
C
p
s
------
⎝⎠
⎛⎞
sa+()C
p
s
2
as K++()
--------------------------------
==
θ
e
t ∞=()Lim
s
sa+
s
2
as K++
---------------------------
⎝⎠
⎛⎞
C
p
0==
so→
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