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How to Design and Debug
a Phase-Locked Loop
(PLL) Circuit
By Ray Sun
Designing and debugging a phase-locked loop (PLL) circuit can be
complicated, unless engineers have a deep understanding of PLL
theory and a logical development process. This article presents a
simplified methodology for PLL design and provides an effective
and logical way to debug difficult PLL problems.
It is difficult to estimate the specifications of a PLL circuit without
simulating it at specific conditions, so simulation should be the first
step of the PLL design process. We recommend that engineers use
ADIsimPLL software to run a simulation based on their system
requirements, including reference frequency, step frequency, phase
noise (jitter), and frequency spur limitations.
Many engineers are confused as to how to choose a reference
frequency, but the relationship between the reference frequency
and the output frequency step is simple. With an integer-N PLL,
the output frequency step is equal to the frequency at the input of
the phase-frequency detector (PFD), which is the reference frequency
divided by the reference divider, R. With a fractional-N PLL, the
output frequency step is equal to the PFD input frequency divided
by the MOD value, so you can use a higher reference frequency
to obtain a smaller frequency step. When deciding whether to use
integer-N or fractional-N, the frequency step can be traded for
phase noise, with a lower PFD frequency providing better output
frequency resolution, but worse phase noise.
and VCO to do a simulation. The time required to edit a library
file for a VCO and reference source will be well worth it when high
simulation accuracy is required.
PLLs use a negative-feedback control system similar to that of an
amplifier, so the concepts of loop bandwidth and phase margin
apply here as well. Generally, the loop bandwidth should be set
smaller than one-tenth of the PFD frequency, and the safe range
for phase margin is from 45° to 60.° In addition, both simulation
and the prototyping on a real board should be performed to
confirm that the circuit will meet specifications given the parasitic
elements on the PCB layout and the tolerances of resistors and
capacitors in the loop filter.
Sometimes suitable values of resistors and capacitors are not readily
available, so engineers must determine whether other values will
work. A small function called BUILT is hidden in the Tools menu
of ADIsimPLL. This function transforms the values of resistors
and capacitors to the nearest standard engineering value, allowing
the designer to rerun the simulation to verify the new values for
phase margin and loop bandwidth.
ADI PLLs provide many user-configurable options to enable a
flexible design environment, but this introduces the challenge
of determining the values to store in each register. A convenient
solution is to use the evaluation software to set up the register
values, even if a PCB isn’t connected to the simulator. Afterwards,
the setup file can be saved to an .stp file or downloaded to the
evaluation board. The simulation results from ADIsimPLL,
shown in Figure 1, recommend register values for parameters
such as the core current of VCO.
As an example, Table 1 shows that if the requirement calls for
a fixed-frequency output with a very large frequency step, an
integer-N PLL, such as the ADF4106, is preferable due to its better
total in-band phase noise. Conversely, if the requirement calls for a
small frequency step, a fractional-N PLL, such as the ADF4153, is
preferable because its total noise is better than that of the integer-N
PLL. Phase noise is an essential PLL specification, but the data
sheet cannot specify performance for all possible applications.
Thus, simulation followed by testing actual hardware is crucial.
Table 1. Phase Noise Determines Choice of PLL
Fixed-Frequency Application
GSM1800 Application
RF = 1.8 GHz,
fosc = 13 MHz,
fixed output frequency
RF = 1.8 GHz,
fosc = 13 MHz,
fres = 200 kHz
FOM + 10log fpfd + 20log N FOM + 10log fpfd + 20log N
= –223 + 10log 13 MHz +
= –223 + 10log 200 kHz
20log 138 = –109 dBc/Hz + 20log 9000 = –91 dBc/Hz
FOM + 10log fpfd + 20log N FOM + 10log fpfd + 20log N
Fractional-N = –220 + 10log 13 MHz + = –220 + 10log 13 MHz +
20log 138 = –106 dBc/Hz
20log 138 = –106 dBc/Hz
Integer-N is better
Fractional-N is better
Even when using ADIsimPLL to simulate a PLL circuit under real
conditions, the results will probably be insufficient unless model
files for the actual reference and voltage-controlled oscillator
(VCO) are included. If not, the simulator will use an ideal reference
Analog Dialogue 47-09, September (2013)
Figure 1. ADIsimPLL simulation software provides
recommended values for setup registers.
Schematics and PCB Layout
Several things should be kept in mind when designing the complete
PLL circuit. First, it’s important to match the impedance at the
reference input port of the PLL to minimize reflections. Also,
keep the capacitance in parallel with the input port small, as it
will decrease the slew rate of the incoming signal and add noise
to the PLL loop. Refer to the input requirements on the PLL data
sheet for more detailed information.
Second, separate the analog and digital power supplies to minimize
the interference between them. The VCO supply is particularly
sensitive, so spurs and noise here can easily couple into the PLL
output. Please refer to Powering a Fractional-N Voltage Controlled
Oscillator (VCO) with Low Noise LDO Regulators for Reduced
Phase Noise (CN-0147) for additional cautions and more detailed
Third, the resistors and capacitors used to implement the loop
filter should be placed as close as possible to the PLL chip using
the values that were recommended by the simulation file. If you
have difficulty locking onto a signal after you have changed the
values of the loop filter components, please try the original values
that were used on the evaluation board.
In terms of the PCB layout, the main principle is to separate the
input from the output, making sure that the digital circuitry does
not interfere with the analog circuitry. For example, placing the
SPI bus too close to the reference input or VCO output will cause
spurs on the PLL output when accessing the PLL registers.
From a thermal design perspective, put a thermal ground pad
under the PLL chip to ensure that heat flows through the pad to
the PCB and heat sink. Designers should calculate all thermal
parameters of the PLL chip and PCB when used in extreme
Effective Use of MUXOUT
At the start of the debug phase, it’s difficult to determine where to
start when the PLL won’t lock. As a first step, use MUXOUT to
see if each internal function unit is operating properly, as shown
in Figure 2. For example, MUXOUT can show the output of the
R-counter to indicate that the reference input signal is fine and that
the contents of the register were successfully written. MUXOUT
can also check to see the detector’s lock status and the output of
the N-divider in the feedback loop. In this way, the designer can
confirm whether the value of each divider, gain, or frequency is
correct. This is the fundamental process of debugging a PLL.
specifications. Be sure to leave enough margin to ensure that the
timing requirements are met under all conditions. An oscilloscope
should be used to check to see that the clock and data edges are
in the right position in the time domain. If clock and data lines
are too close, crosstalk can cause energy from the clock to couple
to the data line via the PCB traces. This coupling results in a
glitch on the data line that occurs on the rising edge of the clock.
Therefore, look at these two lines when writing or reading the
registers, especially if register errors appear. Make sure that the
voltages on the lines are satisfied as shown in Table 2.
Table 2. Logic Inputs
Minimum Typical
Input High Voltage, VINH
Input Current, IINH/IINL
Input Capacitance, CIN
Spectrum Analysis
Issues in the frequency domain are both more frequent and more
complex. With a spectrum analyzer, first check to see if the PLL
output is locked, as indicated by a stable frequency peak. If not,
the tips described above should be followed.
If the PLL is locked, narrow the bandwidth of the spectrum
analyzer to determine whether the phase noise is acceptable or not,
confirming the test result with the simulation result. Measure the
phase noise at several bandwidths, such as 1 kHz, 10 kHz, and
1 MHz, for example.
If the result is not as expected, first review the loop filter design
and check the real values of the components on the PCB board.
Next, check the reference input to see if its phase noise is the
same as the simulation. The simulated phase noise of the PLL
should be similar to the real result unless the external conditions
are different or the registers were written with the wrong values.
The noise from the power supply should not be ignored, even if
a low-noise LDO is used, because both dc-to-dc converters and
LDOs look like noise sources. The LDO data sheet usually shows
a noise spectrum density that will affect noise-sensitive parts such
as PLLs (see Figure 3). Choose a low-noise power source for the
PLL, especially to supply the core current of the VCO.
CBYP = 1𝛍F
CBYP = 1mF
Figure 2. MUXOUT pin aids PLL debugging process.
Time-Domain Analysis
While debugging a PLL, use time-domain analysis to demonstrate
that the data written to the registers on the serial peripheral
interface (SPI) bus is correct. Even though reads and writes
are not done very quickly, make sure that the SPI timing meets
specifications and that crosstalk between different lines is
The timing diagram of the PLL’s data sheet should be consulted
to determine data setup time, clock speed, pulse width, and other
NSD (nV/√Hz)
Input Low Voltage, VINL
Maximum Units
Figure 3. LDO noise spectral density.
Analog Dialogue 47-09, September (2013)
Four types of spurs normally appear at the PLL output: PFD or
reference spurs, fractional spurs, integer boundary spurs, and spurs from
external sources, such as the power supply. All PLLs have at least
one type of spur, and although they can never be eliminated, we
can sometimes improve overall performance by trading spurs of
one type or frequency for another.
To avoid reference spurs, check the rising edge of reference signal.
An edge that is too fast or too large in amplitude will cause strong
harmonics in the frequency domain. Also, carefully check the PCB
layout to avoid crosstalk between input and output.
To minimize fractional spurs, dither can be added to push the
fractional spurs into the noise floor, but this will increase the
noise floor slightly.
Integer boundary spurs are rare, and occur only if the output
frequency is too close to an integer multiple of the reference
frequency such that the loop filter fails to eliminate it. An easy
way to solve this problem is to readjust the reference frequency
plan. For example, if a boundary spur occurs at 1100 MHz with
an 1100.1-MHz output, a 20-MHz reference input, and a 100-kHz
loop filter, changing the reference frequency to 30 MHz will
eliminate the spur.
The process of debugging a PLL requires a deep understanding
of the PLL, and many problems can be avoided by paying close
attention during the design phase. If issues occur during the
debugging phase, please follow the suggestions given in this article
to analyze the issues one by one and tackle them gradually. For
more information, please refer to the wealth of relevant information
that can be found on
Analog Dialogue 47-09, September (2013)
Curtin, Mike, and Paul O’Brien.“Phase-Locked Loops for
High-Frequency Receivers and Transmitters—Part 1.” Analog
Dialogue, Volume 33, Number 1, 1999.
Curtin, Mike, and Paul O’Brien. “Phase-Locked Loops for
High-Frequency Receivers and Transmitters—Part 2.” Analog
Dialogue, Volume 33, Number 1, 1999.
Curtin, Mike, and Paul O’Brien, “Phase-Locked Loops for
High-Frequency Receivers and Transmitters—Part 3.” Analog
Dialogue, Volume 33, Number 1, 1999.
CN0147 Circuit Note. Powering a Fractional-N Voltage
Controlled Oscillator (VCO) with Low Noise LDO Regulators
for Reduced Phase Noise.
Fox, Adrian. “PLL Synthesizers (Ask the Applications
Engineer—30).” Analog Dialogue, Volume 36, Number 3, 2002.
MT-086 Tutorial. Fundamentals of Phase-Locked Loops (PLLs).
Ray Sun [[email protected]] graduated with
a BEng from Wuhan University of Technology in
2002, joined Analog Devices in 2009, and received
an MBA from Huazhong University of Science &
Technology in 2010. A field applications engineer
based in Wuhan, China, Ray currently supports
customers in Central China. In his spare time, Ray is a dog trainer,
a zoophile, and a charter member of Toastmasters in Wuhan.
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