AN575: Introduction to FPGA-based ADPLLs

1. Introduction
1.1. FPGA Approach to Electronics
System architects and board designers must constantly trade off costs, development time, performance, and
supportability. Field Programmable Gate Arrays (FPGAs) are programmable ICs that give the designer many of the
advantages of a highly integrated digital IC without the NRE costs and fabrication time of a custom digital ASIC.
The general functional flexibility of FPGAs invites their consideration in communications, control, and test
applications where different standards or data rates may need to be supported by the same hardware.
Likewise, the frequency flexibility of programmable oscillators such as the Silicon Labs Si514, Si570, and Si598
frequently lands them on the same boards where they are used as independent asynchronous oscillators.
However, these programmable oscillators can also be placed under FPGA control to implement a particular form of
highly configurable Phase Locked Loop. This application note is intended to serve as a brief introduction to this
approach and its advantages.
1.2. Taxonomy of PLLs
Phase Locked Loops or PLLs are electronic feedback circuits which lock an output signal’s phase relative to an
input reference signal’s phase. The signals of interest may be any periodic waveform, but are typically sinusoids or
digital clocks. These circuits are widely used in communications, computers, control, and measurement
applications for frequency synthesis, clock and data recovery, clock distribution, and other more specialized
PLLs are typically divided into the broad categories listed in Table 1, and are described following Roland Best’s
terminology (1997). The hardware PLLs may be implemented in discrete or integrated technology. Software PLLs
will not be described in this application note other than to state that SPLLs can mimic any of the hardware PLLs,
provided there is sufficient sampling, and the instructions can be executed fast enough for the application.
Table 1. General PLL Categories
Phase Detector
Phase Error Signal
Loop Filter
(Linear PLL)
(Digital PLL)
(All Digital PLL)
(Software PLL)
Linear (or analog) PLLs use analog 4-quadrant multipliers, such as mixers, as phase detectors. LPLLs are often
used for frequency translation and are therefore found in frequency synthesizers, radios, and phase noise
The most common PLL in use today is the classic Digital PLL, so-called due to its use of a digital phase detector.
However, these circuits typically follow the digital phase detector with a charge pump whose output is converted to
an analog voltage phase error signal. This analog phase error signal is then filtered and applied to a Voltage
Controlled Oscillator (VCO). A better description might be to refer to this version as a mixed signal or charge pump
PLL. Classic DPLLs and LPLLs have generally been preferred for high-performance (low phase noise), high
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Copyright © 2011 by Silicon Laboratories
frequency applications.
By contrast, PLLs operating only on digital signals are commonly referred to as ADPLLs or All Digital PLLs. The
filtered digital error signal is applied to a Digitally Controlled Oscillator (DCO). The general benefit for considering
this approach are discussed below.
A popular ADPLL option today is to instantiate as much digital circuitry as possible in an FPGA. This application
note will focus on FPGA-based ADPLLs where the DCO is off chip, i.e., not implemented in the FPGA. In particular,
we will concentrate on DCOs, which can be directly controlled by a serial interface. Silicon Labs offers several I2C
oscillators (e.g., Si514, Si570, Si598) which can be used as DCOs for such FPGA-based ADPLL applications.
2. Benefits
2.1. Advantages to ADPLLs
There are a number of reasons to consider an ADPLL as opposed to a more conventional PLL:
Analog components and circuits tend to have more variation across PVT (Process, Voltage, and Temperature)
and to drift or age. They are more sensitive to parasitics compared to digital circuits which also have higher
noise immunity. The Vc or analog control voltage is typically a very sensitive node for example.
An ADPLL may be able to implement loop filter topologies which are impractical to realize using analog
An integrated CMOS digital design is usually lower cost, smaller area, and more easily ported than its analog
counterpart. In short, integrated ADPLLs can take better advantage of Moore’s Law. A digital design flow is also
an easier development than a more analog intensive one.
ADPLLs can be designed to be flexible with respect to loop parameters. Implementations can take advantage of
this flexibility for calibration, digital freeze and hold operations, self test, etc.
ADPLLs can control a DCO as if it has virtually infinite Absolute Pull Range (APR). This capability eliminates the
need for high Kv VCOs, which may have increased phase noise within their modulation bandwidths.
ICs rely on increasingly lower voltages and smaller geometries. It can be argued that modern integrated PLLs
are fundamentally better suited for the ADPLL approach. See, for example, Staszewski and Balsara (2006,
page xiii):
“In a deep-submicron CMOS process, time-domain resolution of a digital signal edge transition is superior
to voltage resolution of analog signals.”
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3. Architecture
3.1. Basic PLL
The principal elements in a basic PLL are illustrated in the PLL block diagram in Figure 1. The dividers may not
necessarily be needed but are included for generality. A reference frequency, fREF, is divided down by the input
reference divider R and applied to the phase detector, PD, here shown as a mixer. Likewise, the output frequency,
fOUT, is divided by feedback divider N and fed back to the phase detector. The phase detector compares the
phase of fREF/R with fOUT/N. The output of the phase detector is an error voltage that is filtered by a low pass
filter (LPF) and presented to the VCO as a control voltage Vc. When the loop is locked, fREF/R = fO/N, and the
error voltage is minimized.
Figure 1. Basic PLL Block Diagram
3.2. Basic ADPLL
3.2.1. Basic ADPLL Block Diagram
The all digital PLL counterpart to the basic PLL substitutes digital signals and blocks as shown in Figure 2. The
output of the digital phase detector is a digital error signal that is filtered and presented to the DCO as a control
word “M” or its logical equivalent.
Figure 2. Basic ADPLL Block Diagram
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3.3. FPGA-based ADPLL
3.3.1. Block Diagram
It is possible to instantiate an ADPLL entirely within an FPGA and, in fact, some FPGAs include built-in system
timing PLLs as resources for the designer. However, the purpose of this application note is to describe custom
FPGA-based ADPLLs, which implement all the blocks in a single FPGA except for the DCO. The previous ADPLL
block diagram is then partitioned as illustrated in Figure 3. If the DCO is controlled serially, then the control word
“M” described previously is typically transferred as a series of register updates.
Figure 3. Basic FPGA-based ADPLL Block Diagram
3.3.2. Advantages
For the ADPLL to be competitive in a given application, it needs to have sufficient accuracy, resolution, and speed
to support the desired loop behavior while yielding a relatively low phase noise (low jitter) output clock with few or
insignificant spurs. For the board or system designer, the FPGA-based ADPLL with external DCO is one approach
to achieving these goals.
Partitioning the ADPLL in this way has a number of potential benefits:
Digital designers can exploit their expertise in DSP to implement a custom PLL, taking full advantage of the
economies of scale, device flexibility, and tools provided by the FPGA manufacturers.
 Since many board designs have one or more FPGAs already, there may be less incremental cost for adding an
FPGA-based ADPLL as opposed to other implementations.
 The DCO, despite being digitally controlled, is a mixed signal device requiring analog and mixed signal design
expertise. In this sense, the “hardest” part of the ADPLL is taken off the chip and the design can be optimized by
vendors specializing in DCO technology.
 Custom FPGA-based ADPLLs can exploit an external DCOs specific performance features such as frequency
range, steps, resolution, and jitter.
 Finally, it may be advantageous from a power supply, switching noise, and crosstalk point of view to “disintegrate” the DCO from the FPGA.
It is also possible to make the functional equivalent of a DCO out of a suitably matched DAC + VCO combination.
(See for example Xilinx App Note XAPP854). This combination would not result in the creation of an ADPLL as
described here; however, it would perform in much the same way. In addition, there might be some relative
advantage regarding loop bandwidth since serial communication would not be a limiting factor. However, this
combination would have to be balanced against the cost, complexity, and likely higher noise contribution of another
component in the signal path.
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3.3.3. An Example FPGA-based ADPLL
A very basic example of an FPGA-based ADPLL is illustrated in the block diagram in Figure 4, where there are six
major Verilog blocks:
1. Reference Input Divider
2. Digital Phase Detector
3. Digital Low Pass Filter (Loop Filter)
4. Command Conversion
5. I2C Master
6. Feedback Divider
Si514/70/98 Programmable XO
Power Supply
Any Frequency 100 kHz to 1417 MHz
Figure 4. Example FPGA-based ADPLL Block Diagram
This example design uses the following major hardware components:
Altera FPGA P/N EP3C25F324 (mounted on Altera Cyclone 3 Starter Kit evaluation board)
Silicon Labs Si514, Si570, and Si598 I2C-programmable oscillators
In this approach, a sampling clock (fs) is supplied to the Flip Flop–Counter phase detector. This version of ADPLL
is a relatively basic example. However, it is sufficient to demonstrate some very important points regarding the
application of ADPLLs. Additional details and performance measurements pertaining to this design will be included
in future versions of this application note.
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4. Additional Reference Resources
Best, Roland E. Phase Locked Loops: Design, Simulation, and Applications. 3rd ed. New York: McGraw-Hill,
1997. See pages 5-6 for Best’s classification of PLLs.
 Staszewski, Robert and Poras T. Balsara. All-Digital Frequency Synthesizer in Deep-Submicron CMOS.
Hoboken, New Jersey: Wiley-Interscience, 2006. The authors refer to the paradigm cited throughout the text.
See discussion on pages xiii and 26.
 Xilinx App Note XAPP854, “Digital Phase-Locked Loop (DPLL) Reference Design”, Justin Gaither, October 10,
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