How to Layout a Phase-locked Loop IC in Your RF PCB
As part of telecom systems, radio systems, and other RF devices requiring frequency synthesis, phase-locked loops play an important role in PCB design. High frequency transceivers and high speed digital devices contain integrated phase locked loops alongside an integrated VCO layout, which provides stable and internally controllable clock signals. However, some PLL ICs are available as discrete ICs, which will include an integrated VCO layout inside the package. In total, a PLL enables some important tasks in your RF PCB design, such as demodulation, phase noise removal, and providing a clean waveform in frequency synthesis.
A phase-locked loop in a PCB can suffer from the same parasitic effects that can plague any other RF PCB, and designers should make some smart layout choices if they are working with a discrete phase-locked loop.
What is a Phase-locked Loop Used For?
A phase-locked loop has a number of important functions in analog (RF) systems and in systems that require precise clock and signal synchronization across a board. Here are some of the basic functions of a phase-locked loop and why they are important in an RF PCB.
- Phase noise removal: A phase-locked loop also be used to remove phase noise from a reference signal by synchronizing with a reference provided by a voltage-controlled oscillator (VCO). In years past, you would use some separate components for these tasks, but today's phase-locked loops integrate the VCO layout into the IC.
- Frequency synthesis: An analog or digital phase-locked loop can also be used for frequency synthesis at higher or lower frequencies than some reference. In terms of digital synthesis, a phase-locked loop can be used to decrease or increase the repetition rate of a stream of digital pulses. In both cases, the oscillation/repetition rate can reach 10’s of GHz with commercially available and experimental phase-locked loops, allowing them to support many RF applications.
- Demodulation of FM signals: If the phase-locked loop is fed with an FM signal, the VCO tracks its instantaneous frequency. The error voltage output from the loop filter stage (see below), which is what controls the VCO, is equal to the demodulated FM output.
At low speeds/low frequencies, phase noise in a given driver is typically low enough that you do not need to take advantage of a phase-locked loop to compensate for it, and the main sources are due to other problems that can be fixed at the PCB layout level.
The Role of a Each Component in a Phase-locked Loop
Phase-locked loops use negative feedback from a VCO in analog applications, or a numerically controlled oscillator (NCO) in digital applications. In analog applications, the frequency of the output from a VCO or NCO depends on its input voltage or a digital input, respectively. In either case, the output from the PLL will be proportional to the phase difference between the reference input signal. When the phase difference (and thus the output) do not change over time, then the two signals are locked at the same frequency.
In an RF system, the output from an analog VCO depends on the input voltage, making it useful for modulating a reference clock signal. Within a phase-locked loop, the VCO effectively locks onto a particular reference through use of a loop filter. In analog phase-locked loops, the loop filter takes some time to lock onto the desired reference signal (reaching ~100 ns).
The output from the loop filter also has a special place within a phase-locked loop. When the VCO is used to lock onto a desired carrier signal, a frequency or phase-modulated signal will generally modulate at a rate that is much faster than the phase-locked loop’s locking time. In this case, the loop filter will output an error signal that is proportional to the instantaneous phase difference between the reference and the VCO signal. When a modulated reference signal is input to the phase-locked loop as a carrier, this error signal is actually the demodulated signal.
Phase-locked loop block diagram
PCB Layout For Your Phase-locked Loop
Phase-locked loop ICs are available on the market that reach low GHz values. Transceivers and modems for higher frequency systems normally include the entire phase-locked loop, including the VCO layout and supporting circuitry, on the die. These may operate at intermediate frequencies to provide a clean output, which is then upconverted and modulated to give a desired RF signal. With a phase-locked loop IC, you'll have RF frequencies being fed into and out of the component and routed around the board, and you'll need to pay attention to signal integrity in the system. Some of the important layout points include:
- Isolation and board gridding: To keep input RF, output RF, and other analog/digital sections from interfering with each other, arrange different circuit blocks into specific regions of the board. Also be sure to use some isolation structures (via fences, ground pour, separate routing layers) to prevent RF sections from interfering with each other and other board sections.
- Power integrity: Power-supply noise requires precise decoupling, so use neighboring power and ground planes as the foundation of your phase-locked loop PDN. Also, treat the circuit as a high speed digital system and place a decoupling network close to the power pins. This will provide steady DC voltage to these ICs and suppress ringing in the power bus or power plane when digital ICs elsewhere on the board switch. Any decoupling/bypassing capacitors should use their own vias to connect back to the ground plane.
- Heat: Place a grounded thermal pad under the phase-locked loop IC to ensure that heat flows back into the PCB ground plane.
- Losses: If you're getting into GHz frequencies, consider a low loss laminate above WiFi frequencies. Rogers or Isola materials are good choices for carrying RF signals with low loss. Try to keep RF traces gridded away from each other, but also keep them as short as possible to prevent interference and excess losses.
- Impedance matching: Like other RF systems, you'll need to carefully impedance match transmission lines and input/output ports on your phase-locked loop IC.
Ever play with a synthesizer? You’re really playing with a VCO
What About a Separate VCO Layout?
This is not common as today's phase-locked loop ICs will contain an integrated VCO layout. That being said, there are some places where a separate VCO layout is used. Higher power RF systems needing a phase-locked loop may need to separate all portions into different board sections (phase-locked loop, VCO layout, amplifier, and other components). In addition, systems using software-defined radio may use a specialty VCO for reference signal generation or direct frequency synthesis. Working with a VCO can be difficult regardless of whether you've built out your own phase-locked loop for the system.
The bandwidth of a VCO will affect its sensitivity to power-supply noise and its own phase noise. Wider bandwidth voltage VCOs may have increased sensitivity to power-supply noise, thus power regulators with ultralow noise are recommended in order to minimize the phase noise on the VCO output. Using a narrowband VCO will only accommodate a narrower range of frequencies, and this should be considered during design.
A VCO can be also used for direct modulation of a carrier signal. The output from a VCO can be used to apply modulation to a carrier signal, which can then be sent to a transmitting antenna. This can be done with a T-section that uses three resistors to match the antenna impedance to the output impedance of the VCO. Parasitics here become problematic at high frequencies as they can interfere with impedance matching and isolation. These difficulties should reveal why a VCO layout is normally integrated into a phase-locked loop.
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