Crystal and Oscillator Layout: Load Capacitance and Jitter

A crystal oscillating at twenty megahertz is a resonator with a very high quality factor, and it will oscillate at whatever frequency the circuit around it presents. The layout is part of that circuit.

How the Oscillator Works

The crystal sits in a feedback loop with an inverting amplifier, and the loop provides the phase shift and the gain that sustain oscillation. The frequency is set by the crystal’s series resonance and by the load capacitance the circuit presents.

The load capacitance appears in series with the crystal, and it is the sum of the two external capacitors and the stray capacitance of the board. The stray capacitance is set by the layout, which is why a layout change can pull the frequency.

The crystal is specified for a particular load capacitance, and a circuit that presents a different value oscillates at a slightly different frequency. The error is usually a few tens of parts per million, which matters for a real time clock and not for a general purpose clock. Our component tolerance notes describe how the budget is assembled.

Placing the Components

The crystal, the two load capacitors and the amplifier must be placed together, with the shortest possible connections. Every millimetre of trace adds capacitance and inductance to a loop that must be precise.

The load capacitors should be placed between the crystal pins and the ground reference, with their ground connections to the same point. A capacitor whose ground return is a long trace has an inductance that changes the effective load.

The traces from the crystal to the amplifier should be short, symmetric and equal in length. Asymmetry changes the phase of the feedback and can shift the frequency or prevent oscillation.

Grounding the Oscillator

The oscillator’s ground should be a quiet reference. Where a switching current flows through the same copper, the voltage drop appears as noise on the oscillator and produces jitter.

The usual arrangement is a local ground area under the oscillator components, connected to the main ground at one point. The connection should be wide and short.

A solid plane under the oscillator is generally better than a divided one, provided the plane is quiet. The question is whether the plane carries digital return currents, and if it does, the oscillator should be moved or the plane divided. Our EMI immunity notes describe how the division is arranged.

Guard and Keep-Out

The oscillator traces should be kept away from any switching node, and a guard ring around the crystal connected to the local ground reduces the coupling of external fields into the resonator.

The area under the crystal should have no traces on any layer, including the layer beneath, because the crystal’s own metal case can couple and interfere with those traces. A cut-out in the plane under a metal can crystal is common practice.

The keep-out should be documented in the layout rules, because a later revision that adds a trace under the crystal introduces a problem that is difficult to diagnose.

Jitter and Its Sources

The oscillator’s phase noise and jitter come from the resonator’s quality factor, the amplifier’s noise and the noise on the supply. Of these, the supply noise is the one the layout can control.

Decoupling the oscillator supply with a small capacitor close to the pin and a series element to isolate it from the rest of the supply reduces the noise reaching the oscillator.

The output trace should be routed away from the oscillator input and should have a series termination where the load requires it. A long output trace with a fast edge is itself a noise source for the input. Our high frequency laminate notes describe the materials used where the frequency is high.

Verification

The verification is a measurement of the oscillator frequency against a reference, and a measurement of the jitter on the output with the rest of the board operating.

The frequency measurement confirms the load capacitance is correct. Comparing the measured value against the crystal’s specification and the specified load gives the margin.

The jitter measurement with the board quiet and busy shows how much of it comes from the supply and the coupling, which is the information needed to decide what to change. Our quality notes describe how such measurements are recorded.

Design Checklist

Place the crystal and its capacitors as close to the amplifier as the layout allows, with equal and short traces and a local ground.

Keep the crystal traces away from switching nodes and from any trace that carries a fast edge. Provide a plane cut-out under a crystal with a metal case.

Decouple the oscillator supply locally and isolate it from the switching supply with a series element. Route the output away from the input and terminate it as the load requires. Our design release checklist notes where these requirements are recorded.

Process Control and Verification

On a design of this kind, grounding is the item that decides how the rest of the board is arranged. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

Process Control and Verification

On a design of this kind, grounding is the item that decides how the rest of the board is arranged. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Process Control and Verification

On a design of this kind, grounding is the item that decides how the rest of the board is arranged. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

FAQ

Does the layout really change the frequency? It changes the load capacitance, which changes the frequency by a small amount. The change is small and it is outside the specification for a timing critical application.

Should the plane under the crystal be removed for every package? For a package with a metal case, yes. For a plastic package, the cut-out is less important and the solid plane is usually better.

What does gopcb provide for oscillator circuits? We provide placement and routing rules for the crystal loop, local ground and supply isolation, plane cut-outs where the package requires them, frequency and jitter measurements on the assembled board, and documentation of the keep-out so that later revisions respect it.