Introduction to the Component
This tutorial centers on the ECS-192-10-36-CKM-TR, a 19.2000 MHz crystal resonator from ECS Inc. in a compact SMD package (3.2mm x 2.5mm). Its key specification is a 10 pF load capacitance, which is a common and convenient value for modern microcontrollers and communication ICs. This crystal is ideal for a hands-on tutorial because its frequency is widely used for USB, Ethernet, and Bluetooth clocks, and the 10 pF load capacitance simplifies the design of the external capacitor network. You will learn a repeatable process applicable to many other crystals.
Design Requirements and Specifications for a Practical Circuit
We will design a Pierce oscillator circuit suitable for driving a microcontroller clock input. The requirements are: a stable 19.2000 MHz output, with frequency accuracy within ±50 ppm at room temperature, and startup time under 5 ms. The supply voltage is 3.3V. The target circuit must use the ECS-192-10-36-CKM-TR and common passive components. The output should be a clean square wave for digital logic, with rise/fall times under 5 ns.
Step-by-Step Design Process with Calculations
Step 1: Determine Load Capacitance (CL). The crystal specifies a load capacitance of 10 pF. This is the total capacitance seen by the crystal from the two external capacitors (C1 and C2) plus any stray capacitance (Cstray) from PCB traces and IC pins. The formula is: CL = (C1 C2) / (C1 + C2) + Cstray. For symmetry, we set C1 = C2. Therefore, CL = (C1/2) + Cstray. Assume Cstray = 3 pF (typical for a well-laid-out PCB). Then, C1 = 2 (CL - Cstray) = 2 * (10 pF - 3 pF) = 14 pF. Use the nearest standard value: 15 pF.
Step 2: Feedback Resistor (Rf). This resistor biases the inverting amplifier inside the microcontroller to operate in its linear region. Typical values range from 1 MΩ to 10 MΩ. For 19.2 MHz, a 1 MΩ resistor is a safe choice. It ensures the crystal starts oscillating reliably without excessive power dissipation.
Step 3: Series Resistor (Rs). To limit drive current and prevent overdriving the crystal, add a resistor in series with the output of the oscillator. Calculate Rs using the formula: Rs = (Vdd / I_max) - (ESR), where I_max is the maximum drive current from the crystal datasheet (typically 100 µA). For Vdd=3.3V and ESR=50Ω (from crystal datasheet), Rs ≈ (3.3V / 0.0001A) - 50Ω ≈ 33,000Ω - 50Ω = 32.95 kΩ. This is too high; a more practical value is 0 Ω to 220 Ω. Start with 0 Ω in simulation, and add if the oscillator waveform is distorted. For this tutorial, use 100 Ω as a starting point.
Step 4: Output Load. The oscillator output typically connects to a microcontroller clock input with an input capacitance of 5 pF. No additional loading is needed.
Component Selection Rationale for the Complete BOM
Capacitors C1 and C2: Use 15 pF ±1% NPO/C0G ceramic capacitors. NPO/C0G has excellent temperature stability and low voltage coefficient, critical for frequency accuracy. Standard 0603 or 0805 SMD packages are fine. Resistor Rf: 1 MΩ ±1% thick film SMD resistor, 0603 size. Resistor Rs: 100 Ω ±1% thick film SMD resistor, 0603 size. Crystal: ECS-192-10-36-CKM-TR. Microcontroller: Any MCU with a built-in oscillator amplifier (e.g., STM32, ATmega). For testing, use a development board with accessible clock pins. Decoupling capacitors: 100 nF and 10 µF ceramic capacitors near the MCU power pins.
Simulation Tips and What to Look For
Use a SPICE simulator (e.g., LTspice, Multisim). Model the crystal as a series RLC circuit: Ls, Cs, Rs (ESR). For 19.2 MHz and 10 pF load, calculate Ls and Cs from the crystal’s equivalent circuit (usually provided by ECS Inc.). For simulation, set Ls = 10 mH, Cs = 0.0069 pF (derived from f = 1/(2π√(Ls*Cs))), and Rs = 50 Ω. Connect C1 and C2 as calculated. Use an ideal inverter with gain (e.g., a voltage-controlled voltage source) or a model of the MCU’s oscillator. Look for: clean sinusoidal waveform at the crystal pins, with amplitude between 0.5V and 1.5V peak-to-peak. Check startup time: the oscillation should stabilize within 1-2 ms. Critical: Verify that the phase shift across the crystal is 180 degrees (inverting condition). If the waveform is clipped or distorted, increase Rs to 220 Ω or reduce C1/C2 slightly.
Prototype Build and Testing Methodology
Fabricate a small PCB with the oscillator circuit and a test point for the MCU clock output. Keep traces short (<10 mm) between crystal, capacitors, and MCU to minimize stray inductance. Solder the components using a reflow oven or fine-tipped iron. Testing: Power the board at 3.3V. Use an oscilloscope with a 10x probe (1x probe adds excessive capacitance) to measure the clock output at the MCU pin. Observe: The waveform should be a clean square wave with amplitude near 3.3V. Measure frequency with a frequency counter or oscilloscope’s frequency measurement function. Startup: Trigger the oscilloscope at power-on to capture the oscillator startup; it should stabilize within 5 ms. Verification: Compare measured frequency to 19.2000 MHz. Acceptable deviation is ±50 ppm (±960 Hz at 19.2 MHz). If the frequency is off, check capacitor values and stray capacitance. Optimization: If the waveform has ringing or jitter, adjust Rs. Increase it to 220 Ω if overshoot is present. If the frequency is low (crystal under-loaded), reduce C1 and C2 slightly (e.g., 12 pF). If high, increase them to 18 pF. Re-test after each change.
Performance Verification and Optimization
Measure the oscillator’s phase noise using a spectrum analyzer if available; a clean 19.2 MHz spike with no spurs is ideal. Temperature stability: Use a heat gun or thermal chamber to vary the temperature from -40°C to +85°C. The frequency should stay within ±50 ppm. If it drifts excessively, the crystal or capacitors may be the cause. Optimization: For best jitter performance, ensure the power supply is well-decoupled. Add a ferrite bead in series with the 3.3V supply to the MCU. Final check: Verify that the oscillator works reliably across multiple PCB samples. If some units fail to start, increase Rf to 2.2 MΩ or reduce Rs to 0 Ω. This tutorial’s design yields a robust, accurate 19.2 MHz clock suitable for most digital applications.
