Every electronic system needs a timing device. And crystal (XTAL) resonators are often the go-to solution. However, oscillators, which pair a resonator with an oscillator IC into one complete integrated timing device, offer several benefits compared to XTALs. These benefits are further extended with MEMS timing technology. System designers no longer need to work around the limitations of XTALs and accept the headaches and risks of designing with crystals.
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1. Plug-and-play oscillators simplify system design
On the surface, oscillator design using quartz crystals might seem straight forward, especially considering the maturity of this technology. But there are a myriad of design parameters to consider when matching a crystal to an oscillator circuit. Among these parameters are crystal motional impedance, resonant mode, drive level, and oscillator negative resistance which is a measure of oscillator gain. Additionally, load capacitance must be considered for parallel resonant mode crystals and it should account for the PCB parasitic capacitance and potentially the on-chip integrated capacitance included in the oscillator circuit.
All of these parameters must be carefully considered to ensure reliable start up and operation of the circuit. Because an oscillator circuit requires close matching of the resonator to the oscillator circuit, crystal vendors cannot guarantee startup of the crystal. By contrast, oscillators are a completely integrated solution. The oscillator manufacturer matches the quartz resonator to the oscillator circuit, thus relieving the board designer of this burden. Because matching errors are eliminated, oscillator start-up is guaranteed by SiTime. In short, oscillators are a plug-and-play solution that greatly simplifies system design.
Design concerns eliminated with MEMS oscillators
Crystal motional impedance and oscillator negative resistance
The oscillator circuit must have enough gain and phase shift to meet the Barkhausen criterion for oscillation. Of particular importance is the motional impedance (ESR) of the crystal and the negative resistance (equivalent to gain) of the oscillator. If the oscillator has insufficient gain to overcome the motional impedance of the quartz resonator, the circuit may not start up. These issues are eliminated with the use of oscillators.
Crystal resonance mode, frequency tuning capacitance, and on-chip oscillator capacitance
Quartz crystals can resonate in either series or parallel resonant mode, but they are typically calibrated for only one of these two modes. If calibrated for parallel resonance, they require a specific load capacitance which is usually specified. However, the proper capacitance is not used, the frequency error may exceed the datasheet specifications. The oscillator IC may or may not have integrated chip capacitance which must be taken into account along with any parasitic capacitance from the printed circuit board connections, bond wires and lead frame of the oscillator IC to ensure the best frequency accuracy.
In contrast, MEMS oscillators integrate the resonator and oscillator/PLL IC into one package, eliminating the need for an external capacitor to tune the resonant frequency.
Crystal drive level
Care must be taken to ensure the oscillator circuit does not overdrive the crystal resonator. Overdriving the resonator can lead to accelerated aging of the crystal resonator and at extreme levels, it can damage the crystal. In contrast, MEMS resonators do not experience aging.
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2. MEMS oscillators offer much better quality and reliability
Quality and reliability are critical &#; not only are company reputations at stake, but re-work can be costly and time consuming. Moreover, systems that are deployed outdoors and exposed to environmental stresses must be especially robust. Quartz resonators, while a mature technology, involve a rather complicated manufacturing process in which each individual resonator is tuned to the desired frequency, usually by ablating the metal electrode with an ion beam. This step occurs before the crystal is encapsulated and causes the resonator to be susceptible to contamination. This process, along with other quartz manufacturing complexities, result in the mean time between failures (MTBF) of quartz to be as low as 14 to 38 million hours. The defective parts per million (DPPM) is up to 50 for the best quartz manufacturers and as high as 150 for tier 2 quartz suppliers.
In contrast to the specialized manufacturing processes of quartz crystals, MEMS oscillator manufacturers use standard semiconductor batch mode techniques. This includes wafer level production of resonators and oscillator ICs, and die bonding to standard lead frames with plastic encapsulation. SiTime MEMS resonator die are made from a single mechanical structure of pure silicon. During the manufacturing of SiTime MEMS, an Epi-Seal process is used to clean the resonator, after which polysilicon is deposited to seal the structure. The ultra-clean hermetic vacuum seal ensures the resonator structure is protected and free of contamination, eliminating aging mechanisms.
As a result, the DPPM and MTBF of SiTime oscillators are about 30 times better than quartz, providing a very reliable technology platform that endures severe environmental stresses and delivers a high quality product for the end user.
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3. MEMS low-frequency oscillators consume 65% less board space
Oscillators are a completely integrated solution and do not require external components such as power supply decoupling caps. SiTime&#;s 1.5 mm x 0.8 mm () footprint is smaller than the smallest quartz crystal footprint at 1.6 mm x 1.2 mm. And when taking into account load capacitors that are needed for the 32 kHz quartz crystal, the total board area or the XTAL solution is over three times larger.
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4. Oscillators can drive multiple loads, reducing costs, BOM and board space
An oscillator is an active circuit with an output driver usually capable of driving 2 to 3 loads depending on the drive strength. This allows the oscillator to replace several crystals and their associated capacitors, further reducing BOM, system cost, and board area.
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5. MEMS oscillators are much less sensitive to EMI
Electromagnetic energy, which is common in most systems, can be picked up by exposed PCB traces that connect the crystal resonator to the IC containing the oscillator circuit. This noise can be coupled into the oscillator circuit and passed to the output, potentially adding jitter and noise to the system. However, integrated oscillators have no exposed PCB connections between the resonator and oscillator IC, and the bond wires or balls that connect the MEMS resonator to the IC are extremely short. This makes MEMS oscillators much less sensitive to EMI. As shown in the following table and plot, SiTime oscillators are up to 11.3 dBm less sensitive (134x on a linear scale) than crystal resonators.
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This test was performed per IEC -2 standard that injects electromagnetic energy into a transverse electromagnetic (TEM) cell where the device under test (DUT) is mounted.
6. MEMS oscillators are much less sensitive to vibration
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Some systems that require a very stable frequency, such as wireless base stations and small cells, can experience system failure and service interruptions due to vibration.
MEMS oscillators are vibration resistant because the mass of a MEMS resonator is approximately 1,000 to 3,000 times lower than the mass of a quartz resonator. This means a given acceleration imposed on a MEMS structure, such as from shock or vibration, will result in much lower force than its quartz equivalent and therefore induce a much lower frequency shift. The figure on page 5 shows that SiTime MEMS oscillators are more than 10 times lower (better) in vibration sensitivity compared to quartz oscillators. Note this figure is based on measurements of quartz oscillators rather than passive crystal resonators, but comparable results are expected on quartz crystal resonators.
7. MEMS oscillators are readily available in any frequency
The quartz supply infrastructure has several constraints which can result in long lead-times, on the order of 12 to 16 weeks or even longer. One constraint is the limited number of ceramic package suppliers. Another constraint is the limited availability of frequency options. With quartz products, every frequency needs a different crystal cut unless a programmable phase locked loop (PLL) is used. Therefore, lead-times for non-standard frequencies can be very long.
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In contrast to crystal resonators, MEMS resonators are based on a standard resonator configuration. The output frequency of MEMS oscillators is generated by programming the PLL to different multiplication values. This enables a very wide frequency range with six digits of accuracy. In addition, silicon MEMS oscillators are manufactured using standard semiconductor processes and packaging. Because MEMS oscillator vendors leverage the very large semiconductor industry infrastructure, capacity is virtually unlimited.
MEMS oscillator samples can be programmed and available within one day, even for non-standard frequencies. By using SiTime&#;s low-cost Time Machine II programmer and field-programmable oscillators, designers can instantly program oscillators in their lab to create a device with any frequency, any supply voltage and any stability within the device&#;s operating range. Production lead-times are only 6 to 8 weeks.
8. One qualification for an entire product family
Qualifying components for end-use (system) conditions can consume significant time and resources. However, qualification efforts can be reduced with MEMS oscillators. SiTime products are based on a programmable platform which allows each device within a base product family to generate a wide range of frequencies, supply voltages and stabilities. If for example, resources have been invested in qualifying a SiTime device at a particular output frequency and a new board design requires a different frequency, the existing qualification data may be extended to a part with a new frequency.
In contrast, each XTAL frequency requires a different quartz blank. And if a design requires frequencies above 60 MHz, a different technology other than fundamental mode quartz is often used. Third overtone quartz crystals are often used for higher frequencies. This mode can introduce additional challenges to ensure reliable start up (i.e. higher motional impedance and different oscillator circuit than fundamental mode) which requires qualification.
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Summary
Despite inherent limitations, crystals have been the standard in electronic timing for several decades. SiTime&#;s MEMS oscillators overcome these limitations and offer many benefits compared to traditional quartz crystal resonators. Designers no longer need to accept the headaches and limitations associated with XTALs.
The top 8 reasons to replace XTALs with MEMS oscillators are:
- Oscillators are &#;plug and play&#; &#; much easier to design, guaranteed startup
- 30X better quality and reliability &#; lowers cost, increases robustness
- Smaller package and no/fewer caps &#; reduces PCB area
- Drives multiple loads, replacing 2 to 3 quartz crystals &#; reduces costs, BOM and PCB area
- Up to 134X lower sensitivity to electromagnetic energy &#; more robust
- 10X lower sensitivity to vibration &#; more robust
- Available in any frequency &#; very short lead-times
- One MEMS product covers a large frequency range &#; reduced qualification effort
There are 7 key factors to understanding successful crystal oscillator circuit design. These include:
- Series circuit
- Crystal load capacitance
- Parallel circuit
- Drive level
- Frequency vs. mode
- Design considerations
- Negative resistance
In this post, we're going to cover the basics of oscillator design and each of the 7 key components of great crystal oscillator circuit design.
In this post, we're going to cover the basics of oscillator design and each of the 7 key components of great crystal oscillator circuit design.
What Is a Crystal Oscillator Circuit? (Oscillator Circuit Basics)
Crystal oscillator circuits consist of an amplifier and a feedback network. The feedback network takes specific output from the amplifier and sends it back to the amplifier input. It looks pretty simple when drawn out...
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... but there's a lot more complexity the deeper we go. Hold on tight!
Two critical conditions must be met for a crystal oscillator circuit to operate effectively:
- The loop power gain must be equal to unison.
- The loop phase shift must be equivalent to 0, 2Pi, 4Pi, etc. radians.
The power directed back to the input of the amplifier must be enough to supply the amplifier input and oscillator yield, and to overcome circuit losses.
The precise frequency of an oscillator is determined by the loop phase shifts within the oscillator circuit. Any change to the phase shift will result in a change in frequency. One of the best ways to reduce net phase shift is to use a quartz crystal in the feedback loop. All of the crystals we manufacture here at Bliley include quartz crystals (quartz crystal oscillators).
Related Read: Understanding the Types of Crystals Inside of Your Oscillators
When a quartz crystal is used in the feedback loop of an oscillator, the frequency output of the oscillator actually regulates itself. The quartz crystal creates a reactance which satisfies the phase loop requirements.
Now that you have a solid understanding of the basics of crystal oscillator design, let's jump into the key considerations for crystal oscillator circuit design.
7 Key Considerations for Crystal Oscillator Circuit Design
1. Series Circuit
A series circuit crystal oscillator uses a crystal that's designed to operate at its natural resonant frequency. There's no need for capacitors in the feedback loop for this type of circuit. Series resonant oscillator circuits are fairly basic and are typically used because of their small component count.
The series circuit may provide feedback paths other than through the crystal. This means that the circuit may continue to oscillate at a subjective frequency even during a crystal failure.
A significant downfall to the series circuit is that you cannot adjust the output frequency if the system requires modification. A series resonant crystal is designed to the preferred frequency, tolerance, and stability, and holds without option for adjustment.
2. Load Capacitance
Load capacitance can play a critical role in oscillator circuit design. You'll see an example of the importance of load capacitance in the next design consideration, but for now, let's take a closer look at load capacitance itself.
Load capacitance is described as the amount of capacitance measured or computed across the crystal terminals in the circuit.
When it comes to the series circuit, there's no capacitance between the connecting points of the crystal circuit. Therefore, there's no load capacitance in the circuit. It's a different story with parallel circuits.
To determine load capacitance in a parallel circuit (described in design consideration #3), use this handy equation:
In this equation, LC1 and LC2 represent the load capacitors. Cs is the circuit vagrant capacitance (usually between 3 and 5 pF).
3. Parallel Circuit
A parallel resonant oscillator circuit is made with a crystal that's designed to operate with a specific load capacitance. This causes a crystal oscillator to operate with a frequency that's higher than the series resonant frequency but lower than the true parallel resonant frequency.
To complete the feedback loop in this type of circuit, you must design routes through the crystal. If the crystal fails, the circuit will no longer oscillate.
So where does the load capacitance come from that determines the oscillator's frequency? This circuit actually uses a solitary inverter with two capacitors in the feedback loop that encompass the load capacitance. If the load capacitance is changed, so will the frequency produced by the oscillator.
With that being said, it's important to note that this circuit type is not ideal for easily adjusting frequency if required. Also, exact frequency control and precise specification of load capacitance is required.
For example, if a 20 MHz crystal with a capacity of 20 pF is placed in a circuit with an assessment of 30 pF, the crystal will be lower than the specified value. But if the circuit has an assessment of only 10 pF, the frequency will be higher than the specified value.
4. Drive Level
The drive level is the amount of power consumed by the crystal while in operation. Power is typically described in terms of milliwatts or microwatts.
Quartz crystals are specified to a maximum value of drive level that can influence the frequency and the mode of operation of the oscillator. It's important to work with your crystal oscillator vendor to determine the maximum drive level that can be sustained by the quartz oscillator.
So what happens if a crystal oscillator exceeds the maximum drive level? It could cause the oscillator to:
- Become unstable
- Speed up age rates
- Cause loss of communication or timing in critical applications
To calculate drive level of a crystal, use this equation (basically just Ohm's Law, but for power):
Drive Level = (Irms2 x R)
In this equation, Irms is the measured RMS current through the quartz crystal and R is the maximum resistance of the quartz crystal.
To measure the actual drive level of a crystal oscillator circuit, you can insert a resistor into it. The voltage drop across the resistor can then be read to calculate the current and power dissipation. Of course, make sure you remove the resistor after this measurement.
5. Frequency vs. Mode
A crystal oscillator's frequency can be restricted by physical dimensions. Sometimes, this might be the length and width for certain applications. Other times, it may be the thickness of the quartz crystal itself. The thinner the quartz wafer, the higher the frequency will be. The thickness of the quartz wafer typically becomes too thin for processing around the 30 MHz level.
If you need an oscillator with a frequency higher than the limiting frequency, you can take advantage of fundamental frequencies. A fundamental frequency is defined as "the lowest frequency which is produced by the oscillation of the whole of an object, as distinct from the harmonics of higher frequency." If a crystal has a fundamental frequency of 10 MHz, it can also oscillate at 3, 5, 7, etc. times the fundamental frequency. Therefore, the oscillator can oscillate at 30 MHz, 50 MHz, 70 MHz, etc. These are the frequency's overtones.
When use of an overtone frequency is needed, a crystal manufacturer must design the crystal to operate at the desired overtone frequency. Never try to order a fundamental mode crystal and then operate it at another desired overtone because crystal manufacturing processes are different for fundamental and overtone crystals.
Related Read: Common Misconceptions About Oscillator Frequency Stability
6. Design Considerations
A handful of design considerations should be followed for best oscillator circuit operation. One thing that's always recommended is that parallel traces be avoided in the circuit. Doing so will reduce stray capacitance. All traces should be kept as short as possible to prevent coupling. Keeping components isolated by using ground planes can also help with this.
7. Negative Resistance
An oscillator must be designed to enhance negative resistance for best performance. Negative resistance is also frequently called oscillation allowance.
Here are six simple steps to help you calculate negative resistance in an oscillator circuit:
- Temporarily install a variable resistor in series with a crystal.
- Set the resistor to the lowest setting (close to zero ohms).
- Power up the oscillator and monitor the output on an oscilloscope.
- Begin to increase the resistance in the circuit using the variable resistor as you continually monitor the oscilloscope signal.
- As soon as the oscillation comes to a stop, take note of the variable resistor to determine the ohmic value.
- Add the maximum resistance value of the crystal (specified by the vendor) to the ohmic value measured in step 5.
This total value calculated is the negative resistance or oscillation allowance. For a general rule of thumb, the negative resistance should be a minimum of 5x the specified maximum resistance value of the crystal to be reliable.
Quartz Crystal Oscillators That Will Take Your Next Project Further
Bliley Technologies has been designing and manufacturing crystal oscillators for almost a century. Browse our full line of frequency products to find one that fits your project's needs, or contact our engineers today to learn more.
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