This article discusses the difference between crystals and oscillators, and the total cost advantage of using oscillators instead of crystals.

When using crystals and MEMS oscillators, we must consider the total cost. Although the unit cost of crystal is usually lower, the situation is quite different once the total cost is calculated.

Like the crystal design problems we often encounter, such as cold startup failures, oscillating circuit problems caused by crystal mismatches, or failure to pass EMI tests. These problems will lead to engineering cost overruns during development and create costly quality issues. In addition, delaying product release date can also result in lost revenue. Under these circumstances, we found several situations where MEMS oscillators help reduce overall costs.

First, let’s briefly introduce some basic knowledge. Crystal is a passive component that needs to be connected to an external oscillating circuit, while an oscillator is an integrated timing solution that contains a resonator and an oscillator IC. A crystal (sometimes called a resonator) is a moving/resonating passive component that connects to the external oscillating circuit in the chip (such as SoC, microcontroller, or processor) so as to offer timing - as shown on the left side of Figure 1 below.

Figure 1: Difference between a crystal (XTAL) and an oscillator (XO) 

Oscillators have the advantages of reducing EMI, programmability, and driving multiple loads. Although the unit cost of crystals is usually lower, oscillators are more cost-effective in terms of total cost, especially when the production volume is low. 

In addition, oscillators are easier to design into a system because they can solve common timing design problems, such as cold start failures, crystal mismatch-induced oscillation circuit problems, and EMI test failures. In some products, oscillators are based on silicon rather than traditional quartz crystals. This architecture enables a rugged and durable “plug-and-play” oscillator that is not only flexible but also very easy to design into a system.

Oscillators are easier to design into a system because their functions and characteristics can solve common timing design problems that are usually difficult to solve. We have listed some total cost examples based on the pricing of XTAL and XO with the same output frequency, stability, and package size from some online electronic component stores. Then, we add the engineering workhours required to solve the problem (based on $100 per hour).

There are different breakpoints for each case based on production volume and engineering time. Generally, when the production volume is high, the cost of using crystals for design is lower, and the design cost can be amortized in large-scale production. Conversely, when the production volume is not in the tens of thousands, the cost of using oscillators is lower. However, things are more than that.

What is not considered in the following examples is the opportunity cost (lost revenue) caused by project design delays, which can be tremendous in some markets. In some cases, external services and testing will cause additional costs, which may also be high. Moreover, there are other additional costs. These include the additional material and component costs required for circuit board re-encapsulation, the cost of load capacitors required for crystals (not oscillators), and the additional circuit board space occupied by capacitors - all of which further tilt the equation towards using oscillators.

For simplicity sake, in the following examples, we only include the cost of timing components and the engineering time of correcting crystal problems.

1. Cost comparison of crystal vs oscillator - cold startup failure

Unlike crystals, MEMS oscillator does not have a startup problem. In the customer's case, it takes 15 hours of engineering work to correct the startup problem of the crystal. Here, through relatively quick repair, when the output reaches or less than 6,500 units, the cost and effectiveness of the MEMS oscillator will appear. In other words, unless the production volume is very large, the use of crystals means paying more in the long run.

2. Cost comparison of crystal vs oscillator - mismatched crystal causes oscillator failure

Because the oscillator is an integrated solution (integrating the resonator and oscillator IC in one package), the matching error is eliminated. The designer does not need to worry about crystal motional impedance, resonant mode, drive level or other pairing considerations. In this customer's case, it takes 40 hours of engineering design to correct the matching problem. When the number is about 17,000 or less, the cost of using an oscillator is lower.

3. Cost comparison of crystal and oscillator -EMI compliance failure

Clocks are usually the biggest factor that generate EMI (electromagnetic interference), which may cause prototypes to fail the compliance testing. MEMS oscillator can provide various of technologies to reduce EMI quickly and easily. One such technique is spread spectrum clocking. Another feature is Flexedge ™, which is a programmable function for adjusting the rise/fall time (slew rate) of the clock signal to reduce EMI.

As a passive device, the crystal does not have these functions to reduce EMI. If the designer needs to use shielding or add a spread spectrum clock generator IC to their crystal, the expense and board space will be increased. In addition, renting an anechoic chamber for additional testing will increase another $3,000 or more. Redesigning boards and retesting may take 50 hours of engineering work, so it is more beneficial to use MEMS oscillator when the output is about 23500 or less. This does not include the cost of additional materials and testing equipment costs mentioned above.

Besides direct costs, there are other factors that affect the cost of using crystals for design. For example, oscillators can drive multiple loads. This means that one oscillator can replace multiple crystals, while multiple crystals can only provide timing signals for one device.

In addition, MEMS oscillators are based on a programmable architecture and can provide any frequency, stability, and voltage within a very wide range. This provides design engineers with great flexibility in optimizing designs. In fact, programmable oscillators can be programmed by major distributors or even by customers in their own lab using Time Machine II.

If the specifications need to be changed, programmability can also reduce the cost of qualification efforts. The reason why cost and time can be saved is that MEMS oscillators (before programming) can generate millions of part numbers. Once the basic components are qualified, they can be configured to support a variety of different specifications.

One of the biggest indirect savings may be the higher quality and reliability. The DPPM of programmable MEMS oscillators is less than 1, and the MTBF (mean time between failures) is more than 2 billion hours, which is 50 times that of typical quartz devices. In addition, compared with quartz crystals, programmable MEMS oscillators have a much higher survival rate under shock and vibration.

The high failure rate of quartz crystals increases costs in many ways, such as additional resource costs for root-cause analysis or extra service and replacement costs. Moreover, quality issues can cause significant and long-lasting damage to a brand’s reputation of a company.

Using oscillators instead of crystals can reduce costs in many ways. Why not eliminate all the trouble and extra expenses to use oscillators? When purchasing work focuses on reducing component costs, please remember that looking at the whole picture is the key to ultimately saving costs in the long run.