Tech Talk

A spread spectrum crystal clock oscillator is an oscillator that has the output frequency intentionally modulated in order to reduce the EMI on the output signal.

Spread Spectrum oscillators can be used in many of the following applications:

• Copiers, FAX Machines
• Set-top boxes, Scanners, Printers
• LCD Displays (not Video CRT)
• Computers: Graphic Cards, Interface Controllers and PCI, CPU and Memory Buses
• Modems, Set-top Boxes, Games

The Ecliptek EPS series of spread spectrum oscillators offer:

• Output frequencies from 14.318MHz to 166.000MHz
• Supply Voltage of 3.3VDC
• High speed HCMOS output with controlled rise and fall times
• Four different packaging options; two SMD and two through-hole
• In factory (at Ecliptek) flash programming of the oscillator for improved delivery
• Tri-state and power down options for high impedance output
• Non-linear, optimized 31.5 kHz modulation profile
• Spread Spectrum enable/disable function (6 pad SMD package only)
• Flash programmable spread spectrum with spread percentages between +/-0.25% and +/-2.00% center spread and -0.5% to -4.0% down spread
• Improved frequency temperature stability through the use of a fundamental mode BAW (bulk acoustic wave) crystal
• Excellent cycle to cycle period jitter noise performance
• Extended commercial temperature range

The EPS series of products features frequencies ranging from 14.318MHz to 166.000MHz.

This product is offered with a supply voltage of 3.3VDC. The supply voltage tolerance is ±5%.

This product has an input current specification of 30mA maximum.

Ecliptek specifies the frequency stability performance of the device inclusive of specific oscillator operating conditions. This is often called the "Inclusive Method". Ecliptek specifies the following parameters for this series of products: Frequency Stability over Operating Temperature Range, Supply Voltage, Output Load, First Year Aging, Shock, and Vibration.

The EPS series offers an extended commercial temperature range of -20°C to +70°C. Certain operating temperature ranges can be provided above and below these temperature ranges on a case by case basis. Please consult the factory with your custom requirements.

Ecliptek offers a ±100ppm frequency stability option (over an operating temperature range of -20°C to +70°C).

Aging is the systematic change in frequency with time due to internal changes in the crystal and/or oscillator. Aging is often expressed as a maximum value in parts per million per year [ppm/year]. The rate of aging is logarithmic in nature. The following factors effect crystal aging: adsorption and desorption of contamination on the surfaces of the quartz, stress relief of the mounting and bonding structures, material outgassing, and seal integrity. At a rated operating temperature of 25°C, this series of products typically age at a rate of less than ±3ppm over the first year, ±1ppm over the following year, logarithmically declining each year there after. The aging parameters can be found on the EPS series specification sheets.

Start-up time for this series is specified at 10mSec maximum. Note: In order to ensure proper start-up, the power supply start-up should have an exponential curve typical of a capacitive charge of a linear voltage ramp. If you have a special voltage start-up profile (i.e. odd ramp steps or shapes), please contact us to discuss possible oscillator performance issues.

Ecliptek offers this oscillator product series with a low voltage HCMOS (LVHCMOS) output. The oscillator output topology is designed so as to optimize circuit load matching and signal performance. Signal integrity is optimized when the low impedance output of the oscillator is driving a high impedance-low capacitance input. Ecliptek specifies a load termination of 15pf maximum with rise and fall times of 2.7nS maximum (20% to 80% of the waveform).

The standard duty cycle specification is 40% minimum, 60% maximum. Tight duty cycle (45% minimum, 55% maximum) is also available for this product.

Period jitter is the measure in the time domain and is specified in picoseconds (pS). Peak to Peak and rms period jitter are commonly specified for non-modulated clock oscillators and are defined over a number of cycles. Due to the output frequency modulation, only cycle to cycle period jitter is specified on spread spectrum clock oscillators. Cycle to Cycle period jitter is measured in the time domain and is defined as the worst case clock period deviation of adjacent cycles.

Because the modulation frequency is substantially slower than the output frequency, the spread spectrum modulation has very little impact upon the cycle to cycle or short term jitter. Ecliptek uses a proprietary design, exclusive processing methods, and a unique ASIC output driver circuit enabling EPS series oscillators to have exceptionally low cycle to cycle period jitter. The cycle to cycle period jitter parameter can be found on the applicable EPS series specifications sheet.

This product offers two different tri-state output logic control function options to facilitate the customer's use of in-process assembly testing or for the use of multiple clocks. The EPS oscillator contains flash programmable power down (PD) and tri- state/output enable (TS) options for power management.

If the power down function is selected, then all active circuitry within the oscillator is shut down when the voltage at the control pin (pin 1) is set to a logic low state. In this condition, the output signal is three-stated (tri- state) with a weak pull down (100k ohm typical) and the input current on the power supply line is negligible (standby current specification of 50µA).

If the tri-state/output enable option is selected, the output is three-stated (tri-state) when the voltage at the control pin (pin 1) is set to a logic low state. In this condition, the oscillator and PLL continue to operate. However, the output signal is now high impedance with a weak pull down (100k ohm typical) and the input current on the power supply line is only slightly decreased from normal operating current (disable current specification of 20mA maximum).

The tri-state output and power down output enable/disable times can be found on the applicable Ecliptek specification sheet.

Because Ecliptek uses a 2 wire 4 pin programming interface, this series product does not offer a 'No Connect' option for pin 1. Only the power down (PD) and tri-state/output enable (TS) options are available. However, the customer can use the EPS oscillator as a non-tri-state oscillator by setting the control voltage on pin 1 to either no connect or logic high. The oscillator has an internal pull up resistor on pin 1 (100k ohms typical).

The term "complementary output", often called a "differential pair", is when one output signal is the logical opposite (complement) of the other output signal. Thus, when the output pin/pad of the oscillator is in a logic high state, the complementary output pin/pad of the oscillator is in a logic low state. Complementary outputs, commonly found on LVPECL and LVDS oscillators, are not available with the EPS series.

The customer should layout their PCB to include proper connections for the tri-state input function. See the recommended solder pad layout for details.

The EPS oscillator series are currently offered in a 5mm by 7mm ceramic four pad SMD package (EPS13D2).

This product consists of a single ASIC and a fundamental mode BAW quartz crystal packaged inside a hermetically sealed package. The EPS13D2 oscillator is manufactured with a ceramic leadless chip carrier that has four gold plated contact I/O pads. The SMD package is seam sealed with a metal cover that is case grounded for improved EMI performance.

It is recommended that the customer use the test fixture specified by Ecliptek. Please see the applicable EPS series test circuit diagrams for the suggested test circuit diagram.

This series of products features frequencies ranging from 14.318MHz to 166.000MHz. Certain frequencies may be able to be provided above and below these frequency ranges on a case-by-case basis. Please contact us with your custom requirements.

Most PLL's are designed to work properly when driven by both modulated and un-modulated clocks. However, one should take extra precautions in circuit design and implementation when using a spread spectrum clock due to PLL tracking rates, loop bandwidths, response times, and phase/jitter performance.

EMI is short for Electromagnetic Interference. EMI is defined as a naturally occurring phenomena when the electromagnetic field of one device disrupts, impedes, or degrades the electromagnetic field of another device by coming into proximity with it. Electromagnetic Interferences (EMI) can cause two or more electronic devices to interfere with each other and affect their performance and operation. For example, when you are using your cordless phone or laptop computer, you do not want it to interfere with your television reception.

Frequency sources such as quartz crystal oscillators, phase lock loop (PLL) synthesizers, and other types of clock signal generation schemes are a major source of EMI in electronic circuits. Therefore, EMI reduction is a major concern for designers of electronic products utilizing these clock schemes.

Conventional methods of EMI reduction include multiple ground and power planes, discrete component filtering and enclosure shielding. These methods are commonly practiced and can have substantial cost impact to the overall product. An alternative to some of these EMI reduction techniques is the implementation of a spread spectrum clock oscillator. The use of such an oscillator can significantly improve EMI and reduce overall product cost.

Crystal oscillators generate an output signal at their intended output frequency. There is a large amount of radiated electromagnetic emission at this fundamental frequency and its harmonics. These oscillators also generate electric signals at frequencies slightly lower and higher than the intended output. These additional signals radiate electromagnetic waves over a frequency spectrum. The range of the frequency spectrum is dependent upon the mechanical and electrical design of the oscillator, power supply regulation, output termination, and printed circuit board (PCB) layout.

EMI is a measurement of radiated energy from a frequency source and is typically measured in dBmV/m (decibel-volts per meter) at a given frequency. This parameter is larger for higher amounts of radiated energy. Thus, the more energy emitted from a frequency source, the larger the resulting electric field and EMI.

As mentioned above, a non-spread spectrum oscillator has radiated emissions over a given frequency spectrum. Thus, when defining the frequency spectrum, one wants to distinguish between the peak electromagnetic emissions and the average electromagnetic emissions. The average emission is defined as the average dBmV/m level over a given frequency spectrum of an oscillator output signal. Peak emission is defined as the maximum dBmV/m level at any frequency over a given frequency spectrum of an oscillator output signal.

Electromagnetic Interference (EMI) is subject to very strict regulations by the US Federal Communications Commission (FCC) and other international regulatory bodies whose goal are to limit the amount of EMI electronic devices emit and to prevent damage to the human body and interference between electronic devices.

The FCC's Class A regulations apply to industrial applications and the Class B regulations apply to residential or consumer applications. Computer and peripheral hardware applications typically are concerned with compliance to Class B regulations.

Today, FCC regulations are primarily concerned with peak emissions at any given frequency, not the average emissions over a given frequency spectrum. Thus, a circuit designer should focus their EMI design efforts with reducing the peak emissions at any given frequency within the frequency spectrum, not the overall average emissions within the spectrum. Figure 2 shows a FCC Class B plot of power (dBµV/m) versus frequency (MHz) for the peak emission requirements (at 10 meters).

Figure 2: FCC Class B Peak Emissions

A spread spectrum crystal clock oscillator is an oscillator that has the output frequency intentionally modulated in order to reduce the EMI on the output signal. Spread spectrum crystal clock oscillators are best used in applications that require a reduction of EMI emissions in order to pass the FCC EMI regulations. Additionally, using a spread spectrum clock oscillator reduces the EMI at the clock source, rather than at locations later down in the clock stream. By reducing the EMI at the clock source, supplemental shielding enclosures and/or filtering components may not be required, reducing overall system costs and improving overall EMI performance.

By modulating the output signal, the EMI on the output signal is 'spread' over a larger frequency 'spectrum'. The total amount of energy is still present, but the spreading of the output power over the frequency band results in a reduction of EMI at any one frequency. As mentioned above, regulatory bodies like the FCC have maximum limits for peak EMI emissions (emissions at any one frequency within the spectrum). Thus, a quartz crystal clock oscillator can be used to pass FCC regulatory EMI test requirements by reducing EMI peak emissions.

An EPS series oscillator consists of a fundamental mode crystal controlled oscillator and a flash programmable integrated circuit. The memory contains programmable functions that control the operating characteristics of the device (i.e. output frequency, modulation frequency, output frequency spread spectrum percentage, power down versus tri-state, and duty cycle). At the heart of an EPS series oscillator is a programmable high resolution PLL (Phase Locked Loop as shown in Figure 3). The PLL consists of a reference counter divider, a feedback counter multiplier, spread spectrum modulation signal, and an output divider. Utilizing a proprietary design and an exclusive programming methodology, the EPS series oscillators are programmed with specific values that define the operating characteristics of the oscillator.

Figure 3: PLL Block Diagram

Figure 4 below shows a plot of output amplitude versus frequency for a modulated and un-modulated center-spread spectrum clock oscillator. As you can see from the figure, there is a large difference between the frequency span and the amplitude for a given modulated and un-modulated spread spectrum clock oscillator. By modulating the output frequency over a frequency spectrum, a reduction in output amplitude can be achieved. This reduction in output amplitude correlates with a reduction in radiated energy (EMI).

Figure 4: EMI Reduction Plot

There are two major factors that significantly affect the amount of peak EMI reduction for a spread spectrum clock oscillator: Output Frequency Modulation Width and Frequency Modulation Profile.

Figure 5 below shows a plot of output frequency versus time for an output of a linear (triangular) modulated spread spectrum clock oscillator. As you can see from the figure, the output frequency has a minimum (FMIN), center (Fc), and maximum (FMAX) frequency. The output frequency is swept linearly though a range of frequencies rather than being held at one constant frequency. This 'range' parameter is often called output modulation width, output frequency spectrum, or frequency spread percentage. The maximum and minimum output frequencies are often stated as a percentage (%) with respect to the center frequency. The Ecliptek EPS series oscillators offer a range of programmable output frequency modulation widths from 0.5% to 4.0% (F_MAX minus F_MIN). The wider the modulation frequency spread percentage, the larger the bandwidth of frequencies over which the energy is distributed, and therefore the more EMI peak reduction.

Figure 5: Output Frequency Modulation Width

There are three major types of frequency modulation profiles: Sinewave, Linear (or triangle), and Non-linear (or optimized). Each of these three modulation profiles result in different EMI reduction performance. An example of each modulation profile and the resultant output is shown in Figure 6 below. The sinewave modulation profile is not typically used in spread spectrum clocks due to its large edge peaking. The linear modulation scheme has improved peaking at the edges, but the small trough in the middle of the profile often reduces the total EMI reduction. The best modulation scheme is the non-linear or optimized modulation profile. This profile reduces edge peaking and troughing, producing the most efficient EMI reduction profile.

Figure 6: Frequency Modulation Profile and Resultant Frequency Spectra

All Ecliptek EPS series spread spectrum oscillators use the non-linear, optimized modulation profile. This profile, often called the 'Hershey Kiss' profile, is shown in Figure 7 below.

Figure 7: Non-Linear Frequency Modulation Profile

Since the frequency modulation width is fixed and independent of the frequency modulation profile, the total radiated EMI is spread over the frequency modulation width. The goal of a spread spectrum oscillator is to spread the EMI energy evenly over the frequency modulation width, so as to eliminate any peaks or troughs. Using a sinwave or triangular profile can result in increases in peak EMI emissions. Using an optimized modulation profile evenly distributes this EMI energy over the frequency modulation width.

As shown in Figure 7, output frequency modulation (F_m) is defined as the inverse of the modulation period. This product series has an output frequency modulation, often called sweep rate, specification of 30 kHz minimum, 31.5 kHz typical, and 33 kHz maximum. Certain modulation frequencies can be provided above and below these frequency ranges on a case by case basis. Please consult the factory with your custom requirements.

Ecliptek offers two output frequency modulation options: Center spread and Down Spread. Figure 8 below shows an example of these two options.

Figure 8: Center and Down Spread Options

The instantaneous output center frequency (Fc) is approximately the midpoint of the minimum frequency and the maximum frequency. The instantaneous output frequency will therefore always vary between F_MIN and F_MAX. The instantaneous minimum (F_MIN) and maximum (F_MAX) output frequencies are stated as a percentage (%) with respect to the center frequency. In Figure 8 above, the center spread diagram provides an example of a device with a +/-1.0% center spread percentage. In this example, if F_O were 100MHz, typical frequencies for F_MIN, F_C and F_MAX would be 99MHz, 100MHz, and 101MHz, respectively.

When a system can not tolerate an operating frequency higher than the nominal frequency (often called over-clocking), then a down spread option should be considered. In Figure 8 above, the down spread diagram provides an example of a device with a +/-2.0% down spread percentage. For this example, if a customer was concerned about over-clocking and had a maximum operating frequency requirement of 100MHz (F_O), typical frequencies for F_MIN and F_MAX would be 98MHz and 100MHz, respectively. For a down spread device, the maximum instantaneous output frequency (F_MAX) is limited to the nominal frequency (F_O).

The disadvantage of down spread modulation is that the average output frequency will be lower than the nominal output frequency. Thus, there is a trade-off between average output frequency, maximum over-clocking and maximum frequency modulation amplitude.

An asymmetric spread is defined as setting the output frequency half way point between the maximum down spread frequency and the center spread frequency. Asymmetric spreading is often used when over-clocking is a concern. The Ecliptek EPS series does not offer an asymmetric spread option. However, carefully selecting the proper center frequency and spread percentage can often accomplish the same design goals.

Utilizing a proprietary design and an exclusive programming methodology, the EPS series oscillators can achieve significant reductions in output EMI emissions. The output frequency, spread percentage, and the measurement harmonic frequency are all variables that determine the EMI reduction. Please contact your Ecliptek sales staff for typical Ecliptek EPS series EMI characterization data.

Ecliptek utilizes a modulation domain analyzer to measure the center and down spread spectrum frequency percentage and a spectrum analyzer for measuring the reduction in output power.

If the part number is specified with the Tape and Reel option (TR), the EPS series oscillators are delivered to the customer in EIA481A compliant tape and reel packaging. Without the TR option, these products are delivered to the customer in industry standard plastic tubes.

This product series is Pb-free as defined in the Ecliptek Pb-free Roadmap and are capable of withstanding industry standard high temperature convection reflow processes. See the Recommended Solder Reflow methods listed on the data sheet.

Please see the Ecliptek Competitor Part Number Crosser.

Ordering on-line is simple. Simply fill in the required information in the part number constructor for the specific EPS series that you would like to order.

The "Environmental and Mechanical Specifications" for this series can be found in the Environmental & Mechanical section of the EPS specification.

Failure in Time (FIT) and "Mean Time To Failure" (MTTF) reliability data is available from the factory. Please contact us with your request for this data.

The engineering staff at Ecliptek can provide applications engineering support or customer technical questions.

Find a representative or distributor and request your desired custom specifications. You can also contact the Ecliptek Global Customer Support Team directly at the factory. Complete the Ecliptek Custom Oscillator Price and Delivery Request page from our website. From this page you will be able to enter custom specifications that are unavailable from the standard part number constructor forms. These parameters will be sent to our Engineering team where they will be evaluated. Upon acceptance, a custom part number will be assigned and an engineering document created to represent your product.