Crystal clear

Crystal clearCrystal oscillators keep frequency-sensitive applications on track in a variety of applications, from telecommunications to computers. Knowing how to specify them will keep design projects in line as well. Nothing looks better to a project leader than the lowest possible cost for the right part, and using these tips when specifying voltage-controlled crystal oscillators (VCXO’s) will ensure a designer gets just that
Q What kind of crystal oscillator is needed?
A Making sure it’s a voltage controlled crystal oscillator your project needs is, of course, the first step. Unlike an ordinary VCO (voltage controlled oscillator), a crystal-controlled VCXO possesses the ability to accurately maintain a reference frequency even in the event of a loss of the control signal (despite changes in the environment such as temperature or supply voltage.This characteristic is especially important in wireless applications (such as satellite and cellular telecommunications(where drop outs can occur. The VCXO will continue to supply a stable reference. Yet, it is of equal value for high-speed wire-line links such as ISDN, ATM, and xDSL, where VCXO’s maintain the essential frequency with minimal noise or jitter. Consumer applications include MPEG video and cable modems.
Q What are my VCXO frequency needs?
A VCXO frequency is the first consideration for all designs. This, of course, is dictated by the application. Each VCXO operates within a certain frequency range, which must suit the reference frequency that will be used in the circuit.
In the past, meeting high frequency demands with a VCXO was difficult. But with new processes for producing high frequency fundamental crystals, manufacturers can offer very cost effective VCXO’s from 32 to 155.52MHz.
Q What output load is required?
A Common output load choices include: true TTL, TTL compatible, HCMOS, ACMOS, or ECL and PECL, where single output or complementary output options need to be selected. This is a good time to plan proper termination and printed circuit board trace layouts. Understanding design requirements ensures that the specified oscillator is right for the application.
Q What supply voltage, control slope, and range?
A 3.3V VCXO’s are becoming more common, replacing some 5V designs. For ECL loads, -5.2V is standard; PECL is available in both 3.3V and 5V.
The control voltage range for the VCXO is generally equal to the supply voltage, minus a ten per cent margin. Therefore, a 3.3V VCXO offers a standard control range of 0.3V to 3V; a 5V part 0.5V to 4.5V.
A positive control slope, where frequency increases with control voltage, is supplied as standard by most manufacturers.
Q What operating temperature do I need?
A Like the other factors, it should be carefully considered and specified to fit the application, as it impacts cost significantly. The wider ranges increase cost, so design engineers should take care to order exactly what they require and no more.
Q How do I differentiate between absolute pull range and variations from nominal?
A A VCXO’s ability to lock onto a frequency is achieved by its capacity to vary its centre frequency under the control of an input voltage. It is important to note that the industry has two disparate methods for specifying a VCXOs’ pullability.
The first method is called Absolute Pull Range (APR), and is defined as the net frequency control range (also known as lock range or capture range) of the VXCO, after all tolerances have been accounted for:
APR = (VCXO pull relative to specified output frequency) – (VCXO frequency stability) – (aging)
The second method considers variations from nominal. Here the engineer must calculate the absolute pull range by starting with the total pull range of the VCXO, and then subtracting the sum of all variations and tolerances such as: calibration, temperature, power supply, and load.
The APR method is preferred because the design engineer can more easily narrow down the requirements to the bottom line, without having to deal with the individual factors leading up to that figure. For example, a VXCO with an APR of 50ppm will track a 50ppm source oscillator under all specified operating conditions.
Q Should I avoid “gold plating” the design?
A Even with the simplicity of APR specification, pullability can be a pitfall for VCXO buyers. Many designers overspecify, attempting to build margin into their designs with high VCXO pullability. Ordering more pullability than is needed is a great way to expand the budget of a design project.
Q Should I determine enable/disable (tri-state) requirements?
A Some designers require an enable/disable feature for in-circuit testing. Yet, some vendors still only offer four-lead packages, so the choices are somewhat limited. However, some vendors now offer five or six-pin packages which can easily accommodate the incorporation of this capability into VCXO’s.
The use of six pins also allows the designer the option of getting enable/disable capability in ECL/PECL VCXO’s with differential complementary outputs.
Q How should the oscillator be packaged?
A Most vendors deliver VCXO’s in dual in-line, through-hole metal cases and in surface mount plastic or ceramic packages. The surface mount package is becoming the more popular. The application’s circuit board form factor and production process determine package type and footprint.
Q What packing method for shipping should I use?
A The last consideration in VCXO specification is delivery method. Surface mount oscillators are typically shipped in industry-standard tape and reel, although they can be ordered in bulk. Through-hole parts, on the other hand, are most often packed in tubes or ESD-control foam. The customer should specify any necessary differences from these shipping methods.
Joshua Wood is a product specialist at Saronix. www.saronix.com


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