10 Quick Tips for VCXO Specification

Crystal oscillators keep frequency-sensitive applications on track in a variety of products from cell phones to computers and appliances with "smart" electronics. Knowing how to specify voltage controlled crystal oscillators (VCXOs) helps keep production costs low and performance high. In order to aid designers tasked with ordering those components which yield the exact performance and control needed within the conditions of their particular circuit, Beat Kocher, Manager of Marketing and Business Development at SaRonix, Inc., Menlo Park, CA, provides information detailing spec requirements.

1 -- Choose a Type of Oscillator

Unlike an ordinary voltage controlled oscillator (VCO), the crystal-controlled VCXO possesses the ability to accurately maintain a reference frequency, even in the event of a loss of the control signal and despite changes in the environment such as temperature or supply voltage.

This characteristic is especially important in wireless applications where drop-outs can occur. Yet, it is of equal value for high-speed wire-line links such as ISDN, ATM and xDSL, where VCXOs maintain the essential frequency with minimal noise or jitter.

2 -- Know Your VCXO Frequency Needs

VCXO frequency is, of course, dictated by the specific application. Each VCXO operates within a certain frequency range which must suit the reference frequency used in the circuit. Owing to new processes for producing high-frequency fundamental crystals, VCXOs are available from 32 to 155.52 MHz.

3 -- Determine the Required Output Load

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 the time to plan necessary termination and printed circuit board trace layouts.

4 -- Specify the Supply Voltage, Control Slope and Range

It is critical to identify supply voltage, as 3.3V VCXOs are increasing in demand, 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, less a 10 percent margin. Therefore, a 3.3V VCXO offers a standard control range of 0.3 to 3V; and a 5V component's range is 0.5 to 4.5V.

A positive control slope, where frequency increases with control voltage, is supplied as standard by most manufacturers. Any design requirements that differ need to be specified.

5 -- Identify the Operating Temperature Range

Operating temperature is one of the most important parameters for VCXOs. Like other factors, it should be carefully considered and specified to fit the application, as it significantly impacts cost. Wider ranges increase cost, so designers should order exactly what they require and no more.

6 -- Differentiate from Absolute Pull Range and Variations from Nominal

A VCXO's ability to lock onto a frequency is achieved by its capacity to vary its center frequency under the control of an input voltage. It's important to note that the industry has two disparate methods for specifying a VCXO's "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 VCXO after all tolerances have been accounted for. In formula form, APR = (VCXO pull relative to specified output frequency) - (VCXO frequency stability) - (aging).

The second method considers variations from nominal. Here, the design engineer must calculate the absolute pull range by starting with the total pull range of the VCXO, then subtract 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 easily narrow the requirements to the bottom line without having to deal with the individual factors leading up to that figure. For example, a VCXO with an APR of ±50ppm will track a ±50ppm source oscillator under all specified operating conditions.

7-- Avoid "Gold Plating" the Design

Even with the simplicity of APR specification, pullability can be a pitfall for VCXO buyers. Many designers overspecify, attempting to build margin into their design with high VCXO pullability. Ordering greater pullability than needed is an excellent way to expand a project's budget.

8--Determine Enable/Disable (Tri-State)
Requirements

An enable/disable feature is occasionally desired for in-circuit testing. While most VCXO packages have four leads, a 5- or 6-lead package permits the accommodation of this function. Six-lead packaging is best suited for providing enable/disable capability in ECL/PECL VCXOs with differential complementary outputs.

9 -- Package It Correctly

VCXOs are presently offered in dual in-line, through-hole metal cases, and in surface-mount plastic packages. The application's circuit board form factor and production process determine package type and footprint.

10 -- Specify Packing Method for Shipping

Surface-mount oscillators are typically shipped in industry-standard tape-and-reel, although they can be ordered in bulk. Through-hole parts are most often packed in tubes or ESD-control foam. Customers need to specify any differences from these shipping methods.

Kocher also reminds all designers to "tap all the resources, such as data sheets and application notes, offered to them by VCXO manufacturers." A website can be especially useful if the manufacturer includes a phone number.

For more information, contact SaRonix, Inc., 141 Jefferson Dr., Menlo Park, CA 94025. 800-227-8974. Circle 430.

Originally published in the January 1999 issue of Medical Equipment Designer.
Please Note: some pictures or diagrams are only available through the printed media.