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A Surface-mount OCXO for Wireless Applications

A surface-mount oven-controlled crystal oscillator (OCXO) capable of covering a wide range of performance using precision AT- and SC-cut srystals

April 1, 1997
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A Surface-mount OCXO for Wireless Applications

Reeves-Hoffman Inc.
Carlisle, PA

The growth of today's electronic communications market is based on increased technological capabilities and making those technologies cost effective. Those same principles led to the development of the surface-mount, oven-controlled crystal oscillator (SMOCXO). The personal communications service (PCS) and paging applications of today require more functionality and higher performance from the frequency sources at lower costs. Higher stabilities, reduced size, electronic correction, increased frequency deviation and better reliability are some of the areas in which the SMOCXO can offer improved performance. Customer cost savings are achieved through reduced PCB space, auto placement and reflow survivability of the SMOCXO, not to mention the reductions in selling price through improvements in automation of the SMOCXO's manufacturing and test processes.

Weighing less than 15 g and measuring less than two-thirds of a cubic inch, the SMOCXO is capable of covering a wide range of performance through the use of precision AT-cut crystals and newer SC-cut crystals, which are available now in miniaturized packages. High stability versions of the SMOCXO, which use the SC-cut crystals (SC-SMOCXO), are able to meet the 50 ppb per year requirements of Global System for Mobile communications equipment and the ±1 ppm for 20 years requirements found in many of today's private branch exchange and central office switch telecom systems. The AT versions of the SMOCXO (AT-SMOCXO) are suitable for microwave and synthesizer applications that require more economy with a high level of performance. Both models are provided with an electronic frequency adjust that allows for calibration or steering of the oscillator manually or through software control. Wide deviation options are available for phase-locked loop applications that require tight stability when free running or out of lock.

The SMOCXO is constructed using high temperature solders and a proprietary low mass design. Thermal modeling of the design during the development process helps ensure that the production units operate within the required ranges even when component and crystal tolerance effects are included. By minimizing the thermal gradients around the crystal, enhanced temperature stabilities can be achieved at lower cost.

The SMOCXO uses an FR4 base. The mechanical design eliminates any coplanarity issues that can result from handling. The use of FR4 material also ensures high solder connection reliability by minimizing the effects of thermal expansion. Edge clips provide additional strain relief, which is critical because of the elevated temperatures produced during reflow and by SMOCXO operation in the system. The FR4 base simplifies SMOCXO construction as it is used as the mounting platform of the oven-controlled circuitry. Proprietary OCXO design and construction techniques are the keys to this low cost and highly reliable assembly. Historically, manufacturing and test labor have been major cost drivers. Extensive automation of the assembly process has occurred, including the use of in-line testing. Aging and temperature testing have also been automated to reduce the labor associated with oscillator testing. Using statistical process control, assembly and test automation, and design for manufacturability techniques, the SMOCXO achieves high yields, high performance and high reliability at significantly lower costs.

The SC-SMOCXOs achieve temperature stabilities of ±5 ppb easily from -40° to +85°C and aging rates can be specified as low as 30 ppb per year. The AT-SMOCXO offers economy with high performance. For the AT-SMOCXOs, the available temperature performance is ±50 ppb from -30° to +70°C with aging rates of 200 ppb per year. Figure 1 shows the temperature stability of a group of 10 MHz SC-SMOCXOs. The SMOCXO has been characterized thoroughly. Attention to the construction details and designing for manufacturability assure the stability of the oscillator is maintained even through the solder reflow process.

Fig. 1: Frequency vs. temperature of an SC-SMOCXO group.

Figure 2 shows typical SC-SMOCXO aging over the first 14 days of operation. The oscillator settles down in three to five days, a time period that is much shorter than the one to two weeks required by more typical SC-cut crystals. The aging performance has been enhanced by SC-cut crystals that have been fabricated using specialized, proprietary processes. Figure 3 shows the projected aging of the SC-SMOCXO out to 20 years using the collected data. The linear extrapolation used represents a worst-case analysis of the expected aging by the oscillator.

Fig. 2: The SC-SMOCXO's initial aging.

Fig. 3: Projected aging.

The SMOCXO is available with two options for phase noise. The close-in phase noise levels are determined primarily by the crystal type. Figure 4 shows the typical phase noise achieved by the SC-SMOCXO. Performance capabilities of the AT- and SC-SMOCXO versions are listed in Table 1 .

Fig. 4: The SC-SMOCXO's typical phase noise.

Table I: Performance Characteristics

 

SC-SMOCXO

AT-SMOCXO

Size (overall) (mm)

25 × 30 × 15

25 × 30 × 15

Current consumption (max) (mA)
at turn on
steady state


500
200


500
200

Warm-up (df within 5 min.)

±5 × 10-8

±1 × 10-7

Temperature stability (df)

±5 × 10-9
(-40° to +85°C)

±5 × 10-8
(-30° to +70°C)

Aging
long term (20 years) (ppm)
per day (at shipment)


±1
±5 × 10-10


±4
±1 × 10-9

Phase noise (dBc/Hz)
10 Hz offset
100 Hz offset
1 kHz offset
10 kHz offset


-115
-135
-145
-155


-100
-120
-140
-155

Frequency adjust
(min) (0 to 5 V) (ppm)

±1

±6

Figure 5 shows the warm-up characteristics of the SMOCXO before and after exposing the oscillator to the typical reflow profile, shown in Figure 6 . The difference in the stabilized frequencies is referred to as retrace. Retrace is a characteristic of all OCXOs and is influenced most by off time and temperature history. The data show that there is no significant frequency offset caused by the surface-mount reflow process.

Fig. 5: Oscillator warm-up characteristics.

Fig. 6: A typical surface-mount reflow profile.

The size, performance range and cost of the SMOCXOs make them a basic building block for the PCS, cellular and paging systems of tomorrow. The unique design and construction allow for easy optimization to meet a wide variety of performance and cost requirements. Depending on the stabilities selected, 1000-piece pricing for the SC-SMOCXO ranges from $140 to $180. The AT-SMOCXO has options as low as $100 in 1000-piece quantities.

Reeves-Hoffman Inc.,
Carlisle, PA (717) 243-5929.

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