Inverted Mesa Fundamental-mode Crystals
Champion Technologies Inc.
Franklin Park, IL
High frequency fundamental-mode crystals are increasingly in demand for applications such as precision high speed clocks, voltage-controlled crystal oscillators (VCXO) and voltage-controlled temperature-compensated crystal oscillators (VCTCXO). However, conventional crystal processing utilizing flat quartz blanks has limited the available frequency range to 55 MHz.
Using higher quality swept quartz, it has long been possible to produce inverted mesa fundamental-mode crystals with frequencies extending beyond 200 MHz. However, the higher cost of materials coupled with slower production throughput has made these resonators undesirable for use in large-volume applications. As telecommunications equipment designers continue to demand oscillators that operate at higher frequencies and still retain the smallest possible size and tightest stability, manufacturers have scrambled to fill this growing market.
Consequently, a means to produce high frequency inverted mesa fundamental-mode crystals without the need to start with swept quartz has been found. The CIM-32 series high frequency crystals were developed with a proprietary processing technology that produces crystals with low series resistance, high pulling ability and excellent unit-to-unit and lot-to-lot repeatability at an affordable price. The new crystals are available in frequencies ranging from 51 to 155 MHz and are targeted for applications in precision high speed, low jitter clocks as well as VCXOs, VCTCXOs and low jitter hybrid modules for data communications.
The New Design
Several basic process steps are involved in manufacturing an inverted mesa-type quartz. A quartz wafer is metallized on both sides with a chromium gold masking mechanism in the shape of a doughnut. The plated wafer is then etched for the appropriate length of time depending on the desired frequency of operation using a proprietary etching process, and the metallization is removed from the inverted mesa wafer. At this point, the wafer thickness is 0.0030" to 0.0033" and the thickness of the middle active area for a 155.52 MHz fundamental-mode resonator is 0.00042". Finally, the resonator is enclosed in an HC-45 resistance-welded package that can be supplied for through hole or surface-mount applications. Figure 1 shows the steps involved in this process.
The general specifications for this type of crystal include a maximum series resistance of 25 W, a standard load capacitance of 32 pF (other options are available) and a typical frequency tunability of 105 ppm. The device is designed for operation over a -40° to +85°C temperature range. Individual specifications include a nominal operating frequency within the range of 51.84 to 155.52 MHz and a frequency tolerance at 25°C of +/-15 ppm. Temperature stability over a 0° to +70°C range is +/-10 ppm referenced to 25°C, and aging for the first year at 25°C is typically 4 ppm. Figures 2 and 3 show typical temperature stability and aging performance, respectively, for a CIM-32-type crystal at 77.76 MHz. Figure 4 shows aging for a similar crystal at 155.52 MHz.
A Typical VCXO Application
One of the more popular applications of this device is in high frequency VCXOs. The example described here illustrates the crystal in a differential pair topology, as shown in Figure 5 . This type of circuit offers several advantages: The amplifier can operate at large-signal amplitudes without serious phase degradation due to the saturation and bias shift present in single transistor designs. Also, the differential pair has a broad linear region with smooth and symmetrical limiting.
Using a proper design, the differential pair can be kept out of saturation to improve phase noise. In addition, the limiting function can eliminate the need for automatic level control. The differential pair configuration can be realized at very high frequencies. The circuit is noninverting and the collector of Q1 is AC grounded, thus eliminating the Miller effect at the base of Q1. The output is taken from the collector of Q2. In this example, R2 provides the negative feedback and the design uses a tank circuit at the collector of Q2, which removes undesired capacitance and permits higher frequency operation. It also constrains oscillation to the crystal's fundamental mode.
The oscillator's performance is typical of what can be achieved using an inverted mesa-type crystal. This example operates at 155.52 MHz +/-20 ppm over a temperature range of 0° to +70°C and produces > -5 dBm from a 5 V DC supply at < 10 mA. The oscillator's phase noise is < -130 dBc/Hz at 10 kHz from the output frequency. The tuning deviation from center frequency is > +/-100 ppm with a linearity of < 10 percent for a control voltage of 0.5 to 4.5 V applied to the varactor diode.
With the introduction of the CIM-32 series inverted mesa crystals, manufacturers of low cost, high volume electronic equipment can specify their VCXOs with higher frequencies, tighter stabilities and smaller packages without breaking their budgets. A full line of high frequency crystal oscillators using this technology is also available.
Champion Technologies Inc.,
Franklin Park, IL