Check the clock oscillators
Engineers at Crystek, a manufacturer of oscillators and quartz resonators, use an obscure oscilloscope feature to test their products.
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 5/1/2007 2:00:00 AM
DEVICE UNDER TEST
Crystal oscillators, voltage-controlled oscillators, and quartz resonators. Frequencies range from 32 kHz to 6 GHz. Most devices are assembled in surface-mount packages, with 7x5 mm being the most common size. The devices come with several output options, including low-voltage differential signaling (LVDS), CMOS, positive emitter-coupled logic (PECL), and clipped sine wave.
THE CHALLENGE
Evaluate new oscillators in the engineering lab prior to production. Measurements include frequency, jitter, phase noise, rise time, fall time, input current, output power, and stability. Perform temperature-stability tests at temperatures ranging from –40C to 85C.
THE TOOLS
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Agilent Technologies: digital multimeter, frequency counter, phase-noise tester, triple-output power supply. www.tm.agilent.com.
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Tektronix: oscilloscope. www.tektronix.com.
PROJECT DESCRIPTION
Crystek (www.crystek.com) manufactures crystal oscillators, voltage-controlled oscillators, and quartz resonators in Ft. Myers, FL. The devices provide timing signals for microprocessors used in embedded systems, for telecom routers, and for other applications. Prior to each new product release, director of engineering Ramon Cerda and a team of engineers evaluate parts.
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Engineers connect an oscillator to an oscilloscope, which connects to a frequency counter. |
The most critical measurements that the engineers make are output frequency, jitter, and phase noise. They use evaluation boards that contain SMA connectors that provide test points and connections to test equipment. The evaluation boards also contain a plastic fixture that holds the oscillator in place.
The engineers measure frequency with a 10-digit RF frequency counter, and they use a 1-GHz, 10-Msamples/s oscilloscope to measure jitter, rise time, and fall time. The frequency counter uses an in-house, GPS-based reference clock. They also measure phase noise with a phase-noise tester.
The difficulty in making these measurements comes from the oscillator’s high output impedance—up to 10 Ù for devices with clipped-sine-wave and CMOS outputs. Connecting two instruments to an oscillator’s output places too heavy a load on the device. “You can’t just use a T connector to route signals to two instruments,” warned Cerda. Rather than build a buffer circuit, Crystek engineers use the oscilloscope’s buffered analog output, which is a replica of the scope’s input signal.
Crystek’s engineers use an active probe to minimize loading on the circuit. The active probe loads the oscillator with less than 1 pF. A 15-pF capacitor on the board provides a known, stable load. A power supply connects to the evaluation board through a digital multimeter (DMM), which measures the oscillator’s input current.
For devices with sine-wave outputs, the engineers calculate the amplitude in dBm based on the peak-to-peak amplitude. For these devices, with outputs to 6 GHz, engineers use a spectrum analyzer and can characterize the device in the frequency domain. For square-wave outputs (output frequencies to 200 MHz), engineers measure rise time, fall time, and duty cycle (45% to 55%). The oscilloscope’s jitter-measurement software measures cycle-to-cycle and periodic jitter.
Engineers perform these and other tests at temperatures beyond the device’s specified range. Temperature cycling ranges from –40C to 85C. The engineers measure frequency, amplitude, and phase noise over temperature. In addition, they perform measurements for varying input voltages, which range from nominal voltage (5 V, 3.3 V, or 2.5 V) to ±10%.
LESSONS LEARNED
“When testing crystal oscillators and other devices with high output impedances,” said Cerda, “test equipment can load the device under test. A buffered output from an oscilloscope is a must.” Cerda warned against using a “T” connector, because both the scope and the counter will load the oscillator’s output, thus changing the output signal. “It’s also important that your oscilloscope has enough bandwidth to let you accurately measure a square-wave oscillator’s third harmonic,” he added.
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