Nikkei Electronics Asia -- March 2009
Insights
Testing with Arbitrary Function Pulse Generators

During the design of electronic components and circuits for computers, peripherals and serial communication devices, pulse pattern generators are likely to be the first tools considered for device characterization. For many applications, modern general-purpose arbitrary/function generators (AFG) frequently present a flexible, versatile and more affordable solution to generate pulses.

AFGs are suited to generating logic, trigger, and clock signals, and via clock multiplier can even support high-speed serial standards such as PCIe and SATA. This article explores typical pulse generator test scenarios based on the AFG3251 and AFG3252 models of the AFG3000 series from Tektronix.

Benefits of Using AFGs

Activating the pulse generator function of the AFG3000 series is as simple as pressing the Pulse button on the front panel. This displays all relevant waveform parameters and a graphical depiction of the pulse waveform on screen to confirm the active settings. 

All pulse related settings (see Fig) are quickly accessible via dedicated shortcut keys on the front panel, and are adjustable on-the-fly via a rotary knob or numeric keypad. During timing parameter adjustments, the output signal remains free from glitches or dropouts, which is important, for example, when characterizing devices over a sweeping clock frequency. Dual-channel models are available to support applications that require more than one input signal.

Since the AFG3000 series is based on direct digital synthesis (DDS), signal shape and frequency can be selected independently in both channels. The signals can also be locked together in frequency and/or amplitude. In this case, an adjustable phase delay between both channels can be introduced, which is extremely useful for measuring channel-to-channel timing differences in semiconductor devices. For stress testing of devices, noise can be added to pulse signals, and jitter to square waves without a separate function generator, as is the case with dedicated pulse pattern generators. Engineers who work on a variety of designs would benefit from the versatility of the AFGs.

Aside from pulse and square waves, the generator can also generate sine, ramp, and arbitrary waveforms, as well as other standard functions. One point to consider when comparing AFGs with dedicated pulse pattern generators is that relative jitter increases with frequency due to the DDS architecture. For some of the pulse generators such as the AFG3251 and AFG3252, the pulse jitter specification of 100ps implies a relative jitter of 0.01% at 1MHz, but 1% at 100MHz.

Measuring Propagation Delay

In logic devices such as buffers and comparators, a parameter of interest is the propagation delay or response time, i.e. the time it takes for the device output to respond to an input signal. To measure this parameter, a pulse generator can be used to stimulate the device input with a pulse signal, and then the device input and the output signal can be measured with an oscilloscope. The signal source can then be programmed to generate pulses of frequency and amplitude within the operating range of the device.

Characterizing Setup, Hold Time

In logic circuit timing, setup time and hold time conditions play a critical role. A logic circuit captures data at the leading edge of the clock. For the data to be captured correctly, it needs to settle a certain time before the clock edge and remain stable for a certain time after this edge. The necessary settling time before the clock edge is known as setup time and the necessary time after the clock edge is known as hold time. These values are specified in the datasheet of the logic IC. They vary with the voltage of the power supply and other conditions. 

The tools necessary to measure setup and hold times are a dual-channel AFG and an oscilloscope. To stimulate the device, one can program the AFG3252 with specific settings, having channel 1 to generate the clock and channel 2 the data. To synchronize data and clock timing, press the phase/delay button and then align phase in the soft menu on the screen. The clock, data and device output signals are then measured with an oscilloscope. To determine setup and hold time, press the delay button on the front panel and vary the delay of channel 1 with the rotary knob while observing the flip flop output signal on the oscilloscope. The delay can be adjusted in increments as small as 10ps via the rotary knob.

Evaluating Slew Rate Performance

High-speed operational amplifiers (op amps) are among the most common analog components in use today. A critical performance aspect of op amps is their transient response or slew rate performance. Op amps used in set-top boxes and security video applications need a high slew rate combined with ultra-low distortion. Slew rate and transient response are also an issue for op amps that drive extremely fine movement in inkjet printers and medical devices. An op amp's transient response may be different for the rising and falling edges of the input signal - behavior known as asymmetrical slew rate performance. It may affect whether the op amp is used in an inverting or non-inverting configuration.

Knowing the timing characteristics of an op amp makes it possible to optimize gain and feedback resistors, or take other measures to achieve the desired circuit behavior.

To characterize an op amp's slew rate performance, one can measure its transient response with an oscilloscope while stimulating its input with a pulse signal with variable rise time, fall time, and amplitude.

Generating Pulses with Jitter, Noise

To assure a reliable operation, digital components and circuits are required to be robust against a certain amount of jitter and noise in clock and data signals. Otherwise, communication errors or system failures could result. To evaluate components and circuits for their jitter and noise tolerance, electronic design engineers need a solution that can generate pulses with controllable jitter and noise. While dedicated pulse pattern generators typically require a separate function generator to add signal distortion, the AFG3000 series provides a one-box solution with jitter and noise generator built into the instrument. Jitter can be added via the built-in phase modulator with selectable modulation frequency, wave shape, and phase deviation. After programming the instrument with appropriate settings, the instrument will output a pulse with 50% duty cycle and well-defined jitter.

To add noise to any generated signal, select the output menu after pressing the desired waveform button and turn on Noise Add. The noise level can then be selected from 0% to 50%. Noise Add reduces the signal amplitude by half to prevent the noise from clipping at amplitude settings close to the maximum.

Creating pulse waves via arbitrary waveform functions works well with DDS-based AFGs as long as the selected pulse repetition rates are well below the instrument's clock rate. With a clock rate of 2Gsps and 500ps jitter (rms), the AFG3252 supports a wide range of applications. However, at higher pulse repetition rates, the waveform point skipping and duplication inherent to DDS-based generators can lead to extra jitter.

Simulating Slow I2C Serial Signals

Microcontrollers and computers in embedded systems often utilize low-speed serial buses such as I2C, SPI, RS-232, CAN and LIN to communicate with specialized devices such as sensors, switches, ADCs, digital potentiometers and displays. To validate and stress test new designs, engineers may need to simulate data and clock signals. I2C bus signals that control a driver for a numerical LED can be easily created and generated with a dual-channel AFG. Channel 1 of the AFG3252 generates the clock and channel 2 the data signal. Data and clock signals are created via the marker functions of the software package ArbExpress. After launching the application, select Standard Waveform in the menu, select DC as function in the standard waveform window that opens and set the wavelength to the desired number of points.

by Zubin Mohta, India AE Manager, Tektronix Inc

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