test  techniques


This is another situation in which a spectrum analyzer will produce more accurate readings than a frequency counter because the amplitude of the frequency of oscillation will usually stand out despite these distortions


In applications outside the general-purpose parametric test environment, test system designers strive to ensure that the impedances of the DUT, transmission lines, and measurement equipment are matched and that there are no open signal paths, thereby minimizing the major causes of AC signal distortion: insertion loss and reflections. However, this is not always possible to accomplish in a DC parametric test environment.


This is another situation in which a spectrum analyzer will produce more accurate readings than a frequency counter because the amplitude of the frequency of oscillation will usually stand out despite these distortions.


Frequency measurement hardware


Some parametric testers, including the S530 Parametric Test System, support characterizing ring oscillators using a frequency measurement option. For the S530, this measurement option is oscilloscope-based and connects to an instrument port on the system’s switch matrix, just like the system’s source measurement units (SMUs), C-V unit, pulse generator units, and digital multimeter (DMM.) Once connected to the switch matrix, the frequency measurements can be switched to any one of the matrix’s DUT pins.


The instrument driver for the frequency measurement option measures the frequency of a ring oscillator signal by using Fourier analysis to convert the signal from the time domain to the frequency domain. The various software commands return the measurement in the form of pairs of results, corresponding to the frequency and amplitude of the strongest signals. The S530’s frequency measurement option can accurately measure AC signals with signal levels above 25mVp-p and frequencies up to 20MHz.


Ring oscillator measurement example To illustrate the process of making a ring oscillator measurement, let’s begin by assuming a ring oscillator like the one shown in Figure 3. The freq _ init, freq _ setup, and freq _ measure commands described previously provide one way to measure the ring oscillator frequency and signal amplitude. For the purposes of this example, let’s assume that


Figure 3.Generalized CMOS ring oscillator


For this example, the following seven-command S530 test sequence could be used to perform this measurement:


vss _ pin = 1


vcc _ pin = 2 vccb _ pin = 3 output _ pin = 4


vcc = 5.0


Defines the voltages required to power and trigger the ring oscillator.


conpin(SMU1, vcc _ pin, Makes the connections to the DUT. vccb _ pin, 0)


conpin(vss _ pin, GND, 0) conpin(SCP1A, output _ pin, 0)


forcev(SMU1, vcc)


init _ status = freq _ init()


setup _ status = freq _ setup( 0e6, 15e6, 1e6)


Powers the device and triggers the oscillation.


Initializes the oscilloscope card. This command is always required to use the scope card!


Defines the frequency scan and resolution


bandwidth. Because the oscillator frequency is 10MHz, it’s essential to make sure that it is within the scan boundaries. In this case, the start frequency is set to 0Hz, the end frequency to 15MHz, and the resolution bandwidth (or scan resolution) to 1MHz. This would give a frequency measurement that is accurate to 1MHz.


meas _ status = freq _ Performs the measurement and returns the


measure(frequency, level) frequency (in Hz) and amplitude (in Vp-p) of the strongest signal found.


Issue IV 2012 www.siliconsemiconductor.net 17 Defines the DUT pins.


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