Agilent Measurement Journal - Issue 4 - 2008 - (Page 63)

Clock accuracy, quoted as parts per million (ppm), indicates frequency drift. In the process of digitizing a signal, it is assumed that the samples are equally spaced in time so any clock inaccuracy will introduce a false frequency shift in the measurement of an input signal. Sampling jitter indicates the random inaccuracies in the sampling of a signal. Normally quoted in picoseconds (ps), jitter introduces random noise into the signal by dispersion of the actual signal in time. Characterizing bandwidth with the Bode plot The analog bandwidth (BW) of a device deﬁnes the difference between the lowest- and highest-frequency signal components that can be measured before the input signal is attenuated to –3 dB of its original value. This is often quoted as a single value in hertz that indicates the device’s upper frequency limit. However, a more useful measure of a digitizer’s frequency response is provided by a Bode plot (or Bode diagram), which indicates gain as a function of frequency (Figure 5). A Bode plot shows the levels of attenuation over all frequencies. Ideally, the curve should be ﬂat (following an asymptotic limit) and then roll off steeply to the bandwidth limit. This is very difﬁcult to obtain and often the Bode plot of a device can show various inconsistencies in gain as a function of input frequency, even for devices that possess the same bandwidth values. 2 0 -2 -4 -6 -8 -3 dB Signal voltage Trigger level Trigger TTI Time Figure 4. Trigger time interpolation allows the precise placement of an external trigger between sample points at a given resolution, normally quoted in picoseconds. Some digitizers include a capability called time-to-digital conversion (TDC) or trigger time interpolation (TTI). Implemented in circuitry within the device architecture, this function allows the precise placement of an external trigger with respect to the internal clock of the digitizer, even when that trigger falls between sample points (Figure 4). TDC performance is quoted in picoseconds and is particularly important when measuring repetitive signals. If no interpolation is used, phase errors may be introduced in the measurement. Tip: A poorly designed digitizer system may need an external reference clock to achieve timing performance comparable to that of a well-designed device. It should also be noted that in some cases, particularly measurement of repetitive signals, the sampling-rate value can be ignored completely if large acquisition memories are present in the system. Gain (dB) -10 -12 0.001 0.01 0.1 1 10 100 Frequency (Hz) 1000 10000 100000 Example of device response Ideal frequency response Figure 5. A Bode diagram shows signal gain or attenuation as a function of frequency. Tip: Note that when measuring the relative amplitude of ﬁxedfrequency signals, digitizer bandwidth and the Bode plot will be of little relevance because input signals of the same frequency will all experience equal attenuation. Bandwidth and the Bode plot have the greatest inﬂuence when measuring signals over a broad frequency range. Agilent Measurement Journal 63

Table of Contents Feed for the Digital Edition of Agilent Measurement Journal - Issue 4 - 2008

Agilent Measurement Journal - Issue 4 - 2008 Delivering Confidence through Compliance with Standards Contents Emerging Innovations Trying Early Device Implementations at the IEEE 1588 PlugFest Achieving Greater Confidence in Measurement Accuracy through Consistency in Calibration Services Overcoming the Challenges of RFID Component Testing 3GPP LTE: Introducing Single- Carrier FDMA Ensuring Reliable Operation and Performance in Converged IP Networks Testing Storage Area Networks and Devices at 8.5-Gb/s Fibre Channel Rates Choosing an Appropriate Calibration Method for Vector Network Analysis Making Traceable EVM Measurements with Digital Oscilloscopes Exploring Terahertz Measurement, Imaging and Spectroscopy: The Electromagnetic Spectrum’s Final Frontier Interpreting Quoted Specifications when Selecting Digitizers

Agilent Measurement Journal - Issue 4 - 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.