![]() ![]() Then we insert the cables and probes and measure the changed response. We measure this signal at the input to the Teledyne LeCroy WavePro HD 804 using the highest bandwidth connection we can, calculate its FFT to get the spectrum, and use this as the stimulus. The method we introduce here uses a Noisecom NC1100 wide-band noise source with frequency components extending from 1 MHz to > 10 GHz. But, the amplitude of each harmonic drops off as 1/f, so there is worse signal to noise ratio (SNR) at higher frequency, where we are more interested in the transfer function roll off. If this is part of a 10 MHz clock, for example, its spectrum would be a comb pattern of peaks at odd multiples of 10 MHz. We could use a very fast step signal as an input. This requires a very flat, high bandwidth sine wave source. We could sweep its frequency from 1 MHz to 10 GHz and measure the amplitude of the sine wave at various frequency steps. However, we could use a sine wave source, with a flat frequency response. Other than in a calibration lab, we can’t pull these pieces out and connect a VNA from the input to the output. We really want to be able to include the scope’s amplifier and whatever DSP equalization is built into the scope’s electronics, in addition to the passive cables and probes. Unfortunately, using a VNA is not one of the. ![]() ![]() There are a number of ways to measure the transfer function of a measurement system. An example of the measured transfer function of a scope with minimal connections to the source, is shown in Figure 1. We could use another figure of merit, like the filter order, to describe how fast the transfer function drops off with frequency. When we use a single figure of merit, like bandwidth, to describe a transfer function, we are making a lot of assumptions, like: the transfer function looks like a low pass filter, the passband region is flat, and the roll-off region is the transition from flat to a constant downward slope. 224 Gb/s Per Lane: Options and Challenges ![]()
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