Rohde & Schwarz Oscilloscopes

An oscilloscope measures the change in voltage level of electrical signals as time passes. An oscilloscope has a screen used as a graphical display, with voltage level shown on the vertical axis (Y) and time on the horizontal axis (X). The curve on the screen (representing the signal waveform), shows the changes in the voltage level, with time. Modern digital oscilloscopes digitize the signal voltage as a series of values (so-called samples), to benefit from the possibilities of digital signal processing. Such digitized waveform displayed on a digital oscilloscope can be analyzed for properties such as amplitude, frequency, rise time, time interval, distortion, and many others.

Most oscilloscopes have several channels, each of which can process a separate signal in parallel. Any signal that can be converted into an electrical signal representing the current signal strength, such as sound or other vibrations, can also be measured using an oscilloscope.

Modern digital oscilloscopes support a multitude of specific measurements and applications for troubleshooting circuits or checking the quality of acquired signals. Examples are, serial bus trigger and decode for debug and compliance tests (such as USB or Ethernet), jitter measurements, frequency response analysis using Bode plot functions, power electronics measurements, digital signal analysis for mixed signal designs, EMI analysis, automotive radar debugging.

The bandwidth is the most crucial characteristic of an oscilloscope. It is measured in Hertz and indicates the range of frequencies that the oscilloscope can accurately measure. Technically, the bandwidth of an oscilloscope corresponds to the analog bandwidth of the oscilloscope’s front-input amplifier which has a low-pass characteristic. The bandwidth is defined as the frequency at which an input signal is attenuated by 3 dB (Decibels), corresponding to an amplitude decrease to about 70.7 % of its original value. This means that signals of this frequency or higher are not accurately measured by the oscilloscope.

1. For non-sinusoidal waveforms like e.g. rectangular clock-signals, the oscilloscope bandwidth should be at least 3 times the clock signal fundamental frequency for decoding or debugging and 5 times the clock signal for compliance testing.

2. For non-periodic signals the “rise time” t_r, i.e. the fastest / steepest edge of the signal, has to be considered. In this case, the required minimum oscilloscope bandwidth f_BW can be approximated using f_BW = 0.5 / f_r.

Sample rate is the number of samples (i.e. discrete values) a digital oscilloscope can acquire per second. The sample rate determines the horizontal resolution of your acquired waveform. Therefore, it defines how many signal details are recorded, i.e. with a small sample rate not all details of a fast-changing signal are measured.

The trigger setting of an oscilloscope determines the time at which the oscilloscope starts to acquire the signal. The basic concept behind the oscilloscope trigger function is that some of the incoming signal is fed into a comparator circuit. When the voltage of the waveform reaches a previously defined trigger condition (e.g. it crosses a threshold level), the data acquisition is initiated. The oscilloscope trigger function also enables repetitive signals to be displayed on the screen as a fixed waveform such as a sine wave.

With a digital trigger this is done behind the A/D converter on the already digitized data. This results in a very low trigger jitter and trigger capability always to full oscilloscope bandwidth. In addition, it enables powerful trigger capabilities such as an adjustable trigger hysteresis.

The key decider will be the frequency of the signals to measure. The oscilloscope bandwidth will determine the maximum frequency that can be measured. The rule of thumb for the bandwidth is three to five time the frequency to be measured. In addition, the measurement scenario plays a major role to find the right oscilloscope. The Rohde & Schwarz oscilloscope portfolio includes solutions for both general purpose measurements and specific industry standards. It covers:

• Entry level oscilloscopes with bandwidths from 50 MHz to 1 GHz to suit budgets from introductory education to professional use, including comprehensive update possibilities, such as the R&S®RTC1000.
• General purpose bench oscilloscopes with deep memory and smart analysis options, such as frequency response analysis using Bode plots, like the R&S®RTB2000.
• High-performance oscilloscopes with bandwidths up to 16 GHz, real-time de-embedding, fast update rates, low noise, and unique high-performance digital triggers, such as the R&S®RTP.