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A Comparison between Oscilloscopes and Spectrum Analyzers
Whether it's exploring the terrain for minerals on Earth or exploring space for alien life, analyzing each signal is as simple as checking information about its timing and frequency. While the oscilloscope displays the signal with respect to time, the spectrum analyzer shows it with respect to frequency. Both of these tools are very important in any signal analysis application. This article explains the difference between an oscilloscope and a spectrum analyzer using examples.
Oscilloscopes are often used to obtain detailed information about the timing of a signal or the timing of several signals. We can use an oscilloscope to find the relative time delay between two signals. On the other hand, to observe the frequency properties of a signal, a spectrum analyzer is needed. For example, we could use a spectrum analyzer to consider the harmonics of a signal. In this article, we explore both the time domain and the frequency domain with an oscilloscope equipped with an accompanying spectrum analyzer and a built-in arbitrary waveform generator.
Time Domain vs. Frequency Domain
First we set the generator to send a sine wave of 5 MHz 1 VPP or 0.707 VRMS on the spectrum analyzer signal. CH1 shows this signal as shown in Figure 1. Aside from its basic shape and amplitude, little else can be learned about the signal with the oscilloscope alone. Now consider a spectrum analyzer. Since an ideal sine wave has only one frequency component, we should expect a pulse at 5 MHz corresponding to 0.707 VRMS of the input signal, with all remaining zero frequency components. Any other pulses are due to the impurity of the sine wave. Figure 2 shows the spectrum analyzer and oscilloscope plots for this signal.
The true Nature of a Signal
However, in reality the nature of the signals is not known in advance. Suppose the signal is instead an FM signal from a radio station with a 5 MHz carrier frequency modulated by a 10 kHz sine wave. At first glance, the oscilloscope, Figure 3, and the spectrum analyzer would show similar results as before. However, by using a spectrum analyzer, we can take a closer look by approximating the signal peak at 5 MHz, which allows us to discover the true nature of the signal as shown in Figure 4. The modulation effect on the 5 MHz band carrier signal is the summation of the peaks on both sides of the base peak at intervals 10 kHz. This discovery can be easily made with a spectrum analyzer, but it is virtually impossible to observe it on an oscilloscope time domain plot.
Characterizing Filters
To further illustrate the differences between the instruments, consider a low pass filter with a cut-off frequency of 300 KHz. To find the settling time or the time response of this filter, we apply a square wave to its input and look at its output. Figure 5 shows the timing behavior of the filter with a settling time of about 16 uS. However, this only gives us filter evaluation with an oscilloscope Fig.5: Evaluating a filter using an oscilloscope. Evaluating a filter with a spectrum analyzer Fig.6: Evaluating a filter using a spectrum analyzer. a single data point regarding the performance of the filter. To fully characterize the filter across different frequencies, we can change the input of the filter to a white noise signal (i.e., inputting all frequencies) and look at it its frequency response as shown in figure 6. Using the spectrum analyzer, we can verify that the filter is indeed a decent 300 KHz low pass filter.
- Real-time spectrum analyzers
- Parallel-filter analyzers
- Fourier (or FFT) analyzers
- Vector Signal Analysis (VSA)
