An Oscilloscope or a Signal and Spectrum Analyzer? - Choosing the Best Instrument for Your Application

Modern oscilloscopes have advanced to the point where they can now directly capture microwave and millimeter wave signals, a task that was traditionally reserved for signal and spectrum analyzers. This development raises the question of whether an oscilloscope can replace an analyzer entirely. So, what are the limitations of an oscilloscope, and in what scenarios is a signal and spectrum analyzer still the preferred instrument?

The latest high-end oscilloscopes now come equipped with high-speed A/D converters that allow them to explore frequency ranges that were once only accessible with a spectrum analyzer. With the integration of a high-bandwidth analog front-end, modern oscilloscope architecture allows for the direct sampling of high-frequency signals without requiring analog down-conversion. This technological advancement allows for unprecedented analysis bandwidth ranges. In fact, some of the oscilloscopes available today have a remarkable bandwidth of up to 16 GHz and are capable of directly acquiring a nominal 8 GHz RF signal with a bandwidth of 16 GHz. Such capability is currently beyond the reach of any signal and spectrum analyzer on the market.

On the other hand, spectrum analyzers can cover frequency ranges of up to 85 GHz and beyond. This extensive range allows them to cater to a vast array of applications in wireless, cellular or satellite communications, radar equipment and IoT devices. In these scenarios, qualities that are exclusive to spectrum analyzers prove to be particularly advantageous. For instance, the high dynamic range of signal and spectrum analyzers enables them to display very faint signals in the vicinity of a strong carrier signal. Additionally, they can also be used for measurements in the time domain, such as assessing the transmitter output power of time-multiplex systems as a function of time.

Moving forward, we will delve into the distinctions between these two instruments and explore the ideal use cases for each.

Signal and Spectrum Analyzers

A spectrum analyzer displays the signal strength as a function of frequency for a chosen resolution bandwidth. With this display, it is possible to measure fundamental signal properties, as well as make estimates regarding the filter settings or frequency response based on the visible signal shape. Other possible measurements include signal-to-noise ratio (SNR) and the detection of spurious emissions, which may require measurements over a broad frequency range.

When operating in swept-spectrum mode, the analyzer focuses on a specific portion of the spectrum at any given time. This frequency selectivity is critical to the analyzer's superior dynamic range, enabling the instrument to measure and display the entire frequency spectrum from 2 Hz to 85 GHz in a single measurement. Furthermore, by utilizing external mixers, it is possible to increase the displayable frequency range by several hundred GHz.

A spectrum analyzer is often the tool of choice when spectrum measurements are required to ensure compliance with standards and regulations. For instance, in mobile radio, the spectrum analyzer is the go-to instrument for measuring spurious emissions, adjacent-channel leakage ratio (ACLR) and spectrum emission masks (SEM). SEM measurements concentrate on single spurs, while ACLR analysis (Figure 1) involves examining the integrated power over the frequency range of neighboring channels of a communication signal. Both require detecting exceedingly small signals in the vicinity of a strong signal, highlighting the need for the spectrum analyzer's dynamic range and frequency selectivity.

Figure 1: ACLR Measurement with a Spectrum Analyzer

Spectrum analyzers are also usually chosen for measuring electromagnetic interference (EMI) during pre-compliance testing. The respective EMI standards require a minimum of spurs to be measured with the appropriate EMI detectors (quasi-peak, CISPR-Average and RMS-Average (CISPR-RMS)). 

In pre-compliance testing, a spectrum analyzer is also the preferred tool for measuring electromagnetic interference (EMI). EMI standards require a minimum of spurs to be measured using the appropriate EMI detectors, such as quasi-peak, CISPR-average and RMS-average (CISPR-RMS). By leveraging the spectrum analyzer's ability to detect and measure these spurs, pre-compliance testing can help ensure that the device or equipment under test adheres to the applicable EMI standards.

Digital Signal Analysis

Modern spectrum analyzers are equipped to process digital as well as analog signals. An input signal bandwidth of up to 1 GHz is common, and some instruments can handle bandwidths up to 8.3 GHz. The analyzer's front end (as shown in Figure 2) down-converts the signal to a low intermediate frequency (IF), samples it with a wide-bandwidth A/D converter and finally digitally down-converts it into the baseband to be equalized. The acquired digital I/Q values contain all the signal information within the bandwidth and dynamic range. Subsequently, the signal can be subjected to further processing using relevant application-specific measurements. Such measurements may be available either on the device or via PC software like R&S VSE (Vector Signal Explorer).

Figure 2: Digital Signal Analysis with Spectrum/Signal Analyzer

As such, spectrum analyzers are used for digital signal analysis in communication systems and radar applications. They measure important signal parameters such as the error vector magnitude (EVM), I/Q offset or imbalance and level ratio of pilot to data channels. Additionally, they can measure the phase, frequency, modulation and level of pulsed signals over the pulse duration in radar applications.

These analyzers can also measure the noise figure and gain of amplifiers, as well as the phase noise of oscillators at the component, module and device level. Furthermore, some high-end instruments can perform very precise measurements, almost down to the thermal noise floor.

In addition to these capabilities, some spectrum analyzers can perform uninterrupted real-time spectrum analysis and uninterrupted streaming of digital I/Q data.

RF Testing with Oscilloscopes

Modern oscilloscopes with their broad analysis bandwidths have a wide range of applications, including radar, where the bandwidth directly determines the radar range resolution. Additionally, for scenarios where the signal of interest is narrowband, the oscilloscope can measure out-of-band signals such as harmonics, neighboring channels and interference signals.

However, when acquiring narrowband signals with a high analysis bandwidth, extra care must be taken to avoid acquiring all possible interference signals from DC up to the maximum frequency of the oscilloscope. Some high-end oscilloscopes allow users to apply digital filters using software tools and importing the filter coefficients. This constrains the analysis window to the signal of interest, improving the signal-to-noise ratio (SNR).

Even with the use of digital filters, however, the achievable capture time for narrowband signals can be limited. The capture time for the BLE signal in the previous example is less than one second, even when the sample rate is reduced to the minimum stated by the Nyquist theorem. Some high-end oscilloscopes with digital down-conversion capabilities can extend the capture time for such signals. For instance, a 2 MHz wide Bluetooth^®Low Energy (BLE) signal at a center frequency of 2.4 GHz can be captured for about 500 seconds with digital down-conversion.

Advanced Trigger System

Compared to signal and spectrum analyzers, oscilloscopes have a more advanced trigger system that enables the accurate detection of short, burst, intermittent or pulse signals. This feature is especially useful in radar applications, where the precise detection of a pulse or chirp start is critical.

Unlike oscilloscopes with a conventional analog trigger, Rohde & Schwarz oscilloscopes come with a fully digital trigger system that works directly on the A/D converter samples. This results in lower trigger jitter and a more flexible trigger sensitivity. Additionally, all trigger types can support the full bandwidth of the oscilloscope.

Phase-Coherent Multichannel Analysis

Multi-antenna designs are increasing in importance for wireless applications, for example, when estimating the angle of arrival (AoA) in radar systems. The multiple tightly aligned channels provided by oscilloscopes are a cost-effective and straightforward solution for testing multi-antenna systems. Unlike spectrum analyzers, they require no additional enhancements to perform phase-coherent measurements, ensuring all channels are constantly phase-coherent.

However, when testing equipment with frequency ranges beyond the maximum bandwidth of the oscilloscope, an external mixer is needed to acquire signals. An oscilloscope with real-time de-embedding can compensate for the losses caused by the additional components in the signal path. Although basic analysis tools for time and frequency domain are often built-in (Figure 3), a dedicated tool such as R&S VSE software may be necessary for in-depth pulse and transient analysis.

Figure 3: Automotive Radar Signal Analysis Using Oscilloscope On-board Tools

Multi-antenna systems are also important to 5G NR communications. Beamforming, which is required to transmit a signal in a specific direction, needs multiple antennas and a precise, consistent phase shift of each adjacent input signal stream. An oscilloscope such as the R&S RTP, which offers up to four input streams and maintains phase coherence, can handle multiple 5G NR input channels. The R&S VSE software allows for a wide range of measurements, including MIMO-specific measurements, such as the phase difference between input signals, that can help characterize the beams and test transmitters in 5G NR base stations or small cells.

System-Level Debugging 

Unlike signal and spectrum analyzers, oscilloscopes are versatile instruments that offer a wide range of measurement capabilities other than RF signal acquisition. Additionally, they come with several options for bus triggering and decoding, as well as power, time and frequency-domain measurements. By providing consistent time-alignment between all these measurements, oscilloscopes enable users to correlate acquired RF signals with other signals, such as the supply voltage or digital bus signals. For example, when developing and debugging automotive radar modules, users can use oscilloscopes to simultaneously capture CAN or Ethernet signals along with the radar signals.

FFT and Zone Trigger

Advanced oscilloscopes typically include FFT capabilities that allow users to correlate signals in the time and frequency domains. For instance, the growing use of the UWB 802.15.4z standard in automotive applications requires the simultaneous examination of UWB signals in both domains. The gated FFT feature on Rohde & Schwarz oscilloscopes offers the option to select a specific portion of a signal in the time domain and display its corresponding spectrum. Spectral measurements such as the channel power and occupied bandwidth are also possible (see Figure 4).

Figure 4: Time and Frequency Domain Measurements of UWB Signal

When investigating electromagnetic interference (EMI) in electronic designs, the pairing of a dedicated near-field probe and an oscilloscope with FFT and trigger capabilities can be beneficial. This allows the EMI to be investigated in both the time and frequency domains, making it possible to locate and analyze the source of the interference.

To aid in EMI debugging, some high-end oscilloscopes also offer a zone trigger feature. By defining multiple zones in the time and frequency domains and combining them through logical operators, it becomes easier to investigate fading effects on a WLAN signal caused by an intermittent or short-lived interference. This can save time and effort when attempting to track down the root cause of EMI-related problems.

Summary

A signal and spectrum analyzer is the ideal tool for applications that require frequency selectivity. It has a high dynamic range and enables standard-compliant ACLR and SEM measurements, as well as phase noise and noise figure/gain measurements and real-time spectrum analysis. With signal demodulation, analyzers can deliver better results, especially for signals with a large bandwidth and a high crest factor. They also offer a high maximum input frequency, continuous sweep and extended, seamless recording times.

On the other hand, oscilloscopes offer superior analysis bandwidth, full signal capture including DC components and multiple phase-coherent inputs. They are preferred for the wideband measurement of signals in the analog baseband, phase-coherent measurements of several sources and time-correlated multi-domain measurements.