Oscilloscope or signal and spectrum analyser?

This article originally appeared in the September'22 magazine issue of Electronic Specifier Design – see ES's Magazine Archives for more featured publications.

Can an oscilloscope replace an analyser? What are the limitations of an oscilloscope, and where does the signal and spectrum analyser remain the instrument of choice?

Modern high-end oscilloscopes contain high-speed A/D converters which allow them to cover frequency ranges that historically could only be investigated using a spectrum analyser. By also integrating a high bandwidth analogue front-end, the modern oscilloscope architecture enables direct sampling of high-frequency signals without analogue down-conversion. This allows unprecedented analysis bandwidth ranges. Some oscilloscopes in the market today have a bandwidth of up to 16GHz and are capable of directly acquiring a nominal 8GHz RF signal with 16GHz bandwidth. This cannot currently be done with a signal and spectrum analyser.

Spectrum analysers, on the other hand, can cover frequency ranges up to 85GHz and beyond. This allows them to address most applications of wireless, cellular or satellite communications, radar equipment, and IoT devices. For these applications, qualities that are quite exclusive to spectrum analysers come to the fore. These include high dynamic range, which allows them to display very small signals in the vicinity of a strong carrier signal. Moreover, spectrum analysers can also be used for measurements in the time domain, such as measuring the transmitter output power of time-multiplex systems as a function of time.

In the following, we will take a look at the differences between both instruments and the use cases, where each of them represents a best fit for the typical requirements of the corresponding measurements.

Signal and spectrum analysers

A spectrum analyser shows the signal level versus frequency at a selected resolution bandwidth. This can be used to measure basic signal parameters, while further parameters such as the filter settings or a frequency response can be estimated from the signal shape visible on the screen. Other measurements possible include signal-to-noise ratio (SNR) and the detection of spurious emissions, which may need to be measured over a large frequency range.

In swept-spectrum mode, the analyser only regards a small part of the spectrum at a time. This frequency selectivity is the key to the analyser’s frequency range can be expanded by hundreds of gigahertz.

A spectrum analyser is usually chosen when spectrum measurements are required to ensure conformity with standards and regulations. In mobile radio, for example, these include spurious emissions, Adjacent-Channel Leakage Ratio (ACLR), and Spectrum Emission Masks (SEM). While SEM measurements are concerned with single spurs, ACLR analysis involves assessing the integrated power over frequency of the neighbouring channels of a communications signal. Both require measuring very small levels in the immediate vicinity of a strong signal, hence benefiting from the spectrum analyser’s dynamic range and frequency selectivity.

Spectrum analysers 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)).

Digital signal analysis

Many of today’s spectrum analysers can also handle digital signals. An input signal bandwidth up to 1GHz is common, while some instruments have bandwidth up to 8.3GHz. The analyser’s front end mixes the signal to a low intermediate frequency (IF) before sampling with a wide-bandwidth A/D converter and then digitally down-converting into the baseband to be equalised. The digital I/Q values obtained contain all signal information within the bandwidth and dynamic range. The signal can then be further processed using appropriate application-specific measurements. These may be available on the device or via PC software such as R&S VSE (Vector Signal Explorer).

Hence the analyser is used when working with communication systems, for example, to measure important signal parameters such as error vector magnitude (EVM), I/Q offset or imbalance, and the level ratio of pilot to data channels. In radar applications, it can help with measurements of interest including the phase, frequency, modulation, and level of pulsed signals over the pulse duration.

Analysers can perform various other measurements at component, module and device level, particularly when used with an appropriate measurement application. These include the noise figure and gain of amplifiers and the phase noise of oscillators. Very precise measurements, almost down to the thermal noise floor, are possible.

Some high-end instruments can also perform measurements such as uninterrupted real-time spectrum analysis and uninterrupted streaming of digital I/Q data.

RF resting with oscilloscopes

With their large analysis bandwidth, today’s leading oscilloscopes can address a wide variety of applications. These include radar, since the radar range resolution is a key parameter and is directly proportional to the available bandwidth. In other applications, although the signal of interest may have a relatively narrow bandwidth, the oscilloscope can measure out-of-band signals such as harmonics, neighbouring channels and interfering signals.

On the other hand, a high analysis bandwidth can call for extra care when acquiring narrowband signals. Consider a 2MHz wide Bluetooth Low Energy (BLE) signal at a centre frequency of 2.4GHz. Acquiring the signal of interest may be relatively easy. However, unless filters are applied, all possible interference signals from DC up to the oscilloscope’s maximum frequency will also be acquired. Some high-end oscilloscopes allow users to constrain the analysis window to the signal of interest by designing digital filters using software tools and importing the filter coefficients.

Although applying an appropriate digital filter improves the signal-to-noise ratio (SNR), the question arises whether the achievable capture time can still be improved for such a narrowband signal. Even if the sample rate is reduced to the minimum stated by the Nyquist theorem, the capture time for the BLE signal in the previous example is less than one second. Digital down-conversion, available on some high-end oscilloscopes, allows the time to be extended to about 500 seconds.

Advanced trigger system

Oscilloscopes are typically equipped with a much more advanced trigger system than signal and spectrum analysers. This allows a very accurate detection of short, intermittent, burst or pulse signals. This is a significant advantage in radar applications, where a precise detection of a pulse/chirp start is essential.

Whereas oscilloscopes with a conventional analogue trigger will split the signal into two paths, instruments from Rohde & Schwarz provide a fully digital trigger system that operates directly on the samples of the A/D converter. This results in a lower trigger jitter and a flexible trigger sensitivity. Moreover, all trigger types support the full bandwidth of the oscilloscope.

Phase-coherent multichannel analysis

In many wireless applications, multi-antenna designs are gaining more and more importance for various reasons. In radar for example, multiantenna systems are state-of-the-art for estimating the angle of arrival (AoA) - the direction from which the surrounding objects are coming - based on the phase difference between multiple receive paths.

Test equipment needs multichannel capabilities to characterise these types of systems and ensure that all channels are constantly phase-coherent. Oscilloscopes typically provide multiple tightly aligned channels. Unlike spectrum analysers, they require no additional enhancements such as time-base and local-oscillator (LO) sharing to perform phase-coherent measurements. Hence, they are a cost-effective and easy-to-use solution for testing multi-antenna systems.

When testing equipment using frequency ranges beyond the oscilloscope’s maximum bandwidth – such as automotive radar in the 77GHz to 81GHz range, or the new 60GHz radar for gesture sensing – signal acquisition is accomplished using external mixers. An oscilloscope that supports real-time de-embedding can compensate for the losses induced by the additional components in the signal path. While tools for basic analysis in time and frequency domain are often built-in, a dedicated tool such as the R&S VSE software may be needed for a more in-depth pulse and transient analysis.

Similarly, 5G NR communications rely on multiple antennas with beamforming to transmit the signal in a desired direction. This is accomplished by generating a defined phase shift of each adjacent input signal stream, which must be kept constant to ensure the generated beam stably points in the wanted direction. An oscilloscope like the R&S RTP, with its multichannel input, allows a phase-coherent measurement of up to four input streams and can therefore handle multiple 5G NR input channels. With the R&S VSE, it can perform a wide range of measurements including MIMO-specific measurements like the phase difference between the input signals for characterising the beams when testing the transmitters of 5G NR base stations or small cells.

System-level debugging

Whereas signal and spectrum analysers are dedicated instruments for RF signal analysis, oscilloscopes are general-purpose instruments that allow multiple measurements besides the acquisition of RF signals. Various options are available for bus triggering and decoding, as well as power, time-, and frequency-domain measurement. The consistent time-alignment between all these measurements can help users correlate acquired RF signals with other signals, such as the supply voltage or digital bus signals. An example may be simultaneously acquiring CAN or Ethernet signals together with radar signals when developing and debugging automotive radar modules.

FFT and zone trigger

Most state-of-the-art oscilloscopes provide FFT capabilities, which enable correlation to time domain. As the UWB 802.15.4z standard is gaining traction in automotive applications, for example, the FFT capability allows simultaneous examination of the UWB signal in both the time and frequency domain. The gated FFT feature provided by Rohde & Schwarz oscilloscopes makes it possible to define a signal portion in the time domain and plot the spectrum of this specific portion. Spectral measurements such as the channel power and the occupied bandwidth are also possible.

In addition, when combined with a dedicated near-field probe, the FFT and trigger capabilities are useful for investigating electromagnetic interference (EMI) in electronic designs.

Some high-end oscilloscopes also provide a zone trigger, which is also useful for EMI debugging purposes. Often, multiple zones can be graphically defined in the time and frequency domain and combined through logical operators to help investigate conditions such as fading effects on a WLAN signal caused by short or intermittent interference, which are otherwise hard to track down.

Summary

The main advantages of a signal and spectrum analyser result from the frequency selectivity. These include the high dynamic range, which allows low-level signals to be analysed in the vicinity of a strong signal. Standard-compliant ACLR and SEM measurements are usually only possible with a spectrum analyser.

With signal demodulation, an analyser can deliver significantly better results, such as EVM values, particularly for signals with a large bandwidth and a high crest factor.

Other advantages include the high maximum input frequency and continuous sweep from minimum to maximum frequency, as well as very long, seamless recording times, depending on the bandwidth.

The main advantages of an oscilloscope result from the superior analysis bandwidth and full signal capture including DC components, as well as the availability of several (typically two or four) phase-coherent inputs.

An oscilloscope is usually preferred for wideband measurement of signals in the analogue baseband, phase-coherent measurements of several sources, and time-correlated multi-domain measurements.