Put this Spectrum Analyzer on Your Shopping List

Sept. 24, 2019
Though labeled a spectrum analyzer, this economical test instrument goes beyond spectral analysis by offering measurement capabilities like signal generation.

As many RF/microwave engineers and technicians can attest, Rohde & Schwarz is known as a supplier of premier high-frequency test-and-measurement equipment. But while the company is certainly synonymous with such high-end instruments, these products also come with a (justifiably) high price tag.

However, to prove that quality test-and-measurement equipment doesn’t necessarily have to be high cost, Rohde & Schwarz now offers what it has branded as “value” instruments. According to the company, this line of test instruments still offers the quality, performance, and engineering that Rohde & Schwarz is known for—but at much lower costs.

A previous Microwaves & RF article provided a general overview of the product line, which covers everything from oscilloscopes to spectrum analyzers to power supplies. In this article, we’ll take a hands-on look at one of these “value” instruments: the FPC1500 spectrum analyzer (Fig. 1).

1. Shown is the FPC1500, which is the focal point of this article.

An Introduction to the FPC1500

The FPC1500 base model covers a frequency range of 5 kHz to 1 GHz. However, customers can extend the frequency range to either 2 or 3 GHz by purchasing additional upgrades. The FPC1500 also features a built-in continuous-wave (CW) source, meaning that it can actually operate as a signal generator as well as a spectrum analyzer. And with a built-in tracking generator, the FPC1500 can essentially perform scalar-network-analyzer transmission measurements, too.

Customers can purchase several optional upgrades to enhance the FPC1500’s functionality. One of them enables the instrument to perform vector-network-analyzer (VNA) measurements. When equipped with this option, the FPC1500 could essentially be described as three instruments in one—a spectrum analyzer, a signal generator, and a VNA.

Other optional upgrades for the FPC1500 include a spectrum-analyzer preamplifier as well as modulation analysis for AM, FM, ASK, and FSK. On top of that, there’s a receiver application, advanced measurement capability, and Wi-Fi connection support.

Getting Down to Business with the FPC1500

Now, let’s explore some of the FPC1500’s capabilities. For reference, the FPC1500 that’s used for these demonstrations comes with an extended frequency range to 3 GHz along with all of the other upgrades.

Pressing the Mode button reveals the different instrument modes available. They include Spectrum, Analog Demod, Digital Demod, Receiver, and Vector Network Analyzer. Of course, Spectrum mode, which we’ll look at first, corresponds to a traditional spectrum-analyzer measurement environment (Fig. 2).

2. The FPC1500 offers the typical functions one would find in a spectrum analyzer.

When operating the FPC1500 in Spectrum mode, the signal-source options can be accessed by pressing the Meas button followed by selecting Source. Users can then choose to have the instrument generate a CW signal, which is accessed through the instrument’s output connector. In addition, the Coupled CW feature generates a CW signal with a frequency that’s always equivalent to the spectrum-analyzer center frequency. That means changing the center frequency of the spectrum analyzer automatically changes the frequency of the CW signal to the same frequency (i.e., the frequency of the CW signal “follows” the spectrum-analyzer center frequency).

In addition, with the Tracking Generator feature, the FPC1500 can generate a CW signal with a frequency that’s coupled to the measurement frequency. Thanks to this capability, users can perform scalar-network-analyzer transmission measurements of components such as filters. As an example, Figure 3 shows a measurement of a bandpass filter in Spectrum mode with the tracking generator enabled. The specified center frequency of this filter is 1.4 GHz.

3. The built-in tracking generator makes it possible to perform scalar-network-analyzer measurements, such as this measurement of a bandpass filter with a center frequency of 1.4 GHz.

Advanced Measurements

As mentioned earlier, the FPC1500 can be upgraded to include advanced measurement capabilities. When equipped with this feature, the FPC1500 offers additional measurement options in Spectrum mode; these can be accessed by pressing the Meas button followed by selecting Measurement Mode. Measurement modes include Channel Power, Third Order Intermod, Spectrogram, Spectrogram Playback, TDMA Power, Harmonic Distortion, AM Modulation Depth, and Occupied Bandwidth.

Let’s dive into some of these advanced measurement features, starting with Harmonic Distortion. This feature is intended to make it easy for users to identify and characterize harmonics. Figure 4 shows a harmonic-distortion measurement of an amplifier that was driven by an 836-MHz CW signal.

4. Thanks to the harmonic-distortion feature, the fundamental frequency and second-harmonic levels were easily determined.

With the harmonic-distortion feature, the fundamental frequency and a user-specified number of harmonics are automatically identified and measured. For the measurement shown in Figure 4, the number of harmonics was specified as 2 (fundamental plus the second harmonic). However, users can specify as many as 6 harmonics (fundamental plus five harmonics). And, when making a harmonic-distortion measurement, the frequency span is automatically adjusted so that all harmonics are visible. Furthermore, the total harmonic distortion (THD) is calculated.

The Channel Power feature makes it possible to selectively measure the power of modulated signals. With this function, users can perform power measurements of specific transmission channels. Therefore, signals at frequencies outside of the specified channel do not factor into the results.

For example, Figure 5 shows a channel-power measurement of a 2-GHz 16-QAM signal. Here, a channel bandwidth of 240 kHz was specified, resulting in a power measurement of −5.5 dBm.

5. This screenshot shows a channel-power measurement of a 2-GHz QAM signal.

The last advanced measurement capability we’ll look at is Occupied Bandwidth. Occupied bandwidth (OBW), which is an important characteristic of modulated signals, is defined as the bandwidth that contains a defined percentage of the total transmitted power. When making this measurement, users can specify this percentage to be anywhere between 10% and 99%. The FPC1500 will then display the corresponding OBW.

Figure 6 depicts an OBW measurement. Once again, the signal being measured is a 2-GHz QAM signal. As shown in Figure 6, setting the power percentage to 99% results in an OBW of 171.743 kHz.

6. Here, the OBW of a 2-GHz QAM signal is measured.

A Spectrum Analyzer with a Built-In VNA

The FPC1500 spectrum analyzer is unique in the sense that it also operates as a VNA when equipped with the optional VNA application. This VNA capability is made possible thanks to the FPC1500’s internal voltage-standing-wave-ratio (VSWR) bridge. The VNA application provides users with several measurement options: Reflection S11, Transmission S21, 1-Port Cable Loss, and Distance To Fault. Among other features available with the VNA application are Smith chart measurement displays.

The Final Word

The FPC1500 offers a wide range of performance features in a single box. This article covered some of the key measurement capabilities of the FPC1500, but the instrument offers more than what was discussed here. In any case, it’s clear that this instrument offers the functionality that should make it a valuable tool for many engineers.

For those who may be considering purchasing an FPC1500, the base unit carries a price tag of only $2,980. At that price level, it appears Rohde & Schwarz has truly created a “value” instrument.

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