Understanding RF Instrument Specifications Part 1

When choosing an RF instrument, it is easy to get lost in the many specifications that vendors use to characterize an instrument's performance. Moreover, in a world where wireless technologies are ever-changing, engineers with little RF experience commonly design and test RF components and devices. This three-part tutorial is designed to help you understand both basic and advanced RF instrument specifications. This tutorial covers generic, generator, and analyzer specifications. In addition, future issues describe specifications such as 1 dB compression point, third-order intercept, dynamic range, and resolution bandwidth.

Understanding RF Instrument Specifications Series

This article was originally published in RF DesignLine on July 25, 2007. A link to the original publication is provided at the end of this document.

 

Part 1: General RF Instrument Specifications
Part 2: RF Signal Generator Specifications
Part 3: RF Signal Analyzer Specifications

 

Introduction

This article describes specifications applicable to both RF generation and analysis. Specifications covered include: real-time bandwidth, frequency range, tuning speed, phase noise, and voltage standing wave ratio (VSWR). Note that many specifications apply to more than just instruments. In fact, all RF devices are subject to the same design rules as RF instrumentation. Thus, the design details and tradeoffs are applicable to a variety of RF devices.

Real-Time (Instantaneous) Bandwidth

Often, the terms "real-time bandwidth" and "instantaneous bandwidth" are used interchangeably to describe the maximum continuous RF bandwidth that an instrument generates or acquires. While a vector signal generator might generate a signal at a center frequency of 2.45 GHz, its real-time bandwidth, and hence signal bandwidth, might only be 20 MHz wide. This means that the device can continuously acquire 20 MHz of RF spectrum without re-tuning the local oscillator (LO).

Real-time bandwidth is largely determined by the RF analog front end of the instrument. To better understand what the real-time bandwidth specification means, it is helpful to understand the basic architecture of an RF instrument. Current technology cannot simply digitize every signal in the GHz range. Thus, RF instruments use a series of LOs, mixers, and filters to bring an RF signal into an intermediate frequency (IF) or baseband frequency range. Figure 1 shows the block diagram of a highly simplified vector signal analyzer.

Figure 1. Real-time bandwidth is determined by the filter and analog to digital converter (ADC).

As the figure illustrates, the vector signal analyzer acquires a portion of RF spectrum by downconverting it to an IF that can be captured by an analog-to-digital converter (ADC). The real-time bandwidth of an RF instrument is determined by two main components, highlighted in blue in Figure 1: 1) the filters implemented in the instrument, and 2) the sample rate and bandwidth of the ADC.

The importance of the instrument's real-time bandwidth depends greatly upon the application. For example, generating a narrow-band FM signal requires only 200 KHz of real-time bandwidth. However, generation and analysis of wide-band signals such as IEEE Standard 802.11g (WiFi) requires at least 20 MHz of real-time bandwidth. In particular, a spectral mask test is performed more quickly when the instantaneous bandwidth is significantly wider than the signal of interest. In the event that a spectral mask test requires more instantaneous bandwidth than the instrument provides, the instrument has to be re-tuned to acquire the frequency information in sections.

Frequency Range

Frequency range is another important RF instrumentation characteristic, and it is perhaps the most non-negotiable. For example, when performing analysis of a component that operates at 900 MHz, the instrument must operate at that frequency range to be useful. A number of components affect the maximum frequency range of an RF instrument, including mixers, input filters, and local oscillators (LOs). However, configuring the instrument to work at a specific frequency is accomplished mainly by tuning the LO. Note that while some instruments use multiple series of LOs, Figure 2 shows a simplified instrument block diagram using a single LO.

Figure 2. Frequency range is determined by the local oscillator.

The LO is mixed with the RF input, which helps convert the RF signal down to an IF. Note that the same frequency synthesis techniques apply to RF signal generators as well.

Frequency synthesis is accomplished using one of two methods. These methods use either a voltage controlled oscillator (VCO), or an yttrium iron garnet (YIG). Historically, RF instruments used a YIG-based architecture as a mechanism for generating the LO. The YIG is a current-controlled oscillator known for its tight phase noise and wide frequency ranges (up to 20 GHz or higher). However, YIG-based instruments consume significant power and can be quite costly. In addition, tuning the YIG from one frequency to the next requires longer tuning times than other methods. As a result, VCO-based LO architectures have recently become more common. While the VCO has a smaller frequency range than the YIG, its tuning speed is much faster.

Again, when choosing an RF instrument, the frequency range is one of the most important specifications to consider. For example, instruments being used to test UHF (ultra-high frequency) radio frequency identification (RFID) tags must operate at frequencies of up to 928 MHz. Similarly, a Wi-Fi test solution requires operation at frequencies of up to 2.5 GHz.

Tuning Speed

Tuning speed is another important specification used to characterize RF instrument performance. This specification measures the length of time required for the LO (series of LOs in a multi-stage architecture) to change from one center frequency to another within a specified accuracy level. When tuning an oscillator to a different frequency, the tuning speed is dictated by the LO settling time.

In typical systems, when tuning from one frequency to another, the LO usually overshoots the desired frequency slightly and then settles on the desired frequency within a certain time period. In most cases, the tuning speed is a function of the frequency hop size. The greater the frequency hop, the longer it takes the LO to tune within a specified range. Figure 3 illustrates the settling time for an example YIG-based LO.

Figure 3. Tuning Speed of YIG-Based Local Oscillator.

Tuning speed is an important specification in several applications, such as an automated production test of an 802.11 g transceiver. Because the WiFi standard specifies that devices function at one of 14 channels between 2.4 GHz and 2.48 GHz, RF instruments must be used to test device operation across a variety of frequencies. The more quickly the test signal sweeps from one station to the next, the more quickly the receiver is tested.

Phase Noise

The next specification, phase noise, usually describes an RF instrument's short-term frequency stability. Phase noise is caused by small, instantaneous LO phase jitter. Moreover, it results in signal power at frequencies adjacent to the carrier.

An easy way to visualize phase noise effects is to analyze a single tone in the frequency domain. Figure 4 represents two simulated carriers, one ideal carrier and the other with phase noise.

Figure 4. Comparison of ideal to non-ideal carrier.

In the left plot, generating a single tone ideally results in a single peak with all the power concentrated at a very precise frequency. However, phase noise produces a slightly different result. As shown in the right plot, phase noise (essentially time-domain jitter) results in a slight periodic spreading of the signal in the frequency domain.

Because phase noise produces a tapered signal outline, it is characterized by measuring the signal amplitude at various offsets from the desired carrier. Figure 4 above, we measure a phase noise of -95 dBc at a 1 KHz offset, and -146 dBc at a 10 KHz offset.

The importance of an RF instrument's phase noise varies from one application to the next. Tight phase noise is required in the detection of low-level blocker signals that are close to a particular signal at interest. When using an LO with significant phase noise, the phase noise translates to additional phase noise in the resulting IF signal. This is illustrated in Figure 5.

Figure 5. Phase noise of LO translated produces phase noise at IF.

LO phase noise translates to phase noise of the resulting IF signal. In this particular application, the two signals' phase noise interferes with one another, making it more difficult to identify the specific blocker signal characteristics.

A second way to illustrate phase noise effects is by visualizing demodulation of a signal with a constellation plot. A signal with significant phase noise shows slight periodic rotations of the constellation plot. Figure 6 compares an ideal phase shift keying (PSK) modulated signal with one subject to significant phase noise.

Figure 6. Constellation plot shows rotation when phase noise is present.

Phase noise affects actual measurements by degrading the error vector magnitude (EVM) performance of an RF instrument. For bit error rate (BER) tests, phase noise actually contributes to higher error rates.

Voltage Standing Wave Ratio (VSWR)

VSWR is closely related to transmission line theory and becomes more important as an instrument's frequency range increases. At a high level, VSWR measures signal reflections that occur as a result of impedance mismatch along a transmission line.

In a perfect world, the impedance of an RF instrument (typically 50 ohms) matches the impedance of each of the cables and the input impedance of the device under test. However, various imperfections such as asymmetric signal traces and part-to-part component variation alter the characteristic instrument impedance. As a result, signal reflections occur in the RF transmission, affecting the signal's amplitude and phase accuracy.

The signal reflection amplitude is dependent both on properties of the material used and on the frequency range. VSWR is most directly caused by impedance mismatch in the transmission line, and is generally more problematic at higher frequencies. For example, a VSWR of 1:1 represents a perfectly matched system. By contrast, a VSWR of 1.1:1 means that up to 10 percent of the signal amplitude (1.1 " 1) is reflected in the transmission line.

Because VSWR is dependent on material properties as well, it can be calculated based on a reflection coefficient, Γ, illustrated in the equation:

VSWR substantially affects a test signal by causing adjustments in its phase or amplitude. Moreover, the generated signal amplitude either increases or reduces depending on the reflection phase. This is illustrated in Figure 7.

Figure 7. VSWR reflections affect signal amplitude.

A reflection that is out of phase with the original signal causes a slight canceling affect. The resulting composite signal shows slightly reduced amplitude. In most cases, VSWR is reduced through attenuator use, either internal or external. Thus, VSWR is reduced by increasing the instrument reference level because this applies internal attenuation.

VSWR is an important RF instrument characteristic because it significantly affects the instrument's amplitude accuracy. Moreover, some applications such as RF filter characterization require the highest amplitude accuracy possible. Because an RF filter is characterized by measuring the amplitude loss according to the stimulus signal frequency, amplitude accuracy of both the stimulus signal and analysis instrument is paramount.

The Big Picture

Understanding RF instrument specifications provides greater insight into how to choose an RF instrument for your application. Remember that many of the specifications apply to all RF devices, and not just to instruments. Thus, you will likely encounter some of the same specifications in your own designs. The next article in this three-part series explains the specifications used to characterize RF generators, including: frequency tolerance, linearity, power output, 1 dB compression point, and third-order intercept.

You can also click here to access NI's  RF Developer's Network, an online resource for tips and techniques to making RF measurements. It is designed to teach basic fundamentals of RF measurements.

Related Links

RF DesignLine

Understanding RF Instrument Specifications Part 1 in RF DesignLine

NI RF Developer's Network

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