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Understanding RF Instrument Specifications Part 3Publish Date: Jan 02, 2014

OverviewRF test instruments are highly complex products with a wide variety of specifications that characterize the performance of each instrument. This article is the third and final part of a series thatdetails common RF instrument specifications. Understanding RF Instrument Specifications Part 1 examines generic RF specifications that apply to all RF instruments. Understanding RFInstrument Specifications Part 2 covers specifications that are significant to RF continuous wave and vector signal generators. This final article focuses on specifications that apply to RF signalanalyzers. Because there are two major types of RF signal analyzers, vector signal analyzers and spectrum analyzers, we first explain the distinctions between both instruments. Then we providean introduction to specifications that apply specifically to spectrum measurements that include dynamic range, averaging methods, and displayed average noise floor.

Articles in this series are divided into the following parts:Part 1: General RF Instrument SpecificationsPart 2: RF Signal Generator SpecificationsPart 3: RF Signal Analyzer Specifications

Table of ContentsTypes of RF Signal Analyzers

Attenuation and Reference Level

Dynamic Range

Averaging Methods

Displayed Average Noise Floor

Conclusion

Related Links

1. Types of RF Signal AnalyzersEngineers are typically interested in signal characteristics such as amplitude, frequency, and phase when acquiring RF signals. Depending on the characteristics you need to analyze, you can useeither a spectrum analyzer or a vector signal analyzer .1

The spectrum analyzer is used to capture only the frequency and power information of an RF signal. The typical output of a spectrum analyzer is a power versus frequency graph.

A vector signal analyzer is capable of the same measurements as a spectrum analyzer but with additional capabilities. You can acquire phase information to produce a constellation plot, shown inFigure 1, as a vector signal analyzer can also capture the time-domain of an RF signal.

Figure 1. Phase and Amplitude Transitions of a Communications Signal

Spectrum analyzers and vector signal analyzers traditionally use different instrument architectures. The traditional spectrum analyzer consists of basic components such as a tunable localoscillator (LO), mixer, bandpass filter, and power sensor. To make spectrum measurements, the traditional spectrum analyzer simply tunes the LO to each frequency bin and makes apower-in-band measurement on the resulting signal. Sweeping through each frequency bin allows the traditional spectrum analyzer, diagrammed in Figure 2, to provide power information across abroad range of frequencies. Some spectrum analyzers still operate in this mode, known as .swept mode

Figure 2. Traditional Spectrum Analyzer Block Diagram

Many modern spectrum analyzers are designed similar to vector signal analyzers. The traditional architecture of a vector signal analyzer, shown in Figure 3, uses a tunable LO mixed with the RFsignal to produce a wideband intermediate frequency (IF) signal. Rather than retuning the LO for each frequency bin, however, the vector signal analyzer performs a fast Fourier transform (FFT)on the IF signal. The FFT can provide power and frequency information across a broad frequency range with a single acquisition. The architecture of a vector signal analyzer is quite similar to thatof a vector signal generator.

Figure 3. Traditional Vector Signal Analyzer Block Diagram

The analog-to-digital converter (ADC) in Figure 3 captures a broader spectrum of data. Acquiring a broader spectrum of data allows the instrument to capture the phase information of the RFsignal as well as perform spectrum measurements with a simple FFT calculation.

2. Attenuation and Reference LevelRF signal analyzers are designed to measure many types of RF signals with the greatest dynamic range possible. One way to maximize the dynamic range over a broad range of signals is to useattenuation to adjust the signal level to the ideal amplitude for a given signal. RF signal analyzers are designed to have a broad range of reference or attenuation levels, specified in decibels (dB).A user typically sets the reference level to a power level that is slightly higher than the maximum expected power. The instrument then applies the appropriate gain or attenuation to the signal.Attenuation or gain is applied as close to the RF front end as possible to maintain a constant signal level at the mixer and to achieve maximum dynamic range on the signal being analyzed.

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Figure 4. Attenuation is Applied to an Input Signal Before the Mixing Stage of an RF Signal Analyzer

Programmable attenuation or gain is significant because it allows an RF instrument to measure signals at a variety of power levels. For example, if you attach a broadband antenna to an RF signalanalyzer, you would notice that many over-the-air wireless communication signals operate at greatly varying power levels. Most FM radio stations can be observed at maximum amplitudes ofaround –50 dBm. Conversely, finding signals in the GSM cellular band higher than –70 dBm is difficult unless you are near a base station. In an even more extreme scenario, GPS signals in the1.57 GHz band might operate at power levels of –157 dBm and below.

Check the range of attenuation the instrument offers when choosing an RF signal analyzer. The combination of maximum attenuation and dynamic range determines the minimum signal level thatcan be analyzed. RF instruments can analyze low-level signals with optional preamplifiers.

3. Dynamic RangeDynamic range describes the maximum and minimum signal amplitudes that you can measure simultaneously. The only factor that determines the maximum signal level is the attenuation appliedto the signal, but many different factors determine the minimum signal level. These factors include noise introduced by the amplifier, spurs and harmonics, or carrier signal leakage (also known asLO leakage). More specifically, dynamic range is the ratio of the largest signal that can be measured relative to the power of the greatest distortion, noise, or spur. Dynamic range is specified indecibels, with a larger range as more desirable.

Spurs and noise can be introduced almost anywhere in the RF signal chain. The nonlinear characteristics of components such as mixers and amplifiers often result in distortion products, each ofwhich can produce spurs in the frequency domain. The bit resolution of the ADC can also affect dynamic range. Generally, the greater the bit resolution of the ADC, the better the dynamic range isof the instrument.

Dynamic range is an important specification for low-amplitude measurements. The specification is even more essential when measuring a low-power signal level next to a high-power signal. Thedynamic range of the instrument determines the minimum signal that it can view next to a high-power signal because the reference level of the instrument cannot be set below the maximum powerof the high-power signal. This concept is illustrated in Figure 5, which shows a low-power signal adjacent to a high-power GSM signal. An RF signal analyzer must have a dynamic range of at least60 dB to measure the smaller signal displayed in Figure 5.

Figure 5. Low-Power Signal Adjacent to a High-Power Signal

4. Averaging MethodsCaution: Averaging can adversely affect the accuracy of carrier-to-noise measurements.

With averaging methods, reducing noise on a signal increases the accuracy of measuring low-level spurs. You can use averaging over several periods of a signal to eliminate random or whitenoise and converge to the real value of the signal. Two methods of complex averaging are described in this section—root mean square (RMS) averaging and peak-hold averaging.

RMS AveragingWith RMS averaging your instrument can detect low-level signals. RMS averaging enables the periodic noise components of the signal to average out, leaving only the desired signal. Todetermine the RMS average, and to average the power or energy of the signal, you can calculate the weighted mean of the sum of squared values. Figures 6 and 7 show the FM band with andwithout RMS averaging, respectively, and demonstrate more accurate detection of low-level peaks.

Figure 6. With RMS Averaging Disabled, Only Three Peaks Greater Than –70 dBm are Visible

Figure 7. With RMS Averaging Enabled, Six Peaks Greater Than –70 dBm are Visible

Peak-Hold Averaging

Peak-hold averaging keeps the peak of each bin through multiple FFT calculations. Peak-hold averaging raises the noise floor as a result because it takes the highest amplitude of all signals

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Peak-hold averaging keeps the peak of each bin through multiple FFT calculations. Peak-hold averaging raises the noise floor as a result because it takes the highest amplitude of all signalsmeasured for many averages. The method also shows peaks of subsequent spectrum measurements on the same graph to enable identification of transient signals. Figures 8 and 9 show the 885MHz GSM cellular band with varying amounts of peak-hold averaging enabled to illustrate this concept.

Figure 8. Traffic Detected with Five Peak-Hold Averages

Figure 9. Traffic Detected with 500 Peak-Hold Averages

5. Displayed Average Noise FloorThe apparent noise floor of an RF signal analyzer depends on much more than the RF system introducing noise, as described in the section. The averaging mode that you useAveraging Methodscan significantly affect the average noise floor. This section describes how the resolution bandwidth (RBW) of the signal can also affect the average displayed noise floor of the instrument. Toillustrate this concept we measured a 20 MHz bandwidth with a single peak. Figures 10 and 11 show that reducing the resolution bandwidth actually lowers the displayed noise floor of theinstrument.

Figure 10. With a 10 KHz RBW, the Noise Floor Appears at Around –70 dBm

Figure 11. With a 100 Hz RBW, the Noise Floor Appears at Around –80 dBm

You can see that the displayed average noise floor (DANF) of the instrument is highly dependent on the resolution bandwidth being used. This specification is significant because it provides anindication of the smallest detectable signal that the instrument can display. The conditions that the measurement was taken in are typically specified along with the DANF, because the DANF isdependent on various settings of the instrument. A typical DANF specification may resemble the following information:

–115 dBm between 1 GHz and 2.7 GHz with RBW set to 1 kHz, with 0 dB input attenuation at 25 ° C.

The noise floor is often normalized to a common RBW (often 1 Hz) because the apparent noise floor of the instrument increases with a wider RBW.

Ensure that both measurements are normalized to the same bandwidth when comparing the DANL between two manufacturers. The easiest technique to make a fair comparison is to normalizeboth instruments to 1 Hz RBW. Subtract from the given noise floor measurement. An instrument that shows a noise floor of –115 dBm at a 1 kHz RBW, for example, calculates to a10 log (RBW)noise floor of –145 dBm at a 1 Hz RBW. Normalizing both instruments to the same bandwidth provides a fair comparison of instrument performance.

Many traditional RF spectrum analyzers normalize measurements to a 6 MHz video bandwidth. You can take any measurement normalized to 1 Hz and normalize it to 6 MHz using simple math.Add , which is 67.8, to the measurement that has been normalized to 1 Hz. A measurement of –145 dBm normalized to 1 Hz is represented by –145 dBm + 68 dBm = –77 dBm.10 log (6 MHz)

The displayed noise floor of an instrument depends on the bandwidth being used. Be sure to normalize the signal level to the appropriate bandwidth when comparing multiple instruments ormaking noise measurements of the device under test.1 A vector network analyzer may also be used to perform analysis. The vector network analyzer is not covered in this article, but you can refer to the for more information about the theory andVector Network Analysis Webcast Seriesoperation of that instrument.

6. Conclusion

Whether you are an RF specialist reviewing instrument specifications or a novice trying to understand RF measurements, we hope you find useful and applicable information in this three-part

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Whether you are an RF specialist reviewing instrument specifications or a novice trying to understand RF measurements, we hope you find useful and applicable information in this three-partseries. Part 1 details general specifications common to all RF instruments. Part 2 and Part 3 focus on specifications for RF signal generators and RF signal analyzers, respectively. Use this seriesfor future reference or help with RF instrument specifications.

Refer to the for more information about making RF measurements.National Instruments RF Developer's Network

7. Related LinksRF DesignLine

Understanding RF Instrument Specifications Part 3 in RF DesignLine

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