1. Types of RF Signal Analyzers
Engineers 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 use either a spectrum analyzer or a vector signal analyzer1.
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 in Figure 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 local oscillator (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 a power-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 a broad 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 RF signal 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 that of 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 RF signal as well as perform spectrum measurements with a simple FFT calculation.
2. Attenuation and Reference Level
RF 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 use attenuation 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.
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 signal analyzer, 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 of around –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 the 1.57 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 that can be analyzed. RF instruments can analyze low-level signals with optional preamplifiers.
3. Dynamic Range
Dynamic 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 applied to 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 as LO 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 in decibels, 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 of which 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 is of 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. The dynamic 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 power of 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 least 60 dB to measure the smaller signal displayed in Figure 5.
Figure 5. Low-Power Signal Adjacent to a High-Power Signal
4. Averaging Methods
Caution: 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 white noise 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.
With 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. To determine 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 and without 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 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 measured 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 885 MHz 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 Floor
The apparent noise floor of an RF signal analyzer depends on much more than the RF system introducing noise, as described in the Averaging Methods section. The averaging mode that you use can 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. To illustrate 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 the instrument.
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 an indication 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 is dependent 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 normalize both instruments to 1 Hz RBW. Subtract 10 log (RBW) 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 a 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 10 log (6 MHz), 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.
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 or making 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 Vector Network Analysis Webcast Series for more information about the theory and operation of that instrument.
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 series. 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 series for future reference or help with RF instrument specifications.
Refer to the National Instruments RF Developer's Network for more information about making RF measurements.
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