Tektronix RSA306: A Compact High Performance Signal Analyzer
Instrument specifications
The RSA306 is a real-time spectrum analyzer, which means it acquires samples with a fast digitizer and continuously updates spectra in the real-time acquisition bandwidth via a Fourier transform. Wider spans are generated by moving the local oscillator frequencies in the instrument and stepping in chunks of the real-time bandwidth to the desired frequency range. The basic performance specifications for the RSA306 are presented in in the table below (full instrument specifications can be found at https://www.tektronix.com/spectrum-analyzer/rsa306).
The displayed average noise level (DANL) and phase noise for various frequency intervals can be found in the table below. The noise floor for the RSA306 is very good and is comparable to a standalone laboratory instrument such as the Tektronix RSA5106B. A mid-range lab instrument such as the RSA5106B, however, has about 25 dB better spur-free dynamic range and phase noise than the RSA306. Some of the design trade-offs in the RSA306 that affect these performance specifications will be discussed in the section below on RF design of the instrument.
Instrument architecture
Probably the most striking change in the architecture of this instrument compared to a conventional real-time analyzer is where the data processing is done. In a typical real-time spectrum analyzer the data is digitally downconverted and stuffed into acquisition memory in hardware. In the RSA306 the raw data is forwarded to the PC across the USB 3.0 interface, and all digital processing is done in software on the PC (Figure 1). Digital down conversion to I-Q data, filtering, Fourier transforms, channel correction and DPX display calculations are all done in software. This necessitates the use of a modern processor (Intel quad-core i7 or equivalent) in order for the software processing to keep up with the high data of 224 MB/s from the instrument.
Figure 1. Data processing paths compared for a laboratory spectrum analyzer (top) and the RSA306 handheld spectrum analyzer (bottom).
The USB 3.0 SuperSpeed connection has a signaling rate of 5.0 Gb/s in each direction. Since USB 3.0 uses 8B/10B coding, the data throughput is limited to 4.0 Gb/s, and when typical protocol overhead and handshaking packets are included the practical data throughput is more like 3.2 Gb/s. The 14-bit digitizer in the RSA306 samples the IF signal at 112 MHz, generating a data stream of 224 MB/s since each data sample is packed in a 16-bit word. Another 0.2% overhead is added to the data for timestamps and trigger information, giving a total information rate of 1.8 Gb/s. While this is well within the 3.2 Gb/s practical transfer rate for the USB 3.0 interface, the bursty nature of transfers using the Microsoft Windows OS could still be an issue. Fortunately, the Cypress FX3 USB 3.0 interface chip has data-path memory buffers to condition the data flow. A set of four memory buffers were set up with the same length as the frame protocol length for our device. The buffers are used in a round-robin fashion to transmit data. In practice we find that with a fast processor we do not drop frames under either Windows 7 or 8 unless substantial processor bandwidth is taken up by unrelated processes.
The software running on the PC controls all aspects of the operation of the RSA306. The software is divided into two parts: the RSA306 API and SignalVu-PC. SignalVu-PC is a user interface that runs on multiple platforms (spectrum analyzers and oscilloscopes) and presents control and data displays in the same format on all devices. The software can also be used standalone without attachment to a hardware platform to play back and analyze captured data. The RSA306 API software is a hardware adaptation layer that translates the control commands issued by SignalVu-PC into hardware commands specific to this spectrum analyzer. The list of API commands is public and can be used by programmers to write their own control and display software for the RSA306.
Multiple data displays can be processed and viewed simultaneously in SignalVu-PC. Displays in the base level software include a frequency spectrum, a frequency spectrogram (scrolling plot of spectral intensity vs. time), time overview, amplitude vs. time, frequency vs. time, RF I and Q vs. time, analog modulation display (AM/FM/PM) and DPX spectrum/spectrogram display. DPX is a color-graded, variable persistence density map that shows the intensity of signals in a given frequency band. The most processor-intensive operation running in software during normal operation is the construction of the DPX data display, but optimization of the code allows for real-time screen updates at 24 frames per second.
Additional displays are available for measurement of signal properties for specific types of signals. These displays are available as software options to the basic displays.
Digital modulation analysis includes the following tools: constellation diagram (27 modulation types supported, including QAM, QPSK, GMSK, FSK, APSK), demodulated I and Q vs. time, eye diagram, EVM (error vector magnitude) vs. time, frequency deviation vs. time, magnitude error vs. time, phase error vs. time, signal quality, decoded symbol table and a trellis diagram.
Pulsed RF measurement tools are pulse trace display, pulse statistics and a pulse table. OFDM analysis tools are the following: OFDM channel response, OFDM constellation diagram, OFDM EVM measurements, OFDM spectral flatness, OFDM magnitude error, OFDM phase error, OFDM power, OFDM signal quality summary and a decoded OFDM symbol table.
WLAN (WiFi) analysis tools are available for 802.11a/b/g/j/n/p/ac and include the following: spectrum emission mask measurements, WLAN channel response, WLAN constellation, WLAN EVM, WLAN spectral flatness, WLAN magnitude error, WLAN phase error, WLAN power vs. time, WLAN signal quality summary and WLAN decoded symbol table.
Public Safety P25 radio measurements consist of MCPR (multiple carrier power ratio) measurements, P25 constellation diagram, P25 eye diagram, P25 frequency deviation vs. time, P25 power vs. time, P25 signal quality summary and a P25 decoded symbol table. Finally, audio analysis tools include audio spectrum display, live audio demodulation and an audio measurements summary.
Mapping software is also available for displaying and recording signal location if the control computer has a GPS receiver. This code is available for any platform running SignalVu-PC.
The SignalVu software available for the RSA306 analyzer also has the capability to act as a 40 MHz bandwidth signal recorder. If the disk on the control PC is fast enough (solid state disk capable of 300 MB/sec write rate required), data can be recorded to disk with no gaps and is limited only by the capacity of the disk. Data can be stored as a single file or as multiple equal-length files for later processing. Matlab software is available for opening the data files for user processing, and instrument corrections for channel flatness can be applied.
Low power operation
The RSA306 spectrum analyzer was designed from the start to be a compact, low- power device that could operate solely from the power supplied over a USB 3.0 interface. These constraints are severe: the USB-attached device is only permitted to consume 100 mA of current upon connection, and can consume a maximum of 900 mA after negotiation at the voltage supplied by the USB 3.0s interface (which might be as low as 4.5 V after cable losses). Thus the device must operate on approximately 4 W of power across the entire range of operating temperatures, which is -10° to +55° C for the RSA306. Since the RSA306 operating current increases with temperature, room temperature operation must be de-rated by about 30 mA to stay within USB 3.0 operating requirements. By comparison, a desktop spectrum analyzer typically consumes 100 to 400 W of power.
The digital portion of the RSA306 design (ADC, FPGA, USB3 I/F, clock source) consumes about 0.75 W of power, including losses in switching power supplies. The remainder of the design comprising the RF signal chain consumes about 3.1 W of power. The RF signal chain (Figure 2) is composed of a switchable input attenuator, switchable preamplifier, fine step attenuator and an additional switchable gain stage (configured automatically as a function of reference level setting to reduce noise and distortion), followed by one of seven analog pre-filters that is selected based on the RF input frequency. The signal then enters the first mixer where it is converted to one of two IF frequencies, 1190 MHz or 2440 MHz, the frequency automatically chosen to minimize spurious signals. After filtering and amplification, the signal goes to a second mixer where it is downconverted the second IF frequency of 140 MHz. The IF frequency is amplified and then filtered with a 42 MHz bandwidth SAW filter followed by a seventh order LC filter before it is sent to the digitizer.
Figure 2. Simplified RF signal path for the RSA306 analyzer.
The RF signal path relies heavily on linearity of the components to avoid generation of spurs during amplification and mixing. For both of these components, linearity generally improves with increasing power. Components were carefully selected for good linearity and excellent noise while operating at low power. Filtering to remove undesired spurs (harmonics and mixing products between the input signal, local oscillators and clocks) allows for a spurious-free dynamic range specification (SFDR) of about -50 dBc over the RF signal input range. To achieve a portable, low-cost design, a single printed circuit board design with miniature shields and a single piece of extruded housing was used. At many frequencies the SFDR meets the design target of -65 dBc; however, the achievable RF isolation between sub-circuits is the main limitation on spurious performance.
Another critical choice of components in the design was the filters used in the IF sections. Low-cost SAW filters with wide bandwidths, low ripple and good out-of-band rejection generally have relatively high in-band loss as well. This necessitates additional amplification, which draws more power and can introduce additional nonlinear artifacts. Rather than make use of a multiple loop frequency synthesizer, the design opts for good phase noise but low voltage synthesizer ICs to keep size and cost small. These were locked to a very low jitter, high-frequency crystal oscillator that is also used as the ADC clock, resulting in decent phase noise that is good enough for EVMs on the order of 1%.
The SFDR and phase noise performance is limited, due to the low-power restrictions for a device that draws power from USB 3.0. Computing standards are continually evolving, however, and USB 3.1 promises both faster data rates and additional power in the near future. In the meantime, laboratory instruments such as the Tektronix RSA5k and RSA6k series have better RF specs since power is not an issue, and are controlled using the same software and user interface.
Typical Applications
The advent of Digital RF is a big enabler of the wireless everywhere phenomenon, coupled with the advent of IOT (Internet of Things).
Digital RF is the merging of digital computing technology in the form of DSP and FPGAs into traditional RF applications. This has led to rapid growth of new technologies such as Bluetooth, Wi-Fi, and WiMAX, leading to new end-user products.
These applications often reside within the same parts of the RF spectrum, requiring complex, time varying signals using adaptive modulation, frequency hopping and bursting techniques to avoid interference.
Because of this signal variability over time, there’s now a need when testing these RF applications to look simultaneously within the time, frequency, and modulation domains.
The portable, lightweight RSA306A with the rich feature set and analysis capability of the SignalVu software lends itself to being a very a cost effective solution in many applications, including
- Interference hunting – with a very wide “stare bandwidth” available of 40 MHz coupled with the real –time DPX , mapping and live spectrum streaming, the RSA306A can quickly isolate and find transient signals as short as 10ms as well easily identify co-channel interference issues .
- Education- the RSA306A offers students and teachers the ability to work with a portable low cost solution that enables multi-domain analysis ( Spectrum, Time, Frequency and modulation ) in a single unit with free software and course material. The interaction of analog, digital and RF parameters can easily be understood and visibly analyzed , ideal for teaching and research labs.
- Network Installation and Maintenance – a portable, ruggedized, handheld and USB 3.0 powered real-time analyser allows the engineer to make all RF measurements in the field and establish the health of the radio / wireless network. Full support for Tetra, P25, and WLAN makes network fault diagnosis fast and convenient as well as allowing full operational verification of Private Mobile Radio networks, Trunked radio systems and deployment of preventative maintenance.
About the author:
Dean Miles is Senior Technical Marketing Manager,Tektronix.
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