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Examining Tektronix RSA306 signal analyser

Posted: 06 Feb 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Tektronix? RSA306? spectrum analyser? RF performance? digitizer?

The Tektronix RSA306 is a spectrum analyser that is designed for portability. It can be used with laptops and tablet PCs that have a USB 3.0 connection, and draws all of its power (less than 4.5 W) from that connection. Data processing, display and storage is handled on the PC, allowing for small size and portability in the analyser itself. The size and power constraints, however, have an impact on achievable RF performance. This article will discuss the trade-offs needed to produce a spectrum analyser in a small form factor (approximately 1" x 5" x 7.5") while retaining much of the performance and usability of much larger instruments.

Instrument specifications
The RSA306 is a real-time spectrum analyser, 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 table 1.

Table 1: The basic performance specifications for the RSA306.

The displayed average noise level (DANL) and phase noise for various frequency intervals can be found in table 2. The noise floor for the RSA306 is very good and is comparable to a stand-alone 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.

Table 2: The displayed average noise level (DANL) and phase noise for various frequency intervals.

Instrument architecture
Probably the most striking change in the architecture of this instrument compared to a conventional real-time analyser is where the data processing is done. In a typical real-time spectrum analyser the data is digitally down-converted 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 analyser (top) and the RSA306 handheld spectrum analyser (bottom).

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