|
(Click here for Part 2, which covers the specs important for signal generators.)
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.
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.
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). (For more information on how mixers operate click here) 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.
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.
|
