Vector Network Analyzers (VNA) Information
Vector network analyzers (VNA) measure the complex transmission and reflection characteristics of two-port devices in the frequency domain. They sample the incident signal, separate the transmitted and reflected waves, and performing ratios that are directly related to the reflection and transmission coefficients of the two-port device. Frequency is swept to obtain amplitude and phase information over a band of interest. Most vector network analyzers use tuned receivers that provide better noise performance than traditional scalar network analyzers. Some vector network analyzers include a 5 ¼” floppy drive or a 3 ½” disc drive. Others include a compact disc (CD) drive for loading programs or storing data. Tape drivers and display options are also available. For example, analog meters display values with a simple visual indicator such as a needle. Digital readouts use numeric or application-specific display. With video displays, data is presented via a cathode ray tube (CRT), liquid crystal display (LCD) or multi-line form.
There are several form factors or instrument styles for vector network analyzers. Portable or benchtop devices can be moved with relative ease and used in a variety of applications. They many include a case or handle, but are not necessarily hand held in use. Fixed vector network analyzers are kept in one location and meant to be used in one place. They are often stand-alone devices. PC-based or "black box" instruments and modules do not include an integral display, but instead interface to a computer. They typically plug into the backplane or motherboard, or otherwise interface directly with the computer bus. For each form factor or instrument style, operating temperature and operating humidity are important considerations.
Performance specifications for vector network analyzers include frequency range, frequency accuracy, frequency resolution, output power range, and nominal input impedance. Typically, applications such a wireless communications require higher frequency capabilities. For example, 900 MHz applications require devices with a high frequency of 10 * 900 MHz for a total of 9 GHz. Other applications must be able to measure lower frequency baseband or intermediate frequency (IF) signals. Frequency accuracy is specified as the sum of several sources of errors, including frequency-reference inaccuracy, span error, and resolution bandwidth (RBW) center-frequency error. Frequency resolution is an important specification for applications that measure close signals that need to be distinguished from one another. Output power is the 1-dB compression point that results in a 1 dB decrease in amplifier gain. Nominal input impedance is the amount of load that an input places on the signal source that drives the load. High input impedance is generally desirable and implies that little change in the signal is expected when the circuit is connected. The most common input impedances for vector network analyzers are 50 and 75 .
There are several interfaces for vector network analyzers. RS232, RS422, and RS485 are common serial interfaces. Universal serial bus (USB) is a 4-wire, 12-Mbps serial bus for low-to-medium speed connections. IEEE 1394 or FireWire® is an interface standard adopted by the Institute of Electrical and Electronics Engineers (IEEE) for very fast digital data transfers. FireWire is a registered trademark of Apple Computer, Inc. The general-purpose interface bus (GPIB) is designed to connect computers, peripherals and laboratory instruments. Small computer systems interface (SCSI) is an intelligent I/O parallel peripheral bus. Transistor-transistor logic (TTL) is a common type of digital circuit in which the output is derived from two transistors. Some vector network analyzers use parallel channels or Ethernet networks. Others use modems or communicate via radio transmissions or telemetry.