Scalar Network Analyzers
Scalar network analyzers measure the amplitude portion of scattering or S-parameters, reflection and transmission coefficients between the incident and reflection waves that describe a device’s behavior under linear conditions at the microwave frequency range. Most scalar network analyzers are used to measure transmission gain, transmission loss, return loss, and standing wave ratio (SWR). Traditional devices use diode detectors to convert a radio frequency (RF) input signal to a proportional DC level. This method is less expensive than the tuned-receiver approach, but inherently scalar in nature. Some scalar 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 S-parameter 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 scalar network analyzers. Portable or benchtop devices can be moved with relative ease and used in a variety of applications. They may include a case or handle, but are not necessarily designed for hand held use. Fixed scalar network analyzers are kept in one location and meant to be used in one place. They are usually 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 scalar 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 scalar network analyzers are 50 and 75 .