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406 Chapter 8

8.3.2 Calibration and Test Units

Many modern and highly sophisticated systems do not have a single source of RF power or
single receiver and are essentially the distributed systems with many generators and receivers.
These include, for example, radars with Active Electronically Steered Array (AESA), in-
building Distributed Antenna System (DAS) that provides seamless wireless communication,
the network-combining power of multiple typically solid-state amplifiers to get the decent high
level of output power, and many other systems. The proper functioning of such complex system
is almost impossible without continuous monitoring, synchronization, and stabilization of
signals in different elements of the system. There are probably more reasons why the system
does not meet specification than why it works perfectly well: output from power amplifiers may
degrade as a function of time or temperature, one or several network elements might fail, low-
noise amplifier sensitivity worse, and so on and so on. Putting multiple directional couplers
with proper detectors like shown in Figure 8.3.1inside each critical system component, we can
transfer all collected information to the system control computer for processing. Loosely
speaking, the rough comparison of incoming data with reference ones in computer database
allow generating multiple error signals. Then these signals should be sent to the corresponding
feedback circuits that are able to put in order power level, phase or frequency. In the case of
severe element detonation or complete fault, the request to take appropriate repair action must
be generated. Consequently, such embedded units of electronic control forms quite a complex
network that should be incredibly smart, fast, accurate, stable and reliable. It is not surprising
that their presence might almost double the system cost.
8.3.3 Power Leveling

Power leveling is often required in a system with a broadband frequency sweep. According to
Wikipedia, frequency sweeps may be used by regulatory agencies (for example, FCC in the
USA) to monitor the radio spectrum, to ensure that users only transmit according to their
licenses. In testing of new electronic devices, a frequency sweep in Network Analyzer is done
to measure the performance of electronic components or systems. For example, RF oscillators
should be measured for phase noise, harmonics, and spurious signals; radars should be tested for
parasitic emission to avoid interference with surrounding radio systems and vice versa. Portable
sweep equipment may be used to detect some types of covert listening device (bugs). In the
system with frequency
hopping, the transmitting
carrier is rapidly switched
among many frequency
channels in some sequence
known on the receiver side
that increases the
Figure 8.3.2 Block diagram of external power-leveling unit communication security.
The broadband frequency
sweep is used in a jammer that intentionally radiates high power signals directionally or spreads
them around to make a broad range of RF devices useless. In all such systems, some form of
signal leveling is virtually mandatory. Although sweep generators are available with leveled
outputs, an external closed loop method of leveling is usually necessary to eliminate
uncertainties introduced by cables and other components (phased shifter, power dividers, etc.)