An RF chain is a cascade of electronic components and sub-units which may include amplifiers, filters, mixers, attenuators and detectors. It can take many forms, for example, as a wide-band receiver-detector for electronic warfare (EW) applications, as a tunable narrow-band receiver for communications purposes, as a repeater in signal distribution systems, or as an amplifier and up-converters for a transmitter-driver. In this article, the term RF (radio frequency) covers the frequency range "medium Frequencies" up to "microwave Frequencies", i.e. from 100 kHz to 20 GHz. The key electrical parameters for an RF chain are system gain, noise figure (or noise factor) and overload level. Other important parameters, related to these properties, are sensitivity (the minimum signal level which can be resolved at the output of the chain); dynamic range (the total range of signals that the chain can handle from a maximum level down to smallest level that can be reliably processed) and spurious signal levels (unwanted signals produced by devices such as mixers and non-linear amplifiers). In addition, there may be concerns regarding the immunity to incoming interference or, conversely, the amount of undesirable radiation emanating from the chain. The tolerance of a system to mechanical vibration may be important too. Furthermore, the physical properties of the chain, such as size, weight and power consumption may also be important considerations. An addition to considering the performance of the RF chain, the signal and signal-to-noise requirements of the various signal processing components, which may follow it, are discussed because they often determine the target figures for a chain.
Parameter sets Each two-port network in an RF chain can be described by a parameter set, which relates the voltages and currents appearing at the terminals of that network. Examples are: impedance parameters, i.e. z-parameters; admittance parameters, i.e. y-parameters or, for high frequency situations, scattering parameters, i.e. S-parameters. Scattering parameters avoid the need for ports to be open or short-circuited, which are difficult requirements to achieve at microwave frequencies.
In theory, if the parameter set is known for each of the components in an RF chain, then the response of the chain can be calculated precisely, whatever the configuration. Unfortunately, acquiring the detailed information required to carry out this procedure is usually an onerous task, especially when more than two or three components are in cascade. A simpler approach is to assume the chain is a cascade of impedance matched components and then, subsequently, to apply a tolerance spread for mismatch effects (see later).
A system spreadsheet A system spreadsheet has been a popular way of displaying the important parameters of a chain, in a stage-by-stage manner, for the frequency range of interest. It has the advantage of highlighting key performance figures and also pin-pointing where possible problem areas may occur within the chain, which are not always apparent from a consideration of overall results. Such a chart can be compiled manually or, more conveniently, by means of a computer program. In addition, 'tookits' are available which provide aids to the system designer. Some routines, useful for spreadsheet development, are given next.
Key spreadsheet topics For the parameters considered below, the chain is assumed to contain a cascade of devices, which are (nominally) impedance matched. The procedures given here allow all calculations to be displayed in the spreadsheet in sequence and no macros are used. Although this makes for a longer spreadsheet, no calculations are hidden from the user. For convenience, the spread sheet columns, show the frequency in sub-bands, with bandwidths sufficiently narrow to ensure that any gain ripple is sufficiently characterized.
Consider the nth stage in a chain of RF devices. The cumulative gain, noise figure, 1 dB compression point and output thermal noise power for the preceding n − 1 devices are given by Gcumn−1, Fcumn−1, Pcumn−1 and Ncumn−1, respectively. We wish to determine the new cumulative figures, when the nth stage is included, i.e. the values of Gcumn, Fcumn, Pcumn and Ncumn, given that the nth stage has values of Gn, Fn, P1n for its gain, noise figure and 1 dB compression point, respectively.
Cumulative gain The cumulative gain, Gcumn after n stages, is given by
G c u m n = G c u m n − 1 × G n {\displaystyle \mathrm {Gcum} _{n}=\mathrm {Gcum} _{n-1}\times G_{n}}
and Gcumn(dB) is given by
G c u m n ( d B ) = G c u m n − 1 ( d B ) + G n ( d B ) {\displaystyle \mathrm {Gcum} _{n}(dB)=\mathrm {Gcum} _{n-1}(dB)+G_{n}(dB)}
where Gcumn−1 [dB] is the total gain of the first n − 1 stages and Gn [dB] is the gain of the nth stage. Conversion equations between logarithmic and linear terms are:
G = 10 G ( d B ) / 10 = exp ( 0.23026 × G ( d B ) ) {\displaystyle G=10^{G(dB)/10}=\exp {\big (}0.23026\times G(dB){\big )}}
and
… excerpt ends here. Continue reading the full article.
