RF chain
An RF chain is a cascade of electronic components and sub-units which may include amplifiers, filters, mixers, attenuators and detectors.
[1] 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 addition, there may be concerns regarding the immunity to incoming interference or, conversely, the amount of undesirable radiation emanating from the chain.
[5][6]: 663 Scattering parameters avoid the need for ports to be open or short-circuited, which are difficult requirements to achieve at microwave frequencies.
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 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.
[3] 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.
The thermal noise power present at the input of an RF chain,[18]: 44 [19]: 435 [20]: 229 is a maximum in a resistively matched system, and is equal to kTB, where k is the Boltzmann constant (= 1.380649×10−23 J⋅K−1[21]), T is the absolute temperature, and B is the bandwidth in Hz.
Two important parameters used in assessing sensitivity performance of a system are[24]: 2.16 [15]: 204 the "probability of detection" and the "false alarm rate".
Tangential sensitivity, (TSS), defines that input power which results in a video signal to noise ratio of approximately 8 dB from the detector.
The TSS level is too low a value for reliable pulse detection in a practical scenario, but it can be determined with sufficient accuracy in bench tests on a receiver to give a quick guide figure for system performance.
(Note: a similar result is obtained by using the equation relating RF and video S:N ratios, given in the previous section[23]: 190 ).
The thumbnail shows the simulated video output (at TSS) corresponding to an RF pulse in wideband noise with S:N = 0.17 and a bandwidth ratio of 500.
[22]: 87 This is a useful figure for use in spreadsheets, and it corresponds to a probability of detection of over 99% for a Swerling 1 target[29][30] (Although lower values of S:N can give acceptable "probability of detection" and "false alarm rate" figures, the measurement of pulse lengths become less reliable because noise spikes on pulses may extend below the chosen threshold level).
As can be seen, if the S:N falls to 15 dB or lower, it becomes difficult to set a threshold level for pulse detection, that is clear of the noise floor, and yet does not result in early termination.
[26]: 30 In general, system sensitivity and pulse detection theory are specialized topics [20]: 12 and often involve statistical procedures not easily adapted for spreadsheets.
Where only first time round echoes are considered (i.e. multiple reflections are ignored), the output response is given by where A typical plot is shown in the thumbnail.
Here, a random selection of path delays are assumed, with α taken as unity and ρ1 and ρ2 taking the typical value 0.15 (a return loss ≈ 16 dB), for the frequency range 10 to 20 GHz For this example, calibration at 50 MHz intervals would be advisable, in order to characterize this response.
The presence of a mixer in an RF chain complicates the spreadsheet because the frequency range at the output differs from that at the input.
Power density, which is in watts per metre squared, can be related to electric field strength ER, given in volts per metre, by The gain of the antenna is related to the effective aperture by[40]: 90 [6]: 746 In practice, the effective aperture of the antenna is smaller than the actual physical area.
Other losses arise from the necessity to include devices to protect the chain from high incident powers.
Similarly, a front end limiter[44] is needed, on a ship, to protect the chain from the emissions of high-power transmitters located close by.
In addition, the system may include a band-pass filter at its input, to protect it from out-of-band signals, and this device will have some pass-band loss.
Square law detectors can give detectable signals at video, in wideband systems, even when the RF S:N is less than unity.
DLVAs[49][22]: 72 have been commonly found in direction finding systems, using multiple channels, squinted antennas and amplitude comparison methods.
If the RF S:N ratio is too low, the output of the longest delay line correlator (which sets the frequency resolution of the IFM) will become degraded and noisy.
As shown earlier, a low-amplitude RF pulse immersed in wideband noise, can be detected by a square-law diode detector.
On the other hand, having excessive chain gain so that the noise floor is unnecessarily high, will result in the loss of dynamic range.
After compression in the signal processor, a high amplitude pulse, whose magnitude is well above the noise is obtained, as shown in the right-hand figure.
