Application to an Analog Communications System

Use of the three-frequency method for a communications system (in which the transmitter and receiver may be moving relative to each other) is similar to the radar already described, and is indicated in Fig. 7.15. Note, however, that only 

Thus far, we have been especially concerned with absorption detectors operating in the optical and infrared regions of the electromagnetic spectrum hvP-kT, where k is the Boltzmann constant and T is the detector temperature). In this case, the intensity of the incoming wave is obtained from the analytic signal and excludes double- and sum-frequency components [7.10, 12-14]. Nevertheless, (7.77) is a general result for intensity detection which applies also to the microwave and radiowave regions hv kT). 

For low frequencies, the intensity is related to the square of the electric field, locE. For a diode mixer which is either operating in the square-law regime or in 

Note that y4 cos (ut-l-d )=( V2)[l+cos(2cut-l-2d))]. Now, since the detector generally does not follow the instantaneous intensity at double- and sum-frequencies (2 Ui, coi +0J2,. ..), only dc and difference-frequency terms remain. Hence (7.121) will in practice reduce to (7.36). The calculations leading to (7.121) will remain correct, provided of course, that we insert the proper relation for (SNR)j in the classical low frequency detection regime. Generally, this is obtained by replacing hv by kTand tj by l/Fj, where Fj is the noise figure of the receiver. 

Once the target is ascertained to be present, a wide bandpass filter can be gradually narrowed about 2 /( — /l or 2I/2 — /l1 and thereby used to obtain Doppler information, Alternatively one could, of course, switch to a conventional configuration. 

---