lifiers oscillator 2nd mixer 2nd I.F. amplifier frequency frequency Detector section changer amplifiers Audio oscillator amplifiers Fig. 2.1-36 Block diagram of a double conversion superhet receiver. Basic features of radio communication systems CHAPTER 2. the local oscillator to produce a signal in the i.f. amplifiers. a masthead unit which is mounted directly on the an- If the desired frequency is 531 kHz, then the undesired tenna. The total gain including r.f. amplification, fre- frequency of 1461 kHz is 2( (fif) or 930 kHz away, i.e. quency conversion and i.f. amplification is about 55 dB. it forms an image on the other side of the oscillator This high order of gain is necessary to compensate for the frequency. This condition is shown graphically in losses which occur in the down-lead coaxial cable to the Figure 2.1-35. satellite receiver which is situated within the domestic There is the possibility that any strong signal or sub- environment. The satellite receiver treats the 1st i.f. harmonics of 465 kHz (fif) might impinge directly frequency band (950-1750 MHz) as a tuning band and fo on the i.f. amplifiers and cause interference. is varied to select the required TV channel which is Any harmonic of the oscillator (fo) could mix with an amplified by the 2nd i.f. section before signal processing. unwanted signal to produce unwanted responses. For example 2.1.11 Summary 2 X 996(fo) kHz - 1527 kHz = 465 kHz The main purpose of this chapter has been to introduce The spurious responses stated above are minimised in you to the radio environment in your home. The superhets by using tuned circuits in the r.f. section of the knowledge you have gained will assist you in un- receiver to select the desired signal and to reject the derstanding basic radio propagation and reception prin- undesired ones. The local oscillator is also designed to be ciples. It will also help you to remedy some of the 'harmonic free'. simpler radio and TV problems which you are likely to encounter in your home. 2.1.10.3 Double superheterodyn