ng between transmitting and receiving 2.1.10 Types of receivers aerials. Attenuation is compensated for by subsequent amplification, but amplifiers add their own inherent in- ternally generated random noise to the signal. Noise There are many types of radio receivers. These include: levels must always be less than the required signal, oth- tuned TRF erwise the required signal will be lost in noise. Some superheterodyne receivers (superhets) means must be provided to specify the level of the signal double superheterodyne receivers (double above noise. This means is called the signal-to-noise superhets) ratio. It is defined as 2.1.10.1 Tuned radio frequency receiver signal-to-noise ratio noise power power (2.1.23) A tuned r.f. receiver (Figure 2.1-33) has three main sections: a r.f. amplifier section, a detector section, and an Notice that S/N is specified as a ratio of power levels. audio amplifier section. Basic features of radio communication systems CHAPTER 2. 1 Tuned radio Audio frequency Detector amplifiers amplifiers Fig. 2.1-33 Main sections of a tuned radio frequency receiver. The r.f. section consists of one or more r.f. amplifiers Circuit instability can be a problem because it is connected in cascade. 19 For efficient operation, all tuned relatively easy for any stray or leaked signal to be circuit amplifiers must be tuned to exactly the same picked up by one of the many r.f. amplifiers in the broadcast frequency and to ensure that this is the case, all receiver. R.F. signal can also be easily coupled from one tuning adjusters are fixed on to a common tuning shaft. r.f. stage to another through the common power supply. Tuning capacitors which are connected in this manner are To minimise these risks, r.f. amplifiers are usually said to be 'ganged' and two- and three-stage ganged shielded and de-coupled from the common power tuning capacitors are common. supply. The detector is usually a conventional AM diode type detector. This type of detector is usually a diode which 2.1.10.2 Superheterodyne receiver d