![]() The selectivity of a receiver is its ability to reject unwanted signals. Portable and other small receivers used only for the broadcast band might have a sensitivity in the vicinity of 150 μV, whereas the sensitivity of quality communications receivers may be better than 1 μV in the HF band. Figure 6-4 shows the sensitivity plot of a rather good domestic or car radio. It is obvious that the noise figure plays an important part. The most important factors determining the sensitivity of a superheterodyne receiver are the gain of the IF amplifier(s) and that of the Transistor RF Amplifier Circuit, if there is one. Under the heading of “sensitivity” in the specifications of a receiver, a manufacturer might quote, “a – 85-dBm 1-MHz signal, 30 percent modulated with a 400-Hz sine wave will, when applied to the input terminals of this receiver through a dummy antenna, produce an output of at least 50 mW with a signal-to-noise ratio not less than 20 dB in the output.” The measurements are made under the conditions described, and the minimum input power is quoted in dB below 1 mW or dBm. Sometimes the sensitivity definition is extended, and the manufacturer of this receiver may quote it to be, not merely 12.7 μW, but “12.7 μW for a signal-to-noise ratio of 20 dB in the output of the receiver.”įor professional receivers, there is a tendency to quote the sensitivity in terms of signal power required to produce a minimum acceptable output signal with a minimum acceptable signal-to-noise ratio. At 1000 kHz, this particular receiver has a sensitivity of 12.7 μV, or – 98 dBV (dB below 1 V). It is seen from the sensitivity curve in Figure 6-4 that sensitivity varies over the tuning band. Sensitivity is often expressed in micro volts or in decibels below 1 V and measured at three points along the tuning range when a production receiver is lined up. The standard output is 50 milliwatts (50 mW), and for all types of receivers the loudspeaker is replaced by a load resistance of equal value. Thus 30 percent modulation by a 400-Hz sine wave is used, and the signal is applied to the receiver through a standard coupling network known as a dummy antenna. For AM broadcast receivers, several of the relevant quantities have been standardized. It is often defined in terms of the voltage that must be applied to the receiver input terminals to give a standard output power, measured at the output terminals. The sensitivity of a radio receiver is its ability to amplify weak signals. In practice Transistor RF Amplifier Circuit have the input and output tuning capacitors ganged to each other and to the one tuning the local oscillator. ![]() Such coupling is used here because of the high frequencies involved. A single-tuned circuit is used at the input and is coupled to the antenna by means of a trimmer (the latter being manually adjustable for matching to different antennas). ![]() Feed through capacitors are almost invariably provided for bypassing at VHF and often have a value of 1000 pF. This arrangement minimizes stray inductance in series with the bypass capacitor. This is surrounded by the dielectric, and around that is the grounded outer electrode. As indicated in Figure 6-3b, one of the electrodes of a feed through capacitor is the wire running through it. Feed through capacitors are used as bypass capacitors and, in conjunction with the RF choke, to decouple the output from the V cc. The medium-frequency amplifier of Figure 6-3a is quite straightforward, but the VHF amplifier of Figure 6-3b contains a number of refinements. Both diagrams in the figure are seen to have an RF gain control, which is very rare with domestic receivers but quite common in communications receivers. The single-tuned, transformer-coupled amplifier is most commonly employed for RF amplification, as illustrated in Figure 6-3. Prevention of re radiation of the local oscillator through the antenna of the receiver (relatively rare).Prevention of spurious frequencies from entering the mixer and heterodyning there to produce an interfering frequency equal to the IF from the desired signal.Better coupling of the receiver to the antenna (important at VHF and above).Improved rejection of adjacent unwanted signals, i.e., better selectivity.The advantages of having an Transistor RF Amplifier Circuit are as follows: The best example of this kind of receiver is a domestic one used in a high-signal-strength area, such as the metropolitan area of any large city. On the other hand, there are some instances in which an Transistor RF Amplifier Circuit is uneconomical, i.e., where its inclusion would increase the cost of the receiver significantly while improving performance only marginally. The receiver having an RF stage is undoubtedly superior in performance to the receiver without one, all else being equal. Reasons for use and functions of RF amplifier:
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