direct detection receiver
The straight-line receiver is a circuit concept used in the early years of broadcast technology in which the reception frequency range is selected and amplified via tunable filters. To increase frequency selectivity and selectivity, straight-line receivers use several identical and tunable filters.
A straight-line receiver consists of several RF amplifiers connected in series, all tuned to the same receive frequency. Since tunable filters have a much lower Q than fixed-frequency filters, selectivity and adjacent- channel rejection can be increased only by connecting several RF amplifiers in series. In addition, the synchronization of the individual filter stages causes great problems during frequency tuning, since all filters must have as identical a frequency response as possible when they are varied in frequency.
Downstream of the RF amplifiers is the demodulator, which demodulates the modulated signal and feeds it to the AF amplifiers.
The principle of the straight-through receiver is used in fixed frequency receivers, but not in tunable receivers. The disadvantages of the tunable straight-line receiver such as the synchronization problems, the inadequate selectivity, the bandpass characteristic changing with frequency or the low constancy of the variable tuning are avoided by the superheterodyne principle, also known as superheterodyne receiver.