You have now studied, circuit by circuit, all the parts of the TRF. You have studied various methods of detection used in the TRF, and in other receivers. You have studied various methods of volume control used in the TRF, and in other receivers. It is time now to bring the various circuits together to form one typical TRF circuit diagram. To help you understand the circuit diagram, the table on page 28 gives a description of each of the circuit components.
The antenna circuit includes a tunable parallel wavetrap for undesirable signal rejection. The antenna is inductively coupled to the input of the first RF amplifier.
There are three RF amplifier stages, followed by a detector stage. Four tuned RF circuits control the inputs to these stages. These tuned circuits are ganged their variable capacitors, C2, C3, C4, and C5 can be adjusted by a single control. Capacitors C6, C7, C8, and C9 are trimmers They are separately adjustable and can be used for tracking the four tuned circuits. These stages, the three RF amplifiers and the detector, are transformer coupled.
The three RF amplifier stages use variable-mu pentode tubes. The detector stage provides diode detection by using a triode tube with plate and control grid connected for diode action.
The output of the detector is capacitively coupled to the audio amplifier, a pentode tube. The output of the audio amplifier is transformer coupled to the headset.
All the amplifiers use cathode bias. A single potentiometer (R4) forms part of the cathode resistance of the first two RF amplifiers. It also forms part of the antenna circuit. Adjustment of this potentiometer provides manual gain control when ganged switch SW2 is in the MGC position. In this position, one contact of the switch grounds the potentiometer so that it can be used. Another contact grounds the AVC circuit so that it can't feed voltage to the grids of the RF amplifiers. A third contact applies the entire voltage drop across the diode load resistor to the AF amplifier. This makes the adjustable part of the manual volume control inoperative.
When switch SW2 is in the MVC-AVC position, one contact grounds the top of potentiometer R4, removing it from the cathode circuit of the first two RF amplifiers, leaving all of R4 across the primary of Tl. Another contact removes the ground connection which prevented AVC voltage from being fed to the amplifier grids. A third contact connects the variable arm of potentiometer R15 (the detector load resistor) to the input of the audio amplifier. Thus, the output of the
detector can be regulated manually, and the automatic volume control circuit is in operation.
Altogether, then, three methods of volume control are used: manual gain control on one position of the switch, and both manual volume control and automatic volume control on the other.
The power supply uses a full wave high vacuum rectifier tube, with a capacitor input pi-type filter. This type of filter develops a high DC voltage output (about 0.9 of the peak AC voltage), but has poor voltage regulation. However, the load current taken by the receiver circuits is fairly constant. Thus, the power supply functions very efficiently. This is the type of power supply used in the majority of AC receivers.
TRF receivers have been widely replaced by superheterodyne receivers. The selectivity and response of the TRF are not uniform over the tuning range. Also, it is difficult to design TRF receivers for satisfactory operation at extremely high frequencies. However, the TRF continues to be useful, particularly in military communications, because it has one advantage over the superheterodyne it does not use an oscillator in its circuit, while the superheterodyne does. Thus, the superheterodyne cannot be used where the enemy has direction finding equipment to detect a receiver's location.
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