Radio 2001, 10
This simple tuned radio frequency receiver (Fig. 1) was designed and constructed in the mid 1990s. The receiver was tested under urban and field conditions, in the heat and cold. This simple circuit has quite high sensitivity, good sound quality and reliability.
VT1, VT3 - KT315B (hfe = 50...350 at Uke=10 V and Ik=1 mA; ft = 250MHz), VT2 - KT361D (hfe = 20...90 at Uke=10 V and Ik=1 mA; ft = 250MHz);
VD1, VD2 - KD503B (Silicon diodes);
WA1, L1, L2 - a standard loopstick antenna; C1 - 5..356pF.
A signal received by a ferrite rod loop antenna WA1 and applied to a resonant tank L1C1, passes through the coupling coil L2 to the two-stage transistor amplifier (VT1, VT2). This is the reflex amplifier. The amplified signal from the resistor R4 is fed to the envelope detector that consists of a diode VD1 and a RC filter R3C4. The diode is biased, the bias depends on the operating mode of transistors. The operating mode may be adjusted by the potentiometer R5. Due to the bias, the detector works well for receiving weak signals.
The audio signal from the detector goes back to the same amplifier (VT1, VT2). From the output of the amplifier the audio signal passes through the low pass filter R6C5 to an audio amplifier based on the transistor VT3. The load of this amplifier is headphones TA-56M (1600 Ohms) connected to the terminals X1.
A negative voltage, proportional to the amplitude of the signal appears at the output of the detector. This negative voltage opens the transistors VT1, VT2 and changes the bias of the detector, reducing its gain. When receiving AM signals, a negative audio feedback across the detector improves the linearity of detection. The result is high quality sound.
In this circuit may be used any suitable loopstick antenna and a variable capacitor from any portable radio receiver. The components are mounted on a PCB, the PCB is placed in any suitable plastic housing. The potentiometer R5 (volume control) is mounted on a side of the housing. In some cases the potentiometer can help to reduce the interferences.
The circuit doesn't require alignment. The voltage of the power supply is 4.5 V, which is formed by serially connecting three batteries of 1.5 V. The consumption current is about 10 mA (when there is no signal).