HAM RADIO SW RECEIVER
Radio, 1985, 11
A simple radio receiver circuit is shown in Fig. 1. This receiver is designed for reception of signals of amateur radio stations operating in the SW band of 10, 15, 20, 40 and 80 meters. This direct-conversion receiver consists of a set of an input bandpass filters tuned to the middle frequency of the amateur bands, a broadband RF amplifier based on a transistor VT1, a diode mixer (VD1, VD2), a local oscillator (VT2) and a three-stage amplifier of audio frequency with high gain based on a transistors VT3-VT5. This amplifier is loaded with a headphones BF1. A switch SA1 is intended to switch between bands, it connects the filters to the input of the RF amplifier, and the mixer - to a corresponding resonant tank of the local oscillator. The local oscillator is tunable by a variable capacitor C27 and it generates an oscillations with a frequency twice lower than the frequency of received signals. A simple voltage regulator is based on the Zener diode VD3 and it is used to reduce a dependence of the local oscillator frequency of a supply voltage variation.
Fig. 1.
VT1, VT2 - BF194; VT3..VT5 - BC149;
VD1, VD2 - BAP749, VD3 - BZP683C7V5 (Zener diode of 7.5 V)
The input signal amplified by the transistor VT1 goes to the mixer (VD1, VD2) through a broadband RF transformer T1. The resulting audio frequency goes through a lowpass filter L16C32 to a volume control - a potentiometer R7, and from this potentiometer the audio signal goes to the AF amplifier.
A decoupling filters R6C18, R9C34, R15C35 and a capacitor C38 id used to prevent the receiver from a self-excitation caused by a stray coupling through a positive power supply rail. For this purpose the wires going to the movable contacts of the switch SA1 is shielded.
Details of the receiver is mounted on a printed circuit board. All coils is wound on a polystyrene tubes with a diameter of 7 mm with a trimmer made of carbonyl iron. The distance between the axes of the coils of the input bandpass filters is approximately 16 mm. The technical data of the coils is shown in the Table 1.
Coil (Band) | Inductance, μH | Number of turns | Wire, mm |
|
L1, L2 (80 m) | 4,6 | 4+26 | 0,2 (AWG 32) |
L3, L4 (40 m) | 2,3 | 3+14 | 0,29 (AWG 29) |
L5, L6 (20 m) | 1,15 | 3+11 | 0,35 (AWG 27) |
L7, L8 (15 m) | 0,57 | 2+6 | 0,62 (AWG 22) |
L9, L10 (10 m) | 0,4 | 2+5 | 0,62 (AWG 22) |
L11 (80 m) | 9,2 | 5+35 | 0,12 (AWG 37) |
L12 (40 m) | 4,6 | 4+26 | 0,2 (AWG 32) |
L13 (20 m) | 2,3 | 2+15 | 0,29 (AWG 29) |
L14 (15 m) | 1 | 2+14 | 0,35 (AWG 27) |
L15 (10) | 1,15 | 2+12 | 0,35 (AWG 27) |
Table 1. All coils are wound with one single layer, turn to turn.
The inductance of a choke L16 is 100 uH (the choke can be of any design). Broadband transformer T1 is wound on a ferrite ring 100NN (the magnetic permeability is 100) with outer diameter of 10 mm. Each of its winding consists of seven turns enameled copper wire with a diameter of 0.3 mm (AWG 29) (the winding is made of three separated wires simultaneously). Between the coils of the bandpass filters and the local oscillator, and also between them and other components of the receiver a brass shielding is installed.
Janeczek A. Nasluchowy odbiornik KF.- Radioelektronik, 1985. N 4. p. 13-14.