Radio direction finding and PJ-80 direction finding machine circuit

The radio direction finding movement originated in the 1960s. It was introduced to my country from Europe in 1960. It's similar to the well-known hide-and-seek game, but instead of looking for a person, you look for the source of the emitting signal. Radio "hide-and-seek" is a combination of modern radio communication technology and traditional hide-and-seek games. The general process is: hide 3 to 10 signal sources (i.e. small transmitters) in advance in natural environments such as wilderness, hilly jungles, or suburbs, parks, etc. ), sending out Morse code signals for different calls at regular intervals. Participants hold radio direction finders to measure the direction of hidden radio stations, and quickly and accurately find these signal sources one by one on foot. The winner is the one who finds the specified number of stations within the specified time and uses the least time. Usually, we compare the signal source that is cleverly hidden in advance to a cunning fox, so this sport is also called radio "fox hunting". At present, there are three main types of radio direction finding activities in our country: one is the short-range 80-meter shortwave band direction finding suitable for popularization and promotion in middle schools, youth science and technology museums (stations), children's palaces, activity centers, etc.; the other is the 160-meter medium wave band direction finding ; The third is long-distance 80-meter shortwave band and 2-meter ultra-shortwave band direction finding that complies with the rules of the international direction finding competition and is suitable for college and middle school students. By participating in this activity, in addition to learning radio direction finding knowledge and technology, you can also learn basic knowledge about circuits and master direction finders and other electronic production skills. Because it is neither a purely technological indoor production nor a single run on a fixed site, but it fully embodies the combination of theory and practice, hands-on and brain use, indoor and outdoor, physical fitness and intelligence, it is very important to promote young people's moral, intellectual, and intellectual development. It is very beneficial to comprehensively develop physical, aesthetic and labor skills and enrich the content of the second classroom in the school. The PJ-80 model is a popular direct-display 80-meter band direction finder. It has a simple circuit, low price, and is easy to install. It is very suitable for the majority of teenagers to use radio direction finders and assembly. The block diagram and circuit principle of the whole machine are shown in Figures 1 and 2. Show. L1 is a magnetic rod antenna, A is a rod antenna, K1 is a one-way and two-way switch, used to determine the direction of the radio station, BG1 and peripheral circuits form a high-frequency amplifier, which amplifies the high-frequency Morse code signal received by the antenna and is coupled and output by B1 . BG3, C14, C15, D2, C16, C17 and C18 form an adjustable deformable capacitor three-point oscillator. Adjusting W2 can change the reverse bias voltage of the varactor diode D2, thereby changing the capacitance of the tube to change the oscillation frequency. The voltage regulator tube D3 is used to eliminate the instability of the oscillation frequency caused by the drop in battery voltage. The oscillation signal is superimposed on the high-frequency signal output by B1, and then mixed by diode D1 to generate a beat signal. The audio signal generated after detection and low-pass filtering is amplified by a low-frequency power amplifier circuit composed of BG2 and LM386. Here D1 plays the role of mixing And the dual function of detection. K2 is the power switch controlled by the headphone socket. K3 is closed when using stereo headphones. Debugging 1. Inspection of the working point: The voltage regulator tube D3 can be 3.5~4.4V. When W1 is placed at the maximum gain, the voltage at both ends of R3 is about 0.4~1V (Ic1 is about 0.4~1mV), and the voltage at both ends of R9 is about 0.4~1mV. The voltage is about 1.5~3V, and the voltage across R12 is about 2~2.5V. 2. Frequency coverage debugging: Place W2 in the center position, set the high-frequency signal generator to 3.5~3.6MHz, and turn the frequency button of the high-frequency signal generator so that the direction finder receives a signal with a pitch change, indicating that the machine is out of beat. The oscillator is working. Set the high-frequency signal generator to 3.55MHz and adjust the B2 core to hear the signal; rotate W2, and you should be able to hear the 3.5MHz and 3.6MHz equal-amplitude signals output by the high-frequency signal generator respectively, with slight noise at both ends. Abundance. If the high-end cannot be heard, but the low-end has a large margin, you can turn the B2 core slightly outward; otherwise, the opposite is true. 3. If the listening frequency is too wide, increase the resistance of R14 appropriately; otherwise, reduce its resistance. 4. Adjustment of the antenna loop: The high-frequency signal generator outputs a 3.53MHz signal, and the position of the coil on C1 and the magnetic rod is adjusted to maximize the sound. 5. Adjustment of the high-frequency circuit: the high-frequency signal generator outputs a 3.57MHz signal, and adjusts the B1 core to make the sound the loudest.