Following the netizen "Universal Gravitation Platform": Further research on DIY mobile phone radio portable antenna
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Following the post by netizen “Universal Gravitation Platform”: DIYRe-study of portable antenna for mobile phone radio I have read the post by netizen “Universal Gravitation Platform” before: [size =14pt]FailedDIYMobile phone radio portable antenna https://en.eeworld.com/bbs/forum.php?mod=viewthread&tid=918130&fromuid=1014845 (Source: I wanted to explore the problem of mobile phone earphones acting as antennas before, but netizens took the lead in exploring it. I had to stand on the shoulders of giants and DIY it again. Some radios on smartphones require headphones to act as antennas. I didn’t understand this before, so I searched on Baidu. Here are the results I got: 1. Why can headphones be used as antennas? Answer 1: FM signals are high-frequency electromagnetic waves of 88~108MHz. When a mobile phone receives this signal, it needs an antenna. However, due to the size of the mobile phone, installing a fixed antenna device will increase the cost and is not beautiful. According to the current characteristics of the wire that can induce high-frequency electromagnetic waves, by using a longer headphone wire and adding a high-frequency path capacitor, the FM signal induced on the headphone line can be sent to the signal input end of the FM radio module to complete the FM signal reception task. Answer2: The radios in mobile phones, MP3, etc. are all FM radios, which require a wire as their receiving antenna. Any metal wire will do, and usually the headphone wire is used instead. To prevent the audio signal on the headphone wire from coming in, a circuit needs to be installed in the middle to isolate the audio signal and only allow the FM band signal to pass through. Therefore, the headphone wire can be used as a radio antenna. Answer3: When the radio signal is not good, put the antenna against the water pipe (metal) and the signal will be much better. The same is true for old TV sets. Because when the signal enters the house, the radio will lose a lot. But the radio can be transmitted in metal. You use your earphones as antennas. The signal will be much different when the earphone wire is rolled up and when it is straightened. I won't talk much about professional knowledge. The radio signal itself is a broadband signal. The wavelength is very long. The earphone wire is equivalent to an unrolled TV signal line. After searching so many answers, I seem to have understood a little bit. Because I am not very smart and don't understand it deeply, I will test it in practice. 1: Experimental materials Metal earphone (audio) connectorAntenna Earphone 2: Experimental steps First, I tested the effect of the radio using earphones as antennas Test results: The effect of the earphones is of course very good! ! ! ! Second, I used the headphone (audio) interface as the radio antenna. Test results: The effect is relatively poor, the radio receives relatively few stations, and the sound quality is very poor, with serious crackling sounds. Third, Based on the second step, I installed an external antenna at the headphone (audio) interface. OK Test result: The effect is much stronger, the number of radio stations has increased, and the sound quality has also improved, but it is not as good as the effect of headphones. I am attaching the test video, and welcome interested netizens to give advice. I cannot upload the video, please go to the resource page to view it. The following content comes from Baidu! ! ! ! ! ! ! Headphone antenna for FM reception The Si484x FM receiver supports headphone antennas. Headphone antennas with a length of 1.1 to 1.45 meters can be used in FM receivers because this length is close to half of the FM wavelength (the FM wavelength is about 3 meters). 1. Design of headphone antenna A typical headphone cable must contain3 or more wires (2 audio wires and 1 common wire). The left and right channel signals output by Si484x are connected to the left and right channel audio wires of the headphone after being driven by the headphone amplifier, while the common wire of the headphone can be used as an audio return path and FM antenna. Other wires in the headphone may be used for microphones, switches or other functions. In some applications, the FM antenna is an independent wire in the headphone cable. Figure 9 shows the application circuit of a typical headphone antenna. Figure 9. Typical headphone antenna application circuit 2. Headphone antenna schematic diagram Figure 10. Headphone antenna schematic diagram The application of headphone antenna requires at least LMATCH, C4, F1, and F2. ESD protection tube and headphone amplifier are also generally required components for radio chip circuits. LMATCH is a matching inductor. Choosing the right inductor value can obtain the maximum voltage gain over the entire FM band. The Q value of LMATCH at 100MHz should be no less than 15 and its DC impedance should be as small as possible. C4 is an AC coupling capacitor, which plays a role in DC isolation. The LNA input capacitance is 4~6 pF. To minimize the loss caused by C4, the value of C4 should be as large as possible. The recommended value is 100pF ~1nF. Ferrite beads F1 and F2 are placed between the headphone amplifier and the headphone to provide a low-impedance audio path and a high-impedance RF path. In addition to FMI and the headphone common line used as an antenna, other headphone wire connections should be added with ferrite beads, such as left and right audio lines, microphone audio lines, switch lines, etc. In the example shown in Figure 10, ferrite beads are added to the connection points of the left and right audio lines. Magnetic beads with an impedance of 2.5 kW or higher at 100MHz should be selected, such as Murata's BLM18BD252SN1. High-resistance ferrite beads at 100 MHz can maximize RSHUNT and RP. Refer to Appendix A "FM Receive Headphone Antenna Interface Model" of "AN383: Si47xx Antenna, Schematic, Layout, and Design Guidelines" for detailed explanations of RSHUNT, RP, etc. If the ESD requirements of the device exceed the ESD levels of the headphone amplifier and Si484x, it is recommended to use ESD protection diodes D1, D2, and D3. The junction capacitance of the ESD protection diodes should preferably be less than 1pF, such as the CM1210 from California Micro Devices. The small junction capacitance of this diode can minimize CSHUNT and CP. If the junction capacitance of D1 and D2 is greater than 1pF, it is necessary to place them between the ferrite beads F1/F2 and the headphone amplifier to reduce CSHUNT. Of course, this will reduce the efficiency of ESD protection. The position of D3 cannot be changed, so the junction capacitance of this diode should be less than 1pF. Each ESD protection diode package includes two diodes to protect against positive and negative ESD strikes respectively. C9 and C10 (125 μF) are AC coupling capacitors that act as DC isolation. R5 and R6 are optional discharge resistors that are used to discharge the AC coupling capacitors C9 and C10 after the headphones are unplugged. C5 and C6 are optional high-frequency bypass capacitors placed on the left and right audio lines of the headphone amplifier output to reduce the digital noise connected to the antenna. The recommended value is 100 pF or greater. When selecting this capacitor value, the designer must confirm that the headphone amplifier has sufficient driving capability to allow such a capacitor to be connected in parallel at the output end. The above schematic uses the National Semiconductor headphone amplifier LM4910. The LM4910 device specification recommends the use of R1~R4, C7, and C8. The gain of the left and right audio channel amplifiers is R3/R1 and R4/R2, and the gain can be changed by changing the resistance values of resistors R3 and R4. According to the electroacoustic gain of the headphones themselves, it is usually recommended to set the gain of the headphone amplifier to 0.6~1.0. The AC coupling capacitors C7 and C8 and the resistors R1 and R2 form high-pass filters to set the low-frequency threshold of the audio amplifier. The inflection point frequency of the high-pass filter of the left and right audio amplifier circuits is calculated as follows: 374918 Formula 2.Calculation of the inflection frequency of the high-pass filter Using the device in Figure 10, according to Formula 2, it can be calculated that the inflection frequency of the headphone amplifier is about 20 Hz. C1 is the power supply bypass capacitor of LM4910. The third pin of LM4910 is the low-level shutdown enable pin. When a low level is added to the third pin, the LM4910 can be turned off. The low-level threshold value of the third pin of LM4910 is 0.4V, and the high-level threshold voltage is 1.5V. The BOM corresponding to Figure 10 is shown in Table 10. For the supplier of resistors and capacitors, users can choose by themselves. 3. Schematic diagram of headphone antenna circuit BOM table 10. Headphone antenna BOM 374919 4. Headphone antenna PCB Layout Matching inductor LMATCH and headphone jack J24 should be placed close to each other and away from noise sources such as clocks, digital circuits, etc. To minimize CSHUNT and CP, the magnetic beads F1 and F2 should be placed as close to the headphone jack as possible. To maximize the protection efficiency of the ESD tube, the diodes D1, D2 and D3 should be placed as close to the headphone jack as possible. If the junction capacitance of D1 and D2 is greater than 1 pF, it needs to be placed between the magnetic beads F1/F2 and the headphone amplifier to reduce CSHUNT. Place the radio chip as close to the headphone jack as possible, reduce the length of the trace from the chip to the headphone jack, narrow the trace width, and keep it as far away from the GND plane as possible to reduce the capacitance CPCBANT of the trace. Also, reduce the number of vias on the trace and only trace on the top or bottom layer. Copper should not be laid near the trace, and it does not need to be designed to be 50W. In order to reduce the digital noise connected to the antenna, high-frequency bypass capacitors C5 and C6 can be placed on the left and right audio lines of the headphone amplifier output. The recommended value is 100 pF or greater. However, when selecting capacitors, users should confirm whether the headphone amplifier has sufficient driving capability to allow such capacitors to be connected in parallel at the output end. 5. Headphone antenna design inspection details The antenna length should be 1.1 ~1.45 meters. n Use matching inductor LMATCH to improve the signal strength of the entire FM band. n Matching inductor LMATCH Q value is not less than 15, and the DC impedance is as small as possible. n Matching inductor LMATCH is close to the headphone jack and away from interference sources. n Si484x is as close to the headphone jack as possible to reduce the length of the trace from the chip to the headphone jack. This can reduce the parasitic capacitance CPCBANT of the line and the interference of noise sources to the antenna, so as to obtain the best FM reception performance. Magnetic beads F1 and F2 should be selected with an impedance of 2.5 kW or more at 100MHz to increase RSHUNT and Rp values. n Magnetic beads F1 and F2 are as close to the headphone jack as possible. ESD diodes D1~D3 should be selected with small junction capacitance. ESD diodes D1~D3 are placed as close to the headphone jack as possible to maximize the ESD protection efficiency. n Optional high-frequency bypass capacitors are placed on the left and right audio lines of the headphone amplifier output to reduce the digital noise connected to the antenna. 5 kW or higher impedance ferrite beads to increase RSHUNT and Rp values. n Ferrite beads F1 and F2 should be placed as close to the headphone jack as possible. ESD diodes D1~D3 should use diodes with small junction capacitance. ESD diodes D1~D3 should be placed as close to the headphone jack as possible to maximize ESD protection efficiency. n Optional high-frequency bypass capacitors are placed on the left and right audio lines of the headphone amplifier output to reduce digital noise connected to the antenna. 5 kW or higher impedance ferrite beads to increase RSHUNT and Rp values. n Ferrite beads F1 and F2 should be placed as close to the headphone jack as possible. ESD diodes D1~D3 should use diodes with small junction capacitance. ESD diodes D1~D3 should be placed as close to the headphone jack as possible to maximize ESD protection efficiency. n Optional high-frequency bypass capacitors are placed on the left and right audio lines of the headphone amplifier output to reduce digital noise connected to the antenna.
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