Keywords: FM transmit circuit
When a personal watching TV, the sound will interfere or affect others to learn or rest, and the wireless headset can be listened separately. Its working principle is to take out the soundtime from the headphone jack or audio output terminal of the television. The audio modulation of the "forwarder" dedicated to the wireless headset is then converted to the audio signal to be converted into a radio frequency signal of a certain frequency point in the band of 88MHz to 108MHz, and emit out in a small antenna. Then be received by a matching receiver (or ordinary FM radio), listening with a headset. This method eliminates the inconvenience and constraints of the wired tolerant mode that is directly inserted with the headset directly into the TV. This wireless headset also combines wireless forwarding such as CD, MP3, etc., and then receives the radio for many people listening. The following is an example with the Xiahua brand wireless headset (see the accompanying drawing of the transponder) as an example. Briefly introduce its principle for reference.
The transponder circuit consists of an audio input, a high frequency oscillation, an emission antenna, and the like. First of all. The loser signal is obtained from the TV sound output (red or white) from the TV. If you take a TV earphone jack. The "RCA / 3.5MM" converter is required. Subsequent signal enters R2 (R3) -r4 appropriate attenuation and voltage matching. The signals above the R4 are also delivered to the high-frequency oscillation circuit composed of Q1 or the like by the C1, L3-R5-C2, and L2 links. Conditioning / emission: The reason why C1, L3-R5-C2-L2 and other components are added. Its purpose is to isolate high frequency signals. The high-frequency signal to prevent Q1 is threatened.
The Q1 high frequency portion is a three-point high-frequency oscillation circuit for a common-base capacitor. This circuit is easy, high frequency, waveform distortion. The C4 capacitance and other elements are bonded to the C4 capacitance by the Q1 tube C-E pole C8 capacitance and the Q1 tube itself. A positive feedback network can be formed to produce continuous oscillation. In the oscillation operation, the C10-L1 component is fixed frequently, which allows the frequency to stabilize in a certain frequency of 88 MHz to 108 MHz band, so that it can correspond to the FM frequency modulation receiver. and. The oscillating tube Q1 has an audio signal on the B-E pole. Thus, the audio voltage will be modulated to Q1 oscillation, and a strong carrier signal radiation voltage can be formed on the oscillator Q1 C. Thereafter, in order to radiate the signal to the space, the radio frequency emission is completed by the transponder or the low-alignment coil L4 by the Q1 C to etch or the low-sensing coil L4. This type is taken after the antenna is added to the launcher. One is the distribution of L4 energy to rearrange the radiation of radiofield energy in the antenna. Thereby to reduce the length of the antenna; The antenna added can also reduce the adverse effects of the antenna on the reflection of high frequency oscillator. Thereby, the high-frequency oscillation circuit has been strengthened.
This radio frequency signal radiation range of this antenna headset repeater. About 8m. Its working current and the corresponding power consumption are small. Even so, the case where the battery is used for long-term use of the battery can still occur, so it is preferred to use a high quality battery. in addition. Sometimes, the reception noise increases, the signal is weak, which is more common to the placement of the repeater, and the repeater should be kept away from the metal to reduce the attenuation and absorption of radiation signals; Adjust, mostly recover normal reception. If the adjustment is invalid. It is common that the oscillation signal is weak, and it should be enhanced positive feedback.
Author: Wang Su Jun
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