Patent Publication Number: US-2022232319-A1

Title: Audio circuit and mobile terminal provided with audio circuit

Description:
The present application claims priority to Chinese Patent Application No. 201910576182.X, filed with the CNIPA on Jun. 28, 2019, disclosure of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present application relates to the field of audios, and for example, to an audio circuit and a mobile terminal having the audio circuit. 
     BACKGROUND 
     As shown in  FIG. 1 , for the circuit design of the smart phone in the related art, a structure including a main circuit board and a sub-circuit board is adopted, and this structure is also referred to as a main and sub-board circuit structure. An antenna circuit, a microphone audio input circuit, a charging interface and the like are designed on the sub-circuit board, and circuits such as a baseband circuit and a radio frequency circuit are designed on the main circuit board. When a maximum transmitting power is applied to a radio frequency antenna circuit of the sub-circuit board, radio frequency radiation common-mode interference current may flow through a reference ground plane of the sub-circuit board. In the related art, a microphone is generally grounded at a sub-circuit board side to solve the influence caused by the interference current. 
     However, certain impedance Z exists between a reference ground plane SGND of the sub-circuit board and a reference ground plane GND of the main circuit board, the radio frequency radiation common-mode interference current is converted into a differential current of the microphone audio input circuit through the ground plane impedance Z, so that an interference voltage is superposed in an input current of the microphone to form the audio noise, and this noise severely affects the audio quality. 
     SUMMARY 
     The present application provides an audio circuit and a mobile terminal having the audio circuit, which provides an effective scheme for eliminating radio frequency radiation common-mode interference current. 
     An embodiment of the present application provides an audio circuit. The audio circuit includes a main circuit board and a sub-circuit board disposed to be separated from the main circuit board. 
     The main circuit board includes an audio codec chip circuit, a radio frequency circuit and a main circuit board reference ground. The sub-circuit board is provided with a microphone circuit, an antenna circuit, a sub-circuit board reference ground, a decoupling capacitor and a first anti-static device. The audio codec chip circuit is configured to provide a bias voltage for the sub-circuit board through a transmission line in the main circuit board. The antenna circuit is electrically connected to the radio frequency circuit, and the microphone circuit is electrically connected to the audio codec chip circuit. The microphone circuit includes a microphone audio differential positive electrode output pin, a microphone audio differential negative electrode output pin and a microphone bias voltage pin. The microphone audio differential positive electrode output pin and the microphone audio differential negative electrode output pin are both connected to the audio codec chip circuit. The decoupling capacitor and the first anti-static device are connected in parallel and then are connected to the transmission line, and the transmission line is connected to the main circuit board reference ground through the decoupling capacitor and the first anti-static device which are connected in parallel. 
     An embodiment of the present application further provides a mobile terminal including an audio circuit, and the audio circuit is the audio circuit of any one of the embodiments of the present application. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram showing structures of an audio circuit provided in the related art; and 
         FIG. 2  is a schematic diagram showing structures of an audio circuit according to an embodiment of the present application. 
     
    
    
     DETAILED DESCRIPTION 
     The technical schemes in the embodiments of the present application will be described below in conjunction with the drawings in the embodiments of the present application, and the described embodiments are only some, but not all, embodiments of the present application. 
     A certain impedance Z exists between a reference ground SGND of a sub-circuit board and a reference ground GND of a main circuit board when respective reference ground planes of the sub-circuit board and the main circuit board are connected by a connecting circuit to perform a circuit signal connection. Due to the existence of the impedance Z, when a common-mode current generated by radiation of a radio frequency antenna flows through the impedance Z, an interference voltage will be generated. However, when an audio signal of a microphone circuit also uses this reference impedance as loop backflow, a generated interference voltage will be superposed on an audio signal loop, and thus the common ground resistance anti-interference is generated. 
     In order to solve the above technical problems, the technical solutions provided in the embodiments of the present application is described below in conjunction with the attached drawings. 
     Referring to  FIG. 2 , an audio circuit provided in an embodiment includes a main circuit board  1  and a sub-circuit board  2  separately disposed with the main circuit board  1 . Herein, the word of “separately disposed” means that the main circuit board  1  and the sub-circuit board  2  are two independent circuit boards. The main circuit board  1  includes an audio codec chip circuit  11 , a radio frequency circuit  12  and a main circuit board reference ground  13 . The sub-circuit board  2  is provided with a microphone circuit  21 , an antenna circuit  22  and a sub-circuit board reference ground  23 . The audio codec chip circuit  11  provides a bias voltage Micbias for the sub-circuit board  2 . The antenna circuit  22  is electrically connected to the radio frequency circuit  12 , and the microphone circuit  21  is electrically connected to the audio codec chip circuit  11 . The microphone circuit  21  includes a microphone audio differential positive electrode output pin (referred as Mic_P) and a microphone audio differential negative electrode output pin (referred as Mic_N), and the microphone audio differential positive electrode output pin Mic_P and the microphone audio differential negative electrode output pin Mic_N are both connected to the audio codec chip circuit  11 . The sub-circuit board  2  is further provided with a decoupling capacitor  24  and a first anti-static device  25 , and the decoupling capacitor  24  and the first anti-static device  25  are connected in parallel and then connected to a transmission line which provides the bias voltage MicBias for the sub-circuit board  2  in the main circuit board  1 . The transmission line is connected to the main circuit board reference ground  13  through the decoupling capacitor  24  and the first anti-static device  25  which are connected in parallel. 
     The microphone circuit  21  is configured to convert a sound signal into an electrical signal. In an embodiment, a microphone in this audio circuit is a silicon microphone. In order to improve the sound processing quality of the silicon microphone, an anti-interference circuit (not shown in the drawings) is further disposed around the silicon microphone, and the anti-interference circuit may be realized by devices such as a resistor, an inductor and a capacitor. The silicon microphone includes a bias voltage pin VCC and a ground pin GND, the microphone audio differential positive electrode output pin Mic_P and the microphone audio differential negative electrode output pin Mic_N described above are pins of the silicon microphone. The audio codec chip circuit  11  includes an audio codec chip and a peripheral function circuit. The audio codec chip includes a bias voltage output pin MicBias, a microphone positive electrode sampling input pin IM+ of the audio codec chip, and a microphone negative electrode sampling input pin IM− of the audio codec chip. 
     In order to simplify a structure of the audio circuit, the microphone audio differential negative electrode output pin Mic_N is connected to the audio codec chip circuit  11  through a first connecting circuit. The decoupling capacitor  24  and the first anti-static device  25  are connected in parallel and then communicate the transmission line with the main circuit board reference ground  13  through the first connecting circuit. In the embodiments of the present application, the first connecting circuit refers to a path connected between the microphone audio differential negative electrode output pin Mic_N and the audio codec chip input pin IM−, after the decoupling capacitor  24  and the first anti-static device  25  are connected in parallel, one end thereof is connected to a bias voltage MicBias transmission line, and another end thereof is connected to the first connecting circuit. Therefore, the bias voltage is connected to the reference ground GND of the main circuit board by means of the first connecting circuit, a simple differential loop is constructed for bias voltage backflow. In order to facilitate backflowing to flow through the reference ground GND of the main circuit board  2 , a grounding resistor R 2  is selected to be 0 ohm. The first anti-static device  25  may select a varistor or a transient voltage suppressor diode (TVS) (referred as RV) as desired. 
     Since the silicon microphone on the sub-circuit board  2  is easy to be damaged or interfered by static electricity, for this reason, the sub-circuit board  2  described in this embodiment is further provided with a second anti-static device  26 . In addition, the first connecting circuit is further connected to the sub-circuit board reference ground  13  through the second anti-static device  26 . In such way, the electrostatic current may be discharged to a ground plane of the sub-circuit board  2  nearby. Herein, the second anti-static device  26  in this embodiment may also select a varistor R 1  or a TVS as desired. 
     The antenna circuit  22  in this embodiment includes an antenna impedance matching circuit and a radio frequency antenna, and the antenna circuit and the radio frequency circuit are connected through a radio frequency coaxial cable  27 . 
     A flexible printed circuit (FPC) may be applied to different spaces, so that the main circuit board  1  and the sub-circuit board  2  may be connected by selecting the flexible printed circuit (FPC). In this embodiment, the microphone circuit is electrically connected to the audio codec chip circuit through the flexible printed circuit (FPC). 
     On the other hand, the present application provides a mobile terminal including an audio circuit, and the audio circuit is the audio circuit of any one of the embodiments described above. For an intelligent mobile terminal, the radio frequency circuit in this embodiment is a radio frequency amplifier circuit applied to an 850 MHz/900 MHz band in a global system for mobile communications (GSM), so that an antenna on the sub-circuit board  2  is a GSM 850 MHz/900 MHz band antenna matched with the radio frequency circuit. 
     In the embodiments of the present application, a bias voltage transmission line of the sub-circuit board  2  is connected to the main circuit board reference ground  13  near the audio codec chip circuit  11  of the main circuit board  1  through the decoupling capacitor  24  and the first anti-static device  25  which are connected in parallel, so that a bias voltage MicBias loop forms a differential circuit, the interference of a radio frequency radiation common-mode current is inhibited, and thus the audio noise introduced by the bias voltage MicBias loop is eliminated. Furthermore, in this embodiment, after the sub-circuit board reference ground  23  is grounded through the second anti-static device  26 , interference such as static electricity on the sub-circuit board  2  may be effectively prevented, introduction of a large-area sub-circuit board reference ground  23  is avoided, the occupied space of the sub-circuit board  2  is reduced, and meanwhile the cost is saved.