Radio frequency power circuit and electronic device

Abstract

The utility model discloses a radio frequency power circuit and electronic equipment, radio frequency power circuit is integrated on the circuit board, connects an antenna, and it includes load resistance, microstrip line coupler, intermediate frequency receiver, amplification filter module and attenuation switch module, microstrip line coupler connects amplification filter module, attenuation switch module, load resistance and antenna, intermediate frequency receiver connects amplification filter module and attenuation switch module, the small signal of intermediate frequency receiver output passes through amplification filter module amplification and filter, and then is output to the antenna emission after passing through microstrip line coupler coupling, microstrip line coupler will monitor the power coupling signal of getting and attenuate after feedback to intermediate frequency receiver through attenuation switch module, load resistance is used for adjusting the directivity of microstrip line coupler, adopts microstrip line coupler to replace the existing coupler component, has saved the purchase cost and production patch fee of existing power coupler component, has solved the problem of high cost of existing power coupler.

Description

Technical Field

[0001] This utility model relates to the field of communication technology, and in particular to a radio frequency power circuit and electronic device. Background Technology

[0002] Currently, 5G communication systems pursue ultra-high data rates, ultra-low latency, and massive connectivity, which relies on complex radio frequency (RF) front-end architectures. Multi-band (Sub-6GHz and millimeter wave), massive MIMO (Multiple-Input Multiple-Output) antenna technology, and beamforming have become standard features, making precise RF power management and signal monitoring crucial. Power couplers, as key passive devices in the RF link, are mainly used for real-time, non-destructive sampling of transmitted signal power, and their application context directly stems from the stringent requirements of 5G for high-performance RF systems.

[0003] The core requirements for power couplers are: First, power monitoring and control to provide feedback to the power amplifier (PA), enabling precise output power control, ensuring regulatory compliance, and optimizing energy efficiency. Second, miniaturization and integration are crucial, given the extremely limited space in mobile phones, requiring highly miniaturized power couplers that can be seamlessly integrated into compact RF modules or packages. Third, low insertion loss is essential to minimize attenuation in the main signal path, ensuring link budget and signal strength. Fourth, wide bandwidth and high-frequency performance are necessary to cover a wide range of 5G frequency bands, especially maintaining stable performance in the millimeter-wave band. Fifth, high directivity and isolation are vital for accurately sampling forward power, suppressing reflected signal interference, and ensuring monitoring accuracy. Sixth, cost-effectiveness must be considered; as a core component of consumer electronics, cost must be strictly controlled while meeting performance requirements.

[0004] Therefore, high-performance, miniaturized, low-loss, and wide-bandwidth power couplers are one of the key fundamental components supporting 5G mobile phones in achieving their design goals. However, power couplers that meet these requirements are very expensive. Due to their small size, some are machine-mounted, but this increases the difficulty of mounting and thus increases production and mounting costs.

[0005] Therefore, the existing technology still needs to be improved and enhanced. Utility Model Content

[0006] In view of the shortcomings of the prior art, the purpose of this utility model is to provide an RF power circuit and electronic device to solve the problem of high cost of existing power coupler devices and high production and mounting costs.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: An RF power circuit, integrated on a circuit board and connected to an antenna, includes a load resistor, a microstrip line coupler, an intermediate frequency (IF) receiver, an amplification and filtering module, and an attenuation switch module; the microstrip line coupler connects the amplification and filtering module, the attenuation switch module, the load resistor, and the antenna; the IF receiver connects the amplification and filtering module and the attenuation switch module. The small signal output by the intermediate frequency receiver is amplified and filtered by the amplification and filtering module, and then output to the antenna for transmission after passing through the microstrip line coupler. The microstrip line coupler attenuates the monitored power coupling signal through an attenuation switch module and feeds it back to the intermediate frequency receiver. The load resistor is used to adjust the directivity of the microstrip line coupler.

[0008] In the aforementioned RF power circuit, the microstrip line coupler includes a first microstrip line routed on the surface layer of the circuit board and a second microstrip line routed on the inner layer of the circuit board; one end of the first microstrip line is a first port of the microstrip line coupler, connected to an amplification and filtering module; the other end of the first microstrip line is a second port of the microstrip line coupler, connected to an antenna; one end of the second microstrip line is a third port of the microstrip line coupler, connected to an attenuation switch module; the other end of the second microstrip line is a fourth port of the microstrip line coupler, grounded through a load resistor.

[0009] In the aforementioned radio frequency power circuit, the two ends of the first microstrip line and the second microstrip line are separated, and the bends between the ports overlap and couple vertically.

[0010] In the aforementioned radio frequency power circuit, the bent portion is convex or S-shaped.

[0011] In the aforementioned RF power circuit, the first microstrip line and the second microstrip line have the same trace width.

[0012] In the aforementioned RF power circuit, the amplification and filtering module includes a power amplifier and a filter. The input pin of the power amplifier is connected to the transmit pin of the intermediate frequency receiver, the output pin of the power amplifier is connected to the input pin of the filter, and the output pin of the filter is connected to the first port of the microstrip line coupler.

[0013] In the aforementioned radio frequency power circuit, the attenuation switch module includes a switch, a first resistor, a second resistor, and a third resistor; The first transmission pin of the switch is connected to one end of the first resistor and one end of the third resistor. The other end of the first resistor is connected to one end of the second resistor and the receiving pin of the intermediate frequency receiver. The other ends of the second resistor and the third resistor are both grounded. The second transmission pin of the switch inputs the RF_TXDET0_LMH signal. The third transmission pin of the switch is connected to the third port of the microstrip line coupler. The control pin of the switch inputs the control signal.

[0014] In the aforementioned radio frequency power circuit, the amplification and filtering module further includes a first inductor, a first capacitor, and a second capacitor; One end of the first inductor is connected to the output pin of the filter, and the other end of the first inductor is connected to one end of the first capacitor and one end of the second capacitor. The other end of the first capacitor is grounded, and the other end of the second capacitor is connected to the first port of the microstrip line coupler.

[0015] In the aforementioned RF power circuit, the amplification and filtering module further includes a third capacitor, which is connected between the third port of the microstrip line coupler and the third transmission pin of the switch.

[0016] An electronic device includes a circuit board and an antenna ANT, wherein a radio frequency power circuit is integrated on the circuit board and the radio frequency power circuit is connected to the antenna ANT; The radio frequency power circuit amplifies and filters the small signal to be transmitted, transmits it to the antenna after passing through a microstrip line coupler, and also monitors the radio frequency power to obtain a power coupling signal, which is then attenuated and fed back to the MRX pin of the intermediate frequency receiver.

[0017] Compared to existing technologies, the RF power circuit and electronic device provided by this utility model integrate the RF power circuit on a circuit board and connects to an antenna. It includes a load resistor, a microstrip line coupler, an intermediate frequency (IF) receiver, an amplification and filtering module, and an attenuation switch module. The microstrip line coupler connects the amplification and filtering module, the attenuation switch module, the load resistor, and the antenna. The IF receiver connects the amplification and filtering module and the attenuation switch module. The small signal output by the IF receiver is amplified and filtered by the amplification and filtering module, then passes through the microstrip line coupler before being output to the antenna for transmission. The microstrip line coupler attenuates the monitored power coupling signal through the attenuation switch module and feeds it back to the IF receiver. The load resistor is used to adjust the directivity of the microstrip line coupler. By using a microstrip line coupler instead of existing coupler components, the procurement cost and manufacturing / mounting costs of existing power coupler components are eliminated, solving the problem of high cost of existing power couplers. Attached Figure Description

[0018] Figure 1 This is a circuit diagram of the radio frequency power circuit provided by this utility model.

[0019] Figure 2 This is a schematic diagram of the microstrip line coupler provided by this utility model.

[0020] Figure 3 This is the application circuit diagram of the amplification and filtering module provided by this utility model. Detailed Implementation

[0021] This utility model provides a radio frequency power circuit and electronic device. To make the objectives, technical solutions, and advantages of this utility model clearer and more explicit, the following detailed description, with reference to the accompanying drawings and embodiments, further illustrates the utility model. It should be understood that the specific embodiments described herein are merely illustrative and are not intended to limit the scope of this utility model.

[0022] Please see Figure 1 The electronic device provided by this utility model includes a circuit board (i.e., a PCB board, printed circuit board) and an antenna ANT. The circuit board integrates a radio frequency (RF) power circuit, which is connected to the antenna ANT. The RF power circuit amplifies and filters the small signal to be transmitted, transmits it to the antenna ANT for transmission after passing through a microstrip line coupler, and also monitors the RF power to obtain a power coupling signal, attenuates it, and then feeds it back.

[0023] In this embodiment, the RF power circuit includes a load resistor R0, a microstrip line coupler 11, an intermediate frequency (IF) receiver 12, an amplification and filtering module 13, and an attenuation switch module 14. The microstrip line coupler 11 is connected to the amplification and filtering module 13, the attenuation switch module 14, the load resistor R0, and the antenna ANT. The IF receiver 12 is connected to the amplification and filtering module 13 and the attenuation switch module 14. The small signal output by the IF receiver 12 is amplified and filtered by the amplification and filtering module 13, coupled by the microstrip line coupler 11, and then output to the antenna ANT for transmission. The microstrip line coupler 11 attenuates the monitored power coupling signal through the attenuation switch module 14 and feeds it back to the IF receiver. The load resistor R0 is used to adjust the directivity of the microstrip line coupler 11.

[0024] By replacing existing coupler components with microstrip line couplers, the procurement and manufacturing costs of existing power coupler components are eliminated, improving system integration and reliability. While reducing costs, it stably and reliably meets the real-time power monitoring and tracking requirements of RF systems, saves terminal costs, and meets the requirements of miniaturization design.

[0025] Please refer to the following: Figure 2 and Figure 3The microstrip line coupler 11 includes a first microstrip line 15 routed on the surface layer (L1 layer) of the circuit board and a second microstrip line 16 routed on the inner layer (L2 layer) of the circuit board. One end of the first microstrip line 15 is the first port 1 (i.e., the main signal input port) of the microstrip line coupler 11, which is connected to the amplification and filtering module 13. The other end of the first microstrip line 15 is the second port 2 (i.e., the main signal output port) of the microstrip line coupler 11, which is connected to the antenna ANT. One end of the second microstrip line 16 is the third port 3 (i.e., the coupled signal output port) of the microstrip line coupler 11, which is connected to the attenuation switch module 14. The other end of the second microstrip line 16 is the fourth port 4 (i.e., the load port) of the microstrip line coupler 11, which is grounded through the load resistor R0.

[0026] The microstrip line coupler is in the form of a microstrip line, which is a set of microstrip lines drawn directly on different layers of the PCB board, with the two microstrip lines overlapping and tightly coupled. Figure 2 To illustrate the shape of the two microstrip lines, they are drawn side-by-side in a staggered manner. In reality, the two microstrip lines are drawn on two separate layers, except that the two ends of the first microstrip line 15 (corresponding to the first port 1 and the second port 2) and the two ends of the second microstrip line 16 (corresponding to the third port 3 and the fourth port 4) are separated, and there are bends between the ports ( Figure 2 The parts that are close together are tightly overlapped; thus, the upper and lower layers are tightly coupled to form a microstrip line coupler.

[0027] In principle, the connection from port 1 to port 2 and from port 3 to port 4 is a direct wire connection. The first microstrip line 15 from port 1 to port 2 is the main signal channel of the microstrip line coupler 11, with the signal flowing in from port 1 and out from port 2. The second microstrip line 16 from port 3 to port 4 is the coupling signal channel of the microstrip line coupler 11. Through the electromagnetic field interaction of the two parallel microstrip lines, directional coupling, distribution, and isolation of signals are achieved. The small signal to be transmitted output by the intermediate frequency receiver is amplified and filtered by the amplification and filtering module 13 and then output to the main signal channel of the microstrip line coupler 11, and finally output to the antenna for transmission. The power coupling signal obtained by the microstrip line coupler 11 from the RF output power monitoring is output from port 3 to the attenuation switch module 14 for attenuation and then fed back to the intermediate frequency receiver. This constructs a closed-loop system to achieve dynamic tracking and closed-loop control of the PA's transmit power.

[0028] To increase the coupling area and thus the coupling degree of the microstrip line coupler 11, this embodiment draws multiple bends between the two ports, resulting in a "convex" shape in the tightly overlapping portion of the upper and lower layers (e.g., ...). Figure 2As shown in the diagram, it can be drawn as an S-shape. In this way, within a finite area, the overlapping coupling parts adopt a "convex" shape, achieving stronger coupling; tests show that the coupling reaches 20dB in the 2.4GHz~2.7GHz frequency band. Within the limited area, it can also be drawn as a pulse waveform or a wave shape to increase the coupling area. It's important to understand that the corners at bends in the circuit layout are blunted and are not 90-degree right angles.

[0029] The two microstrip lines have the same trace width on the upper and lower layers. In practice, adjusting the trace width can adjust the capacitance characteristic of the microstrip line coupler 11, and adjusting the trace length can adjust the inductive characteristic. The trace length and width of the microstrip line coupler are adjusted according to the frequency of the power signal to be monitored. The width and length of the pattern are adjusted through simulation using ADS (Advanced Design System) to maintain the optimal balance between the capacitive and inductive characteristics of the coupler. For example, the operating bandwidth of the microstrip line coupler 11 can be up to 2GHz, applicable from 660M to 2700MHz, and is not limited to the single-frequency design shown in the example. The position of the microstrip line coupler can also be flexibly adjusted. For multi-band power monitoring requirements, a microstrip line coupler can be connected in series at the common terminal of the transmit path TX.

[0030] The load resistor R0 is used to further optimize the directivity of the microstrip line coupler 11, and its resistance range is 55~40 ohms. In this embodiment, it is set to 44 ohms, which is the optimal load resistor value obtained after ADS simulation in the 2.496~2.690GHz (5G NR network N41 band) application, where the directivity is optimal.

[0031] The intermediate frequency receiver 12 is preferably an MT6197. The small signal output from its transmit pin (TX0 pin) is amplified and filtered by the amplification and filtering module 13 and then transmitted to the first port 1 of the microstrip line coupler 11. The output from the second port 2 of the microstrip line coupler 11 is transmitted to the antenna ANT. The power coupling signal output from the microstrip line coupler 11 is transmitted to the receive pin (MRX pin) of the intermediate frequency receiver 12 through the attenuation switch module 14.

[0032] Please refer to the following: Figure 3 The amplification and filtering module 13 includes a power amplifier U1 and a filter SW. The power supply pin (VDD pin) of the power amplifier U1 is connected to the VCC voltage. The input pin (TX_IN pin) of the power amplifier U1 is connected to the transmit pin (TX0 pin) of the intermediate frequency receiver 12. The output pin (TX_OUT pin) of the power amplifier U1 is connected to the input pin (IN pin) of the filter SW. The output pin (OUT pin) of the filter SW is connected to the first port 1 of the microstrip line coupler 11. The ground pin (GND pin) of the power amplifier U1 is grounded.

[0033] The power amplifier U1 is preferably an FX5627YD RF power amplifier, and the filter SW is preferably an F6FC2G600H4PA. The power amplifier U1 amplifies the small signal output from the intermediate frequency receiver 12, and then outputs it after filtering by the filter SW and microstrip line coupler 11 before being transmitted from the antenna ANT.

[0034] The attenuation switch module 14 includes a switch SP (SPDT switch), a first resistor R1, a second resistor R2, and a third resistor R3. The first transmission pin RF1 of the switch SP is connected to one end of the first resistor R1 and one end of the third resistor R3. The other end of the first resistor R1 is connected to one end of the second resistor R2 and the receiving pin (MRX pin) of the intermediate frequency receiver 12. The other ends of the second resistor R2 and the third resistor R3 are both grounded. The second transmission pin RF2 of the switch SP receives the RF_TXDET0_LMH signal (the second coupling signal, which can be the first...). The output of port 3 of the two microstrip line couplers can also be the output of port 3 of a passive power device coupler. When there are multiple couplers in a 5G mobile phone, the data is aggregated through a switch and transmitted to the intermediate frequency receiver. The third transmission pin RF3 of the switch SP is connected to port 3 of the microstrip line coupler 11. The control pin VC1 (also called the control voltage port) of the switch SP receives the control signal CTL_BPI_BUS0 (the BPI control signal output by the intermediate frequency receiver MT6197). The power supply pin VDD of the switch SP receives the VDD voltage, and the ground pin of the switch SP is grounded.

[0035] In this configuration, the first resistor R1, the second resistor R2, and the third resistor R3 form a π-type attenuator. The attenuation value can be flexibly adjusted to prevent the output power of the microstrip line coupler 11 from being too high and exceeding the operating range of the intermediate frequency receiver 12. In this embodiment, the resistance values ​​of the first resistor R1, the second resistor R2, and the third resistor are set to 37.4Ω, 150Ω, and 150Ω respectively, forming a 6dB attenuator.

[0036] Taking dual-channel power monitoring as an example, the switch SP can preferably be a single-pole double-throw (SPDT) switch of model RF1630. When the B41 / N41 frequency band is operating, the intermediate frequency receiver (MT6197) outputs a high-level control signal CTL_BPI_BUS0, connecting the first transmission pin RF1 of the switch SP to the RF3 port of the SP; when other frequency bands besides B41 / N41 are operating, the intermediate frequency receiver (MT6197) outputs a low-level control signal CTL_BPI_BUS0, connecting the first transmission pin RF1 of the switch SP to the RF2 port of the SP. In this way, through the SPDT switch stage combined circuit, the power coupling signal output from the third port 3 of the microstrip line coupler 11 is selected by the switch, attenuated by three resistors in a π-type manner, and then transmitted to the MRX pin of the intermediate frequency receiver 12.

[0037] In practical implementation, the operating frequency range of the single-pole double-throw (SPDT) switch is primarily determined by its suitability for the frequency requirements of the power monitoring signals in all coupled channels. For multiple power coupled channels, switches can be cascaded: for example, a double-pole SPDT switch can be used for two coupled channels, a triple-pole SP3T switch for three coupled channels, and so on; no limitation is imposed on the specific frequency range.

[0038] Preferably, such as Figure 3 As shown, the amplification and filtering module 13 further includes a first inductor L1, a first capacitor C1, and a second capacitor C2. One end of the first inductor L1 is connected to the output pin (OUT pin) of the filter SW, and the other end of the first inductor L1 is connected to one end of the first capacitor C1 and one end of the second capacitor C2. The other end of the first capacitor C1 is grounded, and the other end of the second capacitor C2 is connected to the first port 1 of the microstrip line coupler 11. The first inductor L1, the first capacitor C1, and the second capacitor C2 are used to adjust the impedance matching between the output pin of the filter SW and the input terminal of the microstrip line coupler 11: There must be a section of RF impedance line from the output pin of the filter SW to the input terminal (first port 1) of the microstrip line coupler. Its impedance is ideally 50 ohms on the PCB design drawing. Due to factors such as factory materials and manufacturing processes, there will inevitably be more or less impedance deviation. The actual impedance of this impedance line can be adjusted to match 50 ohms by adjusting the values ​​of L1 / C1 / C2, so as to achieve optimal signal transmission.

[0039] Preferably, the amplification and filtering module 13 further includes a third capacitor C3, which is connected between the third port 3 of the microstrip line coupler 11 and the third transmission pin RF3 of the switch SP. The third capacitor C3 is used to isolate the DC component.

[0040] Preferably, the amplification and filtering module 13 further includes a second inductor L2 and a fourth capacitor C4. One end of the second inductor L2 is connected to one end of the fourth capacitor C4 and the second port 2 of the microstrip line coupler 11, and the other end of the second inductor L2 is connected to the antenna ANT. The other end of the fourth capacitor C4 is grounded. The second inductor L2 and the fourth capacitor C4 are used to adjust the actual impedance of the impedance line between the output port (second port 2) of the microstrip line coupler 11 and the antenna port, so that it is matched to around 50 ohms, thereby achieving optimal transmission of radio frequency signals.

[0041] Preferably, a capacitor for filtering can be connected to ground on the power supply pin VDD and control pin VC1 of the switch SP, respectively.

[0042] In summary, the RF power circuit and electronic equipment provided by this utility model replaces the existing power coupler components by designing two microstrip lines on the circuit board (PCB), thus eliminating the material procurement cost and assembly cost of the coupler. While reducing costs, it realizes real-time monitoring of the RF output power of the power amplifier (PA) transmit path. The monitored power coupling signal is transmitted to the intermediate frequency receiver, thereby constructing a closed-loop system to achieve dynamic tracking and closed-loop control of the PA transmit power.

[0043] The two microstrip lines are directly implemented through PCB etching, saving components and costs while reducing solder joints in the RF link and improving reliability. The microstrip line structure can be integrated with other circuits on the same substrate, significantly saving layout space and increasing design flexibility and customizability.

[0044] By adjusting the width and length of the microstrip line, parameters such as coupling (e.g., -23 to -28 dB) and directivity can be precisely controlled to adapt to different frequency bands (mainly Sub-6 GHz). It also supports ADS simulation for rapid iterative optimization. This RF power circuit design can be used in lightweight applications for power monitoring and tracking in low-cost, small-size Sub-6 GHz terminals (such as IoT devices and mid-to-low-end mobile phones).

[0045] It should be understood that the application of this utility model is not limited to the examples above. Those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. A radio frequency power circuit, integrated on a circuit board and connected to an antenna, characterized in that, It includes a load resistor, a microstrip line coupler, an intermediate frequency receiver, an amplification and filtering module, and an attenuation switch module; the microstrip line coupler connects the amplification and filtering module, the attenuation switch module, the load resistor, and the antenna; The intermediate frequency receiver is connected to an amplification and filtering module and an attenuation switch module; The small signal output by the intermediate frequency receiver is amplified and filtered by the amplification and filtering module, and then output to the antenna for transmission via a microstrip line coupler. The microstrip line coupler attenuates the monitored power coupling signal through an attenuation switch module and feeds it back to the intermediate frequency receiver. The load resistor is used to adjust the directivity of the microstrip line coupler.

2. The radio frequency power circuit according to claim 1, characterized in that, The microstrip line coupler includes a first microstrip line routed on the surface layer of the circuit board and a second microstrip line routed on the inner layer of the circuit board; one end of the first microstrip line is a first port of the microstrip line coupler, which is connected to an amplification and filtering module; the other end of the first microstrip line is a second port of the microstrip line coupler, which is connected to an antenna. One end of the second microstrip line is the third port of the microstrip line coupler, which is connected to the attenuation switch module; The other end of the second microstrip line is the fourth port of the microstrip line coupler, which is grounded through the load resistor.

3. The radio frequency power circuit according to claim 2, characterized in that, The two ends of the first and second microstrip lines are separated, and the bends between the ports overlap and couple vertically.

4. The radio frequency power circuit according to claim 3, characterized in that, The bent portion is convex or S-shaped.

5. The radio frequency power circuit according to claim 4, characterized in that, The first microstrip line and the second microstrip line have the same trace width.

6. The radio frequency power circuit according to claim 2 or 5, characterized in that, The amplification and filtering module includes a power amplifier and a filter. The input pin of the power amplifier is connected to the transmit pin of the intermediate frequency receiver, the output pin of the power amplifier is connected to the input pin of the filter, and the output pin of the filter is connected to the first port of the microstrip line coupler.

7. The radio frequency power circuit according to claim 2, characterized in that, The attenuation switch module includes a switch, a first resistor, a second resistor, and a third resistor; The first transmission pin of the switch is connected to one end of the first resistor and one end of the third resistor. The other end of the first resistor is connected to one end of the second resistor and the receiving pin of the intermediate frequency receiver. The other ends of the second resistor and the third resistor are both grounded. The second transmission pin of the switch inputs the RF_TXDET0_LMH signal. The third transmission pin of the switch is connected to the third port of the microstrip line coupler. The control pin of the switch inputs the control signal.

8. The radio frequency power circuit according to claim 6, characterized in that, The amplification and filtering module also includes a first inductor, a first capacitor, and a second capacitor; One end of the first inductor is connected to the output pin of the filter, and the other end of the first inductor is connected to one end of the first capacitor and one end of the second capacitor. The other end of the first capacitor is grounded, and the other end of the second capacitor is connected to the first port of the microstrip line coupler.

9. The radio frequency power circuit according to claim 6, characterized in that, The amplification and filtering module also includes a third capacitor, which is connected between the third port of the microstrip line coupler and the third transmission pin of the switch.

10. An electronic device comprising a circuit board and an antenna ANT, characterized in that, The circuit board integrates an RF power circuit, which is connected to the antenna ANT. The radio frequency power circuit amplifies and filters the small signal to be transmitted, transmits it to the antenna after passing through a microstrip line coupler, and also monitors the radio frequency power to obtain a power coupling signal, which is then attenuated and fed back to the MRX pin of the intermediate frequency receiver.

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