First, the expected bandwidth of the control line may range from DC to 2 MHz, and it is almost impossible to remove such wide-band noise by filtering; secondly, VCO control The line is usually part of a feedback loop that controls the frequency, and it can introduce noise in many places, so the VCO control line must be handled very carefully. Make sure that the ground underneath the RF trace is solid and that all components are firmly connected to the main ground and isolated from other traces that may cause noise. In addition, to ensure that the VCO's power supply is fully decoupled, since the VCO's RF output tends to be a relatively high level, the VCO output signal can easily interfere with other circuits, so special attention must be paid to the VCO. In fact, the VCO is often placed at the end of the RF area, and sometimes it requires a metal shield. Resonant circuits (one for the transmitter and one for the receiver) are related to the VCO, but they also have their own characteristics. Simply put, the resonant circuit is a parallel resonant circuit with a capacitive diode that helps set the VCO operating frequency and modulate the speech or data onto the RF signal. The design principles of all VCOs also apply to resonant circuits. Resonant circuits are typically very sensitive to noise because they contain a significant number of components, a wide distribution area on the board, and typically operate at a very high RF frequency. The signals are usually placed on adjacent pins of the chip, but these signal pins need to work with relatively large inductors and capacitors to work, which in turn requires that these inductors and capacitors must be placed close together and connected back. A control loop that is sensitive to noise. It is not easy to do this.
An automatic gain control (AGC) amplifier is also a problem that is prone to problems, and both the transmit and receive circuits have an AGC amplifier. AGC amplifiers typically filter out noise effectively, but because handsets have the ability to handle the rapid changes in transmit and receive signal strength, AGC circuits are required to have a fairly wide bandwidth, which makes it easy to introduce AGC amplifiers on critical circuits. noise. Designing AGC lines must follow good analog circuit design techniques, which are associated with very short op amp input pins and very short feedback paths, both of which must be kept away from RF, IF, or high-speed digital signal traces. Also, good grounding is essential and the chip's power supply must be well decoupled. If it is necessary to take a long line at the input or output, then it is better at the output, usually the impedance of the output is much lower, and it is not easy to induce noise. Generally, the higher the signal level, the easier it is to introduce noise into other circuits. In all PCB designs, it is a general principle to keep digital circuits as far away as possible from analog circuits. It is also applicable to RF PCB designs. Common analog grounds and grounds used to shield and separate signal lines are usually equally important, so careful planning, careful component placement, and thorough layout are important in the early stages of design. The line is far from the analog line and some critical digital signals. All RF traces, pads, and components should be filled with as much copper as possible and connected to the main ground as much as possible. If the RF traces must pass through the signal lines, try to lay a layer of ground connected to the main ground along the RF traces between them. If this is not possible, make sure they are crisscrossed, which minimizes capacitive coupling while spreading as much ground as possible around each RF trace and connecting them to the main ground. In addition, minimizing the distance between parallel RF traces minimizes inductive coupling. A solid monolithic ground plane is best placed directly on the first layer below the surface, although other practices are useful when designing with care. On each layer of the PCB, place as much ground as possible and connect them to the main ground. Try to keep the traces together to increase the number of plots in the internal signal layer and power distribution layer, and adjust the traces so that you can connect the ground vias to the isolated plots on the surface. Free ground should be avoided on each layer of the PCB because they pick up or inject noise like a small antenna. In most cases, if you can't connect them to the main land, then you'd better remove them.