8 Rules For Reducing Parasitic Signals On Rf-pcb Circuit Layout

- Sep 15, 2018-

All the currents flowing through the wiring are backflow equal.There are many coupling strategies, of course, but reflux usually flows through adjacent grounding or grounding parallel to the signal line.As the reference layer continues, all coupling is limited to the transmission line, and everything is fine.However, if the signal line switches from the top to the inside or bottom, the backflow must also obtain a path. 

Rule 2: connect the device pad to the top level ground

Many devices use thermal grounding pads at the bottom of the package.On RF devices, these are usually electrically grounded, while the adjacent solder pad has an array of grounded through-holes.The device solder pad can be connected directly to the ground pin and connected to any pouring copper through the top grounding.If there are multiple paths, reflux will be split in proportion to path impedance.The path is shorter and the impedance is lower than that of the pin grounding through the pad. 

A good electrical connection between the circuit board and the device pad is essential.During assembly, unfilled holes in the PCB array may also remove the solder paste of the device, leaving gaps.Filling the through hole is a good way to ensure the welding is in place.In the evaluation, the solder mask layer must be opened to verify that no solder mask is on the circuit board ground below the device, as the solder mask may raise or sway the device. 

Rule 3: no reference interval

The device is surrounded by through-holes.Power network decomposition costs are decoupled and then dropped to the power layer, usually providing multiple through-holes to minimize inductance and increase current-carrying capacity while the control bus can be reduced to the inner layer.All this decomposition will eventually be completely pinched around the device. 

Rule 4: maintain the difference of the differential circuit

Backflow path is very important to signal line performance and should be considered as part of signal path.At the same time, the difference pairs are usually not tightly coupled, and reflux can flow through adjacent layers.Both backflow must be routed through equal electrical paths. 

Rule 5: there is no clock or control line near the RF signal line

The clock and control circuit can sometimes be regarded as an unobtrusive neighbor because of its low speed and even proximity to DC.However, its switching characteristics are almost close to square waves and can generate unique tones at odd harmonic frequencies.The fundamental frequency of the energy emitted by the square wave does not make much difference, but its sharp edges may.In the design of a digital system, the breaking frequency can be estimated by taking into account the highest frequency harmonics: Fknee=0.5/Tr, where Tr is the rising time.Note that it is the rise time, not the signal frequency.However, square waves with sharp edges also have powerful higher-order odd harmonics that may only fall at the wrong frequency and couple to RF circuits, violating strict transmission mask requirements. 

Rule 6: use grounding to isolate high-speed lines

Most of the microwave transmission belts and strip lines are coupled to adjacent strata.Some flux lines are still distributed along the horizontal direction and end to the adjacent track line.The tone on a high-speed line or difference pair terminates on the next trace line, but the grounding irrigation on the signal layer brings a lower impedance endpoint to the flux line, leaving the neighboring trace line free of tone interference. 

Rule 7: do not use RF wiring in a noisy power source

As the tone enters the power layer, it spreads everywhere.If stray tones enter the power supply, buffer, mixer, attenuator, and oscillator, the interference frequency is modulated.Similarly, when the power reaches the circuit board, it is not completely cleared to drive the RF circuit system.The exposure of RF lines to the power layer should be minimized, especially the unfiltered power layer. 

Rule 8: let the decoupling approach the device

Decoupling not only helps prevent stray noise from entering the device, but also helps eliminate the tones generated within the device and avoid coupling to the power layer.The closer the decoupling capacitance is to the working circuit system, the higher the efficiency.Local decoupling is less interfered by the parasitic impedance of the circuit board trace, and the shorter trace supports the smaller antenna, reducing harmful tone emission.Capacitors should be placed in combination with the highest self-resonance frequency, usually the minimum value, the minimum enclosure size, the closest device, and the larger the capacitor, the further away from the device.Under the RF frequency, the capacitors on the back of the circuit board will generate parasitic inductance in the path of connecting the through-hole to the ground, which will lose a lot of advantages of noise attenuation.