They can be integrated on a small PCB board and are used in wireless digital audio, digital video data transmission systems, wireless remote control and telemetry systems, wireless data acquisition systems, wireless networks, wireless security systems, and many other fields.
Potential conflicts between digital circuits and analog circuits arise when the analog circuit (RF) and the digital circuit (microcontroller) operate separately, they may function properly. However, when placed on the same board and powered from the same supply, the entire system can become unstable. This instability is primarily due to the digital signal’s rapid switching between ground and the positive power supply (typically 3V), occurring in nanoseconds. These digital signals have significant high-frequency components independent of their switching frequency. In contrast, analog signals in the RF section, such as those transmitted from the antenna tuning loop to the wireless device’s receiver, are typically less than 1μV. This creates a signal level difference between digital and RF signals of approximately 10-6 (120 dB).
Clearly, without adequate separation between digital and RF signals, the weak RF signals can be corrupted, leading to degraded performance or complete failure of the wireless device.
2. Common problems with RF circuits and digital circuits on the same PCB
Insufficient isolation of sensitive and noisy signal lines is a common issue. As mentioned earlier, digital signals exhibit high swings and contain numerous high-frequency harmonics. If digital signal traces on a PCB run adjacent to sensitive analog signals, these high-frequency harmonics may induce unwanted coupling. Sensitive nodes in RF devices typically include the phase-locked loop (PLL) filter circuit, external voltage-controlled oscillator (VCO) inductors, crystal reference signals, and antenna terminals. These sections of the circuit require meticulous handling.
(1) Power supply noise
Since input/output signals swing several volts, digital circuits generally tolerate power supply noise well (less than 50 mV). Conversely, analog circuits are highly susceptible to power supply noise, especially glitch voltages and high-frequency harmonics. Therefore, when designing PCBs containing RF or other analog circuits, power line routing demands greater attention than standard digital boards, avoiding automated routing. It’s critical to note that modern microcontrollers, designed on CMOS processes, draw substantial current spikes at each internal clock cycle. For instance, at a 1 MHz internal clock frequency, the microcontroller’s periodic current spikes can cause voltage glitches on the power supply line if proper supply decoupling is lacking. Should these glitches reach the power pins of the RF circuitry, operational failures may result. Thus, it’s imperative to ensure separation of analog power lines from the digital circuitry area.
(2) Unreasonable ground layout
RF circuit boards must always incorporate a ground plane connected to the negative side of the power supply. Improper handling can lead to anomalous behavior. This concept may challenge digital circuit designers accustomed to functional digital circuits without extensive grounding. In the RF frequency band, even short traces act as inductors; approximately 1 nH per mm length translates to about 27 Ω for a 10 mm PCB trace at 434 MHz. Without a proper ground plane, extended ground paths fail to ensure desired circuit characteristics.
(3) Antenna radiation affecting other analog components
This issue often goes unnoticed in mixed circuits. Apart from the RF section, PCBs often host additional analog circuits. For instance, many microcontrollers feature built-in analog-to-digital converters (ADCs) for measuring parameters like battery voltage. Proximity of the RF transmitter’s antenna to such components can allow high-frequency emissions to reach ADC analog inputs. Any circuit trace can act like an antenna, potentially causing RF signals to inadvertently self-excite in the ADC’s ESD diode, leading to operational instability.
3. Solution A with RF circuit and digital circuit on the same PCB
Outlined below are some general design and routing strategies applicable to most RF applications. However, adhering to specific RF device routing guidelines remains paramount in practical applications.
(1) Establish a reliable ground plane
When designing PCBs with RF components, a dependable ground plane is indispensable. Its primary function is to establish an effective 0 V reference point across the circuit, facilitating straightforward decoupling of all devices. The power supply’s 0 V terminal should directly link to this ground plane. Due to its low impedance, the ground plane prevents signal coupling between decoupled nodes—an essential consideration given potential signal amplitudes differing by up to 120 dB. On surface-mount PCBs, signal traces reside on one side while the ground plane occupies the opposite. Ideally, the ground plane should blanket the entire PCB (except beneath the antenna). In multi-layer PCBs, the ground plane typically sits adjacent to signal layers. Additionally, unused sections of signal layers should be filled with ground planes connected to the main ground plane via multiple vias. It’s crucial to carefully consider inductance characteristics when positioning and selecting inductance values.
(2) Minimize ground connection lengths
Ground connections to the ground plane must be as brief as possible, with ground vias ideally placed directly at or very near component pads. Avoid sharing ground vias between two signals to prevent potential crosstalk stemming from via impedance.
(3) Implement RF decoupling
Deploy decoupling capacitors in close proximity to pins requiring decoupling. Capacitors should feature high-quality ceramics with “NPO” or “X7R” dielectrics, suitable for most applications. Select capacitor values ensuring their series resonance matches the signal frequency. For instance, a 100 pF capacitor mounted at 434 MHz yields approximately 4 Ω capacitive reactance, akin to via inductive reactance. Series configuration of capacitors and vias forms a notch filter specific to the signal frequency, achieving effective decoupling. At 868 MHz, opt for a 33 pF capacitor. Additionally, include large-value capacitors (e.g., 2.2 μF ceramic or 10 μF tantalum) on power lines for low-frequency decoupling.
(4) Implement star wiring for power supply
Star wiring, a proven analog circuit design technique, ensures each module on the PCB connects directly to a common power supply point. For digital and RF sections, independent power supply lines should undergo separate decoupling in proximity to ICs. This approach mitigates power supply noise from affecting RF components. Modules prone to noise, like RS 232 drivers or switching regulators, may benefit from series inclusion of inductors or resistors (e.g., 10 Ω) in the power line, supplemented by 10 μF tantalum capacitors for effective decoupling.
(5) Optimize PCB layout
Careful arrangement of circuit modules on the PCB minimizes noise module interference with surrounding analog components. Distance-sensitive modules (e.g., RF sections and antennas) should be segregated from noisy counterparts (e.g., microcontrollers and RS 232 drivers) to preempt interference.
(6) Shield RF signal influence on analog parts
Given RF signals’ potential to interfere with sensitive analog blocks like ADCs, integration of RF decoupling capacitors (e.g., 100 pF) grounded appropriately is advised. This precaution proves especially critical at lower frequency bands (e.g., 27 MHz) and higher output power levels.
(7) Considerations for onboard loop antennas
Integrated loop antennas, cost-effective alternatives to traditional whip antennas, ensure mechanical stability. Their design aids in suppressing unwanted signals that might interfere with receivers. However, loop antennas (like any antenna) are susceptible to noise capacitively coupled from nearby signal lines, necessitating careful PCB design to avoid tuning network alterations and reduced transmission efficacy.
4. Conclusion
The evolution of RF integrated circuits presents significant opportunities for engineers designing wireless systems. However, successful RF circuit design demands practical experience and adept PCB engineering capabilities.
Potential conflicts between digital circuits and analog circuits arise when the analog circuit (RF) and the digital circuit (microcontroller) operate separately, they may function properly. However, when placed on the same board and powered from the same supply, the entire system can become unstable. This instability is primarily due to the digital signal’s rapid switching between ground and the positive power supply (typically 3V), occurring in nanoseconds. These digital signals have significant high-frequency components independent of their switching frequency. In contrast, analog signals in the RF section, such as those transmitted from the antenna tuning loop to the wireless device’s receiver, are typically less than 1μV. This creates a signal level difference between digital and RF signals of approximately 10-6 (120 dB).
Clearly, without adequate separation between digital and RF signals, the weak RF signals can be corrupted, leading to degraded performance or complete failure of the wireless device.
2. Common problems with RF circuits and digital circuits on the same PCB
Insufficient isolation of sensitive and noisy signal lines is a common issue. As mentioned earlier, digital signals exhibit high swings and contain numerous high-frequency harmonics. If digital signal traces on a PCB run adjacent to sensitive analog signals, these high-frequency harmonics may induce unwanted coupling. Sensitive nodes in RF devices typically include the phase-locked loop (PLL) filter circuit, external voltage-controlled oscillator (VCO) inductors, crystal reference signals, and antenna terminals. These sections of the circuit require meticulous handling.
(1) Power supply noise
Since input/output signals swing several volts, digital circuits generally tolerate power supply noise well (less than 50 mV). Conversely, analog circuits are highly susceptible to power supply noise, especially glitch voltages and high-frequency harmonics. Therefore, when designing PCBs containing RF or other analog circuits, power line routing demands greater attention than standard digital boards, avoiding automated routing. It’s critical to note that modern microcontrollers, designed on CMOS processes, draw substantial current spikes at each internal clock cycle. For instance, at a 1 MHz internal clock frequency, the microcontroller’s periodic current spikes can cause voltage glitches on the power supply line if proper supply decoupling is lacking. Should these glitches reach the power pins of the RF circuitry, operational failures may result. Thus, it’s imperative to ensure separation of analog power lines from the digital circuitry area.
(2) Unreasonable ground layout
RF circuit boards must always incorporate a ground plane connected to the negative side of the power supply. Improper handling can lead to anomalous behavior. This concept may challenge digital circuit designers accustomed to functional digital circuits without extensive grounding. In the RF frequency band, even short traces act as inductors; approximately 1 nH per mm length translates to about 27 Ω for a 10 mm PCB trace at 434 MHz. Without a proper ground plane, extended ground paths fail to ensure desired circuit characteristics.
(3) Antenna radiation affecting other analog components
This issue often goes unnoticed in mixed circuits. Apart from the RF section, PCBs often host additional analog circuits. For instance, many microcontrollers feature built-in analog-to-digital converters (ADCs) for measuring parameters like battery voltage. Proximity of the RF transmitter’s antenna to such components can allow high-frequency emissions to reach ADC analog inputs. Any circuit trace can act like an antenna, potentially causing RF signals to inadvertently self-excite in the ADC’s ESD diode, leading to operational instability.
3. Solution A with RF circuit and digital circuit on the same PCB
Outlined below are some general design and routing strategies applicable to most RF applications. However, adhering to specific RF device routing guidelines remains paramount in practical applications.
(1) Establish a reliable ground plane
When designing PCBs with RF components, a dependable ground plane is indispensable. Its primary function is to establish an effective 0 V reference point across the circuit, facilitating straightforward decoupling of all devices. The power supply’s 0 V terminal should directly link to this ground plane. Due to its low impedance, the ground plane prevents signal coupling between decoupled nodes—an essential consideration given potential signal amplitudes differing by up to 120 dB. On surface-mount PCBs, signal traces reside on one side while the ground plane occupies the opposite. Ideally, the ground plane should blanket the entire PCB (except beneath the antenna). In multi-layer PCBs, the ground plane typically sits adjacent to signal layers. Additionally, unused sections of signal layers should be filled with ground planes connected to the main ground plane via multiple vias. It’s crucial to carefully consider inductance characteristics when positioning and selecting inductance values.
(2) Minimize ground connection lengths
Ground connections to the ground plane must be as brief as possible, with ground vias ideally placed directly at or very near component pads. Avoid sharing ground vias between two signals to prevent potential crosstalk stemming from via impedance.
(3) Implement RF decoupling
Deploy decoupling capacitors in close proximity to pins requiring decoupling. Capacitors should feature high-quality ceramics with “NPO” or “X7R” dielectrics, suitable for most applications. Select capacitor values ensuring their series resonance matches the signal frequency. For instance, a 100 pF capacitor mounted at 434 MHz yields approximately 4 Ω capacitive reactance, akin to via inductive reactance. Series configuration of capacitors and vias forms a notch filter specific to the signal frequency, achieving effective decoupling. At 868 MHz, opt for a 33 pF capacitor. Additionally, include large-value capacitors (e.g., 2.2 μF ceramic or 10 μF tantalum) on power lines for low-frequency decoupling.
(4) Implement star wiring for power supply
Star wiring, a proven analog circuit design technique, ensures each module on the PCB connects directly to a common power supply point. For digital and RF sections, independent power supply lines should undergo separate decoupling in proximity to ICs. This approach mitigates power supply noise from affecting RF components. Modules prone to noise, like RS 232 drivers or switching regulators, may benefit from series inclusion of inductors or resistors (e.g., 10 Ω) in the power line, supplemented by 10 μF tantalum capacitors for effective decoupling.
(5) Optimize PCB layout
Careful arrangement of circuit modules on the PCB minimizes noise module interference with surrounding analog components. Distance-sensitive modules (e.g., RF sections and antennas) should be segregated from noisy counterparts (e.g., microcontrollers and RS 232 drivers) to preempt interference.
(6) Shield RF signal influence on analog parts
Given RF signals’ potential to interfere with sensitive analog blocks like ADCs, integration of RF decoupling capacitors (e.g., 100 pF) grounded appropriately is advised. This precaution proves especially critical at lower frequency bands (e.g., 27 MHz) and higher output power levels.
(7) Considerations for onboard loop antennas
Integrated loop antennas, cost-effective alternatives to traditional whip antennas, ensure mechanical stability. Their design aids in suppressing unwanted signals that might interfere with receivers. However, loop antennas (like any antenna) are susceptible to noise capacitively coupled from nearby signal lines, necessitating careful PCB design to avoid tuning network alterations and reduced transmission efficacy.
4. Conclusion
The evolution of RF integrated circuits presents significant opportunities for engineers designing wireless systems. However, successful RF circuit design demands practical experience and adept PCB engineering capabilities.
