In PCB design, the unique characteristics of RF circuits are challenging to encapsulate in just a few sentences, nor can they be effectively analyzed with traditional simulation tools like SPICE. However, several EDA software solutions available today utilize sophisticated algorithms, such as harmonic balance and the shooting method, allowing for rapid and accurate simulation of radio frequency circuits. Before diving into these EDA tools, it is essential to grasp the fundamental attributes of radio frequency circuits, particularly the significance of specific terminology and physical phenomena, as this knowledge forms the foundation of RF engineering.

RF Interface

The wireless transmitter and receiver can be conceptually divided into two segments: base frequency and radio frequency. The base frequency encompasses the input signal range of the transmitter and the output signal range of the receiver. The bandwidth of the base frequency dictates the fundamental data transmission rate within the system. This base frequency plays a critical role in enhancing the reliability of the data stream while minimizing the burden placed on the transmission medium by the transmitter at a specific data rate. Consequently, substantial knowledge in signal processing engineering is necessary when designing a base frequency circuit on a PCB. The transmitter’s RF circuit is responsible for converting and up-converting the processed baseband signal to a designated channel before injecting it into the transmission medium. Conversely, the receiver’s RF circuit captures the signal from the transmission medium, converting and down-converting it back to the base frequency.

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1. The transmitter has two primary PCB design objectives:

2. First, it must emit a specified power level while minimizing power consumption.

3. Second, it must avoid interfering with the normal operation of adjacent-channel transceivers.

4. From the receiver’s perspective, there are three core PCB design goals:

5. Firstly, it must accurately restore weak signals;

6. Thirdly, it must eliminate interfering signals outside the desired channel;

7. Finally, like transmitters, it must consume minimal power.

8. **Small expectation signal**

9. The receiver needs to detect faint input signals with high sensitivity.

10. Typically, the input power can be as low as 1 μV.

11. Receiver sensitivity is constrained by the noise from its input circuit, making noise a crucial factor in PCB design.

12. Moreover, the ability to predict noise using simulation tools is essential.

13. The received signal is initially filtered, followed by amplification using a low-noise amplifier (LNA).

14. Next, the first local oscillator (LO) mixes with this signal, converting it to an intermediate frequency (IF).

15. The noise performance of the front-end circuit is largely determined by the LNA, mixer, and LO.

16. While traditional SPICE noise analysis can assess LNA noise, it falls short for the mixer and LO, as their noise is significantly influenced by the large LO signal.

17. Detecting small input signals necessitates substantial amplification, typically requiring a gain of 120 dB.

18. Such high gain can lead to issues if any signals couple back from the output to the input terminal.

19. One key advantage of the superheterodyne receiver architecture is its ability to distribute gain across multiple frequencies, reducing coupling chances.

20. This also ensures the first LO frequency differs from the input signal frequency, helping prevent large interference signals from contaminating small input signals.

21. For various reasons, some wireless communication systems may opt for direct conversion or homodyne architecture over superheterodyne.

22. In this setup, the RF input signal is directly converted to the fundamental frequency in one step.

23. Thus, most gain occurs at the fundamental frequency, aligning the LO frequency with the input signal.

24. In this scenario, understanding the impact of minimal coupling is crucial, necessitating a detailed model of “stray signal paths” such as coupling through substrate, package pins, and bondwires, as well as through power lines.

25. **Big interference signal**

26. The receiver must maintain sensitivity to small signals even amidst large interference signals.

27. This situation arises when attempting to receive a weak or distant transmission while a nearby powerful transmitter broadcasts on an adjacent channel.

28. The interfering signal can be 60-70 dB stronger than the expected signal, leading to substantial coverage in the receiver’s input stage or excessive noise generation, hindering normal signal reception.

29. If the interference source drives the receiver into a non-linear region during the input stage, these issues may arise.

30. To prevent this, the receiver’s front end must exhibit high linearity.

31. Therefore, “linearity” is also a critical factor in PCB receiver design.

32. Given the narrowband nature of the receiver, non-linearity is assessed through “intermodulation distortion.”

33. This involves using two sine or cosine waves of similar frequencies centered in the band to drive the input, measuring the resulting intermodulation products.

34. Generally, SPICE can be time-consuming and resource-intensive since it requires numerous cycles to achieve the necessary frequency resolution for analyzing distortion.

35. **PCB adjacent channel interference**

36. Distortion is also vital for the transmitter.

37. Non-linearity in the transmitter’s output circuit can broaden the transmitted signal’s bandwidth into adjacent channels, a phenomenon known as “spectral regrowth.”

38. Before reaching the transmitter’s power amplifier (PA), the signal’s bandwidth is limited; however, intermodulation distortion in the PA can cause bandwidth to expand again.

39. If bandwidth expansion is excessive, the transmitter may fail to meet adjacent channel power requirements.

40. When transmitting digitally modulated signals, predicting spectrum growth using SPICE becomes impractical, as approximately 1000 digital symbol transmission operations must be simulated to achieve a representative spectrum, coupled with high-frequency carriers.