Most high-frequency PCBs are used in the communication electronics industry. With the rapid progress of the electronics sector, including fields such as air transportation and satellite systems, the application of high-frequency PCBs has become indispensable. So, what are the fundamental requirements for material characteristics in high-frequency PCB substrates when selecting PCB substrates?
1. **Dielectric constant (Dk):** The Dk needs to be small and stable. Generally, a lower Dk results in faster signal transmission speed. The signal transmission speed is inversely proportional to the square root of the dielectric constant of the substrate. A higher dielectric constant can lead to greater signal transmission delay.
2. **Dielectric loss:** Additionally, a lower Dk tends to improve signal transmission quality by minimizing dielectric loss, thereby reducing signal loss.
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1) The thermal expansion coefficient of copper foil should be as consistent as possible, and its water absorption should be minimal. Other crucial factors include heat resistance, chemical resistance, impact strength, and peel strength, all of which require careful consideration.
2) Typically, a high-frequency board is defined as operating above 1GHz. Currently, the predominant substrates for high-frequency PCBs are fluorine-based dielectrics such as Teflon, which are commonly used above 5GHz. FR-4 substrates, on the other hand, are suitable for frequencies ranging from 1GHz to 10GHz.
3) When it comes to frequency characteristics, Teflon stands out as the optimal choice. For products operating at frequencies exceeding 10GHz, Teflon remains the sole viable option. However, the selection of materials for high-frequency PCBs ultimately hinges on specific customer requirements.
(1) Benefits: Both high-frequency PCBs and low-frequency power supply circuits exhibit voltage gain and power output harvesting effects. In the context of resonant differential amplifier circuits, “resonant frequency f0” refers to the frequency at which resonance occurs. For broadband differential amplifier circuits, it denotes a specific frequency range.
(2) Passband: This concept is crucial for low-frequency power supply circuits. In resonant differential amplifier circuits, it signifies the frequency range between the two points where the normalized vibration amplitude drops to 0.707 relative to the resonant frequency f0. For optical fiber broadband amplification circuits, it indicates a corresponding frequency range relative to a specific frequency.
(3) Selectivity: Selectivity primarily pertains to resonant amplification circuits. It characterizes the circuit’s ability to select useful signals and suppress unwanted ones. This is typically evaluated based on the circuit’s resonance characteristic curve, including parameters such as bandwidth and rejection ratio.
(4) Noise figure: During operation of an amplification circuit, fluctuations caused by various factors lead to irregular movement, generating noise within the circuit that degrades signal quality. This effect is quantified as the ratio of signal power (PS) to noise power (PN), known as Signal-to-Noise Ratio (SNR). The noise figure reflects the comparison between input signal noise and output signal noise.
(5) Stability: High-frequency PCB stability refers to the maintenance of its key performance under varying operating conditions or external influences. Changes in environmental temperature or fluctuations in power supply voltage can disrupt the DC biasing of the amplification circuit, alter circuit component parameters, and consequently impact parameters such as gain, center frequency deviation, and resonance curve shape, possibly leading to circuit malfunction.
