Practical RF Hardware and PCB Design Tips

In RF PCB design, critical length refers to the physical length of a signal trace at which transmission line effects become significant and can no longer be ignored. When a trace length approaches a significant fraction (typically one-tenth) of the signal's wavelength, reflections, signal distortion, and timing issues can occur due to impedance mismatches. This threshold is known as the critical length $L_c$, and it is approximately given by the equation $L_c = \frac{v_p}{10f}$, where $v_p$ is the signal propagation velocity in the trace and $f$ is the signal frequency. For microstrip lines, the propagation velocity is $v_p = \frac{c}{\sqrt{\varepsilon_{\text{eff}}}}$, where $c$ is the speed of light and $\varepsilon_{\text{eff}}$ is the effective dielectric constant of the microstrip. For stripline traces, the effective dielectric constant is typically equal to the board's substrate dielectric constant $\varepsilon_r$, so $v_p = \frac{c}{\sqrt{\varepsilon_r}}$. Designers must ensure that any trace longer than this critical length is treated as a transmission line and routed with controlled impedance techniques to preserve signal integrity.

If you use the more conservative definition of critical length:

$$L_c=\frac{v_p}{12f}$$

You're simply saying that transmission line effects start becoming significant at one-twelfth of a wavelength, rather than the common one-tenth. This is often used in very high-speed designs, where even small reflections or delays matter.

Let’s recalculate the critical length at GPS L1 (1.57542 GHz) using:

• $v_p\approx 1.825\times {10}^8\text{m/s (assuming FR4 with }$$\varepsilon_{\text{eff}} = 2.7$)

• $f = 1.57542 \times 10^9 \, \text{Hz}$
  

$$L_c=\frac{1.825\times {10}^8}{12\times 1.57542\times {10}^9}=\frac{1.825\times {10}^8}{18.904\times {10}^9}\approx 0.00965\text{m}=\boxed{{\displaystyle 9.65\text{mm}}}$$

With $L_c = \frac{v_p}{12f}$, the critical length at GPS L1 becomes approximately 9.65 mm.

Now the following picture shows the trace length of 7.62mm between the antenna pad and the GPS chip IC which is below the calculated critical length of 9.65mm.

So while designing RF PCB make sure you know the critical length with the RF circuit component you are working with. One tip is to keep any RF traces and section as short as possible.

The next topic important in RF design that can help you is the PCB stackup. Normally we use two layer PCB stackup. In such PCB design, one layer is carrying the signal traces and the other layer is the reference layer which is ground typically. For a two layer board like 1.6mm PCB board, there can be large height between the trace layer and the reference ground layer. This means that for a 50ohm impedance lines we need to have wider controlled impedance traces. This is because wider controlled impedance traces means better tolerance control during PCB manufacturing. Also with such design, trace width variation will have less impact on the traces impedance. 

To be able to calculate the trace width with certain trace impedance you need to know the PCB stack up and you can use online trace width calculator. Make sure you calculate these parameters when you designing RF PCB boards. 

The next basic thing one can do when designing RF circuit board is the spacing and clearance. RF chips and its surrounding RF circuit should be kept with some spacing from the rest of the circuit like microcontroller or other major ICs. 

A bias tee is a passive RF circuit used to inject DC power into an RF transmission line without disturbing the RF signal. It enables both DC and RF to coexist on the same line while keeping them isolated where needed. This is especially useful in systems like active antennas, low-noise amplifiers (LNAs), or GPS modules, where DC power is supplied to the antenna through the same coaxial cable that carries RF signals.

🧠 How Bias Tee Works

A typical bias tee consists of:

• An inductor (RF choke) that blocks RF and passes DC

• A capacitor that blocks DC and passes RF

Basic Circuit:

📡 Example: GPS Module + Active Antenna

Suppose you have a GPS module that outputs a signal and also provides 3.3V DC to power an active GPS antenna (which contains a built-in low-noise amplifier). A bias tee is needed to combine this 3.3V with the RF signal and send both over the same SMA coaxial cable.

Key Points:

• The inductor connects 3.3 V to the RF line—allowing DC to power the antenna's amplifier.

• The capacitor at the GPS receiver input blocks any DC from returning into the sensitive RF input circuitry, protecting it and ensuring signal integrity.