RF layout and antenna tuning requires a specific concept of RF and needs more attention than traditional circuit layouts. This section describes the basics of RF design, transmission lines, and characteristic impedance.
Need to understand the following concepts and terms to design a valid RF layout.
▪ Transmission line
▪ Feature impedance
▪ Echo loss
▪ Interventional loss
▪ Impedance match
Key factors affecting RF design and simulation design are the impedance of RF circuits. At low frequencies, the load impedance remains unchanged at different distances from the load traces. For most applications, it does not depend on trace width or its uniformity. Therefore, the trace is only represented as a low frequency node. However, when high frequencies, the impedance (Z) of the RF circuit changes when measured at different distances. This change also depends on the size of the substrate and radio traces used. Therefore, the trace is also a design element in the RF schematic.
The transmission line is a medium that transmits electromagnetic energy by defining paths. The RF line between the coaxial cable, the waveguide, and the RF pin and the antenna are transmission lines. Most radio frequency traces are transmission lines such as microstrip lines and coplanar waveguides.
The key characteristic of the transmission is its feature impedance (Z0), which is the amplitude ratio of the voltage and current propagating the wave propagation by the non-destructive transmission line. For applications with a frequency of 2.45 GHz, such as BLE, 50Ω characteristic impedance is widely used in RF traces.
Equivalent model of transmission line
Even if Z0 is a real number, it is not the resistance of the RF trace. Ideally transmitted due to its characteristic impedance does not consume energy or have any loss. The equivalent model of the transmission line is shown in the figure above. It is an attribute that represents the ratio of transmission line distributed series inductance and distributed parallel capacitance.
Where l and c are distributed inductors and distribution capacitors along the transmission line.
The characteristic impedance (Z0) depends on the gap between the PCB material, the substrate thickness, the trace width, the trace thickness, and the gap between the RF traces and the ground filler. These parameters are often ignored in traditional layouts and designs, but they play an important role in RF design.
Impedance measurement settings
The above figure depicts typical measurement settings for measuring RF circuit impedance. The characteristic impedance of the impedance and trace on the radio frequency trace is related to the distance of the load and the load impedance; the calculation method is as follows:
Where Z is an impedance measured at a distance of L, ZL is the impedance measured at load (L = 0), Z0 is the characteristic impedance of the transmission line, and β is a phase constant. J is the reaction portion of the impedance.
Let's take a look at how the impedance changes in a particular situation.
When measured under load, L = 0, so Z is equal to ZL.
When ZL = 0 and L = λ / 4, z = ∞.
When ZL = ∞ and L = λ / 4, z = 0
Therefore, when the distance is measured at a quarter wave (λ / 4), even if the short circuit is, it can be seen as an open circuit, and vice versa. In the conventional circuit design, the length of the trace is never approaching λ / 4, so this operation has not been seen.
When ZL = Z0, for any value L, z = z0.
Therefore, when the load impedance (ZL) is equal to the characteristic impedance (Z0), the measured impedance (Z) is measured at any distance (L) of the distance (L) at the distance (L). For this reason, the impedance transformation of any RF device to Z0 is a common practice before transmitting the RF trace to other devices.
The matching network is a passive circuit for the feature impedance of any given impedance conversion (usually) as the RF trace. To ensure that the maximum power transmission from the source to the load is matched through the RF circuit, the source impedance and load impedance should match.
Since the impedance of the circuit varies with the distance from the circuit, the placement of the element for impedance matches also depends on the distance from the mating circuit. The impedance can be varied even if a short intercept on the radio trace can be used as a capacitor or inductor. For examples of a small short intercept, see the figure below.
Short interception example
An open circuit short interception such as less than λ / 4 is equivalent to the capacitor, and the length of the short circuit short circuit with less than λ / 4 corresponds to the inductor. Therefore, the short interception can be used for components of the radio frequency narrowband application. However, unless intentionally designed, short interception or branches in RF traces will affect impedance matching, resulting in lower RF performance.
Source load and transmission line display in the RF circuit
The validity of the matching network is measured by using parameter echo loss and insertion loss.
The above figure shows a typical radio frequency circuit, one of which is radio frequency power, and one load is loaded with most of the radio frequency power and reflects some radio frequency power. The return loss is the ratio of incident power and reflection power. The insertion loss indicates a portion of the power through the circuit loss before reaching the next phase.
In an ideal matching network, all power is transferred to the next phase, no power is reflected. This will cause zero insertion loss and infinity return loss. In the actual circuit, the required echo loss may depend between 6 dB to 30 dB, depending on the application and usage. In matching network, the echo loss is converted to insertion loss, as shown in the following table.
Echo loss and insertion loss
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