 Hello, in this video we will focus on impedance matching and filtering of the RF part of the STN32WL. The RF circuits between the STN32WL and the antenna can be divided into three main parts, transmitter impedance matching and filtering, receiver impedance matching and balloon, and the antenna matching network. There is a lot of information on this topic which has been divided into several shorter videos. This video covers the analytical description. It shows meaning and basic principle of each block. To illustrate the principle, calculations are based on idle components and conditions. Then there is another series of videos showing practical measurement taken on a real board, in this case STN32WL nuclear board. The series contains several videos, each focusing on a different part of RF circuit. Why is this topic about impedance matching and filtering needed? Impedance matching is needed for optimization of working conditions for the selected band. For minimizing power loss due to impedance mismatch, for example in case of transmitter it improves output power and in case of receiver it improves sensitivity. And to optimize overall energy consumption. In case of filtering, the transmitter is based on a highly efficient, non-linear class of RF power amplifier that generates significant harmonic spectral content at the expense of power consumption. These products must be filtered to pass regulatory certification limits. In this slide, we will focus on the transmitter part. The antenna cannot be connected directly to the transmitter output of the STN32WL due to impedance mismatch and spectral content of the signal. The radiated power would be low and can violate regulatory limits due to unwanted spectral content. If we add matching network, it helps with impedance mismatch. The radiated power will be maximized. The spurious and harmonic products remain. To suppress these products, a filter must be added to the signal path. Now the signal has parameters we wanted and can be transmitted by the antenna. STN32WL has two transmitter outputs, one low power and one high power. If both outputs are used in the design, each needs its own impedance matching and filter. In case of receiver part, we cannot also connect the antenna directly. The receiver input is differential and has different impedance than the antenna. We need to implement Balloon, which transforms single-ended signal to differential under impedance matching network. It minimizes impedance mismatch and improves receiver sensitivity. Typical RF circuit, which implements both transmitter and receiver parts, looks like this. We can see the blocks we discussed earlier for isolated transmitter and receiver. What is new is the RF switch which switches the antenna to the transmitter or receiver part. The system is half-tuplex. It means it can transmit or receive but not at the same time. This structure is general, but there are also some variants. One uses IPD, which means Integrated Passive Device. The IPD is a component that implements all passive components like Balloon, matching network and filters in one component. It's smaller and easier to implement that variant with discrete parts. Another variant doesn't use RF switch. Both transmitter and receiver parts are connected in one point. This is cheaper and easy to implement. This advantage is a worse transmitter and receiver performance. The RF switch normally serves also as an isolator, which isolates both parts. Now this isolation is missing. In this video, we will discuss variant with the RF switch. Okay, let's take a closer look at the transmitter path. As we mentioned earlier, spectral content of the transmitter output can be very rich. With the desired signal, there are harmonic frequencies and discrete spurs. These unwanted products must be suppressed. The signal that we want at the antenna may look like this. The desired signal is maximized. This can be achieved by good impedance matching. All harmonics and spurious products are below regulatory limits. The transmitter path can be divided into several smaller parts. Power amplifier biasing and filtering. Transmitter matching network, notch filter, low pass filter, RF switch and the antenna matching network. These C blocks are needed for the RF switch operation. The RF switch and the antenna matching network are shared with the receiver path. Here is a principle scheme of the transmitter output. Internal power regulator powers the power amplifier transistor through the RF choke. Output signal is not harmonic, spectral content is rich. To get the right signal, it must be filtered and matched. STM32WL has two output pins, which differ in output power. Low power output, which has output power up to 15 dBm and high power output, which has output power up to 22 dBm. Impedance of the output pins is not 50 ohm. It is also not a constant value. The impedance depends on many parameters like package, frequency, transmitted output power and also PCB layout. We measured optimal loading or matching impedances of the output pins to simplify design of the matching network. Detailed measurements can be found in the application node AN 5457. Here is the recommended structure of the power amplifier biasing. A voltage from the internal regulator powers the power amplifier through the RF choke. Impedent value depends on the working band, the two capacitors, filter, the voltage. Their value is fixed. The purpose of this block is to match the power amplifier output impedance to 50 ohms. For example, for frequency 868 MHz at 14 dBm, the optimal loading impedance is this value. We can assume power amplifier impedance as complex conjugated value of the optimal loading impedance. Based on this, equivalent circuit of the power amplifier output can be modeled as a series connection of a resistor and a capacitor with these values. In the block diagram, we can see equivalent circuit of the power amplifier output, power amplifier bias circuit and the matching network structure which match the power amplifier impedance to 50 ohms. Values of the matching network components can be calculated in the Smith chart. If this point is impedance of the power amplifier, then if we use these values of a serial inductor and parallel capacitor, we get 50 ohms at the output. If the distance between the RF output pin and the matching network is too long, we must consider also impact of the transmission line. It means that impedance at the beginning of the transmission line is transformed to another impedance at the end. This depends on the length and impedance of the transmission line. Another block is the notch filter which suppresses the second harmonic. Here is the filter structure. Recommended values can be obtained by the following steps. L2 is about three quarters of L1. Resonance frequency of L2 and C2 is around second harmonic frequency. To have a good matching, C1 and C3 might be tweaked. In this chart are ideal return loss and transmission of the notch filter. It's matched for the fundamental harmonic and suppresses the second harmonic. Matching of the filter can be calculated in the Smith chart. The low pass filter filters the higher harmonics. Recommended structure is P-ledder network. Second filter approximation is third order Chebyshev which is matched for the fundamental frequency. Here is an example of the filter and the simulation results. We can see impedance matching and transmission. Black curves are results with ideal components and the red ones are results with S-parameters of real components. Power capacitors of the filter can be affected by adjacent capacitors. In this case, these capacitors must be adjusted. Capacitors between the notch filter and the low pass filter can be combined into one capacitor. The capacitor close to the RF switch is usually lower due to parasitic capacitance of the RF switch. The RF switch switches shared antenna to the transmitter or receiver circuit. It also isolates transmitter and receiver branches. Typical parameters of the RF switch are listed here. For example, the following RF switches can be used in designs with STM32WL. The DC blocks block DC current to the RF switch. They are mandatory for some RF switches. DC block capacitors might have little effect on the parameters of the low pass filter. When fine tuning the low pass filter, the effect of these DC blocking capacitors must be taken into account. Now let's take a closer look at the receiver path. Calls of the receiver circuit are impedance matching to have good receiver sensitivity and single ended to differential signal conversion. Typical structure of the receiver circuit looks like this. A brief, simplified description of the components is as follows. Inductor L5 matches reactive path of the LNA. This inductor is used for impedance matching together with C11, C12 is a path of so-called balloon light circuit, and C13 can be used for impedance matching or reducing of harmonics. This analytical structure can be simplified like this. This structure is implemented in reference designs with STM32WL. Like the transmitter, also receiver impedance is not 50 ohms or a fixed value. This depends on many parameters like package, frequency or PCB. Recommended matching on the impedances were measured and are described in application note 5457. On the right side is the example of the matching in the Smith chart. We assume QFN package and frequency 868 MHz. Compatible impedance for these parameters is 52 plus J102 ohms. We start matching from the receiver side, so we use complex conjugated value. The goal is to set L5 and capacitors C11 and C12 to get as close as possible to 50 ohms in the Smith chart. Differential transmission line between the STM32WL receiver pins and the nearest passive components has also impact to the optimum values of these passive components. The greater the length, the more it transforms the impedance. This must be taken into account when fine-tuning the receiver matching network. Balloon converts single-ended signal to differential. In designs with STM32WL we don't implement real balloon but so-called balloon-like circuit. It consists of three components. Detailed calculation of these components can be found in the application note 5457. This structure has less losses than the real balloon. The main difference between the balloon and this balloon-like circuit is that this circuit has no exact 180° phase difference. An exact 180° phase difference would only exist if the equivalent input resistance of the LNA was very large. This balloon-like circuit is a good compromise between cost, PCB surface, losses, and performance. Now let's discuss the antenna matching network. The antenna matching network matches the antenna impedance to 50 ohms. Typically, it has p-ladder topology on the PCB but only one half is usually used. The values of the components depend on the specific impedance of the antenna. For example, if the antenna has impedance 20 plus J10 ohms, we can use an inductor L4 and capacitor C7 with these values to get to 50 ohms. C8 is unused. If the antenna matching network is in the right configuration, for example CLC structure, it also behaves like a low-pass filter. This additional filter is needed if the high-power output is used. It's because high-power output generates strong harmonics and in regions where this power is allowed are more strict requirements for harmonics level. The matching and filtering properties of the CLC structure must be fine-tuned together. If the high-power output is used and the antenna matching needs a different structure than a low-pass filter, then additional Pi section for low-pass filter is recommended. It can be used also if better margin from the certification level is needed for harmonics and spurious. Adjacent components can be used as one component if they are of the same type. Few words at the end. For easier explanation, calculations in this video use ideal components, but in the real world additional effects must be considered, for example, behavior of real components, impact of the PCB layout, tolerance of components, temperature, etc. In this picture we can see simulated forward transmission of the low-pass filter. We can see results based on ideal components, real components and the real components with impact of the PCB layout. More accurate results can be obtained by electromagnetic field simulation or measurement on a prototype. More information about this topic can be found in the following application notes. Reference designs can be also useful source of information. Thank you for your attention.