This article explains the RF circuit simulation 4 characteristics, RF interface, small expected signal, large interference signal, and interference from adjacent channels, and gives important factors that need special attention in the PCB design process.
RF circuit simulation of the interface of the RF
Wireless transmitters and receivers in concept can be divided into two parts, fundamental frequency, and RF. The fundamental frequency contains the frequency range of the input signal of the transmitter and the frequency range of the output signal of the receiver.
The bandwidth of the fundamental frequency determines the basic rate at which data can flow in the system. The fundamental frequency is used to improve the data flow’s reliability and reduce the load imposed by the transmitter on the transmission medium at a given data rate.
Therefore, the PCB design of the fundamental frequency circuit requires extensive knowledge of signal processing engineering. The transmitter’s RF circuitry converts the processed fundamental frequency signal, upscales it to a specified channel, and injects this signal into the transmission medium.
Conversely, the receiver’s RF circuitry acquires the signal from the transmission media and converts and downscales it to the fundamental frequency.
Transmitters have two main PCB design goals
They must transmit a specific amount of power while consuming the least amount of power possible.
They must not interfere with the normal operation of the transceiver in adjacent channels.
Receivers have three main PCB design goals
First, they must accurately restore small signals.
Second, they must be able to remove interfering signals outside the desired channel.
Third, the last point is the same as the transmitter, they must consume very little power.
RF circuit simulation of large interfering signals
Receivers must be sensitive to small signals, even when large interfering signals (blockers) are present. This situation arises when trying to receive a weak or distant transmit signal with a powerful transmitter broadcasting in an adjacent channel nearby.
The interfering signal may be 60 to 70 dB larger than the expected signal and can block the reception of the normal signal in the input phase of the receiver with a large amount of coverage or by causing the receiver to generate an excessive amount of noise in the input phase.
Those two problems mentioned above can occur if the receiver, in the input stage, is driven into the region of nonlinearity by the source of interference. To avoid these problems, the front end of the receiver must be very linear.
Therefore, linearity is also an important consideration when designing a receiver for PCB. Since the receiver is a narrow-band circuit, the nonlinearity is measured by cross-tuning distortion (into modulating on distortion) to the statistics.
This involves driving the input signal with two sine or cosine waves of similar frequency located in the center band and then measuring the product of their cross-modulation.
SPI CE is a time-consuming and costly simulation software because it must perform many cycles before it can obtain the desired frequency resolution to understand the distortion.
RF circuit simulation with small desired signals
The receiver must be very sensitive to detect small input signals. In general, the input power of the receiver can be as small as 1 μV. the sensitivity of the receiver is limited by the noise generated by its input circuit.
Therefore, noise is an important consideration when designing a receiver for PCB. Moreover, having the ability to predict noise with simulation tools is essential.
Figure 1 is a typical superheterodyne (superheterodyne) receiver. The received signal is first filtered and then the input signal is amplified with a low-noise amplifier (LNA). The first local oscillator (LO) is then used to mix with this signal to convert this signal to an intermediate frequency (IF). The noise efficiency of the front-end circuit depends mainly on the LNA, mixer (mixer), and LO.
Although the noise of the LNA can be found using conventional SPICE noise analysis, it is useless for the mixer and LO, because the noise in these blocks can be seriously affected by a very large LO signal.
The small input signal requires the receiver to be extremely amplified, usually requiring again as high as 120 dB. At such a high gain, any signal coupled (coupled) from the output back to the input can create problems.
The important reason for using the super outlier receiver architecture is that it allows the gain to be spread over several frequencies to reduce the chance of coupling. This also makes the first LO frequency different from the input signal frequency, which can prevent large interfering signals from contaminating the small input signal.
In some wireless communication systems, a direct-conversion (direct conversion) or homodyne architecture can replace the super-outer-differential architecture. In this architecture, the RF input signal is converted directly to the fundamental frequency in a single step, so that most of the gain is in the fundamental frequency and the LO is at the same frequency as the input signal.
In this case, the impact of a small amount of coupling must be understood, and a detailed model of the stray signal path must be developed, such as coupling through the substrate, coupling between the package footprint and the bond wire, and coupling through the power line.
RF circuit simulation of adjacent channel interference
Distortion also plays an important role in the transmitter. The non-linearity generated by the transmitter in the output circuit may cause the bandwidth of the transmitted signal to be scattered in adjacent channels. This phenomenon is called spectral regrowth. Before the signal reaches the transmitter’s power amplifier (PA), its bandwidth is limited; however, cross-modulation distortion in the PA causes the bandwidth to increase again.
If the bandwidth increases too much, the transmitter will not be able to meet the power requirements of its neighboring channels.
When transmitting a digital modulation signal, it is practically impossible to predict the re-growth of the spectrum with SPICE. Because there are about 1000 digital symbols (symbol) that must be simulated to obtain a representative spectrum and need to be combined with high-frequency carriers, these will make the transient analysis of SPICE impractical.
