Transmitters have two main PCB design goals: the first is that they must emit specific power at the lowest possible power consumption.Second, they do not interfere with the normal operation of transceivers in adjacent channels.For receivers, there are three main PCB design goals: first, they must accurately restore small signals; second, they must be able to remove interference signals outside the desired channel; and last, like transmitters, they must consume very little power.

Disturbance signal of the magnitude of RF circuit simulation.

Receiver must be sensitive to small signals even when there is a large interference signal (obstruction).This occurs when an attempt is made to receive a weak or remote transmission signal with a strong transmitter nearby broadcasting in adjacent channels.The interference signal may be 60-70 dB larger than the expected signal, and can be overwritten in a large amount during the input phase of the receiver, or cause the receiver to produce too much noise during the input phase to block the reception of the normal signal.These two problems occur when the receiver is in the input phase and is driven into a non-linear region by an interfering source.To avoid these problems, the front end of the receiver must be very linear.

Therefore, "linear" is also an important consideration in the design of PCB receivers.Since the receiver is a narrow frequency circuit, the nonlinearity is measured by measuring "intermodulation distortion".This involves driving the input signal using sinusoidal or cosine waves of two frequencies that are similar and within the central band, and then measuring the product of their interactive modulation.Generally speaking, SPICE is a time-consuming and cost-consuming simulation software because it must perform many circular operations to obtain the desired frequency resolution to understand the distortion.

Expected signal of small size for RF circuit simulation.

The receiver must be sensitive to small input signals.Generally speaking, the input power of the receiver can be less than 1 microV.The sensitivity of the receiver is limited by the noise generated by its input circuit.Therefore, noise is an important consideration in the design of PCB receivers.Moreover, the ability to predict noise with simulation tools is indispensable.Fig. 1 is a typical superheterodyne receiver.The received signal is filtered and then amplified by a low noise amplifier (LNA).The signal is then mixed with the first local oscillator (LO) to convert the signal to intermediate frequency (IF).The noise performance of front-end circuits depends mainly on LNA, mixer and LO.Although LNA noise can be found using traditional SPICE noise analysis, it is useless for mixers and LOs because the noise in these blocks is severely affected by large LO signals.

Small input signals require the receiver to have a very large amplification function, usually requiring a gain as high as 120 dB.At such a high gain, any signal that couples from the output to the input can cause problems.An important reason for using the superheterodyne receiver architecture is that it can distribute the gain over several frequencies to reduce the chance of coupling.This also makes the frequency of the first LO different from that of the input signal, which prevents the large interference signal from "polluting" the small input signal.

For different reasons, in some wireless communication systems, direct conversion or homodyne architectures can replace super-heterodyne architectures.In this architecture, the RF input signal is directly converted to the base frequency in a single step, so most of the gain is in the base frequency, and LO is the same as the frequency of the input signal.In this case, the influence of a small amount of coupling must be understood, and a detailed model of stray signal path must be established, such as coupling across the substrate, coupling between the encapsulation foot and the bondwire, and coupling across the power cord.

Interference from adjacent channels in RF circuit simulation.

Distortion also plays an important role in the transmitter.The nonlinearity of the transmitter in the output circuit may spread the bandwidth of the transmitted signal across adjacent channels.This phenomenon is called spectral regrowth.The bandwidth of the signal is limited until it reaches the power amplifier (PA) of the transmitter; however, the "intermodulation distortion" within 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 adjacent channels.When transmitting a digital modulation signal, it is virtually impossible to use SPICE to predict the re-growth of the spectrum.Because about 1,000 transmission operations of digital symbols must be simulated to obtain representative frequencies and to incorporate high frequency carriers, these will make transient analysis of SPICE impractical.