08-06-2013, 03:23 PM
Introduction to RF Simulation and its Application
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The RF Interface
Wireless transmitters and receivers can be conceptually separated into baseband and RF
sections. Baseband is the range of frequencies over which transmitters take their input
and receivers produce their output. The bandwidth of the baseband section determines
the underlying rate at which data can flow through the system. There is a considerable
amount of signal processing that occurs at baseband designed to improve the fidelity of
the data stream being communicated and to reduce the load the transmitter places on the
transmission medium for a particular data rate. The RF section of the transmitter is
responsible for converting the processed baseband signal up to the assigned channel and
injecting the signal into the medium. Conversely, the RF section of the receiver is
responsible for taking the signal from the medium and converting it back down to baseband.
Small Desired Signals
Receivers must be very sensitive to detect small input signals. Typically, receivers are
expected to operate with as little as 1 μV at the input. The sensitivity of a receiver is limited
by the noise generated in the input circuitry of the receiver. Thus, noise is a important
concern in receivers and the ability to predict noise by simulation is very important.
As shown in Figure 1, a typical superheterodyne receiver first filters and then amplifies
its input with a low noise amplifier or LNA. It then translates the signal to the intermediate
frequency or IF by mixing it with the first local oscillator or LO. The noise performance
of the front-end is determined mainly by the LNA, the mixer, and the LO. While
it is possible to use traditional SPICE noise analysis to find the noise of the LNA, it is
useless on the mixer and the LO because the noise in these blocks is strongly influenced
by the large LO signal.
Large Interfering Signals
Receivers must be sensitive to small signals even in the presence of large interfering signals,
often known as blockers. This situation arises when trying to receive a weak or distant
transmitter with a strong nearby transmitter broadcasting in an adjacent channel.
The interfering signal can be 60-70 dB larger than the desired signal and can act to
block its reception by overloading the input stages of the receiver or by increasing the
amount of noise generated in the input stage. Both of these problems result if the input
stage is driven into a nonlinear region by the interferer. To avoid these problems, the
front-end of a receiver must be very linear. Thus, linearity is also an important concern
in receivers. Receivers are narrowband circuits and so the nonlinearity is quantified by
measuring the intermodulation distortion. This involves driving the input with two sinusoids
that are in band and close to each other in frequency and then measuring the intermodulation
products. This is generally an expensive simulation with SPICE because
many cycles must be computed in order to have the frequency resolution necessary to
see the distortion products.
Adjacent Channel Interference
Distortion also plays an important role in the transmitter where nonlinearity in the output
stages can cause the bandwidth of the transmitted signal to spread out into adjacent
channels. This is referred to as spectral regrowth because, as shown in Figure 2 and
Figure 3 on page 5, the bandwidth of the signal is limited before it reaches the transmitter’s
power amplifier or PA, and intermodulation distortion in the PA causes the bandwidth
to increase again. If it increases too much, the transmitter will not meet its
adjacent channel power requirements. When transmitting digitally modulated signals,
spectral regrowth is virtually impossible to predict with SPICE. The transmission of
around 1000 digital symbols must be simulated to get a representative spectrum, and
this combined with the high carrier frequency makes use of transient analysis impractical.
Narrowband Signals
RF circuits process narrowband signals in the form of modulated carriers. Modulated
carriers are characterized as having a periodic high-frequency carrier signal and a lowfrequency
modulation signal that acts on either the amplitude, phase, or frequency of the
carrier. For example, a typical mobile telephone transmission has a 10-30 kHz modulation
bandwidth riding on a 1-2 GHz carrier. In general, the modulation is arbitrary,
though it is common to use a sinusoid or a simple combination of sinusoids as test signals.
The ratio between the lowest frequency present in the modulation and the frequency of
the carrier is a measure of the relative frequency resolution required of the simulation.
General purpose circuit simulators, such as SPICE, use transient analysis to predict the
nonlinear behavior of a circuit. Transient analysis is expensive when it is necessary to
resolve low modulation frequencies in the presence of a high carrier frequency because
the high-frequency carrier forces a small timestep while a low-frequency modulation
forces a long simulation interval.
[attachment=55028]
The RF Interface
Wireless transmitters and receivers can be conceptually separated into baseband and RF
sections. Baseband is the range of frequencies over which transmitters take their input
and receivers produce their output. The bandwidth of the baseband section determines
the underlying rate at which data can flow through the system. There is a considerable
amount of signal processing that occurs at baseband designed to improve the fidelity of
the data stream being communicated and to reduce the load the transmitter places on the
transmission medium for a particular data rate. The RF section of the transmitter is
responsible for converting the processed baseband signal up to the assigned channel and
injecting the signal into the medium. Conversely, the RF section of the receiver is
responsible for taking the signal from the medium and converting it back down to baseband.
Small Desired Signals
Receivers must be very sensitive to detect small input signals. Typically, receivers are
expected to operate with as little as 1 μV at the input. The sensitivity of a receiver is limited
by the noise generated in the input circuitry of the receiver. Thus, noise is a important
concern in receivers and the ability to predict noise by simulation is very important.
As shown in Figure 1, a typical superheterodyne receiver first filters and then amplifies
its input with a low noise amplifier or LNA. It then translates the signal to the intermediate
frequency or IF by mixing it with the first local oscillator or LO. The noise performance
of the front-end is determined mainly by the LNA, the mixer, and the LO. While
it is possible to use traditional SPICE noise analysis to find the noise of the LNA, it is
useless on the mixer and the LO because the noise in these blocks is strongly influenced
by the large LO signal.
Large Interfering Signals
Receivers must be sensitive to small signals even in the presence of large interfering signals,
often known as blockers. This situation arises when trying to receive a weak or distant
transmitter with a strong nearby transmitter broadcasting in an adjacent channel.
The interfering signal can be 60-70 dB larger than the desired signal and can act to
block its reception by overloading the input stages of the receiver or by increasing the
amount of noise generated in the input stage. Both of these problems result if the input
stage is driven into a nonlinear region by the interferer. To avoid these problems, the
front-end of a receiver must be very linear. Thus, linearity is also an important concern
in receivers. Receivers are narrowband circuits and so the nonlinearity is quantified by
measuring the intermodulation distortion. This involves driving the input with two sinusoids
that are in band and close to each other in frequency and then measuring the intermodulation
products. This is generally an expensive simulation with SPICE because
many cycles must be computed in order to have the frequency resolution necessary to
see the distortion products.
Adjacent Channel Interference
Distortion also plays an important role in the transmitter where nonlinearity in the output
stages can cause the bandwidth of the transmitted signal to spread out into adjacent
channels. This is referred to as spectral regrowth because, as shown in Figure 2 and
Figure 3 on page 5, the bandwidth of the signal is limited before it reaches the transmitter’s
power amplifier or PA, and intermodulation distortion in the PA causes the bandwidth
to increase again. If it increases too much, the transmitter will not meet its
adjacent channel power requirements. When transmitting digitally modulated signals,
spectral regrowth is virtually impossible to predict with SPICE. The transmission of
around 1000 digital symbols must be simulated to get a representative spectrum, and
this combined with the high carrier frequency makes use of transient analysis impractical.
Narrowband Signals
RF circuits process narrowband signals in the form of modulated carriers. Modulated
carriers are characterized as having a periodic high-frequency carrier signal and a lowfrequency
modulation signal that acts on either the amplitude, phase, or frequency of the
carrier. For example, a typical mobile telephone transmission has a 10-30 kHz modulation
bandwidth riding on a 1-2 GHz carrier. In general, the modulation is arbitrary,
though it is common to use a sinusoid or a simple combination of sinusoids as test signals.
The ratio between the lowest frequency present in the modulation and the frequency of
the carrier is a measure of the relative frequency resolution required of the simulation.
General purpose circuit simulators, such as SPICE, use transient analysis to predict the
nonlinear behavior of a circuit. Transient analysis is expensive when it is necessary to
resolve low modulation frequencies in the presence of a high carrier frequency because
the high-frequency carrier forces a small timestep while a low-frequency modulation
forces a long simulation interval.