Introduction to RF Design - White Paper

Introduction to RF Design - White Paper
Introduction to RF Design
White Paper
Products:
ı
Spectrum Analyzers
ı
Signal Generators
ı
Vector Network
Analyzers
ı
RF Power Meters /
Sensors
This white paper is the first in a series with a goal of
providing practical knowledge on the entire process
for designing an RF system.
Today there are many wireless technologies that
utilize RF design ranging from mobile phones to
satellite TV to wireless Internet connections and
Bluetooth devices.
10.2014 1.1
Neil Jarvis
White Paper
This material is intended to provide insight into how
these technologies work and considerations during
the design, development and verification process.
Table of Contents
Table of Contents
1 A Simplified Communication System ............................................... 3
2 RF Fundamentals ................................................................................ 4
2.1
Spectrum Allocation ....................................................................................................5
2.2
Wavelength Matters .....................................................................................................6
2.3
Reflections and Interference ......................................................................................7
2.4
Determining Power Using dBs ...................................................................................8
3 System Design Blocks ....................................................................... 9
3.1
Transmitters .................................................................................................................9
3.2
Receivers ....................................................................................................................10
3.3
Antennas .....................................................................................................................11
3.4
Putting It All Together ...............................................................................................12
4 Components ...................................................................................... 13
4.1
Filters ..........................................................................................................................13
4.2
Amplifiers ...................................................................................................................14
4.3
Mixers ..........................................................................................................................15
4.4
Modulators/Demodulators ........................................................................................15
5 Test and Verification Instrumentation............................................. 16
5.1
Spectrum analyzers ...................................................................................................16
5.2
Signal Generators ......................................................................................................16
5.3
Vector Network Analyzers ........................................................................................17
5.4
RF Power Meters ........................................................................................................17
6 Conclusion ........................................................................................ 18
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A Simplified Communication System
1 A Simplified Communication System
Let’s start with the basic blocks of a typical communications system (Figure 1).
Basically any system will include an information source, information processing, the
transmitter, transmission medium, a receiver, and then again information processing,
and information destination. In the early days the signal was primarily voice, but of
course today data is a major part of the communicated information.
The information processing is taking the source information and putting into a format
which can be transmitted over whatever medium is required. In a wireless or RF
system, we’re typically communicating through free space or air, as opposed to
through a wire or a fiber network.
In the early days of RF design it was more common to have a one-way path for the RF
signals. Radio and television are good examples. These signals were transmitted via
large antennas to many radios and televisions. Today’s mobile phone is a great
example of a more common device which is able to both transmit and receive signals.
This paper focuses on transmit and receive portion of an RF communication system,
discusses the key components, and test equipment to ensure performance verification.
Information
Source
Information
Processing
Transmitter
Transmission
Medium
Information
Destination
Information
Processing
Receiver
Figure 1. Simplified Communications System Block Diagram
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RF Fundamentals
2 RF Fundamentals
In the early days of wireless communications signals were basically sine waves.
Remembering back to our school days that a sine wave can be represented with a
frequency, an amplitude, and a phase.
V = Asin (ωt + φ)
where
A = amplitude
ω = 2πf, where f is the frequency
φ = phase
Figure 2 shows two signals in the time domain. In terms of our communications the
intent is to send information from a source to a destination by modifying these sine
waves. Today of course it is more common to have modern digital signals that are
transmitting higher data rates with a signal that is much more complex.
Period = 1/freq
Period = 1/freq
1
0.8
0.6
0.4
0.2
0
0
2
4
6
8
10
12
14
16
-0.2
-0.4
-0.6
-0.8
-1
sin(x)
sin(2x)
Figure 2. Basic sine waves are used to carry information by varying their frequency,
amplitude, and/or phase.
Typically, in terms of RF and microwave signals, we tend to look more in the frequency
domain than in the time domain. Figure 3 shows a basic signal on a spectrum analyzer
display. As the transmitted signals become more complex modulated signals or signals
with more information put on them, the spectrum analyzer displays are excellent for
understanding the multiple frequencies and modulation techniques.
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RF Fundamentals
Figure 3. Spectrum analyzers are excellent tools for evaluating transmitted RF and
microwave signals.
2.1 Spectrum Allocation
The Federal Communications Commission or FCC both defines and allocates the
frequency spectrum in the United States. Similar government bodies around the world
do the same for their countries and regions. Figure 4 shows the spectrum allocation
chart for the US.
The local governing body, such as the FCC, decides who gets to transmit what, over
what frequencies, what power levels and how much bandwidth they get to do it. When
designing a wireless system all radiating devices must adhere to their portion of the
frequency allocation table. Many aspects of the spectrum have been defined for a long
time. Other areas, such as the old analog TV bands have become available as
countries have moved to new digital TV bands. It is important to note that the spectrum
allocation varies across countries. For instance, emergency responder equipment from
one country may not work in another country. Commercial wireless solutions like
Bluetooth and Wi-Fi must be carefully coordinated among countries and manufacturers
to ensure devices work in different regions.
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RF Fundamentals
Figure 4. Each country or region defines and allocates the frequency spectrum for use
by various groups and applications.
2.2 Wavelength Matters
Frequency is a critical parameter in RF design. Early applications were focused on
audio frequencies, now referred to as analog systems. While there still are plenty of
audio devices in use, there has been a broad increase of RF applications such as
mobile phones, Bluetooth devices and Wi-Fi. Today, there are many commercial
applications that use microwave and even millimeter wave frequencies.
Table 1 highlights the fact that as the frequency increases, the wavelength decreases.
The effects of wavelength on a design can have implications on design complexity and
end product costs. First let’s look at how to determine the wavelength:
where
𝜆=
𝑣
𝑓
𝜆 = wavelength
8
𝑣 = phase velocity, which in free space is 3x10 m/s
𝑓 = frequency
Table 1. Examples of Wireless Applications and Their Wavelengths
Application
Frequency
AM Radio
FM Radio
Wi-Fi, 802.11
Automotive Radar
1000 kHz
100 MHz
2.4 GHz / 5 GHz
77 GHz
Wavelength
(metric)
300 m
3m
125mm / 60 mm
4 mm
Wavelength
(English)
968 ft
9.7 ft
5.9 in / 2.4 in
0.15 in
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RF Fundamentals
So how does wavelength affect the RF design? When you consider the device size
relative to wavelength the physical geometry may become an important consideration.
From the signals in Figure 5 we see that the sine wave starts at zero, it goes up to a
maximum, comes back down to zero, goes to a minimum, and come back up to zero
across a single wavelength. From Table 1, we can see that at audio frequencies this
happens across a distance of meters to hundreds of meters. The phase effects of
moving through a typical analog device are therefore minimal. However, as you move
into RF frequencies or higher, the effect of these phase variations becomes a design
consideration. Certain circuit design techniques take advantage of 𝜆 /4 and 𝜆 /2 effects
to optimize or cancel signals, which is really a way to minimize the effects of the topic
in our next section.
2.3 Reflections and Interference
As the sine wave propagates down a transmission line or a cable, what happens when
it hits a discontinuity or some change in impedance? This typically occurs at a
connector or a solder point or even changes in the widths of a transmission line. Figure
5 shows a signal that is propagating down a transmission line and hitting a
discontinuity represented by a green box. While some percentage of that signal will
pass through the green box, some of the signal is reflected back.
This reflected signal will add in and out of phase depending on the phase of the
signals. The red sine wave that’s reflecting back could be completely out of phase with
the incoming signal and could actually cancel out the signal. These effects need to be
analyzed and mitigated when designing your transmission lines and circuit boards at
higher frequencies.
Impedance
Change
Figure 5. Discontinuities in transmission lines often cause reflections which create new
signals that may interfere with and distort the desired signals.
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RF Fundamentals
2.4 Determining Power Using dBs
Typically signal levels for RF and microwave applications are discussed at the power
level and not as voltages and currents. The power in watts is often converted into
logarithms or decibels (dB).
Back in the early days calculators and spreadsheets did not exist. In order to
manipulate signal levels quickly they started using dBs because you can simply add
8
6
4
things. Rather than multiplying a ratio of 10 times 10 and factoring losses of 10 , the
logarithmic components can simply be added. If you know the losses of your
components or gain from an amplifier, one can simply add them up to get an estimated
signal level.
Table 2. Power levels are easier to determine using dBs
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System Design Blocks
3 System Design Blocks
Let’s take a look at the key system blocks in a modern RF communication design. We
will start with the transmitter which is basically our information source. Next is the
antenna that is physically designed relative to the wavelength of the signal so that a
standing wave on that antenna will allow propagation through space. The receiver side
also has an antenna and amplifies the signal to a usable level and then the information
is processed.
How far away are the transmit and receive antennas? How much antenna gain is
needed? How sensitive does the receiver need to be? What are the government
restrictions, if any, on the frequency, bandwidth, and power level? There are many
considerations which will influence the ultimate design needs of the communication
system. Table 3 highlights several key considerations that will influence your RF
design.
Table 3. Potential RF Design Considerations
Link budget?
 How big a signal do I need to
transmit?
 How good a receiver or LNA do I
need?
 What kind of antenna do I need?
Where from to?
What does environment look like?
 Weather
 Obstacles
 Direct Line of Sight
 Spectrum
o What am I sending
Real-time
How fast
o How much data
o Am I moving, stationary, how fast
o Physical limitations
o Size
o Weight
o Power
o
o
3.1 Transmitters
Figure 6 gives a high level view of the key components that make up the transmit side
of a communication system. First the multiplexer is used to route the desired
information into the transmit path. Signal processing is then added to condition the
signal for transmission. Next the signal is modulated into whatever modulation scheme
the application requires. In the early days this may have been Amplitude Modulation
(AM) or Frequency Modulation (FM), today there are a wide range of digital modulation
schemes with many based on Orthogonal Frequency-Division Multiplexing (OFDM)
techniques.
Up to this point the signal is typically at a very low or baseband frequency. A frequency
converter is used to mix the transmitted signal up to the frequency that has been
allocated for the particular application. The signal level is then increased to the
appropriate power level using an amplifier. Next, the signal is passed through a filter to
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System Design Blocks
make sure a clean signal is transmitted and that the signal stays within the allocated
frequency band. Finally the signal is radiated through the air via an antenna.
Signal
Processing
Multiplexer
Power
Amplifier
Frequency
Change
Modulation
Filter
Figure 6. Simplified transmitter block diagram
3.2 Receivers
The receiver side basically has similar components but in the opposite order (Figure 7).
The signal is received by the antenna and then run through a filter to eliminate all of
the signals outside the frequency band of interest. As the signal is at a fairly low level,
a low noise amplifier (LNA) is used to raise the desired signal above the noise floor.
The signal is then down converted to a lower frequency or baseband frequency where
it is then demodulated, processed and directed to the appropriate signal path for the
received information.
Antenna
Frequency
Change
LNA
Filter
Demodulation
Signal
Processing
Demux
Figure 7. Simplified receiver block diagram
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System Design Blocks
3.3 Antennas
Antennas are one of the key aspects of the whole communication system as they
convert the electrical power into the radio waves and vice versa. Some of the key
parameters for antenna include gain, directivity and return loss. These parameters
determine the required size of the antenna and how directional the antenna beam must
be. Many antennas, such as those found on mobile phones and GPS devices, are
isotropic in that they send energy equally in all directions. Satellite dishes and base
station antennas are good examples of fairly focused antenna beams which may be
used over long distances in fixed position applications (Figure 8). Antennas are
evaluated at an antenna test facility where there beam patterns are measured by
rotating the antenna through 360 degrees in both horizontal and vertical planes while
measuring power levels (Figure 9).
Figure 8. Antennas come in all shapes and sizes
Figure 9. Antenna performance is evaluated by measuring the beam patterns in both
the vertical and horizontal planes.
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System Design Blocks
3.4 Putting It All Together
Today, in many cases, one doesn’t have just a transmitter or a receiver. Many devices
are two-way such as your mobile phone, laptop or tablet. Each of these devices
actually contains transmitters, receivers and a series of antenna. These devices create
a new set of challenges in designs that try to minimize signals bleeding into different
sections. Future courses will cover these types of designs in more detail.
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12
Components
4 Components
Next let’s look at a few of the key components that we discussed at the system level.
The design or selection of these components must be clearly understood to ensure
that the system meets the need for the specific application.
4.1 Filters
As we discussed earlier, filters play an important role in both transmitter and receiver
design. On the transmit side it is critical that the radiated signals adhere to the FCC
guidelines for a particular application. On the receive side it is important that all
extraneous signals that are picked up by the receive antennas are filtered out to
achieve optimum signal quality of the desired signal.
There are many different types of filters including low pass, high pass and band bass.
The low/high pass designs basically filter out all frequencies above/below the designed
frequency. A band pass filter restricts frequencies to a particular frequency band
(Figure 11).
Figure 11. Illustration of a band pass RF filter performance characteristics
The In-band or pass band performance is specified to minimize degradation to the
desired frequencies. The bandwidth is defined by either (or both) the 3 dB and/or 1 dB
drop in signal level on either side of the center frequency. The amount of lost power or
insertion loss across the desired frequency range is another critical parameter. Finally,
how much variation or ripple is caused to the magnitude of the signal and the same for
the phase response across the pass band?
For out-of-band performance the critical parameter is with regards to how much the
filtered signal is reduced. There may be different levels of performance based on
distance from the filter’s center frequency.
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Components
4.2 Amplifiers
Amplifiers are also often located in both transmitter and receivers designs, however
they have very different roles and performance requirements.
On the transmitter side, the desired signal has been created and is very well defined.
Power amplifiers (PAs) are used to increase the signal level to the required power
range to allow the radiated signal to be received at the other end, as well as meet the
FCC requirements. The PA may be used in either the linear or nonlinear region. In the
linear region, an increase in input signal yields a defined increase in the output signal,
this is referred to as gain. However, at some point the input signal becomes large
enough that the output starts to increase but at a different rate. This is commonly
referred to as the non-linear region. The rate of roll off in the gain vs. power
measurement is known as the 1dB compression point.
Amplifiers operating in their non-linear region often have their harmonic and spurious
signals specified. Amplifiers designed to operate a specific center frequency, Fc, will
also radiate at multiples of the center frequency - 2Fc , 3Fc and so on. In addition,
spurious signals may be generated from the transmitter components such as the
power supply and amplified by the PA. Amplifier designs try to minimize these effects
and the filters that were previously discussed are also key in minimizing these effects.
The Adjacent Channel Power Ratio (ACPR) is typically specified to ensure that the
radiated signal stays within its given channel and does not spill into the adjacent
channel. The entire allocated spectrum for a technology, such as Wi-Fi is divided up
into channels to increase traffic capacity. It is important that signals stay within their
specified channel and do not leak signals into other channels.
Dynamic range is usually specified to let system designers know what the minimum
and maximum level signal is that can be transmitted.
On the receiver side the antenna is bringing in both the desired signals and also
unknown signals over-the-air. These signals tend to be at lower power levels and may
need to be boosted to separate from the noise floor. These amplifiers are often Low
Noise Amplifiers (LNAs).
Over-the-air there is something that is referred to as thermal noise or kTB, where k is
Boltzmann's constant, T is the absolute temperature of the load (for example a
resistor), and B is the measurement bandwidth. The standard number we use for that
is -174 dBm per hertz, which is the noise floor relative to a one hertz bandwidth. The
goal of the LNA is to raise the signal above this noise level so that it can accurately be
used. One of the key specifications of an LNA is its noise figure. The noise factor is the
ratio of actual output noise to that which would remain if the device itself did not
introduce noise, or the ratio of input SNR to output SNR. The noise figure is the noise
factor expressed in decibels (dB)
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Components
4.3 Mixers
Mixers are the component involved in the frequency up or down conversion in the
transmitter/receiver design. On the transmit side the created signal or baseband signal
is feed into one arm of the mixer. On the other is a local oscillator (LO) that is designed
to mix the baseband signal to the appropriate frequency to be radiated via the third arm
to the antenna. On the receiver side it is the same except for the reverse direction. The
LO is used to mix the RF signal down to baseband.
The mixer produces not only the product of the baseband and LO frequencies but also
the difference of the two frequencies. As the mixer is a non-linear device, it also
creates the harmonics of both the product and difference signals. Based on the design
of the mixer, certain levels of both the input signal and the LO signal may “bleed
through” the mixer and be part of the output signal as well (Figure 12).
While mixers play a critical role of moving signals to the proper frequency range, they
can also be contributors for noise into the desired signal. Filtering plays a key role for
reducing the effects created by unwanted mixer products.
Figure 12. Mixer spectral output
4.4 Modulators/Demodulators
The modulator/demodulator basically adds or extracts the information into the signal
that will be either transmitted or received. Typically the modulation involves adding
information to any or all of these parameters:
▪
▪
▪
Frequency
Amplitude
Phase
In the early days modulation formats were analog, such as TV, radio and the early
mobile phones. Analog modulated signals are continuous variations to the carrier
wave. Modern wireless signals typically use digital modulation techniques. For digital
modulation, signals at different states exist which represent sequences of bits. Further
discussions on modulation techniques will be covered in more detail in a future paper.
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Test and Verification Instrumentation
5 Test and Verification Instrumentation
Throughout the design process it is important to evaluate the performance of your
components and systems. Rohde & Schwarz offers test equipment that will verify your
designs from prototypes through manufacturing and even field operations. This test
equipment can be used to verify not only the total performance of the system, but can
also be used to replace parts of the system which may not be available at test time.
This section provides a brief review of the most common types of test equipment for
performing this verification.
5.1 Spectrum Analyzers
The testing of a transmitting device or subsystem
generally requires a receiver of some sort. Spectrum
analyzers have been designed specifically for this
purpose and are one of the most common pieces of
test equipment that will be found in an RF lab. A
spectrum analyzer is a basic measurement device
that is required if looking at complex signals or
where multiple signals are being used. The basic
measurement is frequency versus power.
Modern versions have many new capabilities for measuring including:
▪
▪
▪
▪
▪
Noise Figure
Group Delay
Phase Noise
Basic Modulation Analysis
Complex Modulation Analysis for:
o Mobile Wireless
o Wireless LAN
o Bluetooth
o Satellite Communications
o RADAR
5.2 Signal Generators
Signal generators are used to represent the transmitter
by creating the required signals with the proper
modulation formats. They are used as a basic
measurement device when required to generate simple
and complex input signals. There are two main types:
analog and vector. The analog signal generator is used to
create basic sine waves at different power levels and frequencies. They typically have
basic modulation capabilities such as AM, FM, phase and pulse. Vector signal
generators are used to create the more complex digitally modulated signals that are
quite common these days.
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Test and Verification Instrumentation
Modern versions have many new capabilities for generating complex signals including:
▪
▪
▪
▪
AM, FM, PM
Arbitrary signals generated mathematically
Frequency hopping signals
Complex Modulation Signals including:
o Mobile Wireless
o Wireless LAN
o Bluetooth
o Satellite Communications
o RADAR
5.3 Vector Network Analyzers
Vector network analyzers (VNAs) are used primarily for
verifying component level performance. The VNA is a
more complex measurement device used to stimulate
and measure amplitude and phase response of high
frequency devices. Basic use is to stimulate a device
such as an amplifier with a sine wave and measure the
amplitude and phase response. Network analyzers
typically measure the Scattering parameters or S-parameters as signal power and
energy considerations are more easily quantified than currents and voltages.
Modern versions have many new capabilities for measuring more complex parameters
or devices such as:
▪
▪
▪
▪
▪
Balance/differential and true differential
Non-linear characteristics
Impedance matching
Mixers or converters
Multiport devices up to 48 ports
5.4 RF Power Meters / Sensors
A power meter is one of the most fundamental measuring
tools that simply measures the power level coming out of a
device. There are typically two types:
▪
▪
Diode Based – high dynamic range
Thermistor Based – more accurate but lower
dynamic range
Power meters do not provide information as to the
frequency content. Newer power meters typically include
sensor with PC software based measurement unit. They can be used in for example in
conjunction with a signal generator to get the basic frequency response of devices.
Many modern versions also have the ability to measure pulsed or bursted signals.
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17
Conclusion
6 Conclusion
This is intended to be the first in a series of papers on the process of RF system
design. RF system design is a complex process beginning with a detailed
understanding of many things such as:
▪
▪
▪
▪
▪
▪
▪
Operating environment
Size, weight, power
What information to be sent
One-way or two way
Stationary or moving
Target cost
Available spectrum or frequency
After a system is conceptualized, it is typically simulated. Finally a prototype needs to
be built and its performance validated.
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About Rohde & Schwarz
Regional contact
Rohde & Schwarz is an independent group of
companies specializing in electronics. It is a leading
supplier of solutions in the fields of test and
measurement, broadcasting, radiomonitoring and
radiolocation, as well as secure communications.
Established more than 75 years ago, Rohde &
Schwarz has a global presence and a dedicated
service network in over 70 countries. Company
headquarters are in Munich, Germany.
North America
1-888-TEST-RSA (1-888-837-8772)
[email protected]
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[email protected]
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[email protected]
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[email protected]
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+86-800-810-8228 /+86-400-650-5896
[email protected]
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Energy-efficient products
ı
Continuous improvement in environmental
sustainability
ı
ISO 14001-certified environmental
management system
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