chapter3 basic concepts in radio communications circuit design
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CircuitTRANSCRIPT
Dr. Cuong HuynhTelecommunications DepartmentHCMUT 1
Huynh Phu Minh Cuong
Department of Telecommunications
Faculty of Electrical and Electronics Engineering
Ho Chi Minh city University of Technology
Chapter 3 Basic Concepts in RF Communication Circuit Design
ELECTRONICS AND COMMUNICATIONS
Dr. Cuong HuynhTelecommunications DepartmentHCMUT 2
ELECTRONICS AND COMMUNICATIONS
Chapter 3 Basic Concepts in RF Communication Circuit Design
Reference: [1] Razavi, RF Microelectronics, Prentice Hall, 2 edition, 2011
[2] David Pozaz, Microwave and Rf Design of Wireless Systems,
Wiley, 2000.
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1. General Considerations
Units in RF Design
• This relationship between Power and Voltage only holds when the input and output impedance are equal
An amplifier senses a sinusoidal signal and delivers a power of 0 dBm to a load
resistance of 50 Ω. Determine the peak-to-peak voltage swing across the load.
Solution:
where RL= 50 Ω thus,
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1. General Considerations
Example of Units in RF
Solution:
Output voltage of the amplifier is of interest in this example
A GSM receiver senses a narrowband (modulated) signal having a level of -100
dBm. If the front-end amplifier provides a voltage gain of 15 dB, calculate the
peak-to-peak voltage swing at the output of the amplifier.
Since the amplifier output voltage swing is of interest, we first convert the received signal
level to voltage. From the previous example, we note that -100 dBm is 100 dB below 632
mVpp. Also, 100 dB for voltage quantities is equivalent to 105. Thus, -100 dBm is equivalent
to 6.32 μVpp. This input level is amplified by 15 dB (≈ 5.62), resulting in an output swing of
35.5 μVpp.
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1. General Considerations
Maximum power transfer between building blocks
Impedance Matching
Noise
Nonlinearity
Dynamic Range
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Maximum power transfer:
2. Maximum Power Transfer
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Taking the derivative of pL and setting it equal to zero, we find that
This implies
which yields
The power delivered when RL = RTh is
In general, if RL and RTh are the impedances, then the load impedance RL
will be the complex conjugate of the source impedance RTh.
2. Maximum Power Transfer
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• Impedance matching is a major problem in RF/microwave circuit design for communication.
• Impedance matching is the process of transforming a load impedance, ZL , in the optimal working impedance of the signal source Z.
• Impedance matching circuits can be implemented using L, C, transformer or transmission line.
3. Impedance Matching
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– In a specific case, this optimal impedance may be the complex conjugate of the source impedance (Zs), assuring a maximum power transfer, as is usual in small-signal amplifiers.
– As an almost general rule, the reactive component of the source impedance must be compensated by a convenient reactance seen at the input of the matching network, so the signal source operates into a purely resistive load.
– Mismatching in RF power amplifiers may cause reduced efficiency and/or output power, increased stresses of the active devices, distortion of the output signal and so on.
3. Impedance Matching
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– If the RF circuit operates at a fixed frequency or over a narrow
frequency band in comparison with the carrier frequency, the above
requirements must be met at only one frequency, and narrowband
matching networks should be used. Obviously, the matching
circuit must contain L and C in order to specify the matching
frequency ω0.
– If the circuit operates over a wide frequency band, the matching
requirements (or at least some of them) must be met over the entire
frequency range. This requires the use of broadband matching
network.
– At low frequencies (HF, VHF and UHF), the narrowband
impedance matching is usually achieved with lumped element
circuits (will be studied in this course). At higher frequencies,
distributed element networks are most often required.
3. Impedance Matching
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Essential revision : Practical components are lossy
3. Impedance Matching
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(Q: Quality factor)
3. Impedance Matching
Essential revision : Practical components are lossy
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• Series to parallel conversion and vice versa:
Assuming that Xs and Xp in the figure are similar
elements (i.e., both are either capacitances or
inductances), the relations between the elements of the
two circuits are given by:
3. Impedance Matching
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Series to parallel conversion and vice versa:
Note that taking into account that the quality factor:
Then
3. Impedance Matching
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• Two-reactance matching networks (L matching network):
(for R < RL) (for R > RL)
3. Impedance Matching
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The L matching networks in the previous slide have several
drawbacks:
a. The design problem has no solution for some
combinations of matched impedances.
b. The values obtained may be impractical; the values of the capacitors and inductors may be too large or too small.
c. There is no design flexibility. Designers may wish to optimize their designs for other parameters of practical
interest, such as harmonic attenuation, power losses, or
bandwidth.
The three-reactance matching networks are most widely used because they are simple and provide flexibility.
Although each network has limitations, one of the circuits
usually meets the design requirements with practical
component values.
3. Impedance Matching
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• Three-reactance matching networks: Pi matching network
This circuit can be used only if:
The pi matching network is widely used in vacuum-tube transmitters to match
large resistance values. For small resistance values, the inductance
of L becomes unpractically small, while the capacitance of both C1 and C2
becomes very large. This circuit is generally not useful in solid-state RF Power
Amplifiers where the matched resistances are often small.
Recommended values of Q usually range from 1 to 10.
3. Impedance Matching
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• Three-reactance matching networks: T matching network
The T matching network in the below figure is applicable to most solid-state RF Power Amplifiers.
Its design equations are:
3. Impedance Matching
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• Three-reactance matching networks: Two-inductance T matching network
Another T matching network with two inductances and is also applicable to many solid-state RF Power Amplifiers.
The design equations are:
3. Impedance Matching
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• Three-reactance matching networks: Three-reactance L matching network
This network is also very useful in solid-state RF Power Amplifiers because it yields practical components for low values of matched resistances.
The design equations are:
3. Impedance Matching
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Impedance or admittance of an RLC circuits is a
complicated function of frequency, and normally has the real
(resistive) and imaginary (reactance) parts.
In some circuits, the imaginary (reactance) part vanishes at
one or more frequencies.
Resonant circuit - resonant frequencies.
In communication systems, resonant circuits are extensively
used to select the wanted signal and reject the unwanted signal.
They may be used in every single circuit in the receiver and
transmitter (LNA, Mixer, VCO, PA . . .)
There are two main resonant circuits: Parallel and series
resonant circuits . Main specs ???
4. Resonant Circuits
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Parallel resonant circuit:
When this circuit is excited by a current source, and the output is terminated with an
open circuit, the transfer function is
The output voltage, Vout, drops from the resonant value by 2 (or 3 dB)
4. Resonant Circuits
1o
LC
Resonant frequency
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The two 3 dB frequencies of the resonant circuit:
The 3 dB bandwidth of the resonant circuit is the difference between the two 3 dB
frequencies:
The resonant frequency is:
and the value of Q given by:
or
where G = 1/R
4. Resonant Circuits
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(Ideal) parallel-tuned circuit: An ideal parallel-tuned circuit is a paralleled LC circuit that provides zero conductance (that is, infinite impedance) at the tuning frequency, f0, and infinite conductance (zero impedance) for any other frequency. When connected in parallel to a load resistor, R, the ideal parallel-tuned circuit only allows a sinusoidal current (with frequency f0) to flow through the load. Therefore, the voltage across the RLC parallel group is sinusoidal, while the total current (that is, the sum of the current through load and the current through the LC circuit) may have any waveform.
A good approximation for the ideal parallel-tuned circuit is a circuit with a very high loaded Q (the higher the Q, the closer the approximation). Note that a high-Q parallel-tuned circuit uses small inductors and large capacitors, which may be a serious limitation in practical applications.
4. Resonant Circuits
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4. Resonant Circuits
Series Resonant Circuit A series RLC resonant circuit is shown below. The input
impedance is
Resonant frequency 1
o
LC
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4. Resonant Circuits
Q =
Quality Factor
Bandwidth
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(Ideal) series-tuned circuit:
An ideal series-tuned circuit is a series LC circuit that provides zero impedance at the tuning frequency, f0 , and infinite impedance for any other frequency. When connected in series to a load resistor, R, the ideal
series tuned circuit only allows a sinusoidal current with frequency f0 to flow through the load. Therefore, the current through the series RLC group is sinusoidal, while the voltage across the RLC group may have any waveform.
A good approximation for the ideal series-tuned circuit is a circuit with a
very high loaded Q (the higher the Q, the closer the approximation). Note that a high-Q series-tuned circuit must use large inductors and small capacitors, which may be a serious limitation in practical applications.
4. Resonant Circuits
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Higher temperature
The average current remains equal to VB/R but the instantaneous current
displays random values
T must be long enough to accommodate several cycles of the lowest frequency.
5. Noise
Noise: Noise as a Random Process
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Noise in Communications Electronic Circuits
5. Noise
Two main sources of Noise: External Internal
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Thermal noise is the most basic type of noise, being caused by thermal vibration
of bound charges. It is also known as Johnson or Nyquist noise.
Shot noise is due to random fluctuations of charge carriers in an electron tube or
solid-state device.
Flicker noise occurs in solid-state components and vacuum tubes. Flicker noise
power varies inversely with frequency, and so is often called 1/ f –noise.
Plasma noise is caused by random motion of charges in an ionized gas, such as a
plasma, the ionosphere, or sparking electrical contacts.
Quantum noise results from the quantized nature of charge carriers and photons;
it is often insignificant relative to other noise sources.
Noise in Communications Electronic Circuits
5. Noise
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Noise in Communications Electronic Circuits
5. Noise
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Noise generated from a resistor
5. Noise
4n
v KTBR
K: Boltzmann's constant
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Noise in Bipolar Transistors
In low-noise circuits, the base resistance thermal noise and the collector
current shot noise become dominant. For this reason, wide transistors biased
at high current levels are employed.
Bipolar transistors contain physical resistances in their base, emitter, and collector regions,
all of which generate thermal noise. Moreover, they also suffer from “shot noise” associated
with the transport of carriers across the base-emitter junction.
5. Noise
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Noise Figure: measures of degradation of the signal-to-noise
ratio (SNR), caused by components in a RF system.
5. Noise
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Noise Figure
5. Noise
No=NinG + Na =KTBG + Na
Na is the added noise power generated from internal components
a
ino in a
in in in
NN
N GN N GNFGN GN N
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3
n
is the noise factor in linear (not in dB) of the n-th stage,
G is the power gain in linear (not in dB), too.
nNF
-1
5. Noise
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Sensitivity
5. Noise
a
ino in a
in in in
NN
N GN N GNFGN GN N
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Required Receiver Sensitivity – A Qualitative View
• To find Receiver NF – Transmit Power – FCC
regulated – Path loss – Receiver sensitivity – govern
by standards and applications – Required SNR – depends on
BER requirement and modulation scheme
– Noise floor – thermal noise or circuit noise limited depending on the modulation schemes
What is the required receiver NF to achieve
a certain level of sensitivity?
Transmit Power
Input Noise Floor (No/G) Required SNRo
Noise Figure
Path Loss
Receiver Sensitivity
Input Noise (Ni)
5. Noise
Dr. Cuong HuynhTelecommunications DepartmentHCMUT
Required Receiver Sensitivity – A Qualitative View
• To find Receiver NF – Transmit Power – FCC
regulated – Path loss – Receiver sensitivity – govern
by standards and applications – Required SNR – depends on
BER requirement and modulation scheme
– Noise floor – thermal noise or circuit noise limited depending on the modulation schemes
What is the required receiver NF to achieve
a certain level of sensitivity?
Transmit Power
Input Noise Floor (No/G) Required SNRo
Noise Figure
Path Loss
Receiver Sensitivity
Input Noise (Ni)
5. Noise
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In this idealized case, the circuit displays only second-order nonlinearity
linear
nonlinear
The input/output characteristic of a memoryless nonlinear system can be
approximated with a polynomial
6. Nonlinear Distortion
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Example of Polynomial Approximation For square-law MOS transistors operating in saturation, the characteristic above
can be expressed as
If the differential input is small, approximate the characteristic by a polynomial.
6. Nonlinear Distortion
Taylor series: f(x)=
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Harmonic generation (multiples of a fundamental signal)
Gain Compression (gain reduction in an amplifier)
Inter-modulation Distortion (products of a two-tone input
signal)
Cross-modulation (modulation transfer from one signal to
another)
AM-PM conversion (amplitude variation causes phase shift)
Spectral regrowth (intermodulation with many closely spaced
signals)
vi vo
6. Nonlinear Distortion
Effects of Nonlinearity
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Effects of Nonlinearity: Harmonic Distortion
Even-order harmonics result from αj with even j
nth harmonic grows in proportion to An
DC Fundamental Second
Harmonic
Third
Harmonic
6. Nonlinear Distortion
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Gain Compression: 1-dB Compression Point
Output falls below its ideal value by 1 dB at the 1-dB compression point
Peak value instead of peak-to-peak value
6. Nonlinear Distortion
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Effects of Nonlinearity: Cross Modulation
Desired signal at output suffers from amplitude modulation
Suppose that the interferer is an amplitude-modulated signal
Thus
6. Nonlinear Distortion
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Nonlinear Distortion – Desensitization and Blocking
6. Nonlinear Distortion
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Effects of Nonlinearity: Intermodulation assume
Thus
Intermodulation products:
Fundamental components:
6. Nonlinear Distortion
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Intermodulation Product Falling on Desired Channel
desired
Interferer
A received small desired signal along with two large interferers
Intermodulation product falls onto the desired channel, corrupts signal.
6. Nonlinear Distortion
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Solution:
Example of Intermodulation
Suppose four Bluetooth users operate in a room as shown in figure below. User 4
is in the receive mode and attempts to sense a weak signal transmitted by User 1
at 2.410 GHz. At the same time, Users 2 and 3 transmit at 2.420 GHz and 2.430 GHz,
respectively. Explain what happens.
Since the frequencies transmitted by Users 1, 2, and 3 happen to be equally spaced, the
intermodulation in the LNA of RX4 corrupts the desired signal at 2.410 GHz.
6. Nonlinear Distortion
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Intermodulation: Third Intercept Point
IP3 is not a directly measureable quantity, but a point obtained by
extrapolation
6. Nonlinear Distortion
is the point where the output power at 1 equals to
output power at (21 - 2 )
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Third Intercept Point: A reasonable estimate
For a given input level (well below P1dB), the IIP3 can be calculated by halving
the difference between the output fundamental and IM levels and adding the
result to the input level, where all values are expressed as logarithmic
quantities.
6. Nonlinear Distortion
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Nonlinear Distortion – Inter-modulation Distortion (IMD)
Input IP is the point where
the output power at 1 equals
to output power at (21 - 2 )
3rd order intercept point : IP3
6. Nonlinear Distortion
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Nonlinear Distortion – Inter-modulation Distortion (IMD)
Determine IP3 by Spectrum Measurement
6. Nonlinear Distortion
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Example of Third Intercept Point
Solution:
A low-noise amplifier senses a -80-dBm signal at 2.410 GHz and two -20-dBm
interferers at 2.420 GHz and 2.430 GHz. What IIP3 is required if the IM products
must remain 20 dB below the signal? For simplicity, assume 50-Ω interfaces at the
input and output.
At the LNA output:
Thus
6. Nonlinear Distortion
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Nonlinear Distortion – SFDR)
7. Dynamic Range
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Nonlinear Distortion – SFDR)
7. Dynamic Range