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Chapter 1
Introduction to CommunicationsCircuits
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Introduction
Radio frequency integrated circuit (RFIC) is one of the fast developing research areas
RFIC circuits were designed as discrete in the past and now integrated into single chip
RFIC finds wide use applications as in cordless phones, cell phones, WLAN, GPS systems remote tags, assets tracking, key less entry for cars, remote sensing and tuners in cable modem
The increasing interest in radio frequency (RF) communications has resulted in an effort to provide components and complete systems on an integrated circuit (IC)
Many researchers aimed at putting a complete radio on one chip which is called System On Chip (SOC)
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Introduction
SOC circuits can be realized using either complementary metal oxide semiconductor (CMOS) technology or bipolar transistor design
The CMOS technology has the advantage of lower power consumption and lower cost compared to bipolar technology
The bipolar technology have the advantage of being older than CMOS and therefore better modeled
The objective of radio communication system is transmit or receive a signal between a source and destination with acceptable quality and without incurring a high cost
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Lower Frequency Versus Radio Frequency Designs
At low frequency, regular circuit theory deign rules can be applied to RFIC design
As the operating frequency increase to the microwave region, the component dimensions became closer to the signal wavelength.
Therefore basic circuit design rules are no longer valid Instead electromagnetic theory need to be employed when
designing the RF circuits The traces connecting between different circuit parts are
treated as transmission lines However advances in technology resulted in the
manufacturing of very small dimensions circuit components (resistors, inductors, capacitors and transistors)
This advances in technology make it possible to deign RFIC with discrete components at RF frequency ranging from (0.1-5) GHz
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Impedance Levels for Microwave and Low-Frequency Designs
In the low frequency design, the input impedance is usually high
On the other hand the output impedance is usually low
For example a given op-amp circuit has almost an infinite input impedance while almost have zero output impedance
The impedance properties of the op-amp makes it good driving device for measurement equipment
On the other hand, if circuits are connected using transmission lines, an input and output matching circuits are required to mach the device I/O impedances of the transmission line
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Units for Microwave and Low-Frequency Analog
Design In microwave circuits, signals, noise, or distortion are
measured with power The typical unit of measure used is the decibels above 1
milliwatt (dBm) Since infinite or zero impedance is allowed in RF circuits,
power levels became meaningless Therefore voltages and current are usually chosen to describe
the signal levels Voltage and current are expressed as peak, peak-to-peak, or
root-mean-square (rms) Power in dBm, PdBm, can be related to the power in watts,
Pwatt , as shown in (1) The voltages when computing power are assumed across 50
ohm resistors
mW
PwattPdBm
1log10 10
(1)
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Units for Microwave and Low-Frequency Analog
Design Assuming sinusoidal signal, the power in watt
is given by
Where R is the resistance where the voltage is developed across
Vrms is related to the peak voltage according to
R
vPwatt rms
2
22pp
rms
vv
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Units for Microwave and Low-Frequency Analog
Design The following table lists different values for vpp,
vrms and the associated power in what and in dBm across 50 Ω resistor
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Blocks of communication transceiver
Any communication transceiver is composed from the transmitter and the receiver
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popular receiver architectures
In the early communications system there were two different receiver architectures
The first was the tuned radio frequency receiver The second was the super heterodyne receiver The third type is the direct conversion receiver
Tuned radio frequency receiver
The tuned radio receiver is composed from three tuned amplifiersThese three amplifiers are tuned to the desired signal frequency before the signal is fed to a detectorThe detector recovers the information signal from the carrier
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Amplifier and filterRF input
Amplifier and filter
Amplifier and filter
Base bandDetector
Tunner
Tuned Radio receiver
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Tuned radio frequency receiver
The tuned radio frequency receiver has the disadvantage of tuning the three tuned amplifiers to the same carrier frequency
It was replaced by the super heterodyne receiver which has a better filter sensitivity
The super heterodyne receiver
The super heterodyne eliminates the need for tuning all the RF amplifiers to the frequency of the RF input
It works by shifting the frequency of the RF input signal to the frequency of the receiver IF filter
This means that a fixed frequency IF filter can built which has better performance compared to the variable tuned RF amplifiers
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Super heterodyne receiver
Amplifier and filter
IF filter Detector Base band
Mixer
LO@ fo
Signal fc1
Signal fc2
RF input
The super heterodyne receiver
The main advantage of the super heterodyne receiver over the TRF is that the same high quality filter can be used for all input signals
The frequency selection is made by varying the local oscillator frequency
The super heterodyne receiver suffers from the image frequency problem
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Super heterodyne receiver
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Image frequency
What is the image frequency? When a signal with carrier frequency fc1 is fed to the
input of a super heterodyne receiver then the frequencies appears on the IF stage are fIF=flo-fc1 and fIF=flo+fc1 (assuming that fc1 is less than the flo)
If another signal of frequency fc2 appears at the same time at the input of the mixer then
Another tow frequency components appears on the IF stage these are fIF=fc2-flo and fIF=fc2+flo (assuming that fc2 is greater than the flo)
By adding the equations fIF=flo-fc1 and fIF=fc2-flo
The result is 2fIF=fc2-fc1
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Image frequency
The previous result shows that if fc2 is spaced in frequency 2fIF from fc1 then the mixer will bring both signals (fc1 and fc2 to the IF stage)
This results in unwanted distortion in the Rx The signal with fc2 is called the image of the signal
with frequency fc1
The purpose of the image reject filter will be to prevent such an action
One of the solutions to this problem is to select the fIF>(fc2-fc1)
Another solution can be realized by the use of tow mixers and tow different local oscillators
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Image frequency example
Consider a receiver with the IF filter centered at 455 kHz. If it is desired to receive a 1 MHz input signal, the local oscillator is tuned at 1.455 MHz determine the image frequency that correspond t the 1 MHz signal.
SolutionThe image frequency is determined from the
relationfimage=fsignal+2fIF OR fimage=fIF+flo
fimage=1 MHz+2*455 kHz or fimage=1.455 MHz+0.455 MHz
=1.91 MHz
The direct conversion receiver
The direct conversion receiver is an immediate extension of the super heterodyne Rx
In the direct conversion receiver the IF section is eliminated
The receiver works by converting the input signal directly to current base band
18Direct conversion receiver
The direct conversion receiver
The conversion is done by setting the local oscillator frequency to the input signal frequency
The mixer output contains signal at the base band frequency and another signal located at the base band frequency+2fLO (high frequency signal)
The high frequency signal can be removed by using the LPF
The advantage of this receiver is that the LPF filter is much easier to build compared with the IF band pass filter
The disadvantage of this receiver is the local oscillator drift
19Direct conversion receiver
The direct conversion receiver
Also this receiver has a DC offset as an another problem
Another problem with such receiver is that a complete synchronization between Tx and Rx local oscillator is needed
Direct conversion receiver finds applications in many battery operated systems
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Transmitters
Transmitter does the following tasks Modulates the information signal by the carrier Does frequency up conversion Amplify the signal using power amplifier Finally route the signal to the antenna prior to transmission
Direct conversion receiver
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A modernCommunications Transceiver A typical block diagram typical super-
heterodyne communications transceiver is shown below
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Reciever building blocks
The communication system is composed from a transmitter and a receiver as shown in the previous slide
The receiver is composed from the Antenna Preselect filter Low noise amplifier Image reject filter Mixer Frequency synthesizer (Local oscillator) IF filter Automatic gain control (AGC) unit Analog to digital converter and DSP processing unit
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Transmitter building blocks
The receiver is composed from the following blocks Base band modulation plus Digital to Analog
(D/A) converter Mixer Frequency synthesizer (Local oscillator) Power amplifier (PA) Antenna
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Description of various transceiver blocks
The transmitter and the receiver are connected to a single antenna through a duplexer
The duplexer can be viewed as a switch or a filter depending the communication standard to be used
The pre-select filter removes the signals not in the band of interest
This may be required to prevent overloading of the (LNA) by out-of band signals
The LNA amplifies the input signal without adding much noise
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Description of various transceiver blocks
LNA is used in the first amplification stage to strengthen the weak signal detected by the receiver
The LNA does not add much noise to the amplified signal
The use of LNA reduces the effect of noise added to the signal by the other electronic components in the receiver
The image reject filter removes the image signals and the noise before the down frequency conversion stage
