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There exists a great demand for wireless devices that achieve a high data transmission rate
for cellular phones and wireless internet services.
The power consumption in such devices has to be low to ensure a longer battery life.
The number of wireless users is increasing exponentially, and the limited frequency
spectrum must be used in an efficient way.
One of the techniques that have been used to service multiple channels in a limited
frequency bandwidth is to place the channels close to each other.
The PA designs that do not have considerable amount of first and third order harmonic
distortion are not very power efficient.
There is a need for PAs with better characteristics that have low distortion and high power
efficiency.
Various linearization techniques are investigated to improve the linearity and facilitate
operation with less back off and high efficiency.
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Three well-known transmitters are considered to
determine the performance using the linearization
techniques., namely
Single step transmitter,
Two step transmitter
PLL based transmitter
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The base-band signal frequency is 50 MHz and the frequency of the voltage controlled
oscillator (VCO) is 1.95 GHz.
The output carrier frequency is approximately equal to local oscillator (LO) frequency.
The input signal is split into an in-phase signal and a quadrature signal.
These signals are fed to the mixer, whose other input comes from the VCO.
The two paths are added in the analog adder before entering the RF PA.
This output signal contains one frequency, which is the VCO frequency minus the input
signal frequency.
The simplicity of the topology makes it attractive for higher level of integration
Single step transmitters have a serious drawback, caused by the disturbance (noise) of the
local oscillator by the power amplifier output.
The feedback from the PA can perturb the VCO because they are at approximately the same
frequency
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Removes harmonicsRemoves unwanted sidebands
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The base-band signal is upconverted to carrier frequency in two or more steps so
that the PA output spectrum is far from the oscillator frequency.
The base-band signals I and Q undergo quadrature modulation at a lower
frequency 1, and is up-converted to 1+2 by mixing and band-pass filtering.
The first band pass filter removes the harmonics of the IF signal and the
second removes the unwanted sideband centered around 12 .
Advantages are
Quadrature modulation is performed at lower frequencies.
Less cross talk between the two input bit streams.
Lower frequency results in better performance of operational amplifier.
Disadvantage
requires filters that may be difficult to implement in integrated form.
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In PLL transmitter ,the output carrier frequency goes through PLL before
entering the PA.Huge reductions of VCO distortion by PA can be achieved if a PLL is used
instead of VCO operating directly at the required RF frequency.
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Supports better functionality to improve the switch voltage and current
waveforms.
The output tank is tuned to resonate at the carrier frequency
In comparison to class E PA, class F stage provides high impedance at
the third harmonic
Hence efficiencies are greater than 85%.
Resonates atcarrier freq
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The input signal Vdrive toggles the switch periodically with approximately 50% dutycycle.
When the switch is ON, a linearly increasing current is built up through the inductor.
When the switch is OFF, this current is steered into the capacitor, causing the voltage
across the switch VS to rise.
The band pass filters then selectively passes the fundamental component of VS to the
load, creating a sinusoidal output that is synchronized in phase and frequency with the
input.
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A linearization technique used for the third order harmonic of a power amplifier is
used to generate a signal, which compensates the non-linear component at the
fundamental frequency of the power amplifier output.
The above method is used along with a phase-locked loop (PLL) transmitter toimprove the system linearity and to reduce first order and third order harmonics.
The linearization techniques applied to the above mentioned transmitter
architectures are
Cartesian feedback,
Feed-forward
Ying linearization
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In Cartesian feedback, the output of the PA is down-converted in a demodulator
circuit, and is compared with the original low frequency signal in a negative feedback
loop
The input signal is at 900 MHz and the oscillator frequency is 1GHz.
The base-band input signals are generated from the quadrature modulator circuit.
The quadrature modulator circuit generates two signals that are 90 degrees out of phase
with each other.
These signals are fed into the differential subtractors in order to generate the error
signals.
The outputs of the differential subtractors are up-converted to RF signals by a VCO.
The resultant RF signals from the two up-converting signals are then added in analogadder to produce a complex RF output signal.
The RF output signal then goes through a PLL loop.
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The power amplifier amplifies the low power RF signal before feeding to the antenna.
The part of the output signal from the power amplifier is attenuated and fed to the down
conversion mixers at a suitable level.
These mixers are supplied with exactly the same oscillator signals because the up-
conversion and down-conversion processes are coherent.
Then the down converted output signals forms the feedback path to the differential
subtractors completing the two feedback loops.
The phase shifter can be used to align the phase of up and down conversion processes
to ensure negative feedback is created and the phase margin of the system is optimized.
PLL transmitter with Cartesian feedback conversion transmitter suffers from the
disturbance of the local oscillator by the power amplifier output.
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The non-linear PA output contains an error (distortion) with
respect to the input signal.
This signal is attenuated, and an error is computed with respect tothe input signal to the PA.
This error signal is subtracted with a proper scale, from the output
signal of the PA to form the final output.
Advantages are:
It has superior performance
Inherent stability
give high inter-modulation distortion (IMD) suppression
Disadvantages are:
its complexity and cost is high
high sensitivity to environmental variations.
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Rf i/p signal
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The RF input signal, running at 1.9 GHz, is amplified by the power
amplifier, which introduces harmonics.
A passive, band pass filter, with a center frequency f0 of 5.7 GHz, filtersnoise outside the third harmonic of the output.
A class F driver with class E load two stage power amplifier was
designed for improved linearity.
A differential subtractor for the subtraction of the synthesized signal
from the PA is employed next to acquire desired output.
A high gain amplifier (HGA) is added to improve the stability of the
feed-forward loop.
Ying linearization technique has achieved a high 1 dB compression point
of 24.51 dBm.
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The 1 dB compression point of PLL transmitter with Cartesian feedback was
21.34 dBm, whereas that with Ying linearization was 24.51 dBm.
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Single step, two step and PLL transmitters are designed in 0.18 m CMOS technology
using Cadence Tools.
The linearity of these transmitters was checked using Cartesian and Ying feedback
techniques.
The class F driver with class E output stage gives better linearity
In PLL transmitter with Cartesian feedback technique, a 1 dB compression point of
21.34 dBm was achieved.
PLL based transmitter with Ying linearization technique achieved a 1 dB compression
point of 24.51 dBm.
This led to an increase of 6dbm in linearity in Ying linearization technique as
compared to Cartesian technique.
Hence Ying linearization is best among other techniques
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Khalid H. Abed, Marian K. Kazimierczuk Shailesh B. Nerurkar Melaka P.
Senadeera, Linearization Techniques In Power Amplifiers For 1.9 Ghz
Wireless Transmitters, 0-7803-9197-7/05/$20.00 2005 IEEE.
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