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CONTENTS
Introduction
Communication system
RF Systems
Direct conversion modulator
DDRF Modulator
Conclusion
References

INTRODUCTION
Increase in number of radio systems in portable devices
High signal bandwidth and narrow channel bandwidth
Need for low cost,low power and low complexity transmitter architecture
Need for adaptation to several different radio systems

Communication system
COMPARISON OF RF SYSTEMS
Direct conversion modulation

Inputs:
In-phase and quadrature components.
DAC
Reconstruction Filter
Modulator
Local oscillator
Phase splitter
RF mixer
Driver amplifier
SAW amplifier
Power amplifier
Antenna




Let x[n] be a signal such that
x[n]=Acos(ωn+Ø)
=AcosØcosωn-asinØsinωn
=xi[n]+xq[n]
xi[n] is called in-phase component
while xq[n] is quadrature component

Reconstruction Filter
A reconstruction filter is used to construct a smooth analogue signal from the output of a digital to analogue converter(DAC) or other sampled data output device.
RF MIXER
Translates frequency by adding or subtracting local oscillator frequency.
Typical characteristics:
-noise figure 12dB
-IP3 +5dBm
-Gain 10dB
-Input impedance 50Ώ
-port to port isolation 10-20dB
DRIVER AMPLIFIER
It includes a voltage level shifting circuit and an Op-amp.
The positive and negative power supply terminal of Op-amp receive a first reference voltage and a second reference voltage outputted from the voltage level shifting circuit.
It keeps a dc voltage level of an output signal to be equal to 0V.
The absolute value of a voltage difference between the first reference voltage and the second reference voltage is equal to VDD.
Thus the circuit operates without risking a high-voltage damage.


Surface Acoustic Wave Filter
It is a small and thin filter using surface acoustic wave(SAW) excited on a piezoelectric substrate.
A SAW filter is an element used for removing noise and interference through selective pass of the bandwidth of a particular frequency.
It uses piezoelectric characteristics that are inter-transducible between electricity and machinery.
A SAW filter is widely used as a band pass filter in communication devices and other electronic devices.
Power Amplifier
Feeds RF signal to antenna for transmission.
Typical characteristics-
∙Output power +20 to +30dBm
∙Efficiency 30% to 60%
∙IMD -30dBc
∙Supply Voltage 3.8 to 5.8V
∙Gain 20 to 30 dB
∙Output harmonics -50 to -70 dBc
∙Power control On-off or 1dB steps
∙Stability factor >1
ADVANTAGES
High integration level.
This architecture is flexible.
No need of external filtering.
Controllable gain.
Eliminates costly external components.
Inherent compactness and ease of integration than heterodyne architecture.
DISADVANTAGES
Carrier leakage.
Low noise performance.
Non-linearity.
Reduced EVM performance.
Increased complexity.
Direct Digital RF modulator
To provide relief from the problem field produced by direct conversion transmitter, this architecture is established.
BLOCK DIAGRAM
COMPONENTS
LPF-Low Pass Filter
DRFC(Digital to RF converter)
-D/A Converter
-Up Converter
Local Oscillator
-DDS
Phase Splitter
RF Mixer
-Gilbert –cell mixer
Power Amplifier
Direct Digital Synthesis
It is a method of producing an analog waveform-usually a sine wave-by generating a time –varying signal digital form and then performing a digital-to-analog conversion.
It has three main parts,
-Phase Accumulator
-Phase to Amplitude Converter
-DAC
Gilbert-Cell Mixer
Mixers shifts the frequency spectrum of an input signal.
Gilbert cell uses a linear time varying circuit to achieve time domain multiplication, and hence frequency shifting.
It is a modified differential amplifier, and so it is an active filter.
ADVANTAGES
Missing of analog baseband eliminates the possibility of using reconstruction filtering after D/A converter.
Due to non linear operation, non idealities can be removed.
Modulator structure relatively immune to affects of transistor mismatches.
Good EVM performance and highly beneficial for WCDMA transmitter.
DDRM can directly drive power amplifier without any high selectivity filtering.
DISADVANTAGES
Quantization noise level.
Digital signal replicas.
Less sensitive to the drawbacks of CMOS technology scaling.
CONCLUSION
Compared different radio systems on the basis of channel bandwidth and RF bands.
Discussed the advantages and disadvantages of Direct conversion modulator and its detailed study.
Understood the need for the DDRF modulator through the detailed analysis.
REFERENCES
[1]A.A.Abidi, “The path to the Software-Defined Radio Receiver,” IEEE J.Solid-State Circuits,vol.42,May2007,pp.954-66
[2]Petri Elorant, Pauli Seppien, Aarno Parssinen “Direct-Digital RF-Modulator: A Multi-Function Architecture for a System-Independent Radio Transmitter” IEEE J.Solid State Circuits,April2008, pp.144-151
[3]P.Eloranta et al., “A Multimode Transmitter in 0.13um CMOS using Direct Digital RF Modulator,” IEEE J.Solid-State Circuits,vol.42,Dec 2007, pp.2774-84