|RF transmitter module|
|Thursday, 18 January 2007|
In cellular systems, a plurality of base stations is distributed across the service area. Each base station services wireless communications within a respective cell. Both base stations and subscriber units include radio frequency transmitters and RF receivers, together they're called "RF transceivers." RF transceivers service the wireless links between the base stations and subscriber units. Transceivers are used in wireless communications to transmit and receive electromagnetic waves in free space. RF receivers generally either convert an input RF signal to an intermediate frequency, or directly mix an input signal to a direct current (DC) signal. The transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The RF transmitter receives a baseband signal from a baseband processor, converts the baseband signal to an RF signal, and couples the RF signal to an antenna for transmission. In most RF transmitters, the baseband signal is first converted to an intermediate frequency (IF) signal and then the IF signal is converted to the RF signal. The data modulation stage converts raw data into baseband signals in accordance with the particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna. The function of the receiver is to detect signals in the presence of noise and interference, and provide amplification, downconversion and demodulation of the detected the signal such that it can be displayed or used in a data processor. The RF receiver receives an RF signal, converts the RF signal to an IF signal, and then converts the IF signal to a baseband signal, which it then provides to the baseband processor. As is also known, RF transceivers typically include sensitive components susceptible to noise and interference with one another and with external sources. The RF receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives an inbound RF signal via the antenna and amplifies it. The one or more intermediate frequency stages mix the amplified RF signal with one or more local oscillations to convert the amplified RF signal into a baseband signal or an intermediate frequency (IF) signal. Typical transmit circuitry includes a feedback loop (often a phase-locked loop, or PLL) that has a voltage-controlled oscillator (VCO) and a loop filter circuitry. Phase locked loops (PLLs) are becoming increasingly popular in integrated wireless transceivers as components for frequency generation and modulation. PLLs are typically used for one of a variety of functions, including frequency translation to up-convert a baseband (BB) signal to an intermediate frequency (IF) or to up-convert a baseband or IF signal to RF prior to amplification by a power amplifier and transmission. Inductive/capacitive (LC) oscillators are important elements of RF transmitters, where the LC oscillators are used as master oscillators, or as receivers where the LC oscillators are used as local oscillators.
In general, the function of a radio frequency (RF) transmitter is to modulate, upconvert, and amplify signals for transmission into free space. An RF transmitter generally includes a modulator that modulates an input signal and a radio frequency power amplifier that is coupled to the modulator to amplify the modulated input signal. The radio frequency power amplifier is coupled to an antenna that transmits the amplified modulated input signal. Power amplifiers are required in radio telecommunication systems to amplify signals before transmitting, because a radio signal attenuates on the radio path. For efficiency, the amplifier is often a non-linear amplifier operated near its peak capacity. To avoid distortion of the transmitted signals due to the non-linearity, the signals are pre-distorted by a predistorter before they are transmitted. The predistortion is required to prevent transmitter from transmitting signals on channel bands other than the band assigned to the transmitter. Digital predistortion may be performed by multiplying the modulated signals prepared for transmission by a set of predistortion values. The predistortion values are chosen such that the product values entering the power amplifier will be distorted by the power amplifier to return to a substantially linear amplification of the modulated signals. A direct conversion transmitter system to produce a transmission signal is generally comprised of a low oscillator (LO), a phase locked loop (PLL), a quadrature generator, a modulator, a power amplifier (PA), and one or more filters. The low oscillator, coupled to the PLL, produces a signal with a frequency that is substantially equal to the frequency of a desired RF transmission signal. The quadrature generator is coupled to the low oscillator and the modulator. The PA is coupled to the quadrature generator, and receives the transmission signal and amplifies it. The amplified signal may go through a filter to reduce noise or spurious outputs outside of the transmission band. High quality RF transmitters typically include bandpass filters, such as surface acoustic wave (SAW) filters provide excellent performance. A typical cell phone may employ a bandpass filter following the power amplifier to reduce undesired noise present at the antenna in different portion of RF spectrum to meet various standards' regulations and specifications. The optimal functioning of a transmitter in a telecommunications system depends upon the suitability of the bandwidth of the transmitted signal to its data rate and modulation type. Cellular phones are designed to operate within the environment of one of several mobile communications networks. Transmitters and receivers for communication systems generally are designed such that they are tuned to transmit and receive one of a multiplicity of signals having widely varying bandwidths and which may fall within a particular frequency range.