A number of functions must be performed in order to transmit voice, data, and other types of baseband signals within a communications system. These functions include filtering, amplifying, and then modulating the signals up to carrier frequencies sufficient to meet system specifications. The type of modulation performed serves as a basis for classifying the transmitter. When modulation (or up-conversion) of the baseband signal is performed in two steps, the transmitter is said to have a dual-conversion architecture. And, when modulation is performed in one step, the transmitter is said to have a direct-conversion architecture.
FIG. 1 shows a transmitter having a conventional dual-conversion architecture. This transmitter includes a modulator which performs up-conversion of a baseband signal in two steps. In the first step, the I and Q components of a baseband signal are converted to an intermediate frequency (IF) based on phase-shifted versions of a local oscillator signal LO1 input into mixers 1 and 2. The IF signals are then combined and converted to a carrier frequency based on a second local oscillator signal LO2 input into mixer 3. Finally, the resulting RF signal is filtered, amplified, and transmitted through an antenna for subsequent demodulation in a receiver.
FIG. 2 shows a transmitter having a conventional direct-conversion architecture. Unlike the dual-conversion transmitter, the direct-conversion transmitter generates an RF transmission signal using only one modulation step. Prior to modulation, digital signals along the I and Q channels are converted to analog signals by DAC 4, filtered in LPF 5, and amplified by VGA 6. The signals are then modulated by respectively mixing them with phase-shifted versions of a local oscillator signal LO in mixers 8 and 9. Because the local oscillation signal is set to the carrier frequency, modulation is performed in a single step. To complete the process, the modulated signals are combined, amplified, filtered, and transmitted to a receiver through an antenna. This specific modulation scheme has come to be known as direct-quadrature modulation.
FIG. 3 shows a transmitter having a third conventional architecture known as a translational-loop or offset phase-locked-loop (OPLL) architecture. Like the dual-conversion transmitter, a translational-loop transmitter uses two PLL circuits to generate an RF signal. However, the translational-loop transmitter uses its PLL circuits in a very different way.
The translational-loop transmitter differs from the dual-conversion transmitter by the way in, which frequency translation is performed. In the architecture of FIG. 1, an intermediate frequency (IF) signal is translated to carrier frequency by a mixer 3, which mixes the IF signal with a second local oscillation signal. In a translational-loop transmitter, this mixer is replaced with a control unit 20 which performs the translation to carrier frequency.
The control unit includes a phase and frequency detector and clock frequency (PFD & CF) unit 22, a filter 24, and a voltage-controlled oscillator 26 situated along a forward signal path of the transmitter, and a mixer 27 and a filter 28 situated along a feedback path. The manner in which the control circuit performs frequency translation will now be explained. First, a baseband signal containing information to be transmitted is input into a first mixer 10. The baseband signal may be in the form of Gaussian Minimum Shift Keying (GMSK) data and the mixer may be one similar to the first mixer of the conventional dual-conversion transmitter. As shown, mixer 10 translates the GMSK data from a baseband frequency to an intermediate frequency using local oscillation signal FLO2 generated by phase-locked-loop circuit PLL2. Once mixed, the IF signal is filtered by a band-pass filter 15 to remove undesirable or so-called mirror frequency components.
The control loop translates the intermediate frequency signal to a carrier frequency in accordance with the following steps. First, the voltage-controlled oscillator (VCO) outputs a signal at a preset frequency FVCO. The mixer 27 mixes this signal with a second local oscillation signal FLO1 generated by phase-locked-loop PLL1. The output of the mixer contains two mirror frequencies FVCO+FLO1 and FVCO−FLO1. The band-pass filter 28 removes the higher-frequency signal and inputs the lower frequency signal into the PFD & CP unit.
The PFD & CP unit determines whether the frequency of the IF signal output from filter 15 matches the frequency of the signal output from filter 28. If these signals do not match, the PFD & CP unit generates a difference signal indicating the amount of frequency mismatch that exists. This difference signal is filtered by filter 22 and input into the VCO to control the frequency of FVCO so that the frequency output from filter 28 will match the IF signal frequency. The IF signal may therefore be referred to as a reference signal since the VCO is adjusted until the output of filter 28 (FVCO−FLO1) matches FLO2.
Once a frequency match exists between these two signals, the PFD & CP unit compares the phase of the signal output from filter 28 with the phase of the IF signal. If there is a mis-match, the PFD & CP unit outputs a difference signal which adjusts the VCO output until the phase of the signal output from filter 28 matches the phase of the IF signal. When the frequency and phase of the output of filter 28 matches the IF signal, the frequency of the VCO will be set to the desired carrier frequency. The VCO then outputs the modulated baseband signal at the carrier frequency to an antenna for transmission.
Each of the aforementioned transmitters has benefits and drawbacks.
Dual-conversion transmitters are desirable because narrowband filtering and gain control may be implemented efficiently at the intermediate frequency (IF) stage. Also, by using two local oscillation frequencies to generate transmission signals, dual-conversion transmitters avoid a problem known as injection pulling, which is a phenomenon that usually occurs in direct-conversion transmitters. Dual-conversion transmitters also have proven to be comparatively less problematic than other types of rf transmitters.
In spite of these advantages, dual-conversion transmitters have drawbacks which make then undesirable in certain instances. Perhaps most significantly, dual-conversion transmitters require more hardware than, for example, direct-conversion transmitters. Most of this hardware is in the form of filters and oscillation circuits used to perform the first (or IF) up-conversion of the baseband signal. Dual-conversion transmitters also use separate phase-locked-loop (PLL) circuits to generate the oscillation signals required for up-conversion. While these drawbacks have proven to be significant in terms of costs and complexity, many code division multiple access (CDMA) and time division multiple access (TDMA) mobile phone systems in use today use this type of transmitter.
Direct-conversion transmitters offer advantages which dual-conversion and translational-loop transmitters cannot realize. For example, as previously discussed, direct-conversion transmitters use less hardware than dual-conversion transmitters because only one local oscillation frequency is used to generate the transmission signal. Consequently, only one PLL is required. This same advantage exists over translational-loop transmitters, which also use two PLL circuits for rf signal generation. Direct-conversion transmitters also do not require the feedback loop found in translational-loop transmitters. Consequently, direct-conversion transmitters use less hardware and therefore are more suitable for use in handsets and other highly integrated applications.
In spite of these advantages, direct-conversion transmitters have a number of significant drawbacks. For example, direct-conversion transmitters use duplex filters to meet specifications for noise reduction in the receiving bands of a communication system. These filters cause several dB of loss to occur in the transmitter which must be compensated for by additional power from a power amplifier. This so-called “back-off” power significantly reduces talk time. Consequently, direct-conversion transmitters are not the optimal choice for many mobile applications. For example, translation-loop filters (which do not use duplex filters) have generally been used in TDMA applications (e.g., GSM) over direct-conversion architectures.
Translational-loop transmitters offer advantages which neither of the previous two types of transmitters can achieve. The PLL used in the feedback loop, for example, minimizes external filtering by acting like a tracking narrowband, band-pass filter. This makes translational-loop transmitters desirable for use in GSM handsets in order to reduce cost and power consumption requirements.
Translational-loop transmitters also realize a low-noise floor. This allows the duplex filter used in direct-conversion architectures to be replaced with a simple switch. As a result, the insertion loss associated with the duplex filter is eliminated, which thereby allows the power amplifier in the transmitter to operate at low output power. Unlike in many other transmitter architectures, class-C power amplifiers may therefore be used which provide good power-added efficiency. This is especially significant in a GSM system, where the modulation is a constant-envelope signal.
An additional benefit of a translational-loop system is that the VCO strips off any residual amplitude modulation (AM) component that may exist. This allows the class-C amplifier to be driven even harder, thereby providing an additional measure of power-added efficiency.
For all their advantages, translational-loop transmitters have a number of drawbacks which make them less than optimally efficient when applied in a mobile communications system. Perhaps most significantly, these transmitters must use multiple PLL circuits to produce the oscillation signals required for translating a baseband signal to carrier frequency. These additional oscillators increase the physical dimensions and cost of the handset, as well as its power requirements. As a result, conventional translational-loop transmitters deplete the charge stored in the battery of the handset at a faster than desired rate.
A need therefore exists for an improved system and method for modulating signals in a translational-loop transmitter, and more particularly which generates modulated signals in a more economical and power-efficient manner compared with conventional translational-loop transmitters and with a more highly integrated architecture which consumes less space when incorporated within, for example, a mobile handset.