Many communication systems modulate electromagnetic signals from baseband to higher frequencies for transmission, and subsequently demodulate those high frequencies back to their original frequency band when they reach the receiver. The original (or baseband) signal, may be, for example: data, voice or video. These baseband signals may be produced by transducers such as microphones or video cameras, be computer generated, or transferred from an electronic storage device. In general, the high transmission frequencies provide longer range and higher capacity channels than baseband signals, and because high frequency RF signals can propagate through the air, they can be used for wireless channels as well as hard wired or fibre channels.
All of these signals are generally referred to as radio frequency (RF) signals, which are electromagnetic signals, that is, waveforms with electrical and magnetic properties within the electromagnetic spectrum normally associated with radio wave propagation. The electromagnetic spectrum was traditionally divided into 26 alphabetically designated bands, however, the ITU formally recognizes 12 bands, from 30 Hz to 3000 GHz. New bands, from 3 THz to 3000 THz, are under active consideration for recognition.
Wired communication systems which employ such modulation and demodulation techniques include computer communication systems such as local area networks (LANs), point to point signalling, and wide area networks (WANs) such as the Internet. These networks generally communication data signals over electrical or optical fibre chanels. Wireless communication systems which may employ modulation and demodulation include those for public broadcasting such as AM and FM radio, and UHF and VHF television. Private communication systems may include cellular telephone networks, personal paging devices, HF radio systems used by taxi services, microwave backbone networks, interconnected appliances under the Bluetooth standard, and satellite communications. Other wired and wireless systems which use RF modulation and demodulation would be known to those skilled in the art.
One of the current problems in the art, is to develop physically small and inexpensive modulation techniques and devices that have good performance characteristics. For cellular telephones, for example, it is desirable to have a transmitter which can be fully integrated onto an integrated circuit.
Several attempts have been made at completely integrating communication transmitter designs, but have met with limited degrees of success. Existing solutions and their associated problems and limitations are summarized below:
1. Direct Conversion Transmitter
Direct conversion architectures 10 modulate baseband signals to RF levels in a single step by mixing a baseband signal with a local oscillator signal at the carrier frequency. Referring to the block diagram of FIG. 1, the in-phase (I) and quadrature (Q) components of the baseband signal are up-converted to RF via mixers MI 12 and MQ 14, respectively. The RF mixing signals are generated using a local oscillator 16 tuned to the RF, and a 90 degree phase shifter 18 which ensures that the I and Q signals are up-converted into their quadrature components. The two up-converted RF signals are added together via the summing element S 20, and filtered via a band pass filter (BPF) 22 having a pass band response around the RF signal to remove unwanted components. Finally, a power amplifier (PA) 26 amplifies the signal to the necessary transmission level.
Generally, a mixer is a circuit or device that accepts as its input two different frequencies and presents at its output:                (a) a signal equal in frequency to the sum of the frequencies of the input signals;        (b) a signal equal in frequency to the difference between the frequencies of the input signals; and        (c) the original input frequencies.The typical embodiment of a mixer is a digital switch, which may generate significantly more tones than those shown above.        
Hence, the disadvantages of this topology are:                the LO signal leaks into the RF signal, since the RF output signal is at the same frequency as the LO signal; and        the output RF signal leaks back into the LO generation elements, causing it to detune. This mechanism is commonly referred to “LO pulling”.        
However, this topology can easily be integrated and requires fewer components than other modulation topologies known in the art.
2. Directly Modulated Transmitter
FIG. 2 presents a block diagram of a directly modulated transmitter 30 in which the baseband signal modulates a voltage control oscillator (VCO) 32, designed to oscillate within the vicinity of the RF frequency. The output of the VCO 32 is then filtered by a bandpass filter (BPF) 34 which has a pass band around the RF frequency to remove unwanted components. A power amplifier 36 then amplifies the filtered signal to the amplitude required.
The disadvantages of this topology are:                the LO signal leaking into the RF signal; and        the output RF signal leaking back into the LO generation element, causing it to detune. Again, this mechanism is commonly referred to “LO pulling”.        
This topology can easily be integrated and requires a small number of components. To maintain stability, the VCO 32 is locked via a phase lock loop in most applications. In some applications, the input to the VCO 32 could be an up-converted version of the baseband signal.
3. Dual Conversion Transmitter
A dual conversion topology solves two of the problems associated with the direct conversion and the direct modulation topologies, specifically, the LO signal leaking into the RF signal and “LO pulling”. In this topology the baseband signal is translated to the RF band via two frequency translations, which are associated with two local oscillators (LOs), neither of which is tuned to the RF signal. Because neither of these LOs are tuned to the desired RF output frequency, the LO leakage problem and “LO pulling” problem are generally eliminated.
The dual conversion topology is presented as a block diagram in FIG. 3. Like the direct conversion transmitter described with respect to FIG. 1 hereinabove, the in-phase (I) and quadrature (Q) components of the base-band signal are first up-converted to RF via the mixers MI 42 and MQ 44, respectively. However, in this case, the RF mixing signal generated using local oscillator 46 is not tuned to the desired output frequency, but is tuned to an intermediate frequency (IF). The 90 degree phase shifter 48 then ensures that the I and Q signals are up-converted into their quadrature IF components. The two-upconverted IF signals are then added together via the summing element S 50, and filtered via a band pass filter (BPF) 52 having a pass band response around the IF signal.
The IF signal is then up-converted to the desired RF output frequency via mixer M 54 and a second local oscillator (LO2) 55, which need not be at the RF output frequency. The signal is then filtered via a second band pass filter (BPF) 56 having a pass band around the RF signal, and is amplified to the desired level using power amplifier (PA) 26.
Though this technique addresses the problems with the direct conversion and the direct modulation topologies, it has disadvantages of its own:                it requires two LO signals;        it requires two filters;        it requires a significant amount of frequency planning; and        it is difficult to integrate all the components into an integrated circuit.4. Offset Conversion Transmitter        
An offset conversion topology 60 such as that presented in FIG. 4, also solves the two main problems associated with direct conversion and direct modulation, that is, the LO signal leaking into the RF signal and “LO pulling”. Like the dual conversion transmitter, this is done by translating the baseband signal to the RF band using two local oscillators, neither of which are tuned to the RF output frequency. As noted above, the LO leakage problem and “LO pulling” problem are avoided, because neither of these LOs are tuned to the RF output frequency.
Referring to the block diagram of FIG. 4, the baseband signal is up-converted to the RF frequency via mixers MI 62 and MQ 64 which are modulated by a combined signal from two separate oscillators LO1 66 and LO2 68. The frequency used to up-convert the base band signal is equal to f1+f2 where f1 is the fundamental frequency component of the local oscillator LO1 66 signal and f2 is the fundamental component of the local oscillator LO2 68 signal. Mixing the signals from oscillators LO1 66 and LO2 68 via the mixer M 70 generates the frequency f1+f2, which corresponds to the RF output frequency. A band pass filter (BPF) 72 is then used to attenuate all frequency components except f1+f2. The 90 degree phase shifter 74 ensures the I and Q signals are up-converted into their quadrature components.
The two-upconverted signals are then added together via the summing element S 76, and filtered via a band pass filter (BPF) 78 having a pass band around the RF signal. Finally, a power amplifier 80 amplifies the signal to the desired level.
The disadvantages of this topology are:                it requires two LO signals;        it requires two filters; and        it requires a significant amount of frequency planning.        
There is therefore a need for a method and apparatus of modulating RF signals which allows the desired integrability along with good performance.