In a typical wireless communication system a receiver part of a receiving station receives radio frequency (RF) signals that have been generated and transmitted by a transmitting station. When a signal is received at the receiving station the radio frequency signal is typically down-converted from the radio frequency to baseband frequency. In a so called direct conversion receiver the radio frequency signal is converted directly to a baseband signal without converting the incoming signal first to one or several intermediate frequencies (IF). Hence the direct conversion receivers are sometimes referred to as zero intermediate frequency receivers.
FIG. 1 shows a block chart for a signal path in a receiver. In a typical multi-frequency band and multi-system direct conversion receiver a radio frequency (RF) signal is obtained by antenna means 1. The signal is then typically conducted through band splitter means to split the signal in appropriate frequency bands. In each of the frequency bands the signal may then be conducted through a band filter. The signal in each of the frequency bands may be amplified at the front end of the receiver by appropriate pre-amplifier means. A gain control function 2 (e.g. a low noise amplifier (LNA)) may be used for controlling the level of the gain. If the received signal is substantially strong, the one or more of the amplifiers may be switched to a smaller amplification level at the amplification step.
The amplified RF signal in each of the frequency bands may then be demodulated or mixed to baseband in-phase (I) and quadrature (Q) signals by mixer means 3. In a typical receiver arrangement only one band of the plurality of bands is active at the same time. Said mixer means (e.g. a I/Q demodulator) typically comprise a mixer pair, one for the 0 degree and one for the 90 degree phase shift. The demodulation functions may be accomplished based on a local oscillator signal from block 4.
After the demodulation at block 3 the baseband signal is amplified and possibly low-pass filtered to attenuate further the out-of-channel signals before the signal is input in an active channel filter. The gain is required in order to enable use of substantially high impedance levels in the subsequent channel filter 6 without worsening the noise performance of the receiver.
Automatic gain control (AGC) 7 is carried out after the channel filtering. After the AGC the I and Q signals can be fed to an analog to digital converter (ADC) 8. The signals may be processed at the ADC 8 with digital signal processing means so that e.g. sound can be reproduced based on the received signal.
The baseband parts of an integrated direct conversion receiver consist of the baseband gain, channel filter and automatic gain control functions. These functions require a number of discrete components. These components require a substantially large area on a circuit board. It could be advantageous if the board area could be made smaller, especially in the field of mobile stations. Use of substantially many discrete components should also be avoided in a receiver in order to keep the cost of the receiver circuitry low. As the more complex circuitry designs are more susceptible for failures, mostly because of the increased number of components and joints between the various components, the number of the components should also be kept as low as possible in order to reduce possibilities for faults.
A problem that is faced when implementing a direct conversion receiver is the control of the so called offset voltage. The term “offset voltage” (or direct conversion offset error) refers to an direct conversion error voltage which has become summed up in the receiver into the signal as an essentially direct voltage. The inclusion of the DC off set voltage in the received useful signal should, however, be avoided as it may introduce error in the reproduced sound. The formation of the offset voltage can be caused by many different factors. The skilled person is aware of the phenomena leading to generation of the offset voltage, and this is thus not described in more detail herein.
In direct conversion receivers the variation in the direct current (DC) level causes problems because the DC cannot be amplified with the same gain as the desired signal. Instead a separate gain needs to be used. In the prior art the separate gain for different frequencies has been provided by discrete components. This has increased further the number of the components and the required board area.
In addition, in the prior art the error in the DC level is charged in large capacitors (see FIGS. 2 and 3). The capacitors have to be substantially big to ensure that the cut in the frequency is not too high. This is so since the RC product has to be substantially large. The value of the resistance is limited in order not to introduce too much noise. Because of the requirement for substantially large capacitors a fully integrated automatic gain control (AGC) has not been considered as a viable possibility.
In the modern receivers the complexity and size of the radio parts has also increased because a receiving terminal may need to be able to handle an increased number of operation bands and/or modes. A receiver may be adapted to be used in a multisystem or multiband environment and/or may need to be co-operative with more than one serving network or system or standard or frequency and so on. An example of a multiband system is a dual-band GSM mobile stations served by both 900 MHz and 1800 MHz frequencies. An example of a multisystem is a dual mode telephone operating e.g. both in the GSM (Global System for Mobile communications) and in WCDMA UMTS (Wideband Code Division Multiple Access Universal Mobile Telecommunications Service) networks.
In an integrated circuit (IC) for a multimode receiver the number of I/O (input/output) pins is typically limited. This is another reason why use of discrete components should be avoided.
The level of the received signal may vary, the lowest received frequency being 0 Hz. In a direct conversion receiver the DC-level in the baseband may thus also vary. Consequently, there may also be variation in the level of the DC-voltage of the receiver. The DC-voltage variation cannot be amplified since it would cause wrong biasing in the baseband and most likely phenomenon known as clipping in the analog to digital converter (ADC). Some of the lowest frequencies can be removed without compromising the reception performance. For example, it is possible to use a highpass filter for the automatic gain control function in some applications.
However, the maximum highpass frequency required from a highpass filter may be substantially different in systems that are based on different modes. For example, the GSM employs Gaussian-filtered Minimum Shift keying modulation and 200 kHz modulation bandwidth whereas in systems that are based on the WCDMA modulation the modulation bandwidth is typically 5 MHz. Therefore the maximum highpass frequency is substantially smaller in the GSM than what it is in the WCDMA. Discrete highpass filter components may thus be required for different modes.
A simple AGC topology is shown in FIG. 2. In a prior art AGC circuit the DC voltage level of the output Vout of the AGC is determined by the DC voltage level of the input Vin thereof. In other words, the DC gain equals one (1). The output DC voltage of the baseband is determined by the ADC circuit to ensure the performance thereof. Therefore a DC level shift circuit (such as the circuit arrangement of FIG. 3) is used in the prior art. The DC level shift circuit is provided with two external capacitors C3 and C4 for each output Vout.
In the prior art the GSM and WCDMA the automatic gain control (AGC) stages are separated. A specific DC-shift circuit is needed to adjust the DC-level of the automatic gain control (AGC) to a desired level. Since the ACC for the WCDMA part of the receiver cannot be used for the GSM, a separate GSM AGC and another DC-shift circuit is also needed.