RADIO RECEIVER

Abstract: There is disclosed a radio receiver architecture which has first and second front end circuits optimised for reception in two fre quency bands. Each front end circuit includes a mixer for downconverting received signals to a respective intermediate frequency. Signals from the two front end circuits are passed to a single analog-digital converter. The sampling rate of the analog-digital converter, and the intermediate frequencies, are chosen such that the received signals appear at the same point in different teeth of the alias response of the analog-digital converter.

Description: RADIO RECEIVER TECHNICAL FIELD OF THE INVENTION This invention relates to a radio receiver architecture, and in particular to an architecture which is suitable for use in a dual-band receiver. BACKGROUND OF THE INVENTION It is becoming a common requirement for portable communications devices, such as mobile phones, to be able to operate in two separate regions of the radio frequency spectrum. For example, it is becoming a common requirement for mobile phones that they should be able to operate at 900 MHz and at 1800 MHz. Such products are known as dual-band products. In the design of such dual-band products, one option is to duplicate the receiver circuitry. US Patent No. 4,584,716 shows a radio receiver, for receiving signals at two different frequencies, in which the receiver circuitry, including radio frequency amplifier, mixer for downconversion to an intermediate frequency, intermediate frequency filter, intermediate frequency amplifier, and analog-digital converter, is duplicated. Clearly, it would be advantageous to avoid such duplication, to reduce the size and power consumption of the device. However, any changes for this reason must also take account of the fact that radio frequency components are typically optimised for use at one particular frequency band. SUMMARY OF THE INVENTION The present invention provides a radio architecture for use in a dual-band product. Circuitry- is provided for receiving signals in two parts of the radio frequency spectrum, and the received signals are downconverted to respective intermediate frequencies. However, there is provided only a single analog to digital converter, which operates at a single sampling frequency. The invention makes use of a known property of analog to digital converters, namely that they have alias responses, at multiples of the sampling frequency. The intermediate frequencies, to which the received signals are downconverted, are selected such that they appear in the alias responses at different multiples of the sampling frequency. This has the advantage that it is necessary to provide only one analog to digital converter in the receiver circuitry, while still allowing the choice of intermediate frequencies which can be close to optimal for the particular received frequency band. BRIEF DESCRIPTION OF DRAWINGS Figure 1 is a block schematic diagram of a receiver architecture in accordance with the invention. Figure 2 is an illustration of the frequency response of an analog to digital converter, for use in explaining the operation of the circuit of Figure 1. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 is a block schematic diagram of a radio receiver architecture in accordance with the invention. As described herein, the architecture is suitable for use in a dual-band GSM mobile phone, operating at other frequencies in the region of 900 MHz and 1800 MHz respectively. However, it will be appreciated that the invention is applicable to such products operating at other frequency bands, and indeed to other devices which receive radio frequency signals in two distinct parts of the radio frequency spectrum. The circuit of Figure 1 includes a first antenna 2, optimised for receiving signals at 900 MHz, and a first RF amplifier 4, for amplifying the received signals. The amplified received signals are passed to a mixer 6, which also receives a first local oscillator frequency LOl, and downconverts the amplified received signals to a first intermediate frequency IF1. The whole of this front-end circuit can be optimised for reception at 900MHz. The downconverted intermediate frequency signals are passed to a first band-pass filter 8, and then to an analog-digital converter ADC 10, operating at a sampling frequency Fs . The output digital signals are then passed to processing circuitry in a conventional way. In a mobile phone as described herein, in which the two frequency bands of operation are designed to receive signals transmitted in the conventional GSM format, the ADC 10 might advantageously sample the signals at the GSM clock rate of 13 MHz . The receiver architecture of Figure 1 further includes a second antenna 12, optimised for receiving signals at 1800 MHz, and a second RF amplifier 14, for amplifying signals received by the antenna 12. The architecture further includes a second mixer 16, which also receives a second local oscillator signal L02 , for downconverting the signals received by the second antenna 12 to a second intermediate frequency IF2. This second front-end circuit can be optimised for reception at 1800 MHz. The downconverted signals at the second intermediate frequency IF2 are passed to a second bandpass filter 18, and the filtered signals are also passed to the same ADC 10. Crucial to the operation of the analog-digital converter 10 is the relationship between its sampling frequency Fs and the frequencies of the received signals. The frequency response of an analog-digital converter is shown in Figure 2, for the purposes of explanation of this relationship. Often, the sampling frequency Fs of the ADC is chosen to be sufficiently high that all of the received signals have a frequency which is less than Fs/2. This is termed Nyquist sampling, and means that all of the signals within that frequency range can be satisfactorily sampled. However, the ADC also produces alias responses, to signals at higher frequencies. Thus, input signals at the sampling frequency Fs itself will appear to the ADC to have zero frequency, and signals at Fs±Fx (where Fx is small compared to Fs) will both appear to the ADC to be at the frequency Fx. Where an input signal has a frequency range from zero to, say, 1.25 Fs, then this is a serious problem, because the alias response means that, in the ADC output signal, it is not possible to distinguish between input signals at 0.2Fs, 0.8Fs and 1.2Fs. Moreover, as Figure 2 shows, there is a similar alias response around each multiple of the sampling frequency Fs . The frequency response shown in Figure 2 is known as a comb response. However, where the input signal lies within a known, relatively narrow frequency band, a technique known as sub-Nyquist sampling can take advantage of this alias response. More specifically, if a receiver circuit can be designed such that it is known that an ADC input signal will appear in a particular region of the ADC frequency response, then the aliasing property of the ADC will mean that the signal is effectively converted down to a frequency which is less than Fs/2. In accordance with the present invention, the dual-band input signals are downconverted to respective intermediate frequencies which appear in different parts of the alias response of the ADC, but which are nevertheless converted down to similar frequencies. More specifically, it is advantageous to arrange for an ADC input signal to have a frequency centered at or close to (N±0.25)Fs, because this allows the signals to vary in the frequency range from N.Fs to (N+0.5)Fs, without causing further aliasing problems. In accordance with the invention, therefore, the ADC input signals can be arranged to appear in two different such regions of the frequency response, namely with different integer values of N. This is illustrated in more detail in Figure 2, which shows the first intermediate frequency IF1 appearing at (nl+0.25)Fs, and the second intermediate frequency IF2 appearing at (n2+0.25)Fs, with nl and n2 being different integers. The difference (IF2-1F1) between the two intermediate frequencies is n3.Fs where n3 is an integer. More particularly, in the case of the dual-band receiver shown in Figure 1, receiving input signals at 900 MHz and 1800 MHz, the optimal intermediate frequency for the 900 MHz signals is in the region of 100 MHz, while the optimal intermediate frequency for signals received at 1800 MHz is in the region of 200 MHz. Therefore, if the ADC 10 has a sampling rate of 13 MHz, as discussed previously, the local oscillator signal LOl can be chosen to give an intermediate frequency IF1 of 94.25 MHz (i.e. 7.25 x 13 MHz) for the signals received at 900 MHz, and the local oscillator frequency L02 can be chosen to give an intermediate ~ frequency IF2 of 198.25 MHz (i.e. 15.25 x 13 MHz) for the signals received at 1800 MHz. Thus, the difference between the two intermediate frequencies is an integer multiple of the ADC sampling frequency. Both signals will therefore appear in the output of the ADC10 centered on 3.25 MHz, namely one quarter of the sampling frequency. There is therefore disclosed an architecture which allows downconversion of signals received by a dual- band receiver, without requiring duplication of the analog-digital converter. Although the invention has been described herein with reference to a dual band receiver, in which two intermediate frequency signals are effectively downconverted in the analog-digital converter, it will be appreciated that the invention is also applicable to architectures in which the downconversion of any larger number of intermediate frequency signals is achieved in the same way.

Claims: CLAIMS 1. A radio receiver, comprising: first receiver front-end circuitry, for receiving signals in a first received frequency band, the first receiver front-end circuitry including a first mixer and means for supplying a first local oscillator signal thereto for downconverting signals in the first received frequency band to a band at a first intermediate frequency IF1; second receiver front-end circuitry, for receiving signals in a second received frequency band, the second receiver front-end circuitry including a second mixer and means for supplying a second local oscillator signal thereto for downconverting signals in the second received frequency band to a band at a second intermediate frequency IF2 ; and an analog-digital converter, for sampling downconverted signals received from both the first receiver front-end circuitry and the second receiver front-end circuitry at a predetermined sampling rate Fs; the first and second local oscillator frequencies being selected such that the intermediate frequencies are given by: IF1 = Fs * (Nl±x) , and IF2 = Fs * (N2±y) , wherein Nl and N2 are respective different integers, 0 < x < %, and 0 < y < . 2. A radio receiver as claimed in claim 1, wherein x = . 3. A radio receiver as claimed in claim 1, wherein y « %. 4. A radio receiver as claimed in claim 1, wherein x = y. 5. A radio receiver as claimed in claim 1, further comprising: a first band-pass filter, for allowing signals at the first intermediate frequency to pass from the first mixer to the analog-digital converter, and a second band-pass filter, for allowing signals at the second intermediate frequency to pass from the second mixer to the analog-digital converter. 6. A radio receiver, comprising: first receiver front-end circuitry, including a first mixer for downconverting a signal received at a first received frequency to a first intermediate frequency; second receiver front-end circuitry, including a second mixer for downconverting a signal received at a second received frequency to a second intermediate frequency; and an analog-digital converter, for sampling downconverted signals received from both the first receiver front-end circuitry and the second receiver fron -end circuitry at a predetermined sampling rate, and having an alias response whereby signals at multiples of the sampling rate appear in the output thereof; wherein the first and second intermediate frequencies are each selected such that, on sampling by the analog-digital converter, the respective sampled signals appear at known points in the alias response thereof . 7. A radio receiver as claimed in claim 6, wherein the first and second intermediate frequencies are each selected such that, on sampling by the analog- digital converter, the respective sampled signals appear at the same points on different teeth of the alias response thereof.

Inventors: BRISTOW ROBERT OWEN, BRISTOW, ROBERT, OWEN
Applicants: BRISTOW ROBERT OWEN, ERICSSON TELEFON AB L M, BRISTOW, ROBERT, OWEN, TELEFONAKTIEBOLAGET LM ERICSSON
Application year: 1999
Main classification: H04B1
IPC codes: H04B1