GB2390241A

GB2390241A - Homodyne radio receiver

Info

Publication number: GB2390241A
Application number: GB0215009A
Authority: GB
Inventors: Andrew John Fox; Roy Peter Henderson
Original assignee: Picochip Designs Ltd
Current assignee: Picochip Designs Ltd
Priority date: 2002-06-28
Filing date: 2002-06-28
Publication date: 2003-12-31
Also published as: AU2003244756A1; GB0215009D0; WO2004004147A1

Abstract

In a duplexerless homodyne receiver, suitable for use in a base station of a mobile communications system, received signals are supplied to a wideband filter 20, and the filtered signals are supplied to a first low noise amplifier 22. The amplified signals are supplied to a first narrowband filter 24. These filtered signals are supplied to a second low noise amplifier 26, and the further amplified signals are supplied to a second narrowband filter 28. These signals, at the received radio frequency, are then supplied to a quadrature demodulator for direct downconversion to baseband. This combination of two low noise amplifiers 22,26 and two narrowband filters 24,28 provides the required attenuation of the interferer to avoid saturating the subsequent quadrature demodulator, while also providing enough gain to ensure that the noise figure of the received signal is not degraded, while using components which are smaller and cheaper than would be the case in attempting to meet the requirements using just one amplifier and one filter.

Description

! 1 RADIO RECEIVER

This invention relates to a radio receiver architecture, and in particular to a receiver 5 architecture which is suitable for use in a base station of a mobile communications system.

According to the Universal Mobile Telephony Service (UMTS) standard, a base station must be able to receive 10 signals within the frequency band 1920-1980 MHz.

Moreover, the receiver must be able to reject interfering signals at a level of -15 dBm at frequencies up to 1900 MHz and above 2000 MHz.

15 It is also necessary for the receiver to be isolated from the transmit circuitry in the base station, in order to ensure that transmitted signals at high power do not swamp the received signals.

20 The receiver must also downconvert the received radio frequency signals to baseband to allow signal processing of the received signals.

Broadly speaking, radio receivers fall into two 25 categories, namely heterodyne receivers and homodyne receivers. According to the present invention, there is provided a homodyne receiver, which does not require a duplexer at 30 the antenna.

More specifically, there is provided a receiver in which received signals are supplied to a wideband filter, the filtered signals are supplied to a first

J ( 2 low noise amplifier. The amplified signals are supplied to a first narrowband filter. These filtered signals are supplied to a second low noise amplifier, and the further amplified signals are supplied to a 5 second narrowband filter These signals, at the received radio frequency are then supplied to a quadrature demodulator, for direct downconversion to baseband. 10 For a better understanding of the present invention, and to show how it may be brought into effect, reference will now be made, by way of example, to the accompanying drawing, in which: 15 Figure 1 is a block schematic diagram of a radio receiver architecture in accordance with the present invention. Figure 1 shows a base station architecture, in 20 particular intended for use in the proposed Universal Mobile Telephony System (UMTS). The base station 10 includes transmit circuitry 12 and a transit antenna 14, which will not be described further herein as they are conventional.

Received signals arrive at a receive antenna 16, and are then supplied to a receiver front end 18.

The front end 18 includes a wideband blocking filter 30 20. For use in UMTS, received signals are in the 1920 1980 MHz frequency band, and the receiver must be capable of rejecting an interfering signal at frequencies up to 1900 MHz or above 2000 MHz at a level of -15 dBm. However, the filter 20 is not chosen to

J ( 3 reject these interfering signals, but to reject interferers which may occur for example at about 1800 MHz or at about 2300 MHz. The filter 20 may therefore have a passband which is several hundreds of megahertz 5 wide. For example, the filter 20 may have a passband which is at least 200 MHz, and preferably 400-450 MHz wide. Choosing a filter with a wide passband enables the insertion loss of the filter to be very low, for example less than ldB. The filter may have 2 or 3 10 poles, which means that it is of relatively low quality, while being small and cheap.

The filtered signals are supplied to a first low noise amplifier 22, and the amplified signals are then 15 supplied to a first narrowband filter 24. The filtered signals are supplied to a second low noise amplifier 26 and, finally in the front-end 18, the further amplified signals are supplied to a second narrowband blocking filter 28.

The first low noise amplifier 22 is chosen to provide sufficient gain to overcome the insertion loss of the filter 24 and the noise figure of the second low noise amplifier 26. However, while taking these points into 25 consideration, the gain of the amplifier is otherwise kept relatively low, since the signal at this point contains interferers in the region of 1900 MHz and 2000 MHz, and it is preferable not to amplify these interferers, which will eventually need to be filtered 30 out of the signal.

The first narrowband filter 24 has a passband which corresponds to the UMTS receive band from 1920-1980 MHz, and is chosen to have very high attenuation of the

interferers at lDoo MHz and 2000 MHz. For example, the filter may have 5 poles, and may have an insertion loss in the region of 3dB.

5 The second low noise amplifier 26 is then chosen so that it provides the remainder of the gain required in the receiver front-end. At this point, the interferers at l9OO MHz and 2000 MHz have been significantly attenuated by the filter 24. As a result, the 10 amplifier 26 can operate without the danger that the interferers will be amplified to a level which will cause compression of the signals.

For convenience, the first and second amplifier blocks 15 22, 26 may contain amplifiers which are effectively identical, but which together provide a degree of amplification which exceeds the total gain required in the receiver front-end. In that case, an attenuator can also be included in the front-end signal path, 20 preferably just after the first low noise amplifier.

The second narrowband blocking filter 28 also has a pas sband which corresponds to the CATS receive band from lg20-1980 MHz, and provides further attenuation of 25 the interferers at 1900 MHz and 2000 MHz. The second narrowband filter 28 can conveniently be of the same design as the first narrowband blocking filter 24.

This combination of two low noise amplifiers 22, 26 and 30 two narrowband filters 24, 28 provides the required attenuation of the interferer to avoid saturating the subsequent quadrature demodulator, while also providing enough gain to ensure that the noise figure of the received signal is not degraded. This arrangement

( 5 allows these requirements to be met, while using components which are smaller and cheaper than would be the case in attempting to meet the requirements using just one amplifier and one filter.

The output signal from the front-end 18 is supplied to two mixers 30, 32. The mixers 30, 32 also receive a local oscillator 34 signal, after it has been passed to a 0 /90 splitter 36.

As this is a direct-conversion receiver, the local oscillator signal 34 is adjusted so that it is at the frequency of the received signals. The downconverted I- and Q- baseband signals from the mixers 30, 32 are 15 then supplied to respective low pass filters 38, 40, and to respective baseband amplifiers 42, 44. The amplified baseband signals are then supplied to conventional baseband processing circuitry 46 in which, for example, the signals are analog-digital converted 20 and passed to digital processing circuitry.

There is therefore disclosed a radio receiver architecture which, in the case of a direct-conversion receiver, avoids the need for a duplexer at the 25 antenna. This means that the size, weight and cost of the receiver can be reduced. The architecture is particularly useful for a base station in picocell of a cellular communications system, since that is a situation in which these factors are especially 30 important.

Claims

( CLAIMS

1. A direct-conversion radio receiver, comprising: 5 means for receiving a radio frequency signal; a first low noise amplifier, for amplifying the received signals; a first narrowband radio frequency filter, for filtering the amplified signals; 10 a second low noise amplifier, for further amplifying the filtered signals; a second radio frequency narrowband filter, for further filtering the further amplified signals; and a mixer stage, for downconverting the further 15 filtered radio frequency signals to baseband.

2. A radio receiver as claimed in claim 1, wherein the means for receiving radio frequency signals comprises an antenna, and wherein the receiver further 20 comprises transmit circuitry and a transmit antenna, and wherein the receive antenna and the transmit antenna are duplexerless.

25

3. A radio receiver as claimed in claim 1, further comprising a wideband radio frequency filter, before the first low noise amplifier.

4. A radio receiver as claimed in claim 3, for 30 use in a base station of a mobile communications system, the receiver being suitable for receiving signals within a receive band of the system, wherein passbands of the first and second narrowband filter correspond generally to the receive band, and wherein a

1 7 passband of the wideband filter is wider than the receive band.

5. A radio receiver as claimed in claim 4, for 5 use in a base station of a UMTS mobile communications system, having a receive band from 1920-1980 MHz, wherein the first and second narrowband filters have respective passbands which correspond generally to the receive band, and substantially attenuate signals at 10 1900 and 2000 MHz.

6. A radio receiver as claimed in claim 4, for use in a base station of a UMTS mobile communications system, having a receive band from 1920-1980 MHz, 15 wherein the wideband filter has a passband which is at least 200 MHz wide, and substantially attenuates signals at 1800 and 2300 MHz.

7. A radio receiver as claimed in any preceding 20 claim wherein the second low noise amplifier has higher gain than the first low noise amplifier.

8. A radio receiver as claimed in any of claims 1 to 6, wherein the first and second low noise 25 amplifiers are chosen such that, together, they provide a desired total gain, and wherein the second low noise amplifier has higher gain than the first low noise amplifier. 30

9. A radio receiver as claimed in any of claims 1 to 6, wherein the first and second low noise amplifiers are chosen such that, together, they provide a desired total gain, and wherein the gain of the first low noise amplifier is chosen such that it overcomes

( 8 any insertion loss of the first narrowband filter and any noise figure of the second low noise amplifier, but such that the second low noise amplifier has higher gain than the first low noise amplifier

10. A radio receiver as claimed in any of claims 7 to 9, wherein the first and second low noise amplifiers include similar amplifier circuitry, but wherein the first low noise amplifier further comprises 10 an attenuator, such that the second low noise amplifier has higher gain than the first low noise amplifier.