WO1992001337A1

WO1992001337A1 - Radio receivers

Info

Publication number: WO1992001337A1
Application number: PCT/GB1991/001102
Authority: WO
Inventors: Andrew Henry John Larkins
Original assignee: C-Com Group Plc
Priority date: 1990-07-09
Filing date: 1991-07-05
Publication date: 1992-01-23
Also published as: GB9015059D0

Abstract

A direct conversion receiver (10) receives a radio signal at a desired frequency and passes it through RF filter (14) and amplification (16) stages to a mixer (18). A local oscillator (20), operating at a subharmonic of the received frequency, provides an oscillator signal to the mixer (18). Baseband signal processing circuitry (26) receives the output of the mixer (18) and provides a demodulated output signal. Operation of the oscillator (20) at a frequency other than the received frequency reduces adverse effects due to radiation from the oscillator. The mixer (18) and signal processing circuitry (26) may be constituted by an integrated circuit.

Description

RADIO RECEIVERS

This invention relates to a direct conversion radio receiver. Such receivers are also known as zero inteπtediate frequency or hcwodyne receivers.

The direct conversion receiver was an early and simple design of receiver, in which the incoming radio signal is converted to a low frequency using a local oscillator (10) and a mixer. The local oscillator is at the same ncπ nal frequency as the desired received radio signal and beats with it to provide an output component at baseband frequency in a single stage.

Problems arise because the screening of the local oscillator can never be perfect, so some of its power is radiated. In particular:

1. The LO signal may be received by the antenna attached to the direct conversion receiver.

When this is mixed with the LO signal in the mixer, a voltage will be produced at the output of the mixer which will depend upon the aπplitude and phase of the radiated signal that is received by the antenna. If the voltage is too great, the output of the mixer will saturate and therefore prevent the reception of any wanted signal. Also, any modulation of the level of the LO picked up in the antenna will appear as a modulation of the voltage at the output of the mixer. This will tend to mask the signal being received.

2. In addition, the antenna will have been designed to receive signals of the frequency of the local oscillator, so it will also radiate any power that reaches it through the receiver circuitry. This may be sufficient to break the limitations on radiated power frcm receivers as laid down by the regulatory authorities. It has been proposed in a direct conversion receiver to use a local oscillator at a sub-harmonic of the received frequency and then to follow this by a frequency multiplier stage to produce a signal at the received frequency for application to the mixer. This means that an oscillator of lower frequency can be employed. However, this does not sufficiently get over the above problems, because the frequency multiplier generates the received frequency.

The present invention is defined in the appended claims to which reference should now be made.

The invention enables the production of a simple receiver which does not su fer from the above-noted disadvantages and which is simple and ecxanomical to manufacture. An example of a receiver embodying the invention is described in more detail below. In this embodiment the major part of the.receiver circuitry is incorporated in an integrated circuit. The local oscillator signal is applied to the chip at a su±ϊ-harmonic frequency of the received frequency and thus the only place where signals are generated at the received frequency is on the chip. The amount of radiation frcm the chip is very small.

Since the local oscillator signal is no longer at the same frequency as the received frequency it is possible to attenuate the LO signal in the main signal path while not attenuating the wanted radio signal. The antenna is no longer tuned to the LO frequency, so it will neither receive ID radiation nor radiate the signal to any significant extent. Also the RILaπplifier can be used to attenuate the ID signal that is received by the antenna.

When an EM signal is received, two local oscillator signals are required which are phase-displaced by 90 degrees at the received frequency. With the local oscillator operating at 1/n of the received frequency, a phase shift of only 90/n degrees is now required. This is readily achievable with a siπple RC phase shift network, thereby very substantially siπplifying the circuitry.

A buffer amplifier may be provided between the local oscillator and the phase shift network. Advantageously such a buffer aπplifier comprises a siπple conmon-base stage which provides good isolation while being very simple in construction.

Embodiments of the invention will now be described by way of example with reference to the drawings,- in which:

Figure 1 is a block circuit diagram of a first embodiment of direct conversion receiver embodying the invention;

Figure 2 shows in more detail the local oscillator and phase shift circuit of Figure 1;

Figure 3 is a block circuit diagram of a second embodiment of direct conversion receiver embodying the invention; and

Figure 4 shows in more detail the local oscillator, buffer circuit, and phrase shift circuit of Figure 3.

Figure 1 illustrates a first embodiment of __1 paging receiver for use on a specified ixed frequency in the range 20MHz to 500MHz to receive digital data. The receiver is of the direct conversion type, that is to say there is no intermediate frequency as is common in the conventional superheterodyne receiver, but instead there is only a single frequency conversion stage.

The receiver 10 comprises an antenna 12 connected through an RF filter 14 and RF amplifier 16 to a mixer 18, which actually comprises two mixer circuits operating in quadrature. As well as receiving the signal from the RF stages, the mixer has inputs for receiving phase-displaced signals derived frcm a local oscillator 20 and phase shift circuit 24. The outputs of the mixer 16 are applied to airplification, filtering and demodulation circuitry 26 which supplies an output 28. The components 16, 18 and 26 are constituted by a single integrated circuit, such as Plessey type SL6639, and are not therefore described here in detail.

The construction of the local oscillator 20, and phase shift circuit 24 is shown in more detail in Figure 2. The local oscillator is itself of conventional type and has a transistor 30 connected to a crystal 32. The detailed components of the oscillator will be apparent to those skilled in the art frcm the figure and are not therefore described here in detail.

The phase shifter circuit 24 can be of very simple construction, comprising a resistor 38 and capacitor 40 connected as an RC circuit. An unshifted output is applied to a terminal 42 and a shifted output to a terminal 44.

In this example, the receiver is designed to work at 150MHz. In accordance with the invention, the local oscillator frequency is not 150MHz, but rather is a sub-multiple of this frequency. In the present case the local oscillator frequency is 50MHz, so that the two frequencies are related by a factor of three. This is because the mixer 18 in this example is found to operate best with an odd harmonic; if it worked better with an even harmonic then a factor of two could be used. Factors of four or possibly five might be appropriate in certain cases. It should be noted that the frequency is factory,set, so no tuning adjustment is required. It will be appreciated, however that the invention is not limited to such receivers.

The mixer 18 is not a theoretically perfect mixer in that it tends to operate in what may be regarded as a switching mode. Thus harmonics are generated in the mixer. Although the signal received by the mixer is at 50MHz, sufficient harmonics will be generated in the mixer to demodulate a received signal at 150MHz. It may be necessary to increase the drive level of the ID signal as applied to the mixer from that normally used, e.g. by a factor of four, though if the mixer were to be specifically designed for harmonic mixing this would not be so necessary. The phase shift network can be of very siπple construction because a lower phase shift is required than the 90 degrees which is usually necessary. Where the ID is at one t_ird of the received frequency the phase shift should be only 30 degrees. This can be produced by the network illustrated using just the single resistor 38 and single capacitor 40, while maintaining a reasonable amplitude match between the two outputs. We have also found that this network, when combined with the oscillator circuit, will provide the DC bias voltage required by the mixer 18 in the receiver integrated circuit.

Instead of the Plessey integrated circuit referred to above, other makes of integrated circuit direct conversion receiver may be used. Some of these have the transistor 30 for the local oscillator incorporated on the chip.

A second embcclLment of EM paging receiver is illustrated in Figure 3. The eπibcdiment of Figure 3 is substantially identical to that described hereinabove with reference to Figure 1 apart from those features described below: corresponding ccmponents are indicated by the same reference numerals and their operation will not be further described.

In the embodiment of Figure 3, a buffer amplifier 22 is provided between the output of the local oscillator 20 and the input of the phase shift circuit 24.

The construction of the local oscillator 20, buffer 22 and phase shift circuit 24 is shown in more detail in Figure 4. As with the first embodiment, the local oscillator 20 is of conventional type having a transistor 30 connected to a crystal 32. A current source 34 is shown although it will be appreciated that this may be replaced by a resistor.

The buffer 22 comprises a transistor 36 connected in common-base configuration. This provides a high degree of isolation, and reduces the effect of any change in the input e

impedance of the mixer 18 on the local oscillator operating frequency.

Claims

1. A direct conversion receiver for receiving a radio signal at a desired received frequency, and comprising: an RF stage for connection to an antenna for receiving a signal at the desired received frequency; a local oscillator for generating a local oscillatory signal at a frequency which is a sub-harmonic of the received frequency whereby the received and local oscillator frequencies are related by an integer factor n; a mixer coupled to the output of the RF stage to receive the received frequency and to the output of the local oscillator to receive the local oscillator frequency and arranged to provide an output baseband signal; and __ baseband signal processing circuitry connected to the output of the mixer to provide a demodulated output signal.

2. A receiver according to claim 1, in which the factor_n is in the range 2 to 5.

3. A receiver according to claim 1, in which the factor n is 2 or 3.

4. A receiver according to claim 1, 2 or 3, in which the mixer and baseband signal processing circuitry are constituted by an integrated circuit.

5. A receiver according to any preceding claim, including an RC phase shifter circuit coupled between the local oscillator and the mixer to provide two signals to the mixer having a relative phase shift of substantially 90/n_degrees.

6. A receiver according to claim 5, in which the RC phase shifter circuit comprises a single resistor means and a single capacitor means.

7. A receiver according to claim 5 or 6, including a buffer circuit between the local oscillator and the phase shifter circuit.

8. A receiver according to claim 7, in which the buffer circuit comprises a buffer amplifier transistor connected in cs_mmon-base configuration.

9. A receiver according to any preceding claim, in which the receiver is operable at a frequency in the range 20MHz to 500MHz.

10. A direct conversion receiver for receiving an EM radio signal at a desired received frequency, and comprising: an RF stage for connection to an antenna for receiving a signal at the desired received frequency; a local oscillator for generating a local oscillatory signal at a frequency which is a sι__-harmonic of the received frequency whereby the received and local oscillator frequencies are related by an integer factor n; an RC phase shifter circuit connected to the output of the local oscillator to provide two output signals having a relative phase shift of substantially 90/_n degrees; a mixer connected to the output of the RF stage to receive the received frequency and to the phase shifter circuit to receive the two output signals therefrom, to provide two baseband output signals; and baseband signal processing circuitry connected to the outputs of the mixer to provide a. demodulated output signal.

11. A receiver according to claim 10 including a buffer circuit between the local oscillator and the RC phase shifter circuit.