GB2030806A - Receiver front end arrangement - Google Patents

Abstract

In a receiver front end in which received signals are fed to two parallel signal paths each path containing a mixer 4, 5 the mixers receiving local oscillator signals 6, 7 which have the same frequency as one another but which are in phase quadrature, a hybrid device 10 is provided either to feed or to be fed from the two parallel paths. The I.F. output is fed to a surface acoustic wave I.F. filter 13. <IMAGE>

Description

SPECIFICATION Receiver front end arrangement This invention relates to a receiver front end arrangement and is particularly although not exclusively applicable to a television or radio receiver.

A known receiver front end arrangement includes a broadband filter for receiving signals at radio frequency, an amplifier for amplifying the received signals, a narrow band filter, a mixer for receiving output signals from the narrow band filter and a local oscillator signal, and for providing output signals at I.F.

frequency, which may be further amplified in an l.F. amplifier state and filtered in an l.F.

filter, conveniently of surface acoustic wave type.

Such an arrangement often suffers from inadequate rejection of image signals. The narrow band filter must track accurately with the local oscillator and this is not easy to achieve. Consequently, front end arrangements of satisfactory image rejection prove difficult to manufacture and align and tend to be expensive.

This invention seeks to provide a receiver front end arrangement in which the above disadvantages are mitigated.

According to this invention there is provided a receiver front end arrangement including two parallel signal paths and two mixers one in each signal path each mixer also being arranged to receive a local oscillator signal, the local oscillator signals applied in operation to the two mixers having the same frequency but phases which are in quadrature to one another and a hybrid device including two ports connected to respective ones of the two paths.

The hybrid circuit is conveniently in the form of a 3dB couple.

In one embodiment of the invention received radio frequency signals are fed in parallel to a respective mixer and intermediate frequency signal outputs from the two mixers are fed to respective input ports of the hybrid circuit, I.F. signals being provided at an output port of the hybrid circuit.

An l.F. filter may be connected to the said output port of the hybrid circuit and in a preferred form this l.F. filter is a surface acoustic wave filter.

In another embodiment, radio frequency signals are applied to an input port of the hybrid circuit and the two mixers are connected to receive signals from respective output ports of the hybrid circuit, output signals from the two mixers being combined to provide an I.F. signal.

A band pass filter may be provided to filter the radio frequency signals prior to application to the hybrid circuit and in a preferred form is a surface acoustic wave filter.

An l.F. signal path filter may be provided to filter the I.F. signal provided by the combined output signals from the two mixers.

In both said one embodiment and said another embodiment the hybrid circuit and the surface acoustic wave filter may be provided as an integral surface wave device.

This invention will now be described further with reference to the accompanying drawings in which: Figure 1 shows an embodiment of a receiver front end arrangement in accordance with the invention and Figure 2 shows an alternative embodiment.

Referring to Fig. 1 an aerail 1 receives signals at radio frequency and these are passed over two parallel signal paths 2 and 3 to respective mixers 4 and 5. Each mixer receives a local oscillator signal at a respective terminal 6 and 7, the frequency of the local oscillator signals being the same but those applied to the terminal 7 being in phase quadrature to those applied to the terminal 6.

The mixers 4 and 5 provide output signals which are applied over the respective signal paths 2 and 3 to respective input ports 8 and 9 of a hybrid coupling circuit 10 provided in the form of a 3dB coupler.

The coupler 10 has output ports 11 and 12, the port 11 including a load and the port 12 being connected to an input of a surface acoustic wave I.F. filter 13 which provides I.F.

output signals at an output terminal 14.

The coupler 10 and the filter 13 are shown within a dashed box 15 to indicate that both are provided as an integral surface wave device. A known surface acoustic wave filter for I.F. filtering includes input and output transducers on a substrate with a directional coupler therebetween. To incorporate the 3dB coupler 10 integrally with the filter 13 it is only necessary to provide a second input transducer on the substrate.

In operation the mixers 4 and 5 provide output signals which are the sum and difference of the input signals and the local oscillator signals. It is the difference signals which provide the desired l.F. signals and the sum signals are rejected by the l.F. filter 13, being well out of the frequency band to which this filter is tuned.

There are, however, two input frequencies which will provide a mixer difference signal output equal to the required l.F. difference frequency and these are frequencies spaced by the l.F. frequency above and below the local oscillator frequency. One of these input frequencies provides the wanted signals whilst the other is an image signal which it is desired to reject.

If the mixer 4 receives local oscillator signals having a phase angle of 0 (zero degrees) and the mixer 5 receives local oscillator signal having a phase angle of + 90* then the mixer 4 will provide output signals at l.F. frequency which have a phase angle of 0 regardless of whether the input signals are of a frequency above or below the local oscillator frequency.

On the other hand the mixer 5 will impart a + 90 phase angle to i.F. output signals generated from input signals of frequency above the local oscillator frequency and a - 90" phase angle to l.F. signals generated from input signals of frequency below the local oscillator frequency.

It is more often required to reject, as an image signal, frequencies above the local oscillator frequency and to do this the output from the mixer 4 is fed to the 0 input port of the coupler 10 whilst the port 9 which receives signals from the mixer 5 is made the 90 input port.

Signals entering the coupler at the zero degree port 8 appear at the output port 12 with the same zero degree phase with which they entered the coupler whilst the signals entering the 90 input port 9 receive a further + 90 phase shift. Consequently, signals travelling over the two paths 2 and 3 and originating from the input signals of frequency below the local oscillator frequency arrive at the output port 12 in phase and reinforce one another whilst signals originating from the input signals of frequency above the local oscillator frequency traverse the paths 2 and 3 to arrive at the port 12 in antiphase to one another and cancel. The image signal is thus substantially rejected.

If it were desired to select the upper of the two possible input frequencies then it is only necessary to reverse the input connections to the coupler 10.

Fig. 2 of the accompanying drawings shows and alternative embodiment of the invention and in this figure like parts bear like reference numerals. The essential differences between the embodiment of Fig. 2 and that of Fig. 1 are that the mixers 4 and 5 are positioned at the output side of the coupler 10 which is reversed so that the ports 8 and 9 become output ports. Prior to passing through the coupler 10, input signals received by the aerial 1 are amplified in an amplifier 16 and band limited by a bandpass filter 17. An l.F.

filter 18 is positioned to receive output signals from the mixer 4 and 5 whose output signals are combined at 19 prior to the filter 18.

In this embodiment the coupler 10 and the band limiting filter are provided as a single surface wave device as indicated by the dashed box 20. This device is of similar construction to that of the embodment of Fig.

1.

The operation of this embodiment is substantially identical to that of Fig. 1 and is not considered to require further detailed description. In respect of performance the embodiment of Fig. 1 is capable of giving a tuning range which is greater than twice the l.F.

frequency whilst the range of the embodiment of Fig. 2 is restricted to a range of less than twice the l.F. frequency in view of the band limiting filter 17.

The above description is given by way of example and modifications may be made without departing from the scope of the invention.

For example the coupler 10 and filter 13 or 17 need not be provided as surface acoustic wave components. Further, the coupler 10 need not be a 3dB coupler but any hybrid coupling circuit which provides the desired phase relationships between input and output ports will suffice.

Claims (11)

1. A receiver front end arrangement including two parallel signal paths and two mixers one in each signal path each mixer also being arranged to receive a local oscillator signal the local oscillator signals applied in operation to the two mixers having the same frequency but phases which are in quadrature to one another and a hybrid device including two ports connected to respective ones of the two paths.

2. A receiver front end arrangement as claimed in claim 1 in which the hybrid circuit is a 3dB coupler.

3. A receiver front end arrangement as claimed in claim 1 or 2 in which received radio frequency signals are fed in parallel to a respective mixer and intermediate frequency signal outputs from the two mixers are fed to respective input ports of the hybrid circuit, l.F. signals being provided at an output port of the hybrid circuit.

4. A receiver front end arrangement as claimed in claim 3 in which an l.F. filter is connected to the said output port of the hybrid circuit.

5. A receiver front end arrangement as claimed in claim 4 in which the l.F. filter is a surface acoustic wave filter.

6. A receiver front end arrangement as claimed in claim 1 or 2 in which radio frequency signals are applied to an input port of the hybrid circuit and the two mixers are connected to receive signals from respective output ports of the hybrid circuit output signals from the two mixers being combined to provide an l.F. signal.

7. A receiver front end arrangement as claimed in claim 6 in which a band pass filter is provided to filter the radio frequency signals prior to application to the hybrid circuit.

8. A receiver front end arrangement as claimed in claim 7 in which the band pass filter is a surface acoustic wave filter.

9. A receiver front end arrangement as claimed in any one of claims 6 to 8 in which an l.F. signal path filter is provided to filter the l.F. signal provided by the combined output signals from the two mixers.

10. A receiver front end arrangement as claimed in claim 5 or 8 in which the hybrid circuit and the surface acoustic wave filter are provided as an integral surface wave device.

11. A receiver front end arrangement substantially as herein described with reference to and as illustrated in Fig. 1 or 2 of the drawings.