This invention relates to radio receivers.

A conventional radio receiver comprises a radio frequency amplifier which is arranged to amplify transmitted radio frequency signals of a selected frequency detected by an aerial; an oscillator for producing an oscillator signal of a frequency differing from the frequency of the selected radio frequency signal by a predetermined amount; a mixer for mixing the oscillator signal with the radio frequency signal to produce an intermediate frequency signal derived from the radio frequency amplifier; an intermediate frequency amplifier for amplifying the said intermediate frequency signal; and a detector for producing an audio frequency signal from the amplified intermediate frequency signal.

In order to avoid interference between transmitted radio frequency signals and the intermediate frequency signals, the radio receiver must be tuned to produce an intermediate frequency signal different from any transmitted radio signals. Under international agreements, broadcasting authorities refrain from broadcasting on selected radio frequencies so that these frequencies can be used by radio manufacturers as the intermediate frequency of the radio receiver. Thus, for example, in the United Kingdom, all radio receivers operate on an intermediate frequency signal of 465 kHZ.

In such radio receivers, the radio frequency amplifier must be re tuned to a different frequency when it is desired to receive a transmission by a different broadcasting station. In a motor vehicle, a driver may be compelled to re-tune the receiver several times during a long journey, particularly if he wishes to obtain such information as local weather or traffic conditions which would be broadcast only on ehort-range radio transmitters.

In a broadcasting system described in "Wireless World", January 1971 pp 78, transmissions from short-range transmitters relating to purely local information would all be broadcast on a single radio frequency, e.g. 526.5 kHZ, so that the driver of a vehicle could receive local information without re-tuning the radio receiver during his journey. Each broadcast would commence and terminate with a signal which could be used to trigger a suitable switching circuit within a receiver so that the receiver would automatically switch itself to the frequency on which the local information as being broadcast. An object of the prsaant invention is therefore to provide a radio receiver which becomes automatically switched to receive a transmission on a desired radio fre- quancy in response to a predetermined radio frequency signal on that fre- quancy

According to the present invention, we provide a radio receiver comprising a radio-frequency amplifier for amplifying transmitted radio-frequency signals of a selected frequency detected by an aerial; an oscillator for producing an oscillator signal of a frequency differing from the frequency of the selected radio-frequency signals by a predetermined amount; a mixer for mixing the oscillator signal with the radio-frequency signal to produce an intermediate-frequency signal derived from the radio frequency amplifier; an intermediate-frequency amplifier for amplifying the signal from the mixer; and a detector for producing audio-frequency signals from the amplified signal from the mixer; characterised in that the intermediate-frequency amplifier is arranged to amplify transmitted radio-frequency signals at the intermediate frequency which have not been amplified by the radio frequency amplifier, and in that means is provided for stopping and starting the supply of intermediate frequency signals derived from the radio-frequency amplifier to the intermediate-frequency amplifier in response to a predetermined radio frequency signal transmitted at the intermediate frequency.

By arranging for the intermediate frequency amplifier to be tuned to the frequency on which the predetermined radio frequency signal is broadcast, rather than the frequency conventionally used in the intermediate stage of a radio receiver, the radio receiver will become automatically tuned to receive the broadcast on that frequency when the predetermined signal is broadcast.

Preferably, the detector is operable in response to the predetermined radio frequency signal to generate a signal pulse which stops or starts the oscillator. For example, the oscillator may be associated with a bi-stable switch which receives the signal pulse from the detector and operates to switch the oscillator on and off in response to successive signal pulses.

In the preferred embodiment of the invention, the intermediate frequency amplifier is arranged to receive signals detected by the aerial at the intermediate frequency by means of a circuit which by-passes the radio frequency amplifier. Desirably, the signals passed along the by-pass circuit are fed to the intermediate frequency amplifier.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the drawing which is a block diagram of a radio receiver in accordance with the invention.

The radio receiver shown in the drawing is intended for use in receiving short-range radio transmissions of short duration, e.g. local weather and traffic resports and which are broadcast on a single radio channel. These broadcasts will be preceded by a signal code which is used to effect automatic tuning of the radio receiver to that channel at the start of each transmission. Each transmission is also followed by a further signal code which is used to effect automatic re-tuning of the radio receiver to the original radio channel, when the transmission terminates. The signal codes are in the form of multiplexed radio frequency signals capable of separation from the main broadcast signal in a decoding circuit.

Referring to the drawing, the radio receiver comprises a radio-frequency amplifier 1 which is connected to a socket connection 2, which is in turn connected to an aerial 3. The radio frequency amplifier 1, which is of conventional construction, is capable of being tuned by a manual control (not shown) to amplify transmitted radio frequency signals detected by the aerial 3. The amplified signals are fed to a mixer circuit 4, also of conventional construction, which mixes the amplified signals with a signal from an oscillator circuit 4. As indicated by a broken line in the drawing, the oscillator circuit 5 is coupled with the radio frequency amplifier 1 to generate signals which always differ in frequency by a predetermined amount from the signals to which the amplifier 1 but which has a frequency equal to the difference between the frequencies to which the amplifier 1 and the oscillator 5 are tuned. Signals from the mixer 4 are fed to an intermediate frequency amplifier 10 which is tuned to the frequency difference to the radio frequency amplifier 1 and the oscillator 5. The amplified signals are then fed to a detector circuit 11 which demodulates the intermediate frequency signals and produces an audio frequency signal which is fed to an audio frequency amplifier 12. The amplified audio frequency signal is then reproduced audibly in a loudspeaker 14.

A by-pass circuit 8 couples the socket 2 with the intermediate frequency amplifier 10 via a switch 9. When the switch 9 is closed, signals from the aerial 3 ar_^ passed directly to the intermediate amplifier 4 without amplification by the radio frequency amplifier ₁.

The detector circuit 11 is also arranged to generate signal pulses in response to a predetermined radio frequency signal. The signal pulses are used to operate a switch circuit 15 which alternately stops and starts the oscillator 5. Thus, for example, the detector 11 may include a conventional decoding circuit which directs signals of a predetermined characteristic to the switch circuit 15 rather than to the audio frequency amplifier 12, and the switch circuit 15 may be in the form of a conventional bi-stable circuit or flip-flop which operates in response to successive signals from the detector 11.

In use, when the switch 9 is open and the oscillator 5 is on, the radio receiver operates in the same way as a conventional superheterodyne receiver. Thus, the amplifier 1 is tuned to a frequency corresponding to a desired radio broadcasting channel. Transmitted signals at that frequency are detected by the aerial, amplified in the radio frequency amplifier and the amplified signals mixed with the signals generated by the oscillator 5. The signals produced in the mixer 4 will have a frequency equal to the difference in the frequencies to which the oscillator 5 and amplifier 1 are tuned. Unlike conventional radio re- ceivershowever, this frequency difference is set to the frequency of local radio transmissions (e.g. 526.5 kHZ) rather than the conventional intermediate frequency setting (e.g. 465 k HZ). The intermediate frequency signal is then amplified in the amplifier 10, and demodulated in the detector 11 and the resulting audio frequency signal is amplified and reproduced in the amplifier 12 and loudspeaker 14.

With the switch 9 closed, the receiver continues to operate in the manner described above until a signal code is detected by the aerial 3, this signal code being transmitted at the frequency to which the intermediate amplifier is tuned. The signal code, which will normally be at a different frequency from that to which the amplifier 1 is tuned, by-passes the amplifier 1 and is amplified directly by the intermediate frequency amplifier 10. The detector 11 then generates a signal pulse which is derived from the amplified radio signal code and this pulse is used to stop the oscillator 5. Thereafter, the radio signals at the frequency which the amplifier 1 is tuned are no longer converted to the intermediate frequency by the mixer 4 so that the signals produced by the amplifier are derived solely from those transmitted at the intermediate frequency. These signals are demodulated by the detector 11 and fed to the audo frequency amplifier ₁2 for reproduction in the loudspeaker ₁₄. As a result, the transmission at the intermediate frequency will be received and reproduced by the radio receiver regardless of the frequency to which the amplifier 1 is tuned.

When the transmission at the intermediate frequency ends, a further signal code is broadcast. A further signal pulse is derived from this signal code by the detector 11, and is passed to the switch 15 circuit which reactivates the oscillator 5 thereby permitting the receiver to continue reproduction of the signals broadcast at the frequency to which the amplifier ₁ is tuned.

It will be appreciated that the precise construction of the switch circuit 15 will depend upon the nature of the signal pulses broadcast before and after each transmission. In the above-mentioned article in "Wireless World", a suitable switch circuit is described for broadcasts in which the substance of the broadcast is transmitted in amplitude- modulated signals, and the signal pulses are in the form of frequency- modulated signals of two different frequencies. The signal pulses are detected by a frequency modulation discriminator, the output of which is fed to two filters each tuned respectively to the frequency of the signal used at the beginning and end of the broadcast. The filters are in turn connected to opposite inputs of a bistable latch which can then be used to operate a switch, as indicated above.