GB2026798A

GB2026798A - Vehicle antenna amplifier

Info

Publication number: GB2026798A
Application number: GB7923615A
Authority: GB
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1978-07-07
Filing date: 1979-07-06
Publication date: 1980-02-06
Also published as: FR2431779A1; DE2830047C2; DE2830047A1; FR2431779B1; IT7923897A0; IT1121962B

Abstract

High-amplitude signals cause excessive interference in active antenna circuits. The antenna signal is supplied to the common input of a plurality of band-pass filters (1) which split the receiver frequency range into respective channels which are amplified by respective amplifiers (2) before being recombined (4). Alternatively reception of an excessively high-amplitude is used to trigger attenuation of the antenna signal before being wide-band amplified. <IMAGE>

Description

SPECIFICATION Vehicle antenna amplifier This invention relates to a vehicle antenna amplifier which is resistant to high-amplitude signals.

As is known, a wide-band amplifier arranged immediately adjacent the base of a vehicle antenna (active antenna) produces a better signalto-noise ratio than arrangements in which the antenna is connected via a cable to a remote receiver.

However, it has proved that active antennae have insufficient resistance to high-amplitude signals. It is in fact possible that a transmitter in the near vicinity will cause so much interference that the reception of other transmitters is no longer ensured. The modulation distortions caused by the strongly received transmitter can be so great that the latter is also unable to provide a useful signal.

According to this invention there is provided an amplifier for a vehicle antenna for inhibiting interference due to a reception of a highamplitude signal at a frequency in the range of the amplifier said amplifier, including an input for connection directly to a vehicle antenna, said input being connected to means arranged for supplying from the input a wave-band signal, in which the high-amplitude signal automatically has its frequency excluded or has its amplitude attenuated, to a wide-band amplifier circuit which has an output for connection to a receiver.

In one embodiment there are at leasthwo bandpass filters assigned to separate frequency bands, and having outputs connected to wide-band amplifier circuits.

In another embodiment there is an antenna amplifier which is connected to, immediately adjacent, the base of a vehicle antenna a sensor connected between the output of the antenna amplifier and a diode, a timer which is connected to the output end of the diode and is arranged to control the response time, to the sensor output, of a threshold value switch arranged to control a first switch for switching an attenuating element into the antenna output, and a second switch controlled by the threshold value switch so as to change said threshold value when said attenuating element is switched into the antenna output.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figs. 1 and 2 are respective block circuit diagrams of two forms of car antenna amplifier embodying the invention.

The VHF band and/or the AM band is split by band-pass filters 1 , which are connected to the antenna A, into at least two sub-bands (see Fig. 1 in which a plurality of the individual sub-bands are shown). Each sub-band is separately amplified in a respective wide band amplifier 2. At the output all the channels are recombined at a collective point 4. Advantageously the wide-band amplifiers 2 are arranged on a single semi-conductor body. When the wide-band amplifiers 2 have appropriate operating characteristics, band-pass filters 3 at the outputs of the band-pass amplifiers 2 are not absolutely necessary and therefore have merely been shown in broken line.

If, in the embodiment illustrated in Fig. 1 , a strongly received VHF transmitter interferes with one of the VHF channels, three other channels are always available for reception (non-overlapping channels).

Referring to Fig. 2 the signal emanating from the antenna A passes via an antenna amplifier V and a sensor amplifier V2 to a rectifying diode D.

The illustrated states of electronic switches S1, S2 are associated with an operating situation involving low-level signals. If a signal now arrives from at least one strongly received transmitter, the diode D produces a d.c. voltage across a capacitor C,. If this d.c. voltage exceeds a reference voltage Uri, a threshold value switch SS emits an output signal which closes the switch S,. The loading by resistor R, considerably reduces the antenna voltage (for example by a factor of 30).

To prevent the threshold value switch SS from then returning to the initial switching state, the reference voltage is switched from ri to U,2 which is considerably lower than the voltage U, (e.g. by a factor of 60).

The reference voltage is changed by a larger voltage ratio than the input voltage in order that in the event of changes in input voltage, which are unavoidable in moving vehicles, the arrangement will not switch frequently from one state into the other. Frequent switch-over can also be avoided by ensuring that the response time constant R2 C,) amounts to about 10 s and the decay time constant (R3 . C,) amounts to about 60 s, for which purpose a resistor R2 is arranged between the diode D and the capacitor C, and a further resistor R3 is arranged between earth and the point of connection of the resistor R2 to the capacitor C,.

In comparison to automatic amplification regulating devices, such as are known in connection with radio relay receivers, the embodiment illustrated in Fig. 2 has the advantage that in the event of rapid amplitude fluctuations of the interference signal, the useful signal amplitude is not regulated upwards and downwards with the same frequency. A further advantage consists in that a simple diode can be used for the electronic switch S, without impairing the behaviour of the high-amplitude signal. On the other hand, conventional amplification regulation units require continuously variable control elements which are problematical in respect of the behaviour of high-amplitude signals.

Other attenuating elements can be used in place of the resistor R1, and in particular a capacitor can be used in place of the resistor R, so as to allow an attenuation which is independent of frequency to be achieved in the AM bands.

In addition it is also possible to obtain the signal for the threshold value switch SS from a point between the antenna and the amplifier V,. If an attenuation element is used in series with the antenna but following the point connected to the sensing input of the switch SS, it is then no longer necessary to switch over the threshold value by means of a switch such as S2. It is also possible to use a plurality of attenuation stages in which case four attenuation stages are particularly advantageous.

Finally it is also advantageous to design each of the individual channels of the embodiment illustrated in Fig. 1, in accordance with the embodiment illustrated in Fig. 2 which produces an optimum car antenna amplifier resistant to high-amplitude signals.

Claims

1. An amplifier for a vehicle antenna for inhibiting interference due to a reception of a highamplitude signal at a frequency in the range of the amplifier said amplifier including an input for connection directly to a vehicle antenna, said input being connected to means arranged for supplying from the input a wave-band signal, in which the high-amplitude signal automatically has its frequency excluded or has its amplitude attenuated, to a wide-band amplifier circuit which has an output for connection to a receiver.

2. An amplifier according to claim 1 wherein said wave-band signal supplying means is arranged for providing a plurality of said waveband signals arranged to cover said amplifier frequency range in a non-overlapping manner.

3. A vehicle antenna amplifier resistant to highamplitude signals and including at least two bandpass filters assigned to separate frequency bands, and having outputs connected to wide-band amplifier circuits.

4. An amplifier according to claim 3 wherein the wide-band amplifier circuits are integrated on a single semi-conductor body.

5. An amplifier according to claim 3 or claim 4 wherein the wide-band amplifier circuits have outputs connected to respective further band-pass filters.

6. An amplifier according to any one of claims 2 to 5 wherein means are provided for selectively attenuating the wave-band signals.

7. An amplifier according to claim 1 wherein said wave-band signal supplying means includes selectively operable means for attenuating the signal from said input, said attenuation being effected as a result of detection of the highamplitude signal from said input.

8. An amplifier according to claim 7 wherein means for detecting the high-amplitude signal has a predetermined threshold level at which it is arranged to effect said attenuation.

9. An amplifier according to claim 8 wherein, during said attenuation, said detecting means is arranged to sense the attenuated signal in order to provide detection of the high-amplitude signal, means being provided for reducing said threshold level during attenuation.

10. An amplifier according to any one of claims 7 to 9 wherein said detection is by means of a circuit having a predetermined response and decay time.

11. A vehicle antenna amplifier resistant to high-amplitude signals and including an antenna amplifier which is connected to, immediately adjacent, the base of a vehicle antenna a sensor connected between the output of the antenna amplifier and a diode, a timer which is connected to the output end of the diode and is arranged to control the response time, to the sensor output, of a threshold value switch arranged to control a first switch for switching an attenuating element into the antenna output and a second switch controlled by the threshold value switch so as to change said threshold value when said attenuating element is switched into the antenna output.

12. An amplifier according to claim 11 wherein the attenuating element is a resistor or a capacitor.

13. An amplifier for a vehicle antenna for inhibiting interference due to reception of a highamplitude signal at a frequency in the range of the amplifier said amplifier being substantially as described herein with reference to Fig. 1 or Fig. 2 of the accompanying drawings.

14. A vehicle radio antenna circuit including an amplifier according to any one of the preceding claims.