WO2003028247A1

WO2003028247A1 - Antenna diversity receiving system

Info

Publication number: WO2003028247A1
Application number: PCT/EP2001/010915
Authority: WO
Inventors: Peter Wegner; Bernd Leinwetter
Original assignee: Receptec Gmbh
Priority date: 2001-09-20
Filing date: 2001-09-20
Publication date: 2003-04-03
Also published as: EP1428329A1

Abstract

An improved antenna diversity receiving system comprising an antenna device including a plurality of FM antennas (5), as well as controllable switching means (33) for sequentially siwthcing through one of said plutality of FM antennas (5) via an antenna cable (9) to a radio set (3) comprising a mulitpath detector (15) coupled to a pulse generator (19) for generating a pulse signal at the detection pultipath detector of multipath interference, a pulse signal to the pulse generator into a pulse signalvomprises the additional features: there is a pilot signal generator (45); the pilot signal generator (45) provides a signal depending on AM/FM switching, there is an additional pilot signal detection witch control (41) controlled directly or indirectly by the pilot signal generator (45) via the cable (9); and via the pilot signal detection and witch control (41) the AM branch in the antenna diversity box will be activated or inactivated or an AM antenna signal will be amplified and/or switched off.

Description

Antenna diversity receiving system

The present invention relates to an antenna diversity receiving system of claim 1. Such a receiver system is known from EP-A-0 792 031. It is specifically suited to be used in vehicles.

This well-known antenna diversity receiving system com- prising an antenna device including a plurality of FM antennas as well as controllable switching means for sequentially switching through one of said plurality of FM antennas via an antenna cable to a receiver upon reception of a switching control signal, the receiver comprising a multipath detector coupled to a pulse generator for generating a pulse signal at the detection of multipath interference .

There is also a system well-known for "scanning diversity" based on a Philips IC, i.e. the TEA6101. Most common practical execution concerns the application of a separate "scanning diversity" box, which is located close to the antenna system (as a rule integrated in' the back screen of the car) . The antenna signal, which is available at the output of the diversity box is supplied to the receiver via a coax cable. Via a second coax connection between radio receiver and the diversity box the selected radio signal is supplied from receiver to diversity box (needed for the quality determination of the received signal) .

The reception of wanted RF broadcast transmitter signal may be disturbed or otherwise deteriorated by various phenomena, such as multipath reception and/or adjacent channel interferences. In general, multipath reception is caused by signal reflections at and/or against environmental physical obstacles, such as mountains, trees, buildings, fences and the like. Due to such signal reflections and RF broadcasting signal may arrive at a certain reception location through various different signal paths, i.e. in various different amplitude and phase conditions. The summation of these multipath signals at the antenna of the receiver results in unpredictable signal amplitude and/or phase distortions, most often effectuating in part or complete cancellation of the useful RF reception signal. These signal cancellations, hereinafter also being referred to as signal dips, strongly depend on the RF carrier frequency of the received RF broadcasting signal and on the location of reception.

In one embodiment as disclosed in EP-A-0 792 031 there are two cables between the antenna diversity box and the radio set installed remote of the antenna diversity box. One cable is used to transmit the RF broadcast signals, whereby the other cable is used to transmit the pulse signals . However, in an other embodiment there is only one single cable used to transmit not only the RF broadcast signals (for an antenna device or receiver) but as well the pulse signal in opposite direction. However, these signals utu- ally interfere, in particularly the pulse signals affected the useful FM RF broadcast signals and may become noticeable in the reproduction of new signals. That is why there has already been suggested an improved antenna diversity receiving system as described in the prior filed but not prior published European patent application with the filing number 97 102 027. The whole content of this prior filed but not prior published European patent application is referred to and is made to be part of the present application.

One of the main objectives of the European patent application referred to above is to provide an improved antenna diversity receiving system using a single antenna cable for the transmission of both useful FM RF broadcast sig- nals and pulsating switching control signals to secure an accurate detection of these pulsating switching control signals and to prevent these from disturbing the processing of the useful FM RF broadcast signals in the receiver.

However, in the European patent application referred to above there is still a drawback that is not still solved. A problem of the described diversity system concerns the fact that the external strong AM signals could trigger a counter in the separate antenna diversity box. Usually that counter may be a Johnson counter. The output value of this counter varies with each switching control signal pulse of the control signal detector within a counting cycle whereby the number of values within the cycle corre- spond to the number of fixed antennas connected to the antenna diversity box. That is why these external strong AM signals might disturb the diversity function in case of simultaneously active FM and AM stages.

This risk can be refused or at least reduced by the choice of a relative high peak-value for the pulse which controls the diversity system. The higher the peak-value, the better the pulse can be distinguished from the amplitude variations of AM signals. However, there is a limit to that.

Therefore, it is a main object of this invention to improve an antenna diversity system in which external strong AM signals are not able to disturb the diversity function in the case of simultaneously active FM and AM stages.

This task is solved in accordance to the features as incorporated in claim 1. Preferred embodiments are referred to in the subclaims.

It is very surprising that it is possible based on the features of the present application to overcome the drawbacks as outlined above. In accordance to the present invention it is no longer possible that external strong AM signals could tricker an counter, especially an Johnson counter, in a separate antenna diversity box "even if the FM and AM stages are simultaneously active" .

This advantage is received by introducing a separate pilot signal which may be switched between "on" and "off" . This separate pilot signal is not able to adversely affect triggering and/or counting pulse signals to function and control the switching means of the diversity signal system.

A preferred embodiment of the antenna diversity receiving system in accordance to this invention is characterized by that the pilot signal is a sine signal (for example, preferred with a signal frequence of 22 kHz) . Based on this pilot signal it is possible to make the AM amplifier active, while the FM diversity is not active. This pilot signal is controlled in the radio set; i.e. is switched between "on" and "off" whereby this pilot signal is transferred from the radio set while the single antenna coax cable to the "antenna diversity box" .

In a especially preferred embodiment of the invention the pilot signal generator provides a low signal if it is activated. Contrary thereto the pilot signal generator provides a high signal if it is inactivated. Is thereby preferred that the pilot signal generator is activated (producing a low signal) then the radio set is switched to

AM receipt. On the other hand the pilot signal generator is producing a high signal if it is switched in inactivated condition if the radio set is switched to receive FM signals .

In the antenna diversity box there is integrated a pilot signal detection and switch control, controlling another switch for activating and inactivating an AM amplifier. The pilot signal detection and switch control works in that way that the "high" signal is produced if AM reception is on.

It is additionally very useful that the pilot signal may be used for diagnostics of system functions, too, together with a pulse generator circuit controlled by the DSP (Digital Signal Processor) in the radio set. In accordance to the invention it is possible that the switching signal (changing form "low" under inactive condition to "high" under active condition) generated by the pilot signal can be used with well-known technics to generate signals for the following items:

- switch on the AM amplifier

- stop the Johnson counter

- reset the Johnson counter

- switch to FM antenna.

In case of wanted diagnostics for the system the signal can be used to set the FM diversity to a defined start position (FM antenna) .

It may be summarized that the invention describes a prac- tical solution for the generation of the diversity control signal, which is needed in case of a "one cable" diversity system. Further, the invention comprises a solution for the problem of diversity function disturbed by strong AM- signals when listening to FM unique elements which are:

The way one control signal is derived from, e.g. the available four control signals as supplied by a diversity IC, for example by the TEA6101; the way the requirement is met that the control signal should have a zero average value (in order to avoid interference by the control signal) ; the way with which by switching via a low frequency sine wave (pilot or encoded) signal technics the mentioned problem with strong AM-signals is solved; the possibility of providing diagnostics for AM/FM car reception system.

Further features and advantages may be apparent with reference to the figures showing a preferred embodiment in accordance to this invention. Herein it is shown:

Figure 1 : a preferred embodiment of an antenna diversity receiving system according to the invention comprising an antenna device coupled via an single antenna cable to a receiver;

Figure 2a a preferred embodiment of the multipath detector and a pulse generator for using in the antenna diversity receiving system of figure 1;

Figure 2b an other embodiment of a multipath detector and a pulse generator for use in the antenna diversity receiving system of figure 1;

Figure 3 : A-D time plots of the multipath detector of figure 1 and

Figure 4a to 4c time plots of output signals of respectively the multipath de- tector, the pulse generator and the pulse shaper following said pulse generator of figure 1.

Figure 1 shows a blockdiagramm of a complete antenna diversity which makes use of the concerned invention.

The antenna diversity system comprises an antenna diversity box 1 and a radio set 3 partly referred hereafter as receiver 3. The blockdiagramm in accordance to figure 1 shows an antenna structure consisting of four FM antennas 5 and one AM antenna 7. Other combinations of number of antennas and summary or/and difference of FM-antennas are in this circuitry possible, too. Each antenna 5 is coupled with an entry 33a, 33b, 33c and 33d, respectively, of an FM-switch 33 via a band pass filter BP .

The output 33e of the FM-switch 33 is connected with an input of an FM-buffer 34 whereby the output 34b of the FM- buffer is intern connected via an AGC-circuitry with the input 34a of this FM-buffer 34.

Further, the output 34b of the FM-buffer 34 is connected via an band pass filter BP with the output la of the an- tenna diversity box.

The antenna diversity box 1, i.e. the output la as well as the receiver 3, i.e. the input 3a, are connected via a single cable 9.

An AM/FM tuner 13 is equipped with a signal level detector which delivers a voltage, which is a measure for the field strength of the tuned antenna signal. As such, amplitude variations in the tuned antenna signal will lead to corresponding variations in the output of the level detector. Both, the output of the level detector and the audio output (the MPX-signal) of the FM demodulator are supplied to the signal quality detection system, which is a part of the antenna diversity IC 15, e.g. the TEA6101. This IC has four DC outputs A, B, C and D from which one has a value "high", this may be featured by an output signal which is "logic 1". The other output has the value "low" (e.g. if the output is "logic 0"). As well-known, each time the signal quality detection system detects simultaneously a dip in the level voltage and the distortion components above the stereo MPX-spectrum, the output pin with value "high" gets the value "low", while the value "high" is taken over by another output pin. The transfer of the value "high" to another pin occurs sequentially (so far A to B to C to D and again to A ) .

The output pins A, B, C and D are connected to a pulse generator 19, which, according to the invention, generates a positive and a negative pulse each time the value "high" of one of the pins A, B, C, and D is transferred to another pin. This pulse-couple (positive and negative pulses together) is supplied to the separate antenna diversity box 1 via the antenna coax cable 9 connection between this box 1 and the antenna input 13a of the receiver 13. The coil LI serves to suppress somewhat the higher frequency contents of the pulse.

The receiver 3 further comprises an IF-Demodulator 21 the input of which is connected via a line 23 with one output 13b of the AM/FM tuner 13 in the audio path. The IF-Demodulator provides a MPX-signal which is provided to one entry 15a of the diversity control system 15. The signal S is led to speakers, e.g. via a not shown signal processor.

On the other hand, a second output 13c of the AM/FM tuner is connected with another input 15b of the diversity control 15 to put a strength signal to this input of the diversity control 15.

An output of the pulse generator 19 is coupled to a pulse shaper 25 comprising signal differentiating R1,L1,C for differentiating the standard rectangular pulse signal waveform of the pulse generator (referred to later on) to convert said rectangular pulse signal waveform into a pulse signal pair comprising a first signal pulse, or a first pulse spike, followed by a second signal pulse, or a second pulse spike, having a signal polarity opposite to the signal polarity of the first signal pulse, and occurring substantially during said first and second signal transients. The overall waveform of the pulse signal pair therewith varies symmetrically around a reference level.

It is noted that the pulse shaper 25 is coupled to a first coil or block coil LI providing a signal suppression within the frequency range of FM RF broadcast frequency band.

As shown in figure 1 the antenna diversity box is also provided with a coil or block coil enumerated with L2. Via this block coil L2 the mentioned pulse couple is fed to a pulse converter 27, which transforms the pulse couple to a one single pulse which triggers a Johnson counter 29. In fact the pulse converter 27 takes care that the Johnson counter 29 is triggered on the positive rising edge of the positive pulse of the mentioned pulse couple (which can be achieved by means of well known technics) . The Johnson counter 29 provides a pulse (1 out of a maximum of 10 outputs) to a switching matrix 31 for antenna combination and is led to an antenna switch 33 by means of choosing sequen- tially another antenna each time the Johnson counter is triggered. The final result is that another antenna is chosen, each time the already mentioned signal quality detection system simultaneously detects a dip in the signal level and the distortion components above the stereo MPX spectrum. This procedure happens continuously in case of bad signal quality reception. In case of good signal quality reception at the antenna, no pulse is generated from the signal quality detection and the switching is stopped, up to a reception signal disturbance occurs.

The negative pulse of the mentioned pulse couple as generated by the pulse generator 19 is needed to maintain the energy average value of the pulse signal equal to zero. If this measure would not be taken, then random fluctuations in the average value of the pulse signal would occur with the risk of interferences.

As indicated in figure 1 the DC supply for the circuitry in the antenna diversity box can be obtained from the radio set 3, i.e. from the DC supply connection 35 via the antenna coax cable 9. For further DC conditioning in an antenna diversity box there is a DC supply circuitry 36 incorporated in the antenna diversity box the input of which is connected with the block coil L2. The output of the DC supply conditioning unit 36 is connected with an switcher

51 for DC supply to the AM buffer 43 referred to later on.

Figures 2a and 2b show the basic principle of the pulse generator 19 according to the invention while figure3 shows an example of the sequential "high" / "low" switching of the pins A, B, C and D. According to figure2a, two identical circuits which comprise an exclusive OR function are connected to pins B and D, while pins A and C are not connected to some circuits. The choice for pins B and D to be connected to the circuits with the exclusive OR functions is arbitrary. One could have chosen pins A and C.

It is obvious for a person skilled in the art that it is only essential that both, the pin, which precedes and the pin, which follows on the connected pin are not connected.

The pulse generator 19 is so designed that only one of the outputs A, B, C or D has a high output voltage of digital "1" value. During such high output voltage or digital "1" value, one of the corresponding four FM antennas is to be switched trough via the antenna cable 9 to the RF receiver input 13a. The description of the function of the Philips IC TEA6101 given so far is sufficient for a proper understanding of the invention. For further details reference is made to the relevant Philips IC Data Handbook of this IC.

The pin connectors B and D are coupled to exclusive OR gates Gl and G2 of the pulse generator 19 (respectively) directly as well as through delay elements R1C1 and R2C2, respectively. Outputs of the exclusive OR gates Gl and G2 are coupled via a R3- or R4-element, respectively, with a summarizing point 39, which in turn is connected to a out- put 19a of the pulse generator 19, which is connected to the pulse shaper 25.

The delay elements R1C1 and R2C2 each comprise an RC cir- cuit delaying the supply of the digital value supplied at the pin connectors B and D over an RC time constant to one

^■ of the inputs of the respective exclusive OR gates Gl and

G2. A high or digital "1" value arising at e.g. pin con- nector D upon detection of a multipath distortions on a point in time tl, will immediately be supplied to the one input of the exclusive OR gate Gl and some time later at the other input of said exclusive OR gate Gl . This results in a pulse shaped signal having a rectangular waveform at the output of said exclusive OR gate Gl, the pulse width thereof being determined by the RC time constant of the delay element R1C1. This is further illustrated in Figure

3 and 4.

Figure 2b shows a very similar embodiment compared with the embodiment of figure 2a. The only difference is that the outputs of OR gates Gl and G2 are coupled to inputs of a non-exclusive OR gate G3 which is connected with the output 19a via an additional OR gate G4 and an R-element R5 and a C-element C4.

In figure 3 it is shown what will happen at the points of time tl and t2 when the antenna diversity box is activated. Figure 4a shows the shape of the signals on the two inputs of the concerned exclusive OR, i.e. of the output of the diversity control 15. The shape difference between both signals is determined by the time constant of the R/C network, which is connected to one of the inputs of the exclusive OR. Due to the shape difference the output of the concerned excl . OR will have the value "high" for a short time, e.g. lμs (dependent on the time constant of the R/C network) thus leading to an output pulse like indicated in figure 4b. So, it is apparent that a "high"/"low" switching from A to B and from B to C leads to the described output pulse of the concerned exclusive OR. In case of "high"/"low" switching from C to D and from D to A a comparable pulse is generated by the exclusive OR circuit, which is connected to pin D.

In figure 2a the outputs of both exclusive OR's are connected to each other by two 1 kOhm resistors (R3 , R4) so that at the concerned connection point 39 the generated pulse is available independent of which exclusive OR generates the pulse. The resistors R3 and R4 in figure 2a decouple the EXOR outputs form each other and reduce the output level of the switching pulses to system used level. The resistors RI, R2 have together with Cl, C2 low pass func- tion to reduce radiation (EMC) of the output rectangle signals at "B" and " of TEA6101. A more elegant method to combine the outputs of the concerned exclusive OR's is given in figure 2b.

By differentiation of the resulting output pulse, a pulse shape like indicated in figure 4c is obtained. This pulse shape then in fact is a combination of a positive and a negative pulse, earlier defined as "pulse couple" . As already explained, the positive pulse is finally used to trigger the Johnson counter 29, which drives the antenna switch 33 while the negative pulse is used to guarantee the average value of the total pulse signal is zero.

The desired differentiation of the output pulse is obtained by a right choice for the values of the resistor RI and coils LI, C4. (see figure 1) . The impedance of the antenna cable at the antenna input of the radio set is roughly determined by tuner input impedance for frequencies in order of 0.1 to 1 MHz, being the frequency range, where the spectral energy of the pulses is concerned. It may further be clear that resistor RI can never be zero (zero would mean a short circuit for the pulses) . A practical value for RI lies in the order of 330 Ohm. Due to this, the AM-sig- nal, supplied by the antenna, should be buffered by an AM- bufferstage 43 as indicated in figure 1. Practically this means an active antenna for AM. For the FM antennas, active or passive is not relevant in this aspect. To avoid unde- sired charge accumulation which may disturb the AM buffer 43 the AM antenna 7 is connected with the AM buffer 43 via an protection circuitry ESD avoiding such an undesired charge accumulation.

As already mentioned, a problem of the described diversity system concerns the fact that the external strong AM-signals could trigger the Johnson counter 29 in the separate antenna diversity box 1, thus disturbing the diversity function in case of simultaneously active FM and AM stages. A fundamental solution is using a pilot signal (for ex. : 22 kHz sine wave signal or other) to make the AM-am- plifier 43 active, while FM-diversity is not active. This pilot signal is transferred from the radio set 3 via the antenna coax cable 9 to the antenna diversity box.

Now a special pilot signal system is discussed.

The pilot signal system comprises two important components:

The first component is the pilot signal generator 45, which is low when activated and high when inactivated (preferred realized by AM / FM switching at the tuner 13) . Location of the transmitter of the pilot signal is in the radio set 3. Only low level is allowed because of activation level of the tuner AGC. As shown in figure 1 the pilot signal generator 45 is switched by a switcher unit 45. If the switcher unit 55 is switched into the state "AM on" the pilot signal generator 45 is connected via the switcher unit 55 with the DC supply connection 35. This connection is interrupted if the switcher unit 55 is switched into the state "FM off" .

In the antenna diversity box 1 is the existence of the pilot signal and the switching pulse couple at the same time during FM reception not wanted, because of possible interference. Therefor the pilot signal is active during AM reception. The pilot frequency should be lower than broadband frequencies AM. This signal is used to switch the diversity box FM antenna 1 generally active, to make sure reception of FM broadband services for ex. RDS during AM reception.

The pilot signal can be used for diagnostics of system functions, too, together with the pulse generator circuit controlled by the DSP (Digital Signal Processor) in the radio set .

In the antenna diversity box is of further importance a pilot signal detection circuitry 41, which detects the pilot signal and handles different functions. The input 41a of the pilot signal detection is connected via the RF coax cable 9 to the radio set 3.

If the pilot signal is transmitted from the radio set 3, the detection circuit detects and generates a switching signal (inactive =>"low"/ active =>"high"). This information can be used with well known technics to generate sig- nals for following items:

- switch on the AM-amplifier 43 (e.g. via the switcher 51) - stop the Johnson counter

- reset the Johnson counter 29 (e.g. via a reset unit 61)

- switch to FM-antennas 5.

In case of wanted diagnostics for the system the signal can be used to set the FM diversity into defined start position

(FM-Antenna 1) . With additional single pulse couple from the pulse generator in the radio set and switching off the pilot signal again, FM-antenna 2 is. switched on. By repeat- ing sending a single pulse couple the next antenna and so on each antenna can be switched on and checked separately.

In case of very strong field strength of low frequency transmissions, for ex. "Submarine Komunication" under AM - broadband frequencies near to the pilot frequency the diversity will be stopped and FM-antenna 1 will be switched on, diversity is then inactive.

Claims

Claims ;

1. Antenna diversity receiving system comprising an antenna device including a plurality of FM antennas (5) , as well as controllable switching means (33) for sequentially switching through one of said plurality of FM antennas (5) via an antenna cable (9) to a radio set (3) upon reception of a switching control signal, the radio set (3) comprising a multipath detector (15) coupled to a pulse generator (19) for generating a pulse signal at the detection of multipath interference, and with a pulse shaper (25) following the pulse generator (19) to convert the pulse signal to the pulse generator into a pulse signal, characterized by the following features - there is a pilot signal generator (45) ;

- the pilot signal generator (45) provides a signal depending on an AM/FM switching;

- there is an additional pilot signal detection switch control (41) controlled directly or indi- rectly by the pilot signal generator (45) via the cable (9) ; and

- via the pilot signal detection and switch control

(41) the AM branch in the antenna diversity box will be activated or inactivated or an AM antenna signal will be amplified and/or switched off.

2. Antenna diversity receiving system in accordance to claim 1, characterized by that the pilot signal generated by the pilot signal generator (45) is a sine wave signal or the like .

3. Antenna diversity receiving system in accordance to claim 1 or 2 , characterized in that the frequency of the pilot signal generated in the pilot signal generator (45) is lower than the broadband frequencies received via the AM antenna (7) .

4. Antenna diversity receiving system in accordance to claim 3, characterized in that the frequency of the pilot signal provided in the pilot signal generator (45) is lower than 50 kHz especially lower than 30 kHz, especially about 22 kHz.

5. Antenna diversity receiving system in accordance to claim 3 or 4, characterized in that the frequency of the pilot frequency generated in the pilot signal generator

(45) is higher than 1 kHz, especially higher than 10 or preferred 20 kHz, especially about 22 kHz.

6. Antenna diversity receiving system in accordance to one of the claims 1 to 5 characterized in that the pilot signal generator (45) is controlled by an AM/FM-switcher (55) .

7. Antenna diversity receiving system in accordance to claim 6, characterized in that during AM reception in the AM/FM tuner (13) the AM/FM-switcher (55) is switched through and/or the DC supply (35) is provided with the pilot signal generator (45) .

8. Antenna diversity receiving system in accordance to one of the claims 1 to 7, characterized in that the pilot signal provided in the pilot signal generator (45) is "low" if the pilot signal generator (45) is "on" whereby the pilot signal is "high" if the pilot signal generator (45) is "off" .

9. Antenna diversity receiving system in accordance to one of the claims 1 to 8, characterized in that the antenna diversity box (1) has a special DC supply or an additional DC supply circuitry (36) for conditioning the DC current provided by the radio set (3) via the single cable (9) .

10. Antenna diversity receiving system in accordance to one of the claims 1 to 9, characterized in that the pilot signal detection and switch control (41) controls the AM buffer (43) in the AM receiving branch via an AM-on/AM-off- switcher (41) .

11. Antenna diversity receiving system in accordance to claim 10, characterized in that the DC supply conditioning unit (36) supplies a DC current via the switcher (41) to the AM buffer (43) only during AM reception and/or if the EM-on/EM-off-switcher (41) is closed.

12. Antenna diversity receiving system in accordance to one of the claims 1 to 11, characterized in that the pilot signal detection and switch control (41) generates signals for switching on the EM amplifier (43) and/or to stop a counter (29) for switching through the FM switcher (33) and/or to reset the counter (29) and/or to switch to the FM antennas (5) .

13. Antenna diversity receiving system in accordance to one of the claims 1 to 12, characterized in that there is an additional reset unit (61) to reset the counter (29) .

14. Antenna diversity receiving system in accordance to one of the claims 1 to 13 characterized in that the pilot signal generator (45) is incorporated in a radio set (3) and the pilot signal detection and switch control (41) is incorporated in an antenna diversity box (1) whereby the antenna diversity box (1) and the radio set (3) are connected by the cable (9) .