Abstract:
Structurally simple transmit/receive circuits for hearing devices are to be able to be deployed for higher frequencies as well. It is therefore proposed according to the invention that at least two PIN diodes should be connected in an anti-parallel manner between the receive oscillating circuit and the amplifier connected thereto to protect the amplifier. A capacitance diode can optionally be expanded so that larger component tolerances can be permitted for the circuit.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of German application No. 10 2006 029 195.6 filed Jun. 26, 2006, which is incorporated by reference herein in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a transmit/receive circuit with a transmit/receive antenna for the inductive transmission of signals, a receive oscillating circuit, which is connected to the transmit/receive antenna, and an amplifier, which is connected to the receive oscillating circuit. In particular the present invention relates to a remote control for a hearing device with such a transmit/receive circuit. 
       BACKGROUND OF THE INVENTION 
       [0003]    The publication DE 103 04 479 B3 is known to be the closest prior art. This describes a device for transmitting and receiving data for remote controls for hearing devices. It shows how a combined transmit/receive unit can be realized with a single transmit/receive antenna, without overloading the downstream receiver. Standard silicon protective diodes are used here, which function well at low frequencies and comparatively high resonant capacitances, their blocking capacitance being negligible. These diodes are necessary in order to limit the voltage at the input of the receive circuit in the transmit instance. They are therefore essential for the combined transmit/receive circuit. 
         [0004]    It is necessary also to be able to deal with higher bandwidths with the transmit/receive circuits of hearing devices. However with the known device for the transmission of higher bandwidths described above, as soon as a switch is made to higher frequencies, smaller inductances and also smaller capacitances are deployed. This means that the blocking capacitances of the protective diodes are no longer negligible and they detune the resonant circuit in the receive element significantly. Moreover the blocking capacitances of the diodes are not known and can vary with the manufacturing process. This makes a clean tuning of the oscillating circuit in the receive element very difficult, so it would have to be carried out manually on an individual basis in each device with a high level of outlay. It is not possible to dispense with said diodes however, as they are absolutely essential as input protection elements for the receive amplifier. Without protection very high voltages would occur at the input of the receive amplifier in the transmit instance, leading to destruction of the receive amplifier. 
         [0005]    The further publication EP 1 301 017 A1 describes a mobile communication device with electrostatic discharge protection. Two PIN diodes are connected in an anti-parallel manner between the high-frequency input of the receiver and ground. They serve as overvoltage protection and are characterized by their minimal effect on high-frequency signals due to their very low capacitance. 
       SUMMARY OF THE INVENTION 
       [0006]    The object of the present invention is therefore to propose a transmit/receive circuit with a single transmit/receive antenna, which is also suitable for the transmission of higher bandwidths. 
         [0007]    According to the invention this object is achieved by a transmit/receive circuit with a transmit/receive antenna, a receive oscillating circuit, which is connected to the transmit/receive antenna, and an amplifier, which is connected to the receive oscillating circuit, with at least two PIN diodes being connected in an anti-parallel manner between the receive oscillating circuit and the amplifier to protect the amplifier. A capacitance diode is connected parallel to the PIN diodes. This allows adequate tuning of the receive oscillating circuit to be achieved, even with larger component tolerances. 
         [0008]    The PIN diodes deployed according to the invention advantageously have lower blocking layer capacitances, so the transmit/receive circuit can also be deployed for higher frequencies. If larger currents are anticipated in the receive circuit, further PIN diode pairs connected in an anti-parallel manner can be connected parallel to the one PIN diode pair connected in an anti-parallel manner. 
         [0009]    A control voltage to trim the receive oscillating circuit can be applied to the capacitance diode. This control also enables the receive oscillating circuit to be compensated automatically if required. 
         [0010]    It is necessary to have a capacitor between the capacitance diode and the PIN diodes, to decouple the DC control voltage at the capacitance diode toward the oscillating circuit. 
         [0011]    A transmitter can also be coupled to the transmit/receive antenna, it being possible to operate said transmitter with reduced power compared with its stipulated transmission at the resonant frequency of the receive oscillating circuit, to tune the receive oscillating circuit. Such excitation of the receive oscillating circuit allows said receive oscillating circuit to be better tuned by measuring the amplitude in the receiver. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The present invention is described below with reference to the accompanying drawing, which shows a circuit diagram of an inventive transmit/receive circuit. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    The exemplary embodiment described in more detail below represents a preferred embodiment of the present invention. 
         [0014]    The transmit/receive circuit for a hearing device shown in the figure has a transmit/receive antenna L 1 , configured as a coil. A second coil L 2 , serving to transmit a transmit signal of a transmit amplifier V 1  in an inductive manner, is coupled loosely to it. To this end the transmit amplifier V 1  is coupled by way of a resistor R 1  to the coil L 2 . The coil L 2  is connected to a supply voltage VCC on the other side. The transmit amplifier V 1  is also supplied with the supply voltage VCC at its supply voltage pin VDD. It is connected to ground at the pin GND. It receives a transmit signal to be amplified SG by way of its input IN. The transmit amplifier V 1  is only active when transmitting and is deactivated in the receive instance. 
         [0015]    The receive circuit is connected to the transmit/receive antenna L 1 . The antenna coil L 1  forms a resonant circuit together with a main adjusting capacitor C 1  connected by way of a resistor R 1 , said resonant circuit being excited to oscillate when receiving. The signal of the resonant oscillating circuit is amplified by an amplifier V 2 , which has little background noise, so a correspondingly amplified receive signal ES results. 
         [0016]    Since the receive element (in the figure on the right next to the transmit/receive antenna L 1 ) also oscillates during transmission, with very high field strengths and correspondingly high voltages then occurring, the receive amplifier V 2  must be appropriately protected. A PIN diode array consisting of the PIN diode pairs D 2 , D 3 , D 4  and D 5  is therefore connected parallel to the receive oscillating circuit L 1 , C 1  here. The individual diode pairs here each consist of two PIN diodes connected in parallel. The PIN diode pairs D 2  and D 3  are connected in an anti-parallel manner, as are the PIN diode pairs D 4  and D 5 . In principle a simple PIN diode pair connected in an anti-parallel manner could be deployed instead of the PIN diode array, if the occurring currents are correspondingly small. The number of PIN diodes should be adjusted accordingly in the case of larger currents. 
         [0017]    In the receive instance, when the signal is very small and not above the switching threshold of the PIN diodes, the PIN diode array blocks. In contrast in the transmit instance, when the signal is correspondingly large, the PIN diode array forms a short circuit, which protects the amplifier V 2 . 
         [0018]    A varactor diode is connected parallel to the PIN diode array, in other words parallel to the input of the amplifier V 2 , by way of a capacitor C 2 . In the receive instance the capacitor C 2  together with the capacitance of the varactor diode D 6  compensates for the loss of the coil L 2 . 
         [0019]    So that the capacitance of the varactor diode D 6  can be adjusted correspondingly, the connecting point between the capacitor C 2  and the varactor diode D 6  is subjected by way of a resistor R 3  to a setting voltage V Tune . The connecting point between the capacitor C 2  and the varactor diode D 6  is connected to the one input terminal of the amplifier V 2  by way of a coupling capacitor C 4 . The other input terminal, which is connected to the anode of the varactor diode, a common contact of the PIN diode array and a terminal of the transmit/receive antenna L 1 , is connected to ground. 
         [0020]    The mode of operation of the inventive transmit/receive circuit is described in more detail below. Since PIN diodes are deployed here instead of standard diodes, detuning of the oscillating circuit in the receive instance is very slight even at higher frequencies, as these PIN diodes have very small blocking capacitances. In addition, to compensate for this relatively small residual error, a further capacitance or varactor diode D 6  can be deployed, which can compensate for any residual errors. This varactor diode D 6  can either be subjected manually to an appropriate control voltage V Tune  or the trimming process is carried out automatically by measuring the oscillating circuit amplitude at resonant frequency and an interactive compensation takes place. This reduces the tuning process to the setting of a single trimming potentiometer or it can even be carried out fully automatically. The capacitance diode should only be deployed in conjunction with the PIN diodes, since the blocking layer capacitance of standard diodes is generally too high and could not be fully compensated for by a capacitance diode. 
         [0021]    Automation of the compensation method is advantageous in particular because the circuit as a whole is subject to an aging process and temperature fluctuations. Therefore automatic compensation is available at every switch from transmit to receive or vice versa. Automation can be achieved by activating the transmitter to the resonant frequency with minimal power (e.g. reduced by pre-resistor) and measuring the receive voltage behind the low-noise receive amplifier V 2 . Frequency tuning is optimal, when the maximum voltage is reached. Alternatively the receive voltage can simply be compared with a reference voltage and the transmit power is varied. At the point when the reference voltage is no longer reached, the transmit power is too low. The power must then be increased a level. The tuning can be changed, when the reference voltage is reached/exceeded during interim tuning. 
         [0022]    The inventive transmit/receive circuit with the PIN diodes can advantageously also be deployed at higher frequencies up to several MHz. The capacitance diode D 6  also means that larger component tolerances can be permitted. This means reduced component costs. In addition the oscillating circuit can be tuned precisely in the receive instance. This allows high quality levels to be achieved, which means good receive characteristics with low susceptibility to interference, thereby resulting in long receive ranges. The transmitters can frequently be operated with less energy without losing range.