Patent Application
20070298712
The present invention is described below with reference to the accompanying drawing, which shows a circuit diagram of an inventive transmit/receive circuit.
The exemplary embodiment described in more detail below represents a preferred embodiment of the present invention.
The transmit/receive circuit for a hearing device shown in the figure has a transmit/receive antenna L1, configured as a coil. A second coil L2, serving to transmit a transmit signal of a transmit amplifier V1 in an inductive manner, is coupled loosely to it. To this end the transmit amplifier V1 is coupled by way of a resistor R1 to the coil L2. The coil L2 is connected to a supply voltage VCC on the other side. The transmit amplifier V1 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 V1 is only active when transmitting and is deactivated in the receive instance.
The receive circuit is connected to the transmit/receive antenna L1. The antenna coil L1 forms a resonant circuit together with a main adjusting capacitor C1 connected by way of a resistor R1, said resonant circuit being excited to oscillate when receiving. The signal of the resonant oscillating circuit is amplified by an amplifier V2, which has little background noise, so a correspondingly amplified receive signal ES results.
Since the receive element (in the figure on the right next to the transmit/receive antenna L1) also oscillates during transmission, with very high field strengths and correspondingly high voltages then occurring, the receive amplifier V2 must be appropriately protected. A PIN diode array consisting of the PIN diode pairs D2, D3, D4 and D5 is therefore connected parallel to the receive oscillating circuit L1, C1 here. The individual diode pairs here each consist of two PIN diodes connected in parallel. The PIN diode pairs D2 and D3 are connected in an anti-parallel manner, as are the PIN diode pairs D4 and D5. 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.
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 V2.
A varactor diode is connected parallel to the PIN diode array, in other words parallel to the input of the amplifier V2, by way of a capacitor C2. In the receive instance the capacitor C2 together with the capacitance of the varactor diode D6 compensates for the loss of the coil L2.
So that the capacitance of the varactor diode D6 can be adjusted correspondingly, the connecting point between the capacitor C2 and the varactor diode D6 is subjected by way of a resistor R3 to a setting voltage VTune. The connecting point between the capacitor C2 and the varactor diode D6 is connected to the one input terminal of the amplifier V2 by way of a coupling capacitor C4. 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 L1, is connected to ground.
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 D6 can be deployed, which can compensate for any residual errors. This varactor diode D6 can either be subjected manually to an appropriate control voltage VTune 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.
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 V2. 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.
The inventive transmit/receive circuit with the PIN diodes can advantageously also be deployed at higher frequencies up to several MHz. The capacitance diode D6 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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2006 029 195.6 | Jun 2006 | DE | national |
