| 摘要 |
1,187,705. Radio signalling. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 21 Feb., 1968 [24 Feb., 1967], No. 8458/68. Heading H4L. In a radio receiver for distress signals consisting of a carrier modulated alternately by two different predetermined frequencies, the demodulated signal is supplied to terminal E, Fig. 2, and the two frequencies are selected by filters 2 and 3 respectively. The outputs of the filters are combined at S1 and supplied as an input E1 to the normally blocked output stage 13 of the receiver, Fig. 3. The outputs of filters 2, 3 are also amplified by transistors T2, T3 respectively and supplied to detectors 6 and 7 which produce positive square-wave pulses corresponding to the pulses of modulation frequency the mean value of which is fed back to the base of transistors T2, T3. This adjusts the working point of the transistors so that they are saturated at the positive peaks of the input signal whereby the amplitude of the output signals is maintained substantially constant. When the distress signal is superimposed on an interfering signal the mean value fed back alters the working point again and only the distress signal is amplified. The pulse outputs of detectors 6, 7 are also applied to rectifiers and voltage-doublers 8, 9 respectively supplying RC circuits 10, 11 having a time constant which exceeds the pulse duration of the modulation of a channel. In the presence of a distress signal substantially constant voltages appear at the outputs of these circuits which are applied to the bases of normally cut-off transistors T4, T5 respectively of an AND circuit 12 to cause them to conduct so that the potential at terminal S2 falls and unblocks the output stage 13 via terminal E2. With no input signal transistors T4, T5 are cut off by the positive voltage across diode D9 and under these conditions input to only one of the transistors will have no effect. The amplifier 13 is of known type comprising complementary NPN-PNP transistors T8, T9 and class A transistor T7. In the absence of a distress signal (terminal E2 at its maximum positive value) transistor T6 is driven into saturation so that diode D10 is cut off. A switch may be provided to put a fixed potential on terminal E2 corresponding to the open state of the AND- gate. |
