| 摘要 |
1291516 Radio receivers; semi-conductor circuits; line transmission systems INTERNATIONAL STANDARD ELECTRIC CORP 10 Sept 1970 [18 Sept 1969] 43288/70 Headings H3Q H3T and H4R [Also in Division G4] In a device for detecting the presence of a given frequency in a signal, the signal is fed to a ring modulator. MO. A multivibrator Gf1 tuned to the given frequency is coupled to the input so that if the given frequency is present in the input the multivibrator locks on to the frequency and phase of the input aided by coupling capacitor CK, and if the given frequency is present in the input the multivibrator biases the ring modulator so that it full wave rectifies the signal. The output of the ring modulator feeds a summing circuit SU which in turn feeds a threshold device SD. If the given frequency is present an output is obtained at AS. An initial limiting amplifier may be used and the summing circuit and threshold device may be replaced by a delay line. If more than one frequency is to be detected, a series of multivibrators Gf 1 -Gfn may be connected in turn to the modulator MO by means of switch AT1, Fig. 3a. The outputs of the modulator for the different multivibrators are fed to summing circuits SU1-SUn by switch AT2. The outputs of the summing circuits are fed in turn via switch AT3 to threshold device FD, the output of which is fed via AND gates U1-Un, to output terminals AS1-ASn. Summing circuit SUb monitors the whole signal bandwidth and prevents errors arising at the commencement of the signal during short -measuring times. |
