Patent GB-135635-A (GB)
IPC: H03H7/01
Abstract: 135,635. Western Electric Co., (Western Electric Co.). Dec. 6, 1918. Wireless signalling.-An arrangement of circuits for filtering out currents of certain frequencies from currents of various mixed frequencies, comprises a plurality of sections having either resistance, capacity, and no inductance, or resistance, inductance, and no capacity. One type of filter, described as an infra-filter, operates to suppress currents of a frequency below a certain limit, and another type, the ultra-filter, suppresses currents of a frequency above a certain limit. By a combination of the two types of filter, only currents between the two frequency limits are transmitted. When used in a wireless receiving-apparatus, the aerial is not in tune with the signalling-waves, and oscillations set up in the aerial by atmospherics are suppressed by means of the filters, while the signal currents are transmitted and amplified. Fig. 3 shows a simple form of ultra-filter. The input circuit 18, energized by currents of various frequencies, includes a resistance 19 and capacity 20. The output circuit is connected across the capacity 20, and is shown as comprising a volt-meter 21. The proportion between the voltages across the capacity and resistance will he greater, the lower the frequency, and in the output circuit the currents of lower frequency will therefore predominate. If the output circuit had been connected across the resistance 19, the filter would operate as an infra-filter. In the form of filter shown in Fig. 6, an inductance 30 replaces the capacity 20, and the device operates as an infra-filter or an ultrafilter according as the output circuit 31 is connected across the inductance 30 or the resistance 29. Fig. 1 shows a plurality of filters F1 with interposed thermionic amplifiers which serve to prevent reaction of the output circuit on the input circuit. The input circuit 1 includes a blocking condenser 14 and a shunt resistance 3, and is connected across the grid 5 and filament 8 of the valve 2, the grid being given a normal negative potential by the battery 4. The plate circuit is energized by a source 6 through a high resistance 7, and includes a series resistance 10 and a shunt capacity 11. A further filter secton F' is connected across the capacity 11, this arrangement operating as an ultra-filter as described with reference to Fig. 3. After passing through a series of such filters, the transmitted currents of lower frequency are amplified by the valve A. By interchanging the positions of the resistance 10 and capacity 11, the arrangement becomes an infra-filter. A form of ultra-filter, shown in Fig. 4, comprises a valve 23 with an inductance 22 and a battery 25 in its plate circuit. A blocking condenser 24 is interposed between the battery 25 and the next filter section F<4>, and current is taken off across a resistance 26. Fig. 7 shows a combination of an ultra-filter composed of a series resistance 10 and a shunt capacity 11, and an infra-filter, composed of a series capacity 16 and a shunt resistance 15, for the purpose of transmitting only a band of frequencies. For minimizing static disturbance in wireless reception, the aerial is connected to the detector through a series of filters of both types and amplifiers, so as to transmit most efficiently a narrow band of frequencies in the neighbourhood of the signalling frequency. The aerial is out of tune with the signal waves, and oscillations set up therein by static impulses or waves are substantially eliminated bv the filters. The natural frequencies of the various sections of the filter system are extremely high, owing to the fact that either the inductance or the capacity of each section is negligible. Such frequencies are practically eliminated by the filters.