Patent GB-426873-A (GB)
Title: Improved devices for the automatic reception and selection of wireless telegraphic signals
IPC: H04L15/26
Filed: 1933-10-11
Abstract: 426,873. Selective calling apparatus. ELFORD, E. N., BOULDING, R. S. H., and ROBINSON, E. H., Gambrell Works, Merton Road, Wandsworth, London. Oct. 11, 1933, No. 28063. [Class 40 (iv)] [See also Groups XXXVIII and XL] Apparatus for the transmission or reception of signals of predetermined duration and spacing comprise two timing devices each comprising a condenser and one or more resistances associable with the grid circuit of a thermionic valve so that an anode relay operates at predetermined time intervals. In one arrangement, one condenser with alternative resistances times the minimum and maximum dash periods while another condenser and resistance times the maximum space period. In another arrangement the maximum space and dash periods are timed by the two devices, the minimum dash period being timed by a further condenser, resistance and valve. Means tor compensating variations of voltage supply and for reducing the sensitivity of a receiver during interference are also described. Reception of correct dashes. On receipt of the first dash relay A operated by signal current removes the shunt on condenser N1 which charges over R1, and energizes C to connect the positive plate of the condenser to the grid of valve M which is normally biassed negatively. When the charge on N1 has built up to a value corresponding to the minimum dash period, anode current operates B, magnet K moves holding pawl J into operative position and holds itself, while magnet H moves the switch E one step. A contact of H discharges the condenser, B falls back and D pulls up, but H maintains itself until A falls back on termination of the signal. The circuits are now in original condition except that the pawl magnet K remains operated. Two more correct dashes step switch E to close contacts in the alarm circuit which also includes contacts of H and K to ensure that the circuit is not closed until the end of the third dash and then only if this dash is correct. Rejection of incorrect signals. When the condenser N1 has been discharged by H at the end of the minimum dash period as described in the preceding paragraph, the operation of relay D immediately permits it to recharge over resistance R2 to time the period of tolerance. If the prescribed period is exceeded, B pulls up, short-circuits pawl magnet K and switch E returns to normal when the dash ceases. The reception of incorrect spaces will now be considered. When a signal ceases (all relays and magnets except K de-energized), condenser N2 charges over resistance R3 and is connected to the valve grid, and if the maximum space period is exceeded, B pulls up and releases the pawl magnet. It will be seen that short spurious signals arriving during the space period merely switch the grid of M from condenser N1 to N2, but N2 is not discharged until H operates on receipt of a dash of at least the minimum length. Hence the receiver responds if there is a space however short providing it is not too long and is followed by a dash within the tolerances. The holding circuit of H includes resistance R6 so that the holding current is the minimum required and H falls back quickly on relapse of the signal relay A which is also quick to release. Modifications. The step - by - step alarm operating switch may be replaced by a relay chain operated in pairs on successive operations and releases of H and released if K is released by an incorrect signal. A modification in which the alarm controlling switch is stepped in response to spaces as well as dashes is shown in Fig. 3. Valve V1 is normally negatively biassed and V2 positively so that E is energized and pawl magnet F operated to release pawl P. On receipt of a dash, A pulls up followed by C so that the positive bias on V2 is removed, E falls back followed by magnet F to make the holding pawl effective, while condensers K1, K2 commence to charge over R1, R2. Condenser K1 times the minimum dash period at the end of which B is operated by valve V1 and magnet G moves switch Q one step. If the dash exceeds the maximum period as timed by K2, valve V2 operates E and magnet F releases the switch, while B is short-circuited to release magnet G. If the dash ceases within the prescribed tolerances, A and C fall back, Kl, K2 are negatively charged and B falls back, but as E is unoperated, magnet H is operated to record a space by stepping on the switch Q. It will be seen that the teeth engaged by the dash and space pawls J, L are such that Q must be moved one step by J after each dash before it can be stepped by L. This procedure continues until Q is stepped by an accepted signal to a position in which the alarm circuit is closed, Q being then held by a manually releasable pawl U. During a space, K3 is charged up over R3 and if the space is too long, E operates to release the switch Q as before. It will be noted that the condensers, when not timing a signal are rapidly given a negative charge whereas when timing they are positively charged over the resistances, the charging potentials being determined by potentiometer R4. Automatic control of sensitivity of the wireless receiver. Under conditions of severe interference equivalent to a long continuous dash, it is desirable to reduce the sensitivity of the receiver by automatically biassing a valve as shown in Fig. 4. When the selector is quiescent, contact c controlled by magnet H, Fig. 1, is open and condenser K is charged to a potential determined by the tapping on battery B which also governs the bias on valve V. When a long dash comes in, contact c closes, K is short-circuited over R3 and the negative bias on V slowly increases to a value determined by potentiometer R1, R2, R3. If the signal is an acceptable one, c is closed for relatively short periods and the sensitivity of V is not seriously affected, but with prolonged interference, the bias appreciably reduces the sensitivity. When c opens the bias returns slowly to normal to prevent rapid alternations in the condition of the valve. Arrangements for providing constant operating potentials. The potentials for the selector may be governed by a potentiometer having parts of different temperature coefficients. Another method shown in Fig. 5 comprises arrangements for compensating variations in the supply potential v2. It is shown that by adjusting the relationship between a constant voltage supply vl, resistances r1-r4, resistance of relay B, the current required to operate it, and the amplification factor of the valve, the timing circuit comprising the valve, relay B condenser K and resistance R is not affected by variations of v2. Transmitter, Fig. 6. This is similar to the receiver shown in Fig. 1, the pawl magnet K being used solely as a relay. When the voltage supply is connected, condenser N1 charges up and relay C connects it to the grid of 1\1. Relay B ultimately operates and energizes K which holds itself and closes the transmitting circuit. Relay K also releases C so that N2 which is now charging is connected to the valve and B, which relapsed, pulls up to terminate the mark by releasing K. A fresh cycle then commences.