Patent GB-688420-A (GB)
Title: Improvements in or relating to circuit arrangements for generating signals for use in telephone systems
Inventor: DAVIDSON ALAN
IPC: H04M19/02
Filed: 1950-10-17
Abstract: 688,420. Automatic exchange equipment. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd. Oct. 15, 1951 [Oct. 17, 1950], No. 25216/50. Class 40 (iv). [Also in Group XL (c)] A circuit for generating the various tone signals required in telephone systems comprises a multi-vibrator one of whose valves has at least five electrodes, the two nearest the cathode being in the multi-vibrator circuit and the outer control grid being fed from a source of tone frequency. Output of tone frequency interrupted by the multi-vibrator pulses is taken off at the anode of the aforesaid valve. The multivibrator may be associated with two further multi - vibrators having progressively increasing recurrence frequencies. In the example shown, a back-coupled oscillator V<SP>2</SP> generates a continuous 400 c/s. tone which may be taken off at TRB and utilized as an indication that a desired number is unobtainable. This tone is also fed to the outer coritrol grid of a pentode valve V<SP>3</SP>, the screen and inner control grid of which are associated by the usual condenser-resistor couplings with like electrodes of a valve V<SP>4</SP> to form a multivibrator pair. The couplings are such that the valve V<SP>3</SP> alternately conducts for 1 sec. and is blocked for 2 sec. During conductive intervals the tone frequency is fed from the anode of the valve V3 through a transformer TRC to provide the normal busy tone. A second reciprocally-coupled pair V<SP>5</SP>, V<SP>6</SP> operates to render the valve V<SP>5</SP> conductive for 0.4 sec. and blocked for 0,2 sec., so that two current pulses occur in valve V<SP>5</SP> during the 1 sec. interval when valve V<SP>3</SP> is conducting; during the 2 sec. interval when valve 4 is conducting the valve V<SP>5</SP> is blocked by positive voltage. on its cathode derived from the cathode resistor R<SP>13</SP>. At each cut-off of the valve V<SP>5</SP>, a positive-going pulse is applied from its screen through a condenser C<SP>13</SP> to the control grid of valve V<SP>4</SP>. The first of these is ineffective but the second unblocks the valve V<SP>4</SP> and so synchronizes the throw-over of the pair V<SP>3</SP>, V<SP>4</SP> with the termination of the second pulse of the pair V<SP>5</SP>, V<SP>6</SP>. A third pair of valves V<SP>7</SP>, V<SP>8</SP> operates symmetrically at 16.6 c/s. and is rendered inoperative whenever the valve V<SP>6</SP> is unblocked. In this way, the current in the valve V<SP>7</SP> comprises the sequence 0.4 see. on, 0.2 sec. off, 0.4 sec. on, 2 sec. off, the on periods being interrupted at 16.6 c/s. 400 c/s. tone is supplied from the oscillator V<SP>2</SP> to the outer control grid of the valve V with the result that the anode current of 400 c/s. is interrupted in the above sequence to provide ringing tone at the output transformer TRD. The screen grids of the valves V<SP>7</SP>, V<SP>8</SP> supply the intermittent 16.6 c/s. current in push-pull to a valve pair V<SP>9</SP>, V<SP>10</SP> feeding a centre-tapped transformer TRE for the supply of ringing current, the 400 c/s. current being shunted to earth by condensers C<SP>16</SP>, C<SP>20</SP>. Negative feedback for the valves V<SP>9</SP>, V<SP>10</SP> is provided by capacities C<SP>26</SP>, C<SP>27</SP> and resistors R<SP>37</SP>, R<SP>38</SP>. A further back-coupled oscillator V<SP>1</SP> operating at 400 c/s. and arranged to interrupt itself at 33.3 c/s. provides dial tone at the output transformer TRA. Specification 668,430 is referred to.