Patent GB-854201-A (GB)
Title: Improvements in or relating to receiving apparatus for frequency-shift telegraphy
Inventor: ROBIN HAROLD KILNER
IPC: D21F1/52
Filed: 1957-06-18
Abstract: 854,201. Code telegraphy. NATIONAL RESEARCH DEVELOPMENT CORPORATION. June 17, 1958 [June 18, 1957], No. 19147/57. Class 40(3). [Also in Group XL(c)] In a frequency-shift telegraphy system 32 tones are used for the transmission of information, being spaced at intervals of 90 c.p.s. in the frequency range 1À8-4À59 kcs. In addition, the C.W. signal is 25% amplitude modulated by a 90 c.p.s. tone used for synchronizing. The synchronizing signal is separated out by a demodulator (not shown) and applied to the grid of valve A of a monostable flip-flop V1 in which valve B is normally conductive. The positive-going half-cycles of the 90 c.p.s. tone trigger valve A conductive and B non-conductive, positive-going rectangular pulses appearing at the anode of valve B and negative-going at that of valve A. The latter are differentiated by Cl, R1, and the positivegoing "spikes" produced by the trailing edges are utilized to trigger a second flip-flop V2 following a time-delay of 1À3 m/s. The negative-going rectangular pulses produced at the anode of valve V2A are differentiated by C2R2 and the positive-going "spikes" produced by the trailing edges are utilized to trigger a third flip-flop V3 following a further time-delay of 5 m/s. The positive-going rectangular pulses produced at the anode of valve V3B are differentiated and the negative-going "spikes" produced by the trailing edges are passed by diode V5B to trigger a further flip-flop V6, following an additional time-delay of 3À11 m/s which is adjustable by variation of resistor R8. In the flip-flop V6 valve A is normally conductive and holds negative the grid of a pentode V7 which is thus cut off, so that condenser C9 is normally charged. The negative pulses drive A cut off, releasing the bias on valve V7 so that this becomes conductive and discharges condenser C9. The charge on condenser C9 controls the cathode potential of 32 thyratrons such as V11, 12 the grid potential of each of which is controlled by a corresponding tone generator, e.g. V10A, B respectively. Each tone generator, e.g. V10A comprises a cathodecoupled oscillator so damped by resistor R14 as to be normally just non-oscillatory: the incoming tones are applied to the grids of all via line L2 and cause oscillations to build up linearly in that oscillator corresponding to the tone being transmitted, the rate of build up being proportional to the amplitude of the transmission. The oscillations in the anode circuit of the tonegenerator which is operative are rectified to produce a rising positive potential which at a certain point - the greater the tone amplitude, the earlier - triggers the corresponding thyratron. When a thyratron conducts its anode current is drawn through a common resistor R11 and also the primary winding of one of 32 transformers, e.g. P1, the output from the secondary of which indicates the received tone frequency. The capacity C9 is rapidly recharged, raising the cathode potential of all the thyratrons and ensuring no other one conducts. A negative going pulse is generated across R11 which is differentiated and applied via diodes V5A and V9A to the grids of valves V6B and V8B respectively. Valve V6B becomes cut off and V6A conducts, so cutting off the valve V7 again. The conducting thyratron becomes cut off, and at the same time valve V8B; valve V8A conducts, causing its anode voltage to fall. The valve V8A is cut off at the beginning of the tone period following by a negative-going pulse obtained from the anode of valve VIA via diode V1B. Since the time at which the valve V8A becomes conductive comprises the total delay of 9À41 m/s plus the time for a thyratron to become conductive, which is shorter the greater the amplitude of the transmitted tone, the width of the negativegoing rectangular pulse at its anode is greater, the greater the amplitude of the transmission. This pulse is rectified by diodes D1, 2 and smoothed to constitute the A.G.C. voltage: it is applied to the audio stages of the receiver. At the beginning of each tone period a negative-going quench pulse is applied from the anode of valve V1A via line L1 to the control grids of all the tone generators V10A &c. to quench any oscillations therein: at the same time a positive-going pulse from the anode of valve V1B is applied through a differentiating circuit to the screen grid of a thyratron V13. This causes it to conduct and earth, via line L3 the condensers, e.g. C11, 12 in the rectifying circuits associated with the tone generators.