Patent GB-975622-A (GB)
Title: Improvements in and relating to frequency-dividing circuit arrangements
IPC: H03B21/02
Filed: 1963-09-09
Abstract: 975,622. Frequency dividers; colour television. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. Sept. 9, 1963 [Sept. 12, 1962], No. 35477/63. Headings H3F and H4F. The invention relates to a frequency divider of the type in which one input to a mixer 1, Fig. 1, comprises the signal fi(= mfo) to be divided and the other a heating frequency (m - 1 )#o derived by multiplication in a multiplier 4 from the desired divided frequency fo which occurs in the output from the mixer 1. Starting of the arrangement is initiated by means of a starting signal fh or fh<SP>1</SP> which is injected at a suitable point between the output and second input to the mixer 1; according to the invention the time delays in the filter networks (1) item 2 in the signal input to the mixer 1; (2) item 6 in the starting signal input path (or 6<SP>1</SP> or 6<SP>11</SP>); (3) item 3 between the mixer 1 and multiplier 4 and (4) item 5 between the multiplier 4 and mixer 1, are so chosen that the phase of the divided signal fo bears a predetermined relationship to that of the input signal fi. Mathematical expressions are given in the Specification for the relationships involved. The arrangement is more particularly intended for use in a colour television receiver using as display means an indexing tube, the screen of which is composed of a number of groups of colour strips and additional indexing strips from which information is derived as to the colour strip struck by the scanning electron beam; in the receiver the subcarrier on which the incoming colour signals are modulated is replaced by the indexing signal, the signal so modulated being then fed to a control electrode of the indexing tube. The starting signal for the dividing circuit is obtained from a small number of " run-in " indexing strips provided on that side of the screen of the display tube where the horizontal scan starts; for a division of X 3 groups of three run-in indexing strips may be used, spaced apart as the indexing strips proper, but with one strip in each group omitted. In an embodiment the signal 3fo to be divided is fed via amplifier 9, Fig. 2, to the tuned primary of a transformer 10 feeding bridge modulator 1. The output of this is fed via transformer 13 with a tuned secondary to amplifier 14, the anode circuit of which comprises the tuned primary winding of a transformer 15. The balanced secondary winding of this constitutes, with diodes 16, 17 a frequency doubling circuit which feeds frequency 2fo to the bridge modulator via amplifier 11 and transformer 12, which has a tuned primary winding. The starting signal, of frequency fo is fed in via transformer 18 with a tuned secondary winding, amplifier 19 and transformer 20 with a tuned primary winding, to the grid of the valve 14. In order that the starting signal may be suppressed as soon as the indexing signal proper is available, the latter is supplied from a winding 21 on transformer 10 to a diode 24 which produces a D.C. voltage across load resistor 23 which biases valve 19 to cut-off. The Specification also refers to a second embodiment (Fig. 3, not shown), in which a push-pull valve modulator is employed.