Patent GB-1483247-A (GB)
Title: Search tuning circuit
IPC: H03J7/26
Filed: 1974-12-17
Abstract: 1483247 Stop on signal receivers PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 17 Dec 1974 [20 Dec 1973] 54440/74 Heading H3Q In a search tuning circuit when an operating signal is applied at one of two operating signal inputs 2, 2<SP>1</SP>, Fig. 1, a search direction signal for one of two search directions and a starting signal can be written in a search direction signal memory 8 and a starting signal memory 5, a gate circuit 3, 31, 4, 17, 6, 6<SP>1</SP>, 7 and 7<SP>1</SP> is connected between the operating signal inputs 2, 2<SP>1</SP> and the starting signal and search direction signal inputs of the memories 5 and 8 so that for simultaneous operating signals at the two operating signal inputs 2, 2<SP>1</SP> only a starting signal occurs at the starting signal input of the starting signal memory 5 without a search direction signal being generated. The search direction is selected and started by a switch 1 which provides an operating signal to input 2 or 2<SP>1</SP>. The output of the starting signal memory 5 enables an AND gate 9 or 9<SP>1</SP> to be controlled by an output of the search direction memory 8 so as to cause a current source 10 or 10<SP>1</SP> to discharge or charge a capacitor 11 providing the search tuning voltage U A . When a station is received a stop signal is applied to disable gates 9 and 9<SP>1</SP> to switch off the current source 10 or 10<SP>1</SP>. If the received station satisfies certain requirements such as input field strength above a certain threshold value, pilot signal present then a stop signal is applied to reset the starting signal memory 5 and prevent further control of the current sources 10 or 10<SP>1</SP> from the gates 9 or 9<SP>1</SP>. If the received station does not satisfy these requirements then the stop signal is removed from gates 9 and 9<SP>1</SP> and the current source 10 or 10<SP>1</SP> is switched on to continue the signal search operation. AFC is applied to the current sources 10, and 10<SP>1</SP> for maintaining a frequency once adjusted constant. The elements 2-10 may be formed as an integrated circuit. When during station searching the turning voltage reaches its upper or lower maximum value an input to OR gate 7 or 7<SP>1</SP> changes the state of the search direction memory 8 to change the direction of station search. To restart the station search in the same direction as set by switch 1 an automatic start signal or manual switch 14 is operated to turn on a transistor 12 and apply simultaneous operating signals to inputs 2, 2<SP>1</SP>. The output from OR gate 4 resets the starting signal memory 5 and a "0" output from an exclusive OR gate 17 provides a "0" output from AND gates 6, 6<SP>1</SP> so that the contents of the search direction memory 8 is not changed. Due to difficulty in obtaining simultaneous signals at 2, 2<SP>1</SP> due to unequal response characteristics of diodes 13, 13<SP>1</SP> and not exactly rectangular output voltage from the transistor 12 this may cause the output of OR gate 17 to change the search direction of the memory 8. This may be avoided by using a transistor circuit (Fig. 2, not shown) for providing the elements between 2, 2<SP>1</SP> and the inputs to AND gates 9, 9<SP>1</SP>. With no control signal applied to 2,2<SP>1</SP> transistors (28, 30, 31, 28<SP>1</SP>, 30<SP>1</SP>, 31<SP>1</SP>) are off and (20, 23, 20<SP>1</SP>, 23<SP>1</SP>) are on. A search direction signal 0 applied to 2<SP>1</SP> by switch 1 causes transistors (20 and 23) to turn off and turn on a transistor (311) of OR gate 7<SP>1</SP> to select the state of the search direction memory 8. When a search restart signal is applied from transistor 12 due to capacitive input at the base of the transistor 12 the input signal to (20 and 20<SP>1</SP>) is delayed in dropping from a positive voltage to 0. This enables the transistors (28, 30 and 28<SP>1</SP>, 30<SP>1</SP>) to switch on before the transistors (20, 23 and 20<SP>1</SP>, 23<SP>1</SP>) turn off so that outputs (24, 24<SP>1</SP>) to the memory 8 remain the same and no change in the state of the search direction memory 8 is obtained.