GB971805A - Improvements in frequency dividers - Google Patents

Abstract

971,805. Frequency dividing circuits. COMPAGNIE FRANCAISE THOMSON-HOUSTON. Nov. 24, 1961 [Nov. 24, 1960], No. 42136/61. Heading H3F. In a frequency divider of the regenerative type means are provided for automatically maintaining the input at a low level, so that amplitude variations in the input are reproduced in the output. This renders the arrangement suitable for use in a frequency-modulation receiver for compressing the deviation and so reducing the I.F. bandwidth necessary. The signal to be divided is applied via wideband circuit 4 to the grid of a pentode amplifier 3, the anode load of which comprises a further wideband circuit 5. This valve feeds signal to the inner grid of a four electrode mixer valve 8 via capacitor 6 and grid leak 7. The mixer 8 is provided with cathode bias by means of circuit 9 and feeds wide band circuit 10 tuned to the output frequency, which is a third of the input frequency. A secondary winding provides an output at terminals 11. A bifilar tertiary winding feeds a push-pull input to a double triode frequency doubler 12 which is biased by a common cathode resistor 13. The anodes of frequency doubler 12 are connected in parallel to a wideband circuit 14 tuned to two-thirds the original signal frequency, across which is connected a series-tuned trap circuit 15 tuned to one-third the original signal frequency. From this the second input is supplied to the mixer 8, being applied to the outer grid via capacitor 16 and grid leak 17. This signal is also applied via capacitor 18 to a diode rectifier included in a double diode triode 20 which provides delayed automatic gain control of the input; i.e. the D.C. output from the diode is smoothed and applied as a negative bias to the grid of the triode portion. A resistor 21 connects the cathode of the valve to a source of negative potential, the cathode potential thus going negative as the input to the diode increases. Delay is provided by the second diode, the anode of which is earthed via resistor 22. This diode is non-conductive until the cathode potential becomes negative to earth; it then is conductive and applies a negativegoing potential via line 23 to the grid of amplifier 3 so as to reduce the gain. The arrangement is such that when the signal to be divided is first applied the gain is at a maximum and a large signal is applied to the inner grid of mixer 8; this is the condition for frequency division to commence, but the output is not proportional to the input. As the output builds up the automatic gain control comes into action and reduces the input to the inner grid of mixer 8 to a small value, which gives the desired condition of proportionality between input and output. A limiting device (not shown) is desirable in the path between the multiplier 12 and the outer grid of mixer 8. Modifications referred to in the Specification include the connection of diodes between one and/or the other of the grids and the cathode of the valve 8, the use of two mixers in push-pull, or of one in which both inputs are applied to the same grid. Starting of the divider may be facilitated by the connection to a suitable point in the frequency multiplying chain of an oscillator injecting the correct frequency at that point and which is automatically rendered inactive or is disconnected upon normal operation of the circuit; alternatively, it may be kept in circuit and synchronized with the output of the frequency multiplier. An auxiliary non-proportional frequency divider may be provided instead. It is also stated that transistors may be employed in the arrangement. Specification 912,317 is referred to.