GB787826A - Variable band width intermediate frequency system - Google Patents

Abstract

787,826. Variable selectivity apparatus. COLLINS RADIO CO. Dec. 29, 1955, No. 37299/55. Class 40 (5). In a variable band width amplifier an input signal is applied in turn to mixers 11, 17, 24, Fig. 1, separated by highly selective band-pass filters 16, 23, the mixers 11, 24 being fed with oscillations from oscillators 12, 19 while the mixer 17 receives oscillations from a difference mixer 18 receiving oscillations from the oscillators 12, 19. The maximum (or minimum) frequency passed by the circuit may be lowered (or raised) by decreasing (or increasing) the frequency of either oscillator and by both maximum and minimum may be effected by adjustment of both oscillators in opposite directions. The filters are preferably of the mechanical type as described in Specifications 722,645 and 722,664, [both in Group XL (b)]. Assuming the input signal has a frequency of 470 kc/s. and the oscillator 12 a frequency of 720 kc/s., then the output passes through the filter 16 having a centre frequency of 250 kc/s. and a band-width of 4 kc/s. Assuming the response of filter 16 to be according to curve A, Fig. 2, then adjustment of the oscillator 12 to a frequency of 719 kc/s. will displace the output from the mixer 11 to the form of curve B, Fig. 2, so that part of the response is lost. Due to the change in frequency of oscillator 12 the output of the difference mixer 18 rises to 206 kc/s., but since the signal received at mixer 17 is only 3 kc/s. wide it passes unaffected through filter 23 to mixer 24 where it is restored to the input frequency. By variation of the frequency of oscillator 19 the band-width of the system may also be varied, Fig. 3 (not shown). The mixers 11, 17 and 24 are preferably of the heptode type, Fig. 4 (not shown), energized from oscillators of the cathodecoupled type, Figs. 5 and 6 (not shown).