430,787. Wireless receiving-apparatus. HAZELTINE CORPORATION, 15, Exchange Place, Jersey City, New Jersey, U.S.A.-(Assignees of Wheeler, H. A. ; 18, Melbourne Road, Great Neck, New York, U.S.A.) Sept. 25, 1934, No. 27502. Convention date, Oct. 3, 1933. [Class 40 (v)] In a radio receiver responsive only to a narrow frequency band, indicating means are provided to give a clear indication when the carrier frequency is adjusted to either edge of the band. Fig. 4 shows a radio frequency amplifier 32, frequency changer 33 and intermediate frequency amplifier 34, the couplings 160-162 of which select a single sideband the response being flat over say 4 kc. extending from the carrier to either the highest or lowest sideband frequency required. Delayed automatic volume control is obtained by signal energy from the coupling 79 amplified by the two stage amplifier 42 and applied to a diode detector 43. Delay is obtained by the bridge circuit 101-105 and the diode 44 which earths the point 107 so long as the anode is positive with respect to the cathode. In the absence of a signal point 157 is at the same potential as point 102 and current flows in the diode 44. Signal voltage rectified by the detector 43 renders the point 157 negative with respect to point 102 and reduces the current in diode 44 to zero and applies automatic volume control voltage over the lead 45. This voltage varies the current in the amplifier 34 which passes through resistance 76 to decrease the negative bias applied to amplifier 42, thus applying reversed automatic control to the amplifier 42 feeding the gain control diode 43 and accentuating the control. The output is suppressed and a visual indication of tuning given by diode 47 and the amplifier 48. The latter lights incandescent lamp 50 which is maintained at point of incipient incandescence by the source 116, and removes a prohibitive bias from the amplifier 36. Signal suppression and visual signal operation can be rendered inoperative by opening a switch 112 which is preferably operated when variation of resistance 75 is completed, the latter serving as a control of the signal strength necessary to overcome suppression. The timing of the automatic gain control is such that it is lower than the period of the lowest audio-frequency modulation and may be À025 second while the quieting system timing, effected by the combined resultant of resistances and condensers 114, 115 and 110, 111 gives an overall time constant 40 per cent greater than that of the automatic gain control. Audio-frequency output is controlled by potentiometer 86 associated with network 87-90 so that fidelity is obtained by accentuating the middle frequencies at high volume levels and attenuating them at low volume levels. This control is preferably ganged with coupling control, which by axially displacing coil 80 gives a uniform reduction of output in decibels with displacement. The oscillator, in operation, is tuned so that the mid-point of the response curve of the intermediate frequency amplifier lies at the middle of one or other of the sidebands, the intermediate frequency amplifier then accepting the carrier and only one sideband. A clear indication that the tuning is such that these conditions are obtained is obtained by the co-operation of the automatic gain control, the signal suppressing circuit and the intermediate frequency trap circuit 41. The latter having a response curve as shown in Fig. 11 so that the gain in the receiving amplifiers 32, 33, 34 is increased as the oscillator tuning is varied to bring the intermediate frequency carrier to one or other edge of the response curve of the intermediate frequency amplifier. The signal suppressing circuit renders the indicator 50 and the amplifier 36 operative only in these tuning positions. The high selectivity necessitates accurate ganging of the signal and oscillator circuits which is obtained by adjusting the size of the oscillator tuning inductance and the series and padding condensers, and utilizing a fourth degree of freedom obtained by providing resistances 68, 69 across coils fairly closely coupled to the oscillator tuned circuit. The selectivity is adjustable by co-axial closed turns 74, 741 associated with the transformer windings and preferably in a co-axial cylindrical shield. Specifications 431,121 and 431,631 are referred to.