Patent GB-514319-A (GB)
Title: Improvements in or relating to multi-wave band radio receivers and the like
IPC: H03J3/10
Filed: 1938-05-02
Abstract: 514,319. Wireless receiving circuits. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. May 2, 1938, No. 13064. Convention date, April 30, 1937. [Class 40 (y)] Relates to a multi-waveband receiver which is designed to cover a wide frequency-range in a number of successive bands, including a number of subsidiary bands of substantially-equal width, the tuning scale or pointer being movable over its whole range of movement for each subsidiary band so as to obtain adequate band-spread over all ranges. The invention consists in the provision of a simplified switching arrangement for the wave band circuits. Fig. 1 shows the radiofrequency amplifier 5, frequency changer 6 and local oscillator 145 of a superheterodyne receiver arranged for reception on three principal wavebands: A (530-1,720 kc.-the normal broadcast band) ; B (2,300-7,000 kc.); C (7,000- 22,000 kc.). In addition, provision is made for four subsidiary bands, corresponding to 49, 31, 25 and 19 metres wavelength. The entire tuning scale is used for each subsidiary band by - fixing the tuning of the radio frequency stages at the mid-frequency of each band, making each band-width approximately 300 kc., and utilizing the oscillator to tune over the band. Thus the input circuits of the radio-frequency amplifier 5 and the mixer 6 comprise inductances 31, 32, 33, 85, 86, 87 tuned by condensers 42 and 56 over the three main bands A, B, and C, and further inductances 34, 88 tapped to resonate at the mid-frequency of 300 kc. bands corresponding to 49, 31, 25 and 19 metres. By means of the rotary switches 13, 89, the contacts of which are shown in a developed view, the condensers 42, 56 are operative when any of the three bands A, B and C are in use through contacts 30, 44 and 100. As the switch member moves upward, the primary windings 36, 37, 38 are progressively short-circuited together with their secondaries. In the positions of switch 89 for the B and C bands, coils 85 and 86 are shunted by a fixed condenser 130, whereby they are tuned below the response range of the next adjacent band. The switches 13, 89 are conjointly operated with the oscillator, waveband switch 158, and the tuning condensers 42, 56 and 58 are also unicontrolled. The oscillator 145 is a pentode operating with a Hartley circuit, the oscillations being taken to the mixing stage from the cathode over lead 137. The anode is earthed over condenser 201 for alternating currents, whereby a connection is made to the grid-circuit inductances 151 ... 157 inclusive by earthing one terminal of each coil. The coils are successively tunable over each range by the condenser 58 which has two stators 166, 167 separately switched ; both sections are employed for the A, B and C bands but only the smaller section 167 for the subsidiary bands. On the A band the cathode circuit is not connected to the inductance 151, but is coupled thereto by coil 180, while on the remaining bands the cathode' is connected to tap 197 on the B band inductance 152. Fixed tuning condensers 174, 175, 176 are respectively switched in series with the tuning condenser 58 on the A, B and C bands, while the tuning range of the subsidiary bands is kept substantially constant by additional padding condensers 203, 208, 209 which have a negative capacity-temperature coefficient to compensate for increase in inductance of the tuning coils with temperature. The A band coil 151 and the subsidiary band coils 154 to 157 have adjustable magnetic cores 217, 218, the coil 151 being provided with connections 220 for automatic frequency control. An additional cathode tap may be desirable on coil 154 where the B and C bands are dispensed with in the receiver design. Should either section of the tapped coil 152 resonate in any of the tuning ranges of the coils which are switched in shunt thereto, an additional cathode tap is made on the next lower inductance to the coil in whose range the stray resonance falls (Fig. 2, not shown). Additional back coupling for the oscillator may be provided on the higher frequency range as by an auxiliary coil in the oscillator anode, coupled to coil 157.