US 2511327 A
Description (OCR text may contain errors)
June 13, 1950 E. J. H. BUSSARD BAND-PASS INPUT CIRCUIT 2 Sheets-$heet 1 Filed Jan. 3, 1949 INVENTOR.
fM/VEE) J 5055420 My; 7%; %4 m Arm s June 13, 1950 E. J. H. BussARD BAND-PASS INPUT CIRCUIT Filed Jan. 5, 1949 2 Sheets-Sheet 2 GAIN DECIBELS I200 I400 FREQUENCY- KILOCYCLES INVEN TOR. [MMEEI 7-K 51/554 213 Patented June 13, 1950 UNITED STATES PATENT OFFICE BAND-PASS INPUT CIRCUIT Emmery J. H. Bussard, Cincinnati, Ohio, assignor to Avco Manufacturing Corporation, Cincinnati, Ohio, a corporation of Delaware Application January 3, 1949, Serial No. 68,771
The present invention relates to antenna input circuits for radio receivers.
The primary object of the invention is to provide a radio frequency input circuit which requires no manual tuning operations but which covers a wide frequency band, such as the AM broadcast band. (540 kc.-l600 kc.) with high voltage gain for desired signals and great-attenuation of undesired signals outside the broadcast band.
Another object of the invention is to provide a fixed-tuned radio frequency input circuit which has substantially uniform transfer constant and voltage step-up characteristics over a 'wide frequency band, and which at the same time provides desired selectivity and discrimination against image signals and signals of intermediate frequency.
A further object of the invention is to provide an antenna input circuit comprisingan autotransformer, in which a part of the transformer not included in the primary is employed in conjunction with a shunt capacitor as an anti-resonant wave trap to discriminate against undesired signals.
For a better understanding of the invention, together with other and further objects, advantages, and capabilities thereof, reference is made to the following detailed description of the accompanying drawings, in which there are disclosed two illustrative R. F. antenna input circuits in accordance with the invention.
In the drawings:
Fig. l is a circuit diagram of a preferred form of radio frequency antenna input circuit in accordance with the invention;
Fig. 2 is a circuit diagram of a modified form in which the antenna loop is employed as the primary;
Fig. 3 is a performance curve showing the response characteristic of the Fig. 1 embodiment; and Fig. 4 is a perspective View of a transformer suitable for incorporation in the Fig. 1 embodiment.
As is well known to those skilled in the art, the radio frequency section of a receiver includes the coupling from the antenna to the grid of the first tube, such coupling, being generally referred to as the antenna input circuit. The chief purposes of this circuit are to provideselectivity against image and intermediate frequency and other interfering signals and at the same time to afford voltage gain between the antenna and the grid-cathode input circuit of the first R. F. amplifier tube. It is customary to provide a tuned radio frequency stage between the antenna input circuit and the mixer or frequency converter, or a tuned antenna input circuit coupled to the converter. Capacitors or inductors in both the antenna input circuit and the oscillator tank circuit areadjusted to select the desired signal channel. However, when push-button tuning or equivalent automatic selector arrangements are employed, it is desirable to omit the adjustments of the-antenna input circuit and to provide a band pass antenna input circuit which passes si nals within the entire band under consideration, such as the AM broadcast band, with substantially uniform voltage step-up characteristics over the entire band, and also with the required selectivity and discrimination performance characteristics.
To provide such a, circuit has for a long time posed a very difiicult problem, and the need for a simplified push-button tuning system has prompted many endeavors to solve it.
Referring now specifically to Fig. 1 of the drawings, there is shown a low impedance loop antenna II and a converter tube Hi. The purpose towhich the present invention is directed is to couple the loop to the control electrode of converter tube ill in such a way as to eliminate the conventional antenna input variable tuning and at the same time to meet the performance requirements mentioned above. The performance equivalent of an over-coup1ed double-tuned transformer has been provided by the invention. Loop I I is shunt-connected to primary I3 of autotransformer l6, and this section of the input systein is resonated to about the mid-frequency range by means of fixed capacitor 52. Specifically, the loop I i and its series tuning capacitor I2 are coupled to the primary [3 of an auto-transformer I6. The primary I3 is tuned approximately to the center of the AM broadcast band, for example, by capacitor l2 in series with low impedance loop I l and by the distributed capacitance ll of the loop and primary. The secondary l3, l4, l5 of transformer i6 is tuned by shunt capacitor l8 approximately to the center frequency of the AM standard broadcast band. There are shown in association with the circuit the usual coupling capacitor [9 and a grid resistor 28 leading to source of AVG voltage (not shown). The elements I9, 29 couple the antenna input circuit to the control electrode of converter I0. Local oscillator voltage is injected in conventional manner through a capacitor 25 coupled to the output of an oscillator (not shown). The low voltage terminals of the loop, coil I3, capacitor I8, resistor 22, and the cathode of tube 10 are grounded at 23.
Portions i4 and I5 of the transformer secondary, together with shunt capacitor 24, comprise a trap circuit which is anti-resonant at the I. F. frequency. In a particular embodiment of the invention which was successfully reduced to practice, the receiver was of the double superheterodyne type and the first I. F. frequency was 5825 kilocycles. The trap circuit comprising secondary portions [4, l5 and capacitor 24 provides very effective discrimination against unwanted.
signals of I. F. frequencies. The other parameters employed in this illustrative embodiment were as follows:
Inductance of loop ll micro-henries 15.65 Capacitor l2 micro-micro-farads '750 Distributed capacitance of coil l3 do 11 Capacitor l8 do 220 Inductance of coil [3 micro-hem'ies 30 Sum of inductance of coils l3, l4, and
it: in series and twice the mutual inductance between the primary and secondary micro-henries 215 Mutual inductance between primary l3 and secondary i3, l4, l5 micro-henries 38 Shunt tuning of loop H ki1ocycles 2550 Distributed capacitance of loop ll micro-micro-farads 63 Resistor 22 ohms 5600 Tube l0 Type SAC? Resistor 23 megohm 1 Capacitor 2| micro-micro-farads 10 The gain and selectivity of. the system can be materially improved by resonating the loop near to the high end of the desired range. 2550 kilocycles proved satisfactory in the example cited. That is, loop H and shunt capacitance I! are preferably resonant at about 2550 kilocycles. Loop H and capacitor I2 and primary 13 are resonant at the center of the broadcast band. Resistor 22 is employed to limit the peak gains in such a manner that the gain ratio of peaks to valley is not excessive.
Referring now specifically to Fig. 3 of the drawings, the performance curve there shown illustrates the band pass input gain characteristic of a receiver having an antenna input circuit in accordance with the invention and employing the illustrative parameters above mentioned, as compared to a like receiver employing a typical manually tuned input circuit. The response curve is plotted on a. system of Cartesian coordinates with frequency in kilocycles as abscissae and gain in decibels as ordinates. It will be observed that the response characteristic is indicative of a double-tuned over-coupled system, one hump occurring at about 660 kilocycles and the other at about 1250 kilocycles. This gain characteristic curve indicates a uniformity which is commercially acceptable throughout the standard broadcast band. Such results cannot be achieved by conventional methods of coupling. A coupling coefficient on the order of 0.55 is. re-
quired for coverage of the AM broadcast band. for example. I 4
4 Sensitivity measurements of a receiver with these illustrative parameters give the following results:
Frequency in Kilocycles Input Output Micro- In this illustrative receiver the band pass input transformer, as shown in Fig. 4, was wound with 15-strand No. 44 Litz wire in three coils l3, i4, 15, each /64 inch in width, spaced about inch, on a form 25 having a inch diameter. These coils are identical, each having 35 turns.
Referring now specifically to Fig. 2 of the drawings, there is shown an embodiment in which loop 28 is common to both input and output of the coupling network. This coupling arrangement includes loop 28 and series inductors 29 and 30 as, the secondary. Loop 28 is the primary of an effectively over-coupled double-tuned system. Like reference numerals are employed to designate elements which are the same as those shown in the Fig. 1 embodiment. In the Fig. 2 embodiment the secondary portions ti] and 29 are shunttuned by their own distributed capacitance to provide the anti-resonant trap circuit for I. F. frequency rejection. The loop is tuned by a shunt capacitor 3|. The primary is tuned by capacitor 31, and the secondary by capacitor l8, each to the approximate mid-frequency of the band to be received.
The auto-transformer feature provides the required coupling for peak displacement and for uniform voltage stepup. The invention is of particular utility in a double superheterodyne receiver in which the first intermediate frequency is substantially higher than the band to be received. Its use permits the antenna tuning condenser to be omitted, avoids tracking problems, and permits complete range tuning with a very small variable capacitor.
While there have been shown and described what are at present considered to be the preferred embodiments of the present invention, it will be understood by those skilled in the art that various substitutions of equivalents may be made therein without departing from the true scope of the invention as defined by the appended claim.
Having fully described and disclosed my invention, I claim:
In a radio receiver, an antenna input circuit adapted to afford a relatively uniform response throughout the AM broadcast band without manual tuning, comprising a vacuum tube having an input and an output circuit, an input network including a low impedance loop, a high-Q autotransformer having primary and secondary windings, a first capacitor coupling said low impedance loop across the primary of said autotransformer so as to form a series resonant network tuned to the mid-frequency of said band, a second capacitor means connected in shunt across that part of the secondary of said autotransformer which is not common to the primary to form a rejector network anti-resonant at the receiver intermediate frequency, a third capacitive means coupled across the end terminals of said autotransformer to series resonate at the mid-frequency of said band the loop circuit in which the third capacitor means is included, and means coupling the input circuit of said vacuum tube across said third capacitive means, whereby a large gain between the low impedance loop and the input of said vacuum tube and a wide band relatively uniform response characteristic is realized, and whereby undesired signals of intermediate frequency are rejected by said anti-resonant rejector circuit.
EMMERY J. H. BUSSARD.
REFERENCES CITED The following references are of record in the file :of this patent:
UNITED STATES PATENTS