US2233706A - Radio receiving circuit - Google Patents

Description

H. P. KALMUS RADIO RECEIVING CIRCUIT Filed Nov. 1, 1939 March 4, 1941.

e m W O V Ill \nm iiu v2 6 :15 22% IN VEN TOR.

. Henry P-K'almua Afi'arrwgfi.

Dnshmc Patented Mar. 4, 1941 This invention. relates to improvements in radio receiving circuits and particularly those employed in super-heterodyne radio receiving An object of this invention is the provision of an improved super-heterodyne receiving circuit whereby it is adapted to receive frequency modulated signal waves.

An important object of this invention is the provision of an improved form of circuit of the above type which has been simplified while improving the eillciency thereof.

A still further object of the invention involves the simplification of the so-called discriminating and limiting portions of a super-heterodyne radio receiving circuit adapted to respond to frequency modulated carrier waves.

A more specific object ofthe invention is to provide in an arrangement of .the above type a single vacuum tube device and associated circuits for performing both the limiting and discriminating functions in frequenc ing circuits.

These and other objects as will appear from the following disclosure are successfully secured by means of the invention herein described.

This invention resides substantially in the combination, construction, arrangement and relative location of parts, all as will be detail below.

In the accompanying drawing:

Figure 1 is a diagrammatic illustration of that portion of a super-heterodyne radio receiving circuit adapted to receive frequency modulated waves illustrating the subject matter of this invention; and Figure 2 is a chart of the potential distribution within the vacuum tube of this invention which is employed in accordance therewith to 40 accomplish both the limiting and discriminat-' ing functions of such apparatus.

As is known in the art any radio receiving circuit adapted to reproduce the modulating current, which is of course the current representative of the signal, of a frequency modulated carrier, commonly employs portions of the circuit for limiting the amplitude of the signal wave and for demodulating the limited wave to produce an audio -frequency signal current which is of course the original modulating current. In the prior art arrangements for accomplishing this purpose the output of the intermediate frequency amplifier of a super-heterodyne receiver is fed into a pentode vacuum tube, the output current of which has beenlimited in amplitude. This y modulated receivdescribed in full UNITED STATES PATENT OFFICE A 2,233,706

l I RADIO RECEIVING CIBQUI'I Henry P. Kalmus, New York, N. Y., minor to "Emerson Radio & Phonograph Corporation,

New York, N. Y., a corporationof New York Application November 1, 1939, Serial No. 302,285

portion of the circuit distinct from the limiter o portion referred to above.

In accordance with this invention substantial simplification of the discriminating portion of the circuit is accom'plshed. A simple resonant circult and a suitable multi-grid vacuum tube takes 15 the place of the pentode previously used and the limiting and the discriminating functions are performed thereby with further attendant ad-' vantages hereinafter to be described.

At I is diagrammatically illustrated a portion 20 of the last vacuum tube of the intermediate frequency amplifier commonly employed in a superheterodyne receiving set. The output of this tube is fed through a suitable radio frequency transformer 2 to the input circuit of the multi- 25 grid tube generally indicated by the reference numeral 1. It may be assumed for purposes of description that the intermediate frequency is 3 megacycles. It follows therefore that the input circuit will be tuned to a frequency of 3 meg- 3 acycles. The input circuit of the multi-grid tube includes the secondary of the transformer 2 shunted by a suitable capacitor 3 with the values thereof fixed to render the combination resonant at 3 megacycles. One terminal of this com- 35 bination of inductor and capacitor is connected by Wire 4 through a capacitor 5 to the control grid 6 of the multi-grid tube 1. The other ter-- minal of the resonant circuit is connected by the wire: 8 to the cathode return lead 9. The 40 cathode l3 of the tube is connected by a wire In through a suitable resistor H to the cathode return 9. The resistor H is shunted by a capacitor I2 of suitable value to by-pass the radio frequencies present. A suitable grid leak resistor 34 is connected between the cathode I3 and the control grid 6.

As diagrammatically illustrated the multigrid tube 1 may include a grid ll, adapted to be connected by a wire l5 to the positive side of a suitable potential source so as to maintain this grid positive with respect to the cathode in accordance with usual practice. A suitable high frequency by-pass circuit is'provided' between the lead l5 and the cathode return 9 by means 66 of a properly proportioned capacitor I8. The multi-grid tube 1 has a third grid I1, a fourth grid I8, and a fifth grid I9. The grids I| and I9 are provided with a common lead 26 which 5 is connected to the positive terminal of a suitable potential source and a by-pass capacitor 2I is provided between the lead 28 and the cathode return 9. The grid I8 is connected to the return lead 9 by a lead 22 througha resonant'circuit comprising the inductor 28 and the capacitor 24. This circuit is tuned to resonance'at 3 megacycles'- It may be noted, as indicated diagrammatically, that the grids 6, I4, I'l, I8 and, I9 are spatially distributed within the envelope of the multi-grid'tube in the manner illustrated. The distributionof the elements and the pitch of the The plate or anode 25 of the tube 'l'connected by a wire 26. through a suitable resistor 21 of the order of 50,000 ohms to the positive side of the plate current source and lead 26 is connected througha suitable by-pass capacitor 28 to the cathode return lead 9. Wire 26 'is connected by a wire through a capacitor 29, a high resistor 30, and a resistor 3| of lower value to the return lead 9. The common point of resistors 38 and 3I is connected through a capacitor 32 to the return lead 9. A suitable adjustable connection 33 togetherwith the return lead 9 provide the output terminals for this apparatus which would be coupled in any suitable manner to an audio signal responsive device such as a loud speaker or an audio frequency amplifier if amplification is desired.

Figure 2 illustrates diagrammatically the potential distribution in the multi-grid tube I betweenthe cathode and plate along a theoretical line which does not cut any of the turns of the grids. When grid I8 is negative the electrons 40 passing through grids 6 and "are partially impeded with the result that a virtual cathode is formed before the grid I8. During the time grid 8 is positively charged this virtual cathode causes a bending in the curve of potential between grids I1 and I8 as indicated in Figure 2. During the,

sentsthe potential distribution from cathode to plate'when grid 6 is positive and for difierent positive values thereof with the variation indicated in the dotted line in the region between grids I1 and I8 during the negatively charged time of grid 6. This chartserves to further demonstrate that the space charge periodi-callyformed before grid I8 periodically charges that grid.

' .65 If it is. imagined that grid I8 is connected to the cathode through a condenser this induced charge causes a periodic charging of the condenser and a simultaneous alternating current voltage between grid I8 and the cathode. changing at the. same rate as the space charge and the alternating current voltage on grid 6.

When this is at maximum positive value most of the electrons are crowded before grid I8, and this grid becomes negative if there is a capacity 75 reactance between grid I8 and cathode. There- This voltage is I fore, the alternating current voltage introducedbetween grid "and the cathode is Just out of phase with the alternating current voltagebetween grid 6 and the cathode. As stated another way, it can be imagined that there is an 5 apparent negative capacity between grids 8 and I8. There is no such apparent negative capacity in the opposite direction between grids I8 and 6. Therefore, the described effect is comparable with a' negative capacity in one direction. By reason of these characteristics it is possible by connectinga resonantcircuit proper- 1y tuned between the grid I8 and ground to perform the discriminating functions desired in ap paratus of this type when adapted to the reception of frequency modulated signal waves.

In order to understand the operation of this circuit the action which goes on in the multigrid tube needs some description. Before giving this description it will be noted, however, as 20 is apparent to those skilled in the art, that all of the circuit connections for the portion of the circuit shown have not been completed because their completion is apparent to those skilled in the art, and by omitting certain portions the disclosure is thereby simplified. As ndted above, the grid 8 is the control or input grid of the tube, which gn'dis initially grid leak biased so as to be negative with respect to the cathode. The grids I4, I1 and I9 are maintained positive with respect to the cathodes, whereas-the grid 3 I8 is made negative with respect to the cathode by resistor II. A space charge is present between grids I1 and I8 depending on grid 6 potential. This space charge is varying in accord-' 35 ance with the alternating current voltageon grid 6. This variation of the space charge produces an alternating current voltage on grid I8 by capacitive induction. If there is a capacity reactance between grid I8 and cathode return lead 9 this capacity is charged and discharged in the same phase as the space charge varies at grid I8. In other words, the alternat ing current voltage produced on grid I8 of the same frequency as that present on grid 8 is.180 degrees out of phase with grid 6 voltage, thereby decreasing the slope of, the characteristicof the plate current, as well as the plate current of the tube. If an inductive reactance is present" a between grid I8 and cathode return lead"9 the voltages on grids 6 and I8 will be in phase, thereby, increasing the slope of'the characteristic of the plate current and therefore the plate current of the tube. On theother hand, if resistance is present between grid I8 and lead 9 the voltages of grids'Ii and I8 are ninety degrees out of phase so that. no change occurs in the slope of the plate current characteristic. Therefore, if a resonant circuit including an inductor and acapacitor is inserted between the'grid I8 and return lead 9,, depending upon the relationship of the resonance frequency thereof and the frequency ap-. plied to grid 6, the'useful results of this invention may be secured.

With such an arrangement the plate current changes progressively with the frequency of the voltage variation of grid .6, i. e., the plate current changes proportionately'and in :the same. direction asthe input frequency. Hence, if, with. the frequency characteristics of the resonant circuit 23-24 fixed, the control grid frequency changes, the plate current changes in the samev way. The result is that the output of the circuit, as illustrated in Figure 1, is the same as the output-of the conventional discriminator previously mentioned. The slope of the plate current characteristic, however, when plotted against the input frequency is much greater by this arrangement, with the result that this device is more sensitive and has greater output.

The arrangement also can act as a limiter, cutting oil, so to speak, the tops of the frequency modulated carrier, as well as all portions of ex-. traneous noises such as static above the amplitude of cut-off providing the desirable limiting function oi a circuit of'this type. Limiting is accomplished by grid circuit cut-oft due to condenser and grid leak 34 and by plate circuit cut-oi! due to proper choice of voltage on and geometry of grids I 4 and II, as is well known in the art.

From the above description it will be apparent to those skilled in the art that I have devised certain new and novel circuit combinations which produce advantages of practical importance and that the embodiment oi the invention herein disclosed for the purpose of illustrating the invention is capable of variation without departure from the novel scope of the subject matter herein' disclosed. I do not, therefore, desire to be limited bythis disclosure but rather by the scope of the claims granted me.

What is claimed is:

1. In a combined radio frequency carrier operated limiting and discriminating circuit, the combination including a multi-electrode vacuum tube including a cathode, an anode and a plurality 01 control electrodes, a tuned input circuit including one of said control electrodes and said cathode, an output circuit including said anode and said cathode, means for biasing the control electrode independently of the other electrodes, means for biasing others of said control electrodes, the biasing of said biased electrodes being selected to limit, the response to the amplitude of the input, and a resonant impedance network connecting another of said control electrodes with said cathode whereby the current in said output circuit is varied by and in accordance with changes in the frequency on the first control electrode.

2. In a radio circuit combination, a vacuum tube having a cathode, an anode and a plurality of grids, an input circuit connected to one of said grids and said cathode, an output circuit connected to said anode and said cathode, means connected between said cathode and another of said grids for causing the current in the output circuit to change progressively with changes in frequency in the input circuit, and means for biasing one or more of the grids to limit the response to the amplitude of the input.

3. In a combined radio frequency carrier operated limiting and discriminating circuit, the combination including a multi-electrode vacuum tube including a cathode, an anode and a plurality of control electrodes, a tuned input circuit including one of said control electrodes and said cathode, an output circuit including said anode and said cathode, means for biasing the control electrode independently oi the other electrodes, means for biasing others of said control electrodes, the biasing of said biased electrodes being selected to limit the response to the amplitude.

of the input, and a resonant impedance network having a resistance component connecting another of said control electrodes with said cathode whereby the current in said output circuit is varied by and in accordance with changes in the frequency on the first control electrode.

4. In a radio circuit combination, a vacuum tube having a cathode, an anode and a plurality of grids, an input circuit connected to one of said grids and said cathode, an output circuit connected to said anode and said cathode, means connected between said cathode and another or said grids for causing the anode current to change progressively with changes in the frequency on the first grid, and means for self biasing the input grid to limit the response to the amplitude oi the input.

5. In a radio circuit combination, a vacuum tube having a cathode, an anode and a plurality of grids, an input circuit connected to one oi said grids and said cathode, an output circuit connected to said anode and said cathode, a resonant impedance network connected between said cathode and another of said grids for causing the current in the output circuit to change progressively with changes in the frequency in the input circuit, and means for biasing the input grid, this biasing being selected in relahon to the shape and spacing of said grids to response to the amplitude of the input.

Images