US2233339A - Radio detecting system - Google Patents

Description

Feb. 25, 1941. R W JR, 2,233,339

RADIO DETECTING SYSTEM Filed Sept. 24, 1932 INVt'NTOR' Patented Feb. 25, 1941 UNITED- STATES RADIO DETECTING SYSTEM Reynolds D. Brown, Jr., Mount Airy, Pa.,

assignor to Philco Corporation Applicationseptember 24, 1932,-Serial No. 634,661

9 Claims.

I This. invention relates to radio receivers having diode .or grid leak detectors, particularly those having automatic volume control.

The object of this invention is to improve the 5 performance of radio receivers of this nature,

particularly by reducing the response of the receiver to signals of very low intensity, including noises present when no sustained carrier wave is being received.

Other objects will be apparent as the description of the invention proceeds.

In the drawing Figure 1 illustrates one embodiment of my invention as applied to a simplified radio receiver,

Figure 2 shows a modification of the device shown in Figure 1, and

Figure 3 is a chart comparing the performance of a device according to my invention with a similar device not including my invention.

In the figures, II! is a carrier frequency transformer, adapted to be supplied with energy from an antenna, preferably through an amplifier. The secondary of transformer I may be tuned by variable condenser II to any desired fre- 25" quency within its range. The tuned circuit includes fixed condenser I2, the purpose of which will be set forth below.

Voltages set up in the tuned circuit I0, II, I2

are applied between the control electrodes of amplifier I3. Amplifier I3 may be any voltage controlled amplifying device, and in present day practice would preferably be a screen grid vacuum tube. The anode circuit of tube I3 may be energized by battery I 4. Tube I3 amplifies the energy applied to it and supplies it to the primary of transformer I5. The secondary of trans former I5 may be tuned by variable condenser I6, through condenser H, which serves another purpose, as will later appear. The voltage across the secondary of transformer I5 is applied to the input terminals of detector I8, through condenser IB.

Were it not for. the unidirectional voltage of the polarity shown across condenser I9, and occurring by reason of the connections of my invention, a portion of the positive half of each wave arriving at the control electrode of tube I8 would be conducted, thus setting up across condenser I1 and resistor 2|, a unidirectional voltage of the polarity shown and having a component derived from the modulation of the carrier. In the normal operation of this system, the above described action occurs when strong and moderately weak carrier waves are being rew /ceived,v but my systems differ from. those previously used in that when very weak signals are being received, no rectification occurs and no voltage-is built up across condenser H.

The secondary of transformer I5 presents a very low impedance to the wave derived from the modulation as do condenser I9 and resistor 26, and substantially the full voltage across resistor 2| isapplied tothe control electrodes of tube I8. The control electrodes control the fiow of current from battery l4 3 through the primary of output transformer 23, filter resistor 22, the vacuous space of tube I8; and time circuit I 5!, 20. Thevarying component of the anode circuit voltage issubstantially all applied to the primary 23, while a-substantial-portion of the unvarying portion is applied across resistor 29, thus providing the voltage previously mentioned. Impedance 22 is shown as a resistor, and serves to filter carrier frequency currents out of the detector anodecircuit. Resistor 25, in conjunction with condensers I! and I2, prevents the transfer of radio frequency currents through the conductor in which it is inserted.

In the embodiments shown, the unidirectional voltage appearing across resistor 2| when rectification occurs, is utilized to bias the control electrode of amplifier I3. Automatic control of volume occurs because carrier waves of increasing amplitudes applied to detector I8 generate increasingly negative voltages across resistor M, from which they are transferred to the control electrodes of amplifier I3 to decrease its amplification.

The resistor 20 also serves to prevent excessive plate current in detector I8 when no carrier is being received, as compared with similar circuits whenthe resistor is not employed. Thus a high voltage battery I 4 may be used to increase the possible output of the detector plate circuit before overload occurs. The grid bias on the detector, instead of I shifting from approximately zero, to the plate current cut-off point, will shift only from the voltage set up across resistor 20 to the same cut-off point. As there is less bias change, the internal plate impedance change is less, and impedance matching to a power conset to a high value, another valuable feature obtains. Although the signal must be increased to a moderately strong level, before output response occurs, as soon as the signal peak exceeds 5 the bias voltage across resistor 20, the grid is supplied with a negative bias from the rectified current across resistor 2!, and the plate current decreases. As a decrease in plate current decreases the opposing bias across resistor 20, further rectification occurs. It finally results from this process, that the increased unidirectional bias across resistor 2| becomes much larger than the decreased bias across resistor 20, which is the suit able condition for proper rectification and amplification of carriers having a high degree of modulation. However, when the carrier level is reduced, it continues to be rectified and the modulation component amplified, until rectification can no longer occur due to the carrier becoming less than the decreased bias across resistor 20. Thus if a weak carrier becomes strong enough to provide an output, it will not disappear until it again fades until it is well below the noise level. This prevents the annoying continuous response and silence-which would occur were the bias at element 2!) constant. Figure 2 shows a modification of the device of Figure 1 wherein the detection and amplification occur in separate sets of thermionic elements. Among other advantages, --the voltage supplied to the grid of the amplifier is substantially free of high frequency energy, which would tend to overload the amplifier, and to produce an undesirable source of high frequency voltage which might be fed back to an earlier amplifier and cause annoying oscillation. As shown, the filter impedance 22 may therefore be omitted. The other changes are that detector 26 and amplifier 21 are substituted for detector I8, and that the control grid of the amplifier is -connected directly to condenser I1 directly instead of through secondary l5. It is convenient to incorporate the elements of detector 26 and amplifier 27 in a single evacuated envelope, preferably using a common cathode. It should be understood that while the invention has been illustrated in connection with a given circuit arrangement, it is readily adaptable to other systems without departure from the spirit thereof and that the invention should only 50 be limited by the scope of the appended claims.

I claim: 1. A carrier wave detecting system comprising a source of modulated carrier waves, a diode rectifier network including a cathode, an anode 55 and a resonant circuit connecting the cathode and anode, said circuit being tuned to a desired carrier frequency, a pair of load resistors connected in series with each other and connecting said cathode and anode, an audiooutput net 60 work including an electron discharge device, the space current circuit of said device including the load resistor nearest said diode cathode whereby the space current of said device traverses said last load resistor, a control electrode disposed in 65 the space current path of said device and being connected to the diode anode side of the load resistor nearest the diode anode, and means elec+ trically associated with the last named resistor for preventing the impression of the carrier com- 70 ponent of rectified waves upon said control electrode. 1

2. In a modulated carrier-current signaling system employing a carrier-current amplifier and rectifier, which rectifier produces a modulated 75 "unidirectional voltage, a direct current connection from said rectifier to anelement of said amplifier whereby the amplification is automatically regulated, and a connection from said rectifier to a modulation current amplifier whereby the signal is further amplified, and an additional co-n- 5 nection from the output of the modulation current amplifier to the output electrode of said rectifier for varying the potential of said rectifier output electrode in dependence on the intensity of received carrier current. i g 10 3. A carrier wave detecting system comprising a source of modulated carrier waves, a rectifier network connected to said source, a resistor and a non-linear impedance serially included in said rectifier network, a modulation-signal output net- 15 work including an electron discharge device, the space current circuit of said device including said resistor, whereby a threshold bias voltage is set upacross said resistor to prevent operation of said rectifier network on signals whose amplitude is less than the bias, and a control electrode disposed in the space current path of saiddevice and connected ,to said rectifier network, whereby signals of suflicient amplitude to overcome said bias are applied to said control electrode thereby decreasing the space current through said resistorand'decreasing the bias voltage accordingly, said control electrode being otherwise functionally independent of said non-linear impedance.

4. A carrier wave detecting system comprising 30 charge'device, the space current circuit of said 40 device including said resistor, whereby a threshold bias voltage is set up across said resistor to prevent operation of said rectifier network on signals whose amplitude is less than, the bias,

and a' control electrode disposed in the space current path of said device and connected to said rectifier network, whereby signals of sufficient amplitude to overcome said bias are applied to said control electrode thereby decreasing the space current through said resistor and decreasing the bias voltage accordingly, said control electrode beingotherwise functionally independent of said non-linear impedance.

5. In 'a modulated carrier-current signaling system, a rectifier, which produces a modulation signal, a modulation signal amplifier connected to 'saidrectifier, and a connection from the output of the modulation signal amplifier to the output electrode of said rectifier for varying the potential of said rectifier output electrode in de- 00 pendence on the intensity of received carrier I current.

6. In a radio receiver, a diode rectifier, a signal input circuit connected to said diode rectifier to provide a rectification network, a modulationsignal amplifier network including a space discharge device connected to said rectification network, and a biasing impedance in the cathode circuit of said device connected to said diode rectifier for rendering the latter inoperative circuit connected to said "diode rectifier to provide a rectification network, a modulation-signal amplifier network including a space discharge device connected to said rectification network, a biasing impedance in the cathode circuit of said device connected to said diode rectifier for rendering the latter inoperative when signals of less than a predetermined amplitude are impressed on said input circuit, and a direct current connection between said rectification network and a gain control electrode of said firstmentioned amplifier.

8. In a modulated carrier-current signalling system, a rectifier for producing an output signal of modulation frequency, a modulation frequency signal amplifier connected to said rectifier, and a connection from an output circuit of said amplifier to an electrode of said rectifier for varying the potential of said electrode in dependence upon the intensity of at least one component of the output of said amplifier.

9. In a receiver of the type including at least one carrier frequency amplifier, space discharge means including a triode section and a diode section whose anode lies outside the space current path of said triode section, an audio frequency network connected to the output electrodes of said triode section, a carrier signal input circuit connected to the diode section to provide a rectification network, a connection between said rectification network and the triode grid, and a biasing impedance in the cathode circuit of the triode section connected to said diode section for supplying negative bias to said diode section and the triode grid to prevent rectification of signals below a certain minimum carrier level, said impedance having a value such that said bias undergoes a substantial decrease in magnitude as the carrier level increases from said minimum value, said diode functioning as a rectifier for carrier signals whose amplitude exceeds said minimum level.

REYNOLDS D. BROWN, JR.

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