US2203498A

US2203498A - Frequency discriminating network

Info

Publication number: US2203498A
Application number: US166495A
Authority: US
Inventors: Winfield R Koch
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1937-09-30
Filing date: 1937-09-30
Publication date: 1940-06-04
Anticipated expiration: 1957-06-04

Description

`June 4,1940.. v w. R. nourri 2,203,49

FHEQUENY DISUBIMINNI'ING NETWORK Filed Sept 3U, 19:57

s sheets--sheet l 3 :mentor June 4, 1940. -W, R, KOCH 2,203,498

FREQUENCY DISCRIMINATING NETWORK Filed Sept. 30, 1937 5 Sheets-Sheet 2 June 4, l.

w R. KOCH 2,203,498

FREQUENCY DISCRIMINATING NETWORK Filed sept. 5o, 19:57

:s sheets-sheet 5 :inventor Cttorneg Patented June 4, 1940 UNITED STATES FREQUENCY DISCRIMINATING NETWORK Winfield R. Koch, Haddonfield, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application September 30, 1937, Serial No. 166,495

10 Claims.

This invention relates to frequency discriminating networks for radio signaling systems including automatic frequency and volume controlling means, and has for its `principal object the provision of an improved apparatus and method of operation whereby automatic control of the frequency and signal volume in such systems may be effected without the `expense and disadvantages heretofore incident to the construction and operation of such devices. i

Automatic frequency control circuitsheretofore available have usually included a center tapped winding, separate primary and secondary windings, a blocking condenser and choke coil, a separate detector circuit, a tube capable of performing a plurality offunctions or combinations of certain of these features.` Such circuits are expensive both because of the accurate center tapping required and because of the choke coil involved and are not altogether satisfactory for the reason that the diode used for securing audio frequency potentials has impressed on it the signal potentials from both the primary `and the secondary of the coupling transformer. Under these circumstances,'not only is the full selectivity of the transformer not utilized but the voltage impressed on the diode varies throughout the desired frequency band, or channel, instead of remaining substantially constant. i

` In accordance with the present invention', these difficulties are minimized or avoided by the use of means including a pair of diode tube elements so interconnected as (l) to detect the modulation or audio component of the signal, (2) to provide suitable automatic volume control potentials, and (3) to provide suitable automatic frequency control potentials. More specifically stated, the modulated signal or intermediate frequency impulses are supplied through a coupling transformer to one diode which detects the modulation component and affords one component of a resultant unidirectional voltage which is'utilized in effecting automatic control of the amplitude `ofthe carrier frequency. lAnother diode is subjected to the resultantof the coupling transformer primary and secondary voltages for producing another unidirectional potential which varies with change in the phase relation between the primary and secondary voltages, and which iscombined with the substantially constant unidirectional component of the detected potential for automatically regulating the carrier frequency. For the automatic volume control potential derived from the output of the signal detector, suitable filters are provided to exclude (Cl. Z-20) Figure 1 is a wiring diagram of a preferred embodiment of the invention wherein the automatic frequency control potential is derived from series- I connected resistors,

Figures 2 to 4 are `explanatory diagrams relating to the operation of the invention,

Figure 5 illustrates thev application of the invention to a tuned radio receiver including groundedrotor condensers, the automatic frequency potential being in this case derived from a bridge network instead of series-connected resistors,

Figure 6 illustrates a modification differing from those of Figs. 1 and 5 in that substantially constant component of the automatic frequency control potential is derived from the transformer primary instead of `the transformer secondary, thus permitting the following tube to function either as a detector or amplifier,

Figure 7 illustrates a modication which differs from the previous modifications in that the automatic frequency control potential derived from the network has the opposite polarity at any frequency from that derived from the network in Fig. 1. This is desirable for some types of frequency control means,

Figure 8 exemplifies a modification in which only part of the primary voltage is utilized, and

Figure 9 illustrates a modification including a voltage doubler detector arranged to produce the automatic frequency control potential through a bridge network.

The circuitof Fig. 1 includes an input transformer I0, an intermediate` frequency amplifier tube II, a coupling transformer I2, a detector I3, a `coupling resistor I4 and an audio frequency amplifier tube I5. As hereinafter referred to, the relative frequency response between the primary and secondary voltages within the pass band of the transformer I2, which is of the tuned type, is shown in Figure '2. Connected between the primary winding IB and the secondary winding I1 of the coupling transformer I2 is a circuit including capacitors I8 and I9 and a diode 20 which is shunted by a pair of resistors 2| and 22. Automatic frequency control potential dependent on the unidirectional potential drops of the resistors I4 and 22 is applied to the lead 23, impulses of audio frequency being excluded from this lead by lter means such as the capacitor I9 and a resistor 24 interposed between the resistors I4 and 22. Impulses of both radio and audio frequencies are further excluded from this lead by the capacitor-resistor filter 25-26. The capacitors I8 and I9 afford direct current isolation for the diode 2U, thus permitting the series connection of the

resistors I

4 and 22.

Automatic volume control potential is applied to a lead 21 from the output ofthe detector I3 through a resistor 28 which functions with a capacity 29 to form a filter whereby audio and radio frequency impulses are excluded from this automatic Volume control lead 21. The resistor 24 also prevents radio frequency impulses froml getting into the automatic volume control circuit.

The operation of the circuit of Fig. 1 will be readily understood. yW'here exact tuning is in the center of the `frequency band, the primary and secondary voltages of the transformer I 2 are displaced in phase by approximately 90 degrees for the reason that they provide two coupled circuits, tuned to the same frequency. The resistors 2I and 22 on which the resultant of the primary and secondary voltages is impressed are so proportioned that, at exact tuning, the potential drop of resistor 22 is equal to and opposed to that of resistor I4. Under these conditions, the Aautomatic frequency control potential is zero at exact tuning. SinceY the unidirectional potential drop of the resistor I4, that is, the rectified secondary voltage, is substantially independent of change in frequency in the region of exact tuning due to the band-pass characteristic of the transformer I2 that is, band pass tuning response in a relatively narrow band about the mean frequency of 460 kc. as indicated by the response curves of Figure 2, and the drop of resistor 22 changes rapidly with change in frequency, there is applied to the autor matic frequency control lead 23 a potential which is too low and negative when the frequency is too high or vice versa, according to the frequency adjusting element to be controlled, and depending on the direction of the coupling between the primary I and the secondary I1 of the coupling transformer.

The rectified primary or secondary voltage is balanced against the vector sum of the primary and secondary voltages rectified thereby to produce a composite output potential. At resonance the drop in resistors 22 and I4 are equal and the automatic frequency control or composite output voltage is Zero. When the primary and secondary voltages are in phase the voltage in resistor 22 is zero and the voltage in resistor I4 is positive as applied to the output or AFC circuit 23. When the primary and secondary voltages are fully out of phasethe potential drop in resistor 22'is equal to twice the drop in the resistor I4 and the automatic frequency or composite output voltage in resistor I4 is nega-tive as applied to the output lead 23.

As indicated by the curve of Fig. 4, this automatic frequency control voltage is substantially linear in the region .of exact tuning which in this particular case is assumed to be in the neighborhood of 460 kilocycles. This control potential ltroltuning capacitor 3U.

, sumed to occur at 460 kilocycles.

Fig. 5 illustratesq the application of the invention to atune'd radio frequency receiver including a grounded-rotor;,multiple-section uni-con- Where these circuits are tuned tothe incoming signal frequency, a men chanical frequency control device or an electronic reactance across each tuned circuit would ordinarily'v'beyrequired. Where the adjustable frequency circuits form the intermediate frequency amplifier', theA invention may be used to regulate the frequency -of`thelocal oscillator. Tov this end, the substantially frequency independent and frequency'responsive control voltages are produced inthe resistors I4 and 22 as previously indicated and the resultant of these voltages is applied to the automatic frequency control lead 23 'through resistors 32'and 33 which are interconnected with the resistors I4 and 22 to form a bridge network which will be balanced at exact tuning,"to give vzero frequency controlv potential. By proper choice of circuit constants, the resistor inthe l'ead"'23 of Fig) l'may be omitted as shown in Fig. 5I In1thiscase, filteringz is effected by theirss'to'rs 32 and' 33." 1

The" arrangement o'fFig. Gdiffers from the previous modifications in that' a separate detectoror amplifier 34 is supplied from the secondary of the transformer l`2'and a separate diode 35 in place of the diode I3- in Figure 1, coupled through a capacitor; iSd to thetransformer primary I 6 instead of to the secondary, isutilized to produce in'a resistor ytti the automatic volume'control potential andthe y"frequency independent com-- ponent of the automatic frequency control potential. This connection hasthe advantage' that somewhat 'better' automatic volume control'is secured' sincey the primary is more broadly tuned than the secondary. Fig. 6 'also illustrates an arrangement wl'iicl'i differs from that of Fig. 1 in thatl the frequency'independent component of the automa-tic frequency control potential is derived from the,automatic volume control lead 21, instead of from? the anode end of the filter resistor 28. This assures thaty'no'audio frequency voltage will beappliedy to the diode ZIJ. This potential is then balancedwith the vector sum of the rectified primary and secondary voltages by conneyction through a resistor 24 to the diode 20 outputrresistor 22, las in Figure l. l In the 'circuit ofFig. '7, the automatic volume control potential applied to `the lead 21 is amplilfied byv an `'amplifier '31 'interposed between the double diode' andthe control grids of the controlledtubes suchas the tube iI.` This requires that the amplifier grid controlr potential derived from the resistor I4` be positively polarized for the reason that the/tube 3l effects a reversal of polarity and the control grids of the controlled tubes such as the tube II, lmay be subjected to a negative bias potential.v 4ln this case the automaticfrequency control potential derived from the .resistorsM and22 goes from plus tominus as the'frequencyincreases. This would permit the use of an amplifier similar to 31, for amplifying 40 ence is the derivation of the automatic frequency the automatic frequency control potential, if desired, or permit the use of a frequency control `means requiring opposite control voltages.

Fig. 8 illustrates an arrangement that is similar to that of Fig. l, but differs therefrom in that only a portion of the primary voltageis impressed on the cathode of diode 2|] by reason of the tapped capacitor connection 3I-3I. Where very loose coupling is used between primary I 5 and secondary II of the coupling transformer I2,

l the primary voltage may be several times as large as that 'of the secondary. Also, under these conditions, the capacitive reactance of the diode 20 age divider 3 I -3I shown is in the capacitive pori tion of the primary tunedcircuit, it is readily apparentthat voltage division maybe accomplished in the inductive portion ofthe tuned primarycircuit if desired. The circuit of Fig. 8 also differs from that of Fig. 1 in that the `frequency independent component of the automatic frequency control .potential is derived from the automatic volume control lead 21 instead of the output resistor I4 of the detector I3.

'Ihe circuit of Fig. 9 differs from that of Fig. 1 in that the single diodes I3 and`20 are replaced respectively by diode pairs 35i-4D and 4|42 so as to secure voltage doubling as disclosed by Schade Patent No. 2,072,278. A further differcontrol potential from a bridge circuit formed by the resistors I4 and 22 and a pair of resistors 43-44 to the common terminal of which the lead 23 is connected. The resistors 43 and Mare not only chosen to attenuate alternating voltages from the lead 23 but are also so proportioned that, at exact tuning, the potential drops of resistors I4 and `22 are exactly balanced. As in the previous arrangements, mistuning results in a control potential which is applied through the lead 23 to a suitable tuning control device `such as a local oscillator or the like.

I claim as my invention:

l. `An automatic frequency control circuit responsive to an amplitude modulated carrier wave `including a transformer provided with primary and secondary windings, a detector connected with said transformer to produce a unidirectional potential component which is substantially independent of the frequency of the impulses sup-n plied to said circuit, means connected with said transformer to produce a unidirectional potential component which varies with change in said frequency, and means providing a series connection between said last-named means and said detector for deriving the resultant of said potential components and the amplitude modulation output.

2. The combination of an automatic frequency control circuit including a transformer having input and output terminals, a detector connected to the output terminals of said transformer,

. means for deriving from said detector a unidirectional potential which is substantially independent of frequency within arelatively narrow pass band, means for deriving from the input and output terminals of the transformer a resultant unidirectional potential as the vector sum of input and output potentials whichis equal and opposed to said detector unidirectional potential only at exact tuning, and means for combining said unidirectional potentials to provide a controlling potential for said circuit. l

3. In a radio signal conveying system, the combination of a coupling transformer having primary and secondary windings tuned to substantially the same signal frequency within a relatively narrow pass band of frequencies, a diode signal. rectifier device connected with the ksecondary winding, means for applying modulated signals through said transformer to said diode rectifier, means for deriving from said rectifier the detected modulation component of said signals, and a resultant direct current potential which isV substantially constant with changes in `frequency and phase between said primary and selcondary windings within `lsaid narrow pass band, a seconddiode rectifier device coupled to the primary and `secondary windings of said transformer to receive the vector sum o-f the potentials in said primary and secondary windings, means forwderiving from said second rectifier device a second direct current potential which varies with frequency and the phase relation between the primary and secondary voltages, a control circuit connecting said potential-deriving means for` said rectifiers in series relation to each other, to derive `a resultant control potential therefrom. l

4. An automatic frequency control circuit including artransformer provided with a primary and a secondary winding, a detector connected with said transformer to produce from one of said windings aunidirectional potential component which is substantially independent of the frequency of the impulses supplied to said circuit within a predetermined relatively narrow pass band, means connected with said primary and secondary windings to produce `from both of said windings a unidirectional potential component which varies with change in said frequency within said band, and means for combining said potential components to provide a' re- Sultant controlling potential.

5. An automatic frequency control circuit including a transformer provided with tuned pril mary and secondary windings, a detector connected with one of the windings of said transformer to produce a uni-directional potential component which is substantially independent of the frequency of the impulses supplied to said circuit within a relatively narrow pass band of frequencies, rectifier means connected with both of said windings to produce a unidirectional potential component which varies withchange in said frequency from a mean frequency within said pass band,` means connected with said detector and rectifier to combine said potential components to produce a third unidirectional potential component which changes in polarity in response to departure of said frequency in different directions from exact tuning to said, mean frequency, and circuit means for deriving the resultant of said potential components.

6. The combination of an automatic frequency control circuit including a transformer having input and output terminals, a detector connected to the output terminals of said transformer, means for deriving from said detector a unidirectional potential which is substantially independenty of frequency within a relatively narrow pass band, means including a mid-tapped tuning capacitor for deriving from said transformer a resultant unidirectional potential as the vector sum of input and output potential which is equal and opposed to said detector unidirectional potential only at exact tuning, and means for combining Vsaid unidirectional potentials.

7. lin a signal conveying system, the combination of a transformer having an input circuit and an output circuit, means for tuning said input and output circuits whereby at resonance `to a received signal the potentials therein are substantially 90 out of phase with, one another, means for detecting signals in the output circuit of said transformer, additional means for detecting a resultant vector sum of signal potentials in the input and output circuits of said transformer, and means for combining the deytectedl signal output of said detecting means.

8. In a signal conveying system, the combination of a transformer having an input circuit and an output circuit, means for tuning said input and output circuits whereby at resonance to a received signal the potentials therein are .sub-

stantially 90 out of phase with one another,

means for detecting signals lin the output circuit of `said transformer, additional means for detecting a resultant vector sum of signal potentials in the input and output circuits of said trans` former, and means including a bridge network lcombining the between said first and second-named means for signal output of said detecting means. f 9. In a signal receiving system, the combination ofa transformer having an input circuit and an outputA circuit, means for'tuning said 'input and output circuits whereby at resonance to a received signal the potentials therein are substantially`90 out of phase with one another, means for detecting signals in the output circuit of said transformer, additional means including a frequency doubler for detecting a resultant Vectorv sum of signal potentials in the input and outputy circuits of said'transformer, and means for combining the signal output of said detecting means. f

10.` In a signal conveying system, the combination of a transformer havingl anY input and an output circuit,means for tuning said input and Output circuits whereby at resonance to a received signal the potentials therein are substantially 90 out of phase with one another, means for detecting signals in the output circuit of said transformer, additional means including a fre,- quency 'doubler for detecting a resultant'vect'o-r sum of signal potentials inthe input and output circuits v-of said transformer, and means'A including ya bridge network for combining the signal output of said'detecting means; H Y

WINFIELD Ru KOCH 30