US2308280A - Radio receiver - Google Patents

Description

2 sheets-sneei 1 INVENTOR` ALFRED L. GREEN w ATTORNEY Jan. 12, 1943. 'A.,l GREEN RADIO RECEIVER Filed July 8.` 1941' [IN 1 n wm, IW.. n vm m mm @4| Jr .T v n NVM" u M" www ma N mm.. A uw n 4 4 111 A .mv nl.. m A m Tr x Q MW .Snug I l .mi mw E Jan. 12, 1943. A. L.' GREEN RADIO RECEIVER Filed July 8, 1941 2 Shets-Sheet 2 lNvE-NITOR ALE/E0 L. GREEN ATTORNEY Patente-cl Jan. l2, 194

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2,308,280 nuoro nncnivnn Alfred Leonard Green, Ashel'd, Sydney, New

South Wales, Australia, assigner to Amalgamated Wireless (Australasia) Limited, Sydney, New South Wales, Australia, a company of New 'South Wales,Australia t application .tu1y s',lf1941, serial' No'. 401,406 A In Australia July 26,1940

' "claim's'l- (ci. 25o-zo) This invention relates to radio receivers and more particularly to radio receivers of the type adapted to receive signals over a plurality of wave length ranges or bands.

In receivers of this type, Where for economical or other reasons only a limited number of valves are to be employed, the problem arises of provid-` ing adequate amplification for the weak signals which are customarily obtained on thev higher frequency or so-called short-wave ranges.v

This problem is accentuated by the lfact that the sensitivity of receivers of this type varies from band to band, the gain being lower on the higher frequency bands than on the lower or medium frequency broadcast bands.

In order to obtain comparable performance 'on all bands the sensitivity of the receiver mustbe made higher on the high frequency bands than Qnj the lower. This requirement limits the minimun; number of valvesY which may be employediniany receiver of the type referred to, to that number which will provide the desired sensitivity on the short wave bands. j

This is not a very satisfactory arrangement because to obtain sufcientreceiver. gain in the` short are introduced in controlling this surplus gain.

One suggestion `for overcoming the diii'iculty of providing sufficient gain on the. high frequency bands, without aiecting the normal design of the receiver on the broadcast band, has been to combinewith the wave selector switch, means for switching into operation an additional amplifying stage only when the wave selector switch is adjusted to provide reception on the H. F. bands. This arrangement is quite effective in achieving its purpose but is uneconomical inasmuch as it necessitates the introduction of an additional valve and associated circuit equipment over that normally required to provide satisfactory reception on the medium wave broadcast band.

It has also been suggestedto increase the sen-l sitivity in a receiver employing a limited number of valves by the use of a circuit arrangement in which the signal is amplified both before and after detection in the same amplifier valve or valves. This is the well-known reflex circuit and whilst it provides a means of economically realisf ing the added sensitivity required on the short wave bands, it also increases the sensitivity on the medium wave band and as a result introduces undesirable defects into the receiver output on that band. The most noticeable defect is the inability-to control vme output' voiume to zeroes@ the marked increase in distortion as the receiver output volume is reduced; f

These defects are introduced by reasonof thev fact that in practice itis not usually possible'to secure a valve for use in the reflex circuit, which has linear amplifying characteristics over Yafwide range of input potentials. Consequently an un',- desirable rectification takes place within this valve whenjthe stronger input potentials, customarilyl received on the medium waveband, are subjected to vthe increased ramplification brought about by the reflex circuit arrangement. K g t I These defects are not particularly evident on the short wave band because the input potentials on this band` are usually of much smaller ampli'- tude and the sum of the R1.' F. and'A. F. potentials applied -to the input of the reex valve is''wi'thin the linear operating characteristics of l'the' valve;

It is an object ofthis invention to provide an improved receiver of the type referred to in which satisfactory performance characteristics maybe economically obtained on all ban-ds whenusing only the minimum number of valves requiredhto providesatisfactcry reception on the medium' wave 'broadcast band. n A

A further object or" the invention is to provide an improved lcircuitV arrangementf'or a receiver of the type referredto in which the sensitivity of the receiver when operated on the Shortwave bands is greater than that employed for the re` ception of signals on the medium wave broadcast ban-d.

A still further object of the invention is to proi vide means whereby the essential circuit components employed for increasing thes'ensitivityvof the receiver on the short wave band may be utilized to increase the selectivity on the broadcast band. f

The labove objects are achieved, in accordance with this invention, by animproved 'circuit arrangement; for -a receiver of the typel referred to, and comprising means whereby-a valve, employed to amplify only L. F. energy on the broadcast band, is utilized to amplify both high and L. energy on theshcrt wave band.Y Y

In carrying out the invention, the audio'frequency amplifying valve, which ampliiiesthe demodulation output of the carrier frequency rectifier on the broadcast band, is employedr in a reflex circuit for amplifying both high and L. F. energy on the short wave band, the arrangement being su-ch that an additional radio frequency transformer, necessitated by the reex ccnnecf tion on the short wave band, forms part of aband pass filter network for increasing the selectivity of the receiver on the broadcast band.

For a more complete understanding of the invention and the manner in which it is to becarried out, reference will now be made to the following description in conjunction with the accompanying drawings.

In order to simplify the description, only that portion of the circuit of a multi-wave superhcterodyne receiver, which is essential to convey an understanding of the invention, is shown in the said drawings in which Figures l and 2 are circuit diagrams illustrating by way of example two embodiments of the invention.

Referring now to Figure l of the said drawings,

modulated intermediate frequency energy is fed to the terminals 2-4 from the preceding stages 5 of the receiver.

The detail circuit arrangement of the tuning,

amplifying and detecting portions of ther preceding stages 5 are so well known that further description is considered unnecessary. It is suincient for the purpose of this invention to indicate that wave selector means 3 are provided in the preceding stage 5 to cover a plurality of wave bands.

The switching devices 8, 9, IG, which are provided for `the purpose of effecting the required circuit alterations in accordance with this invention, are preferably gangedwith the wave selector means 6 for simultaneous rotation therewith as indicated by the link 1. Whilst this is the preferred arrangement it is to be clearly understood that if desired, the switching devices 3, 9, I!! only may be ganged together for operation independ'- ently of the wave selector switch 6. l

When the wave selector switch 3 isl adjusted for reception of signals in the broadcast band, the

switching devices 8, 9, I are adjusted to the position MW shown in the drawing. In this posi- I I, I 2 are tuned to resonate at the desired intermediate frequency by means of the condensers 28, 3| and 29, 30 respectively.

The output secondary winding I4 of the trans.. former II is coupled to the input primary winding I5 of the transformer I2 across the coupling condensers 32, 33, the switch section 9 and the lead 34. By the use of this filter -network the receiver selectivity is considerably increased. The value chosen for condensers 32 and 33 determines the degree of coupling between the two transformers Ii, I2. Increasing the value of 32 and 33 decreases the coupling whereby the selectivity is increased and the sensitivity reduced. The amount of sensitivity required therefore usually determines the final value of 32 and 33.

Intermediate frequency signals in the secondary circuit I6, of the transformer I2 are supplied to the demodulating rectifier 6U. The latter is of the diode rectifier type comprising an anode electrode 2| and a cathode I9. The diode rectifier 60 is provided with an output resistor 36 included in series between the low potential terminal 43 of the secondary circuit I6, 3!) and a lead I8 is connected with the cathode I9. The output resistor 3S is provided with a suitable intermediate frequency by-pass capacitor 31 and an intermediate frequency filter comprising a series filter resistor 38 and a second by-pass capacitor 33 to the lead I8 from the terminal 40.

The circuit for the demodulator may be traced from the anode 2I, which is connected with the high potential terminal 4I of the secondary circuit IG, 30, through the secondary circuit to the terminal 40, thence through the filter resistor 38 and the output resistor 3S to the cathode lead I8 and the cathode I9.

Intlie present example the diode rectifier S0 is shown as one of a pair of independent diode anodes 2li- 2l which are included in the envelope 21, common also to an amplifying valve having a cathode I9 in common with the diodes 23, 2|

' and having a control grid 22, a screen grid 23, a

suppressor grid 25 and an output anode 23. The diode anodes 20, 2-I are disposed adjacent a portionof the cathode I9 which only provides an electron stream to said anodes 20, 2l. The multiple valve described is the well known double diode pentode.

Although the amplifier and rectiiiers housed within the single envelope23 are shown. as a pentode and diodes respectively, it is to be understood that any suitable form of amplifier or rectifier other than a pentode or diode may be utilised to constitute the multiple assembly. Furthermore, separate valves may be employed if desired without affecting the scope of this invention.

As is well understood, modulated intermediate frequency signals received through the filter network II, I2 are applied to the diode rectifier B0 through the input circuit I6, 33 and the audio frequency components thereof appear across the output resistor 33. The output resistor 36 is arranged as a volume control potentiometer having a variable contact 42. Audio frequency -signals from the volume control contact 42' are ap plied to the grid 22 through a coupling capacitor 43, which is connected to the junction point 44 of the input coupling resistors 46, 45 whose other ends are connected to grid 22 and ground 48 respectively.

Biassing potentials are applied to the grid 22 through the resistors 45, 46M from any suitable source,'such as the self-bias resistor v49, in the cathode return lead I8. The self-biassing resistor 49 is shunted by a suitableradio frequencyand audio frequency by-pass condenser 5B.

The suppressor grid 25.. is joined to the cathode I9Y through the lead 5I whileA the screen grid 23l is connected at 51 to a suitable positive potential supply source (not shown) in known manner.-

The anode 26 is connected through the anode load resistor 52 to the moving arm of the switch I 0. With the switch I3 in the position MW shown in the drawings, i. e. for reception on. the broadcast band, operating potentials are applied tothe anode 26 through the switch I0 and the load resistor 52 from the positive terminal 54 of a po'- tential supply source (not shown) v Amplified audio frequency potentials appearing across the anode-load resistor. 52 may be applied through the R.. F. choke 6I and the lead 55 ulated I. F. beingapplied to said rectifier, through a lter network, Awhereby the. selectivity of the receiver is considerably increased.

When the wave selector switch 6 is adjusted for reception on the shortwave band, the switches 8, 9, IU are adjusted to the alternative positions SW shown in the drawing. With the switches in this position the circuit arrangement of the portion of the receiver indicated in thev drawing is altered so as to cause the amplifying section of the valve 21 to amplify the signals both before and after rectification. Thus the sensitivity of the receiver on the short-wave bands is considerably greater than the sensitivity it would be possible to obtain if the circuit arrangement for these rbands was maintained the same as that employed for reception on the medium wave broadcast band. More specically when the wave selectorswitch 6 is adjusted for reception on the short Wave band the switches 8, 9, I are also adjusted to the alternative positions SW. Modulated I. F. energy applied to the terminals 2, 4 is passed through the transformer II across the switchu and the blocking condenser 41 to the grid 22. The amplified energy appearing in the output circuitzt is applied through the R. F. by-pass condenser 53, the switch section IB and the'coupling transformer I2, to the demodulating rectier '55. The A. F. signal potentials across the output load resistor 35 are applied to the grid 22 from the variable contact 42 through the coupling condenser 43 and the grid coupling resistors 45, 45 via junction point 44. A

The A. F. potentials applied to the grid 22 are simultaneously amplified with the high frequency potentials in the amplifier and appear in amplied form across the anode load resistor 52 and are passed to the utilization means (not shown) through the R. F. choke 6|- and thev lead 55 in the same manner as that employed on the medium wave broadcast band. v

Operating potentials are applied to the anode 25 in this arrangement of the circuit from the positive terminal 56 of a suitable potential supply source (not shown), through the primary winding I5 of the coupling transformer I2, the switch section lil, the load resistor 52 and the lead 24. The condenser 58, which is connected between the low potential end of the primary I5 andrground 48, provides an effective by-pass for I. F. energy at that point.

From the foregoing description of the circuit alterations effected by operation of the switches to the SW positions, it will be seen that the coupling transformers II, I2, instead of forming a single highly selective lter network as on the medium wave broadcast band with said switches set .at the MW positions are now employed as individual interstage coupling transformers in a well-known reflex circuit. The coupling condensers 32, 33, in the altere-d (SW) circuit arrangement, function as blocking condensers to prevent short circuiting of the high voltage potentials supplied to the anode 26 from the terminal 5t.

From the foregoing description it will be seen that in the SW position of the switches 8, 9, I0, increased sensitivity is obtained with a limited number of valves by the use of a reflex circuit in which the amplifier valve 21 is caused to function as an amplier of the signal energy both before and after rectification.

In the MW position of the switches, the reflexing function is removed, thus eliminating a stage of I. F. amplication and causing the valve 21 to function as an A. F. amplifier only. In order to compensate for the loss in selectivity occasioned CII `rangement described.

by the elimination of the stage of I. F. ampli'- cation, the I. F. coupling transformer l2,l which was employed in the reiiex circuit, is utilised in this position of the switches to form part of a selective filter network.

As only one diode 2l is required for detection, the second diode (2G) may be fed with H. F. potentials from a suitable point in the H. F. amplifier circuit and used to provide A. V. C. bias for preceding carrier frequency stages. On the other hand, the two diode anodes 20, 2I may be conductively coupled together and the desired A. V. C. potentials obtained from a suitable point in the diode load circuit in known manner.

Furthermore, it will be appreciated that the scope of the invention is not limited to-the ar- For example, if. the switches 8, 9, III are ganged together for operation independently of the wave selector switch S, it is'possible to utilize the added sensitivity which is obtainable in the SW position to receive weak medium Wave broadcast signals. Also, if with said switches 8, 9, I0 in the SW position, strong signals are received which produce dirstortion due to thevreflex connection, said distortion may be avoided by moving said switches to the MW position therebyv removing the reiiexed.

connections.

The circuit arrangement shown in Figure 1 has the disadvantage that the I. F. potential difference across the primary winding I5 of the vtransformer I2 is applied across the input circuit of the output valve (not shown) and necessitates the use of the radio frequency choke 6 I.

The arrangement illustrated in Figure 2 however removes this disadvantage by providing the switch I5 with an additional section IIIziv and arrangingthe circuit connections to the individual sections Iilzand Ica so that, in the SW position of theswitch, the relative positions of the winding I5and the anode resistor 52 in the output circuit of the valve 21 are reversed to those shown lfor'the corresponding position of the switch I0 in Figure 1.

In Figure 2 the R. F. bypass condenser 53 is removed from the simple shunt connection across the anode resistor 52, (as shown in Figure 1) and is now connected between the primary coil I5 and earth 48, thus improving the fidelity of the receiver when the respective switches are in the MW position.

On account of the new connection of the anode resistor 52 in Figure 2, the desired coupling between the coils I4 and I5, when the switches are in the MW position is effected by means of the coupling coils 320., 33a and the double link connecting leads 34, 34a. This necessitates the use of an additional section 5a in the switch 9 so that both leads 34, 34a may be broken in the SW position of the switches. The coupling coils 32a and 33a may consist of a few turns of wire magnetically coupled to the tuned coils I4, I5 respectively.

With the exception of the diiferences outlined above, the arrangement of the circuit of Figure 2 and the method of operation agrees with that already described in connection with Figure 1.

What I claim is:

1. A radio receiver comprising in combination, a source of modulated carrier frequency energy, a demodulating detector having an input and an output circuit, an amplifying Valve having an input and an output circuit, means for coupling the input of the said amplifying valve to the output of said demodulating detector to receive audio frequency energy therefrom, a rst radio frequency coupling transformer having a primary and ay secondary winding, the primary winding being connected to receive energising potentials from said source of modulated carrier frequency, a second radio frequency coupling transformer having a primary and a secondary winding, the latter secondary winding' being coupled to the input ofthe said demodulating detector, and a plurality of switching devices, mechanically i linked together for simultaneous operation, constructed and. arranged, in one operating position, to connect the secondary of said lirst radio frequency transformer to the primary of said second radio frequency transformer through a common coupling impedance, and in a second operatingI position, to connect the secondary winding of said first mentioned transformer to the input of said amplifying valve and to connect the primary winding of said second radio frequency transformer in series with the output circuit of said amplifying valve.

2. A radio receiver as claimed in claim 1, adapted for operation in a plurality of bands, characterised in that the said switching devices are mechanically linked with a wave range selector switch for simultaneous operation therewith, the arrangement being such that the said second operating position of said switching devices coincides with the selection of the short wave range by said wave range selector switch.

3'. A multi-band modulated-carrier signal receiver tunable over a high-frequency band and one or more low-frequency bands comprising, a multi-band carrier-frequency selector system, an amplifier, a signal-translating stage, first and second transformers, a link circuit coupling said transformers and means for selectively coupling the output of said selector system to said translating stage, in the order named either through said rst transformer, said link circuit, said second transformer and said amplifier and for simultaneously conditioning said selector system for operation in said .low-frequency. band o r bands, or through. said rst transformer, 'said amplifier, said second transformer and saldampliiier and for simultaneously conditioning said selector system for operation in said high-frequency band. l

4. A multi-band modulated-carrier signal receiver tunable over a high-frequency band vand one or more low-frequency bands comprising, a multi-band carrier-frequency selector system, an amplifier, a demodulator, a signal-translating stage, first and second transformers, a link circuit coupling said transformers, and means for Vselectively coupling the output of said selector system to said translating stage, either through said first transformer, said link circuit, said second transformer, said demodulator and said amplifier and for simultaneously conditioning said selector system for operation in said low-frequency band or bands, or through said first transformer, said amplifier, said second transformer, said demodulator and said amplifier and for simultaneously conditioning said selector system for operation in said high-frequency band.

5. A multi-band superheterodyne signal receiver tunable overy a high-frequency band and one or more low-frequency bands comprising, a multi-band carrier-frequency selector system, an amplifier, a demodulator, a signal-translating stage, iirst and second coupling means, switch means for selectively conditioning-said receiver for operation in any of said bands, switch means for selectively coupling the output of rsaid selector system to said translating stage, in the order named either through said first coupling means, said second coupling means, demodulator and amplifier for operation in said low-frequency band or bands, or through said iirst coupling means, said amplifier, said second coupling means, demodulator and amplifier for operation in said high-frequency band, and unicontrol means for operating said switch means.

ALFRED LEONARD GREEN.

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