US2129020A - Modulated carrier wave receiver - Google Patents

Description

` Sept. 6, 1938.

I IMODULATED CARRIER WAVE RECEIVER F. M. G. MURPHY Filed April 8, 193e ATTORNEY Patented Sept. 6, 1938 UNITED STATES PATENT oEEi-eE 2,129,020 MODULATED CARRIER WAVE RECEIVER Francis Money Graham Murphy, Chelmsford, England, assigner' lto Radio Corporation of America, a corporation of Delaware Application April 8, 1936, Serial No. 73,286 In Great Britain April 10, 1935 Claims. (Cl. Z50- 20) Thisl invention relates to modulated carrier through which the said signals pass must allow Wave receivers and more particularly to receivers for an additional range determined by the ambit. for receiving energy transmitted from a so-called New, if in addition to ther single sideband transsingle-sideband transmitter. mited some proportion of the equivalent carrier 5 In systems of single sideband transmission, it is frequency is also transmitted, this carrier will, 5 well known that for satisfactory reception it is after the first detection stage, be subject to the necessary to supply local oscillatory carrier ensame ambit as the signals at that point, and ergy of the same frequency (though not necesprovided that this ambit does not approach the Sally the Same phase) as the equivalent carrier value of the frequency interval between the employed for the transmission and owing to the carrier and that side of the signal frequency band 10 di-fculty in practice in securing the necessary nearest thereto, it will be possible to select the close agreement in frequency between the local carrier component from the signal band after the oscillations Vand the transmission carrier (an first detection stage.` The thus selected (inter` agreement of twenty periods per second or closer mediate) carrier can after suitable amplifica-- is required if degradation of the signals is to be tion be applied to the second detector and may 15 avoided) it has been proposedto supplement the constitute the second set of local oscillationsin transmission by the additional transmission of other words, a separate second local oscillator the equivalent carrier frequency or some fremay be dispensed with-so that the required quency bearing a predetermined relation thereto, agreement between the transmitted carrier and f-' the supplementary transmission being at a lower the sum' or difference of the two local oscillations 20 power than the main single si-deband transmisis automatically obtained. In practice,`however, sion. f there is the serious difficulty that a highdegree The present invention provides improved modof frequency discrimination is necessary in order ulated carrier wave receivers suitable for coopto select out the intermediate carrier obtained erating with transmitters transmitting a single after the first stage of detection while further 25 sideband and supplemental carrier frequency the pass band of the intermediate frequency filter energy as just described. More specifically, the (for passing the single intermediate frequency invention provides an improved single sideband sideband) must be increased above that of the receiver for use in cooperation with a transmit'- signal range by the amount of the ambit of the ter as above set forth and which is ofthe so-called equivalent carrier at the point in question. 30 frequency changing type, i. e.,yof the type The present invention enables these difliculwherein the received modulated carrier frequency ties to be avoided. f is changed to another modulated carrier fre- According to this invention, a single sideband quency in a first detector before demodulation. receiver of the frequency changing type, and

In order that the invention may be the better for use for recevng from a single sideband transunderstood consider rst a known type of somitter which also transmits supplemental carrier called double detection single sideband receiver energy comprises at least two local oscillators wherein frequency changing is resorted to. In one of-which supplies its output to the demoduthis known type of receiver, there is a local oscillating detector of the receiver and the other of Li0 lator provided for each stage of detection and which is used to beat received supplementary car- 40 these local oscillators are so designed that the sum rier frequency to a frequency which is mixed or difference of the two local oscillation frewith local oscillations from the first mentioned` q uencies is equal to the equivalent carrier. In local oscillator to produce a further frequency; such a receiver, the local oscillators must be such which is in turn mixed with oscillations from the' that the sumor difference between the two local second mentioned local oscillator to produce a oscillation frequencies remains to a close degree still further' frequency differing from the freas predetermined. The frequency of the signals quency of the rst mentioned local oscillator by obtained in the output from the first detector an amount equal to the received carrier frestage will depend upon the degree of constancy quency, and this still further frequency is used as 5,0 of the difference between the first local oscillator the local oscillation frequency for the frequency 50..

frequency and the equivalent transmitted carchanger detector of the receiver. The rst menrier frequency and the said signal frequency will tioned local oscillatorshould be of a frequency accordingly have an ambit determined by this outside the rangeof frequencies occupied' by the degree of constancy whence it follows that the single sideband to be recei-ved.V

ffilter or other frequency discriminating circuit The invention is `illustrated in the accompany- 55;

ing drawing which shows in block diagram form one embodiment thereof.

Referring to the drawing, the receiver installation therein shown comprises the usual receiving aerial I and high frequency selection and amplifying means 2, said means being followed in cascade by a first (frequency changing) detector 3, an intermediate frequency amplifier and filter 4, a second or demodulating detector 5 and such audio frequency amplification at 6 as may be required. Energy from the output end of the apparatus at 2 is fed to a first control detector 'I which is followed in cascade by a first control filter 8, a second control detector 9, a second control filter I0, a third control detector II and a third control filter I2. Output from the third control filter I2 is fed as local oscillations to the first (frequency changing) detector 3. Two sources I3, I4, of local oscillations are provided, the source I3 supplying local oscillations to the second control detector 9 and to the demodulating detector 5 of the receiver, and the other source I4 supplying local oscillations to the first control detector 'I and to the third control detector II. For convenience in description, call the incoming sideband fIiS (corresponding to an audio frequency S) and the incoming supplemental carrier frequency ,fI and let the frequencies of the first and second local oscillators I3, I4, be termed f3 and f2 respectively. f3 is selected outside the range fIi-S. One of the frequencies in the output circuit of the first control detector 'I will accordingly be fI-f2 and this is selected by the first control filter 8 and passed on to the second control detector 9 whichalso receives from I3 local oscillations of frequency f3. Therefore, the output from the second control detector will contain the frequency fl f2 -l-f3 and this is selected by the second control filter I and passed to the third control detector II which receives from I4 local oscillations of frequency f2. The third control detector will accordingly give, inter alia, an output frequency fbi-f3 and this is selected by the third control filter I2 and passed to the first L (frequency changing) detector 3 of the receiver.

The output from this detector 3 will accordingly contain the band f3iS and this is selected by the intermediate frequency filter 4 and passed on to the second or demodulating detector 5 which, since it also receives from I3 local oscillations of frequency f3 will give a demodulated output consisting of the modulation signals iS.

Now with this arrangement, it will be observed that the difference between the local oscillation frequencies applied to the first (frequency changing) and second (demodulating) detectors 3 and 5 respectively, is always equal to the carrier frequency fl despite such ambit of these frequencies as may be experienced, while furthermore, the ambit of the intermediate frequency signals will only be that of the local oscillation frequency f3. The oscillator supplying the frequency f3 operates at a relatively low frequency of the same order as the intermediate frequency and accordingly may readily be constructed to have a high constancy. This involves that the pass band of the intermediate frequency filter 4 can be made small and a consequent gain in signal to noise ratio obtained. Furthermore, the necessary selection of the supplemental carrier fl from the sideband is a fairly simple matter for this selection is effected by the first control lter 8 at a point at which the frequency to be selected is fI-f2 and the sideband signal is fl i-S-f2. In other words,

there is a frequency interval S between the frequencies to be separated and the frequency fl f2 may readily be chosen at such a value that the percentage frequency discrimination is large and accordingly frequency discrimination fairly easy. There is, of course, a limit to the lowness of value which can be adopted for fI-f2 this limit being set by the ability of the second control lter I0 to discriminate between fI-f2-l-f3 and f3, there being also an upper limit set by the fact that it is necessary to discriminate by means of the said second control filter I 0 between the frequency fI-JZ-i-f3 and the frequencies 2f3 and 2fI-2f2. However, notwithstanding these limits it is a relatively simple matter as compared with known receivers of the type in question to effect the necessary frequency discrimination. It Will be noted that the discrimination required of the third control lter I2 is rather less than that of the high frequency amplifying and selecting means at 2.

Another important advantage of the receiver just described arises in connection with signal reception in systems in which large deliberate changes in frequency of the transmitted carriermay occur from time to time. Such changes are often made between different service hours in commercial radio telephone and other systems, and, of course, such changes must be met by receiver adjustments. In the described receiver, only three changes will be necessary, namely, change (at I4) in the value of f2, change of the pass range of the third control filter I2 and change in the tuning of the high frequency amplifying and selecting means at 2. The necessary changes in the third control filter I2 and in the high frequency amplifying and selecting means at 2 are such that they can readily be effected by gang controlled tuning means.

Receivers in accordance with this invention are well adapted for incorporation of so-called automatic gain control means for the sideband energy does not appear in the control part of the receiver after the first control filter 8 and accordingly the energy level at any point between the said first control filter 8 and the first (frequency changing) detector 3 can be employed to effect gain control, e. g., to control the amplification of a high frequency amplifier and/0r of an intermediate frequency amplifier as shown by the broken lines of the drawing.

What is claimed is:

l. A single sideband receiver of a frequency changing type and adapted for use in cooperating with a single sideband transmitter which also transmits supplemental carrier energy, said receiver comprising a high frequency selecting apparatus, a frequency changing detector and a demodulating detector connected to said high frequency selecting apparatus, a first and a second local oscillator having different frequency outputs, the rst local oscillator having its output coupled to said demodulating detector, and the second local oscillator having its output coupled to control detectorsI and control filters to beat a received supplementary carrier frequency to a frequency which is mixed with local oscillations from the first local oscillator to produce a further frequency which is in turn mixed with oscillations from the second local oscillator to produce a still further frequency differing from P the frequency of the first local oscillator by an amount equal to the received carrier frequency, the further frequency being employed as the local oscillation frequency for the at a detector.

received frequency 2. A single sideband receiver of the frequency changing type and adapted for use in cooperation with a transmitter which transmits a single sideband (fl :L-S) and supplemental carrier frequency (fl), said receiver comprising a local oscillator generating a frequency (f3) outside the band fl is: a second local oscillator generating a frequency (f2) high frequency selecting apparatus having an input and output circuit for selecting the incoming frequencies fl and fl iS; a cascade chain coupled to the output circuit and adapted to be energized therefrom and including, in the order stated, a frequency changing detector, a filter having an output pass range ,'f3iS, and a demodulating detector; a second cascade chain also energized from the output circuit of said selecting apparatus and including, in the order stated, a first frequency changing control detector, a filter having an output pass range fI-2, a second frequency changing control detector, a lter having an output pass range fl-fZ-l-f, a third frequency changing control detector, and a lter having an output pass range fi +f3; a circuit for applying the output from the last mentioned lter as local oscillation energy to the frequency changing detector in the first mentioned cascade chain; circuits for supplying energy from the first mentioned local oscillator to the second frequency changing control detector and to the demodulating detector; and circuits for supplying energy from the second mentioned local oscillator to the first frequency changing control detector and to the third frequency changing control detector.

3. A receiver as claimed in claim 1 and wherein the rst local oscillator is of a relatively 10W frequency of the same order as an intermediate frequency obtained by being connected from the output of the frequency changing detector of the receiver.

4. A receiver as claimed in claim 1 and comprising auto-matic gain control means operated in dependence upon carrier energy which is derived by connecting the automatic gain control means to a point in the circuit preceding the frequency changing detector of the receiver whereat sidepand energy is' not present.

5. A receiver comprising radio frequency se lecting and amplifying means having input and output circuits, a frequency changing detector and a control detector connected to the output circuit of said selecting and amplifying means, an intermediate frequency amplifier and lter, a demodulating detector and audio frequency output means coupled to said frequency changing detector, a control filter, a second control filter and a third control lter connected in series with said control detector, coupling means between said frequency control detector and said third control filter, two local oscillators having frequency outputs, the output of o-ne of said local oscillators being coupled to said first control detector, and the output of the other of said local oscillators being coupled to said second control detector and said demodulating detector.

FRANCIS MONEY GRAHAM MURPHY.

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