US2173145A - Single side-band transmitter - Google Patents

Description

3 Sheets-Sheet 1 W. H. WIRKLER Filed Nov. 26, 1937 Sept. 19, H939.'

SINGLE SIDE-BAND TRANSMITTER Sept. 19, T939. w. H. WIRKLER 2373,145

SINGLE SIDE-BAND TRANSMITTER FiledJlQLi. 1957 3 Sheets-Sheet 2 UUU Sept. 19, H939. w. H. wlRKLER I SINGLE SIDE-BAND TRANSMITTER Patented Sept. 19, 1939 STATES PATENT OFFICE Walter H. Wirkler, Garnavillo, Iowa, assigner to Collins Radio Company, a corporation of Iowa Application November 26, 1937, Serial No. 176,670

2 Claims.

This invention relates to a single side band transmitter, and more particularly to means for generating a high frequency electrical wave, varying in accordance with a low frequency signal wave, comprising only a single side band.

One feature of this invention is that it enables a signal to be readily and simply transmitted by a high frequency wave comprising only a single side band; another feature of this invention is that it does not necessitate the use of band-pass filters for higher radio frequencies; yet another feature of this invention is that it does not require an excessive number of modulators and carriers of different frequencies to achieve the desired single side band at high radio frequencies; a further feature of this invention is the generation of the desired single side band high frequency electrical wave by the direct combination of audio frequency signal waves with a pair of carriers of the frequency to be transmitted; still another feature of this invention is the provision of simplified means for securing a plurality of audio frequency signal waves having a predetermined desired phase relationship with respect to each other; other features and advantages of this invention will be apparent fromthe following specification and the drawings, in which:

Figure 1 is a schematic diagram of one form of my invention; Figure 2 is a circuit diagram of a transmitter embodying the form of my invention shown schematically in Figure 1; and Figure 3 is a schematic diagram of another form of my invention.

The present invention provides means for gen- J erating a single side band output wave directly at the nal desired radio frequency without the use of tuned circuits for separating out or rejecting one side band at this frequency. In order to accomplish this, a plurality of modulators are supplied with carrier waves of the desired radio frequency, these carriers all being identical (that is, of the same frequency, wave form, etc.) but having a predetermined phase relationship with respect to each other, and with modulating or signal waves, also identical but having a predetermined phase relationship. The output of the modulators is then combined, and if the phase relationship is properly chosen, one set of side bands, as the inferior side bands, is cancelled out upon such combination. Where it is desired to eliminate the carrier as well as one side band, balanced modulators may be used.

It is thus apparent that in order to secure the desired single side band output with two modulators it is necessary to supply the modulators with identical frequency carrier waves ninety degrees out of phase, and with identical audio or signal waves also ninety degrees out of phase. It

is a comparatively simple matter to split a high frequency carrier Wave into two portions, one 5 having the normal phase, let us say, and the other being displaced ninety degrees with respect thereto. This may be done by a simple phase shifting network, since the frequency is constant. It is a much more diilicult matter, however, to secure a signal wave exactly ninety degrees out of phase with another identical signal or audio frequency wave, since here the frequency varies through a range of quite a few octaves, as for example, from thirty to five thousand cycles per second. Not only does the fundamental tone vary through this range, but also the usual speech or music transmitted contains not only the fundamental but many higher order harmonics, and these must be maintained in their proper phase relation during the shift in order to prevent distortion.

I have found that the desired ninety degree phase shift in the audio frequency wave can be achieved by combination of the audio waves with two carriers ninety degrees out of phase. To state this more fully, the audio frequency wave is rst combined with a relatively low frequency carrier in a balanced modulator to provide a pair of side bands having a frequency several times that of thel signal wave; one of these side bands is then eliminated by any convenient method; the side band thus produced is then split into two portions and recombined with two carriers, these carriers being ninety degrees out of phase with each other; and at the same time the carriers are eliminated, as by performing the combination in a detector, so that the resultant output is, in one case, the original audio frequency wave, and in the other case an identical o wave ninety degrees out of phase therewith. Another side band is produced by this combination, of course, but it would be of a frequency many times that of the audio signal, and it is very easily eliminated, as by a low pass filter.

In the particular embodiment of this invention illustrated schematically in Figure 1, a microphone l0 is adapted to translate sound waves into an audio frequency electrical signal wave, which may be then amplified by the audio amplifier Il and passed through the low pass filter l2 to eliminate frequencies over five thousand cycles, in accordance with conventional practice. This signal wave is then passed into a balanced modulator I 2, where it is combined with a carrier Wave generated by the low frequency oscillator I4 to produce a pair of side bands, one of these side bands being cancelled or eliminated by the band pass filter I5. For the purpose of convenience in explanation, let us assume that a pure two thousand cycle per second note is received by the microphone, and a twenty-thousand cycle per second low frequency carrier generated by the oscillator I4. The two side bands which would comprise the output of the modulator I3 would then be eighteen thousand cycles per second and twenty-two thousand cycles per second, respectively. Assuming the band pass filter I5 eliminates the superior side band, the eighteen thousand cycle side band would be introduced to the detector I8, and at the same time the twenty thousand cycle carrier from the oscillator I4 would be reintroduced. The output of detector I8, therefore, would be a two thousand cycle wave and a thirty-eight thousand cycle wave, the latter being very easily eliminated by the low pass filter I1. At the same time, a portion of the inferior or eighteen thousand cycle side band would be diverted to the other detector I8, where it would be combined with a reintroduced twenty thousand cycle carrier from the oscillator I4, this latter carrier passing through the phase shifting network 29 and being ninety degrees out of phase with the carrier supplied to the detector I6. Again the output of the detector I8 would be a two thousand cycle wave and a thirty-eight thousand cycle wave, the higher frequency wave being eliminated by the low pass filter I9. Thus the input to the two audio amplifiers 20 and 2| would be a two thousand cycle signal wave identical with that received by the microphone I0, but the two waves being ninety degrees out of phase with each other. These two `signal waves would then be introduced into the balanced modulators 22 and 23, where they would be combined with two.high or radio frequency carrier waves, these waves also being ninety degrees out of phase. These latter carriers would be produced by the oscillator and amplifier 24, the carrier waves being introduced directly into modulator 23, and passed through the network 25 before being introduced into the modulator 22 in order to secure a ninety degree phase shift. The output of the' two balanced modulators would then be arithmetically combined, as in the wire 26, passed through a linear amplifier 21, if desired, and supplied to the aerial 28 for transmission. The radio frequency wave transmitted by the aerial 28 will thus be seen to consist only of a single side band, the other side band and the carrier band being eliminated in the construction illustrated in this particular figure.

A wiring diagram of the particular embodiment of my invention described in general in Figure l is shown in Figure 2. Here the sound wave received by the microphone 30 is converted into an electricalwave, passed through a simple audio amplifier 3| and low pass filter 32, and developed in the primary of transformer 33. The secondary of this transformer is connected in pushpull to one set of grids of the tubes 34 and 35 so arranged as to provide a 'balanced modulator. An oscillator tube 36 with a plate circuit comprising a tuned portion 31 develops a low frequency carrier wave and supplies it, through the coupling condenser 38, to a pair of grids of the tubes 34 and 35, these latter grids being connected in parallel. The plates of the tubes 34 and 35 are connected in push-pull to the primary of the transformer 39, plate voltage being supplied in accordance with conventional practice, as by the battery 40 connected to a center point of such primary. The connection of the carrier wave grids in parallel and the plates in push-pull results in elimination or cancellation of the carrier from the current developed in the secondary of the transformer 39, so that this current consists only of the two side bands, one then being eliminated by a conventional band pass filter 4I, the single side band remaining being supplied to a pair of grids, arranged in parallel, of the detector tubes 42 and 43. That is, these tubes are of such a type, and are supplied with the necessary bias voltage as by a battery 44, that the tubes act as detectors in accordance with conventional radio practice. Part of the carrier wave generated by the oscillator tube 36 is supplied to the detectors by the lead wire 45, this wire being connected directly to a grid of the tube 43, and to a corresponding grid of the tube 42 through a phase shifting network comprising, for example, avariable inductance 48 and variable condensers 41 and 48, these latter being so adjusted that at the particular carrier frequency being used the wave supplied to the tube 42 is exactly ninety degrees out of phase with that supplied to the tube 43. The outputs of these two tubes 42 and 43 are connected, respectively, to conventional low pass lters 49 and 50, the plate circuits including impedances, coupling condensers, and a source of plate voltage. As discussed above, the outputs of these filters 49 and 50 would then be audio frequency or signal waves ninety degrees out of phase with each other.

The signal wave from the filter 49 is coupled, through a conventional audio amplifier 5I and reversing switch 52, to a transformer 53, the secondary of this transformer being connected in push-pull relation to the grids of a pair of tubes 54 and 55, these latter tubes being so arranged as to form a balanced modulator. Similarly, the output of filter 50 passes through a conventional audio amplifier 56 and is coupled in push-pull to the grids of tubes 51 and 58 forming another balanced modulator.

A high frequency carrier wave, ofthe radio frequency at which the transmitter is designed to operate, is generated by the oscillator 59, which may be controlled by the crystal 60. This high frequency carrier wave may be amplified, as by the tube 6I and its associated circuits, and coupled in push-pull to the same grids of the tubes 54, 55, 51 and 58, the high frequency wave being prevented from getting back into the audio ampliers by appropriate choke coils. The high frequency carrier is transferred to these last-mentioned tubes through an intermediate circuit containing a phase shifting network 62, so that the carrier supplied the tubes 51 and 58 is ninety degrees out of phase with that supplied the tubes 54 and 55.

The plates of all four of these last modulator tubes are connected in parallel to the lead wire 63, and coupled in a conventional manner to a linear amplifying tube 64, which tube supplies its output to the aerial 65. When these last modulator tubes are properly balanced the carrier wave is eliminated and one of the side bands cancelled out, as previously explained, so that the wave transmitted by the aerial 65 consists only of a single side band. Where it is desired to transmit the carrier along with the single side band it is only necessary to unbalance one of the modulators.

My system of developing a single side band output wave is particularly capable of ready adaptation to the use of an inverted side band, as is frequently done in transoceanic radio telephony. Referring more particularly to the embodiment of my invention illustrated diagrammatically in Figure 3, the audio wave developed in microphone 'l0 is passed through an audio amplifier 'H and low pass lter 12 removing everything above five thousand cycles, for example. This audio frequency signal wave is then supplied to the balanced modulator 13, where it is combined with a ninety kilocycle carrier wave from the oscillator 14. Again assuming, for convenience, a pure two thousand cycle signal note, the output of the balanced modulator 13 would be eighty-eight and ninety-two kilocycle side bands, and these bands would be split into two portions and supplied to the detectors l5 and 16. In this embodiment, however, instead of being combined with the reintroduced original carrler, a one hundred kilocycle wave is supplied by the oscillator 11, phase shifting network 18 resulting in the carrier supplied to the detectors 15 and 16 being ninety degrees out of phase. The output of detectors 'l5 and 16 would then comprise an eight thousand cycle wave, a weak four thousand cycle Wave (if the one hundred kilocycle carrier is strong the beat note between the two side bands would be quite weak compared to the beat notes generated by a combination of either side band and the carrier), and a plurality of waves having a frequency above ten kilocycles, the latter being eliminated by the low pass filters 19 and 80 designed with a cut-off at ten kilocycles. Thus the signal wave supplied to the audio amplifiers 8| and 82 would be a low frequency wave, eight thousand cycles, being inverted or changed from the original two thousand cycle note (the very weak four kilocycle wave can be neglected for practical purposes, or eliminated by a band pass filter immediately following the modulator 13, if desired). These two eight kilocycle signal waves would then be introduced into the balanced modulators 83 and 84 and combined with a radio frequency carrier wave supplied by the oscillator 85, in one case through the phase shifting network 86. In order to prevent any possibility of the combination of the outputs of the balanced modulators 83 and 8d affecting these modulators, or unbalancing them in any way, blocking means is inserted in the form of linear amplifiers 81 and 8B before the side bands are combined in the lead wire 89, where the carrier is eliminated and one side band cancels out. The remaining side band may then be again amplified by the linear amplifier 90 and transmitted through the aerial 9|.

In this case, of course, provision would have to be made at the receiving end for again inverting the signal so that intelligible signals might be received. This may be readily done by merely having the reintroduced carrier at the receiving end ten kilocycles different in frequency from the carrier generated by the oscillator 85. That is, assuming that the carrier generated by the oscillator 85 had a frequency of 720 kilocycles per second, and the signal wave supplied to the modulators 83 and 84 was 8 kilocycles, the side band transmitted through the ether would be 728 kilocycles, assuming the inferior side band to be cancelled out. If the carrier introduced at the receiver and adapted to heterodyne with the received side band had a frequency of 730 kilocycles, the resultant signal would be two thousand cycles, corresponding to the original sound wave received by the microphone 10.

While I have described and claimed certain embodiments of my invention, it is to be understood that it is capable of many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention as disclosed in the appended claims in which it is my intention to claim all novelty inherent in my invention as broadly as permissible in view of the prior art.

I claim:

1. The method of generating a plurality of Varying identical inverted low frequency electrical waves having a predetermined phase relation with each other, comprising combining a carrier wave with a Varying low frequency wave, eliminating said carrier and one of the side bands produced by such combination, combining a portion of the remaining side band with a carrier wave of a frequency different from that of the aforesaid carrier wave and another portion with another carrier wave identical with said carrier wave of different frequency but out of phase with respect thereto the predetermined amount.

2. Means for generating a plurality of varying identical inverted low frequency electrical waves having a predetermined phase relation with respect to each other, comprising a source of varying low frequency waves, an oscillator, a modulator connected with said source and said oscillator, and means for selecting one of the side bands produced in said modulator; a second oscillator operative at a frequency dlferent by a predetermined amount from the frequency of operation of the aforesaid oscillator, means for producing separate oscillations in phase quadrature from the last said oscillator, and separate means for combining the selected side band with each of said oscillations for producing varying identical low frequency waves in phase quadrature and of frequency different from the frequency of the varying low frequency waves from said source dependent upon the difference in frequency of operation of said oscillators.

WALTER H. WIRKLER.

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