US2036022A - Method of and means for receiving signals - Google Patents

Description

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AT TO RNEY INVENTOR JAMES w. CONKLIN BY 7r% W36. J. W. CONKLIN METHOD OF AND MEANS FOR RECEIVING SIGNALS Filed Jan. 25, 1954 March 3L Patented Mar. 31, 1936 UNITED STATES PATENT OFFICE METHOD OF AND MEANS FOR RECEIVING SIGNALS tion of Delaware Application January 26, 1934, Serial No. 708,373

4 Claims.

This invention relates to a method of and means for supplying local carrier to a received signal modulated Wave to reduce the effects of U selective fading on heavily modulated signals 5 and the effects of waxing and waning in carrier strength on the signal energy resulting from demodulation of the wave.

More in particular this invention relates to a simplified method of and means for locally supplying a synchronized wave to supplement or enhance the carrier wave in a modulated telephony signal. The artificial or supplied carrier is of a steady value and preferably is stronger than the modulated carrier and therefore reduces the distorting effect of selective fading and in general improves the quality of the signal resulting from demodulation by reducing the apparent percentage modulation.

Systems have been known in the prior art which filter out the carrier from the incoming signal modulated wave, amplify the said carrier separately and recombine it with the original energy to replace or supplement the normal carrier. In general such systems work very well but often necessitate careful adjustments of tuning controls and require very selective filtering devices. Such filtering devices are necessarily costly and from a practical standpoint may not be applicable to standard broadcast receivers as known today. In these known systems crystal filters are usually used, said filters being the most selective type available for this purpose. The filters, of course, have the drawback that their fixed characteristics limit their use to one particular wave or to a particular intermediate frequency where a superheterodyne type receiver is used. With present day broadcast practice of using high percentages of modulation on the carrier, even small amounts of selective fading increases the appar- 40 ent percentage modulation of. the wave above one hundred percent, and consequently introduces into the signal bad distortions. The high percentage modulation will, of itself, produce noticeable second harmonic distortions in the general run of detector circuits in use even without selective fading. For the above reasons where high quality of reproduction is desired, some device should in accordance with my invention be incorporated in the receiver to supplement the received carrier wave.

In accordance with my invention, I provide this supplemental carrier by means of a vacuum tube oscillator to which a small amount of the received signal modulated wave is fed prior to detection thereof for the purpose of holding the local oscillator in step. The local oscillator is tuned as closely as possible to the carrier frequency, in which state a very little energy is required to entrain it or to lock it in step as to frequency and phase with the incoming carrier. 5' In practical broadcast receiver the tuning for such an oscillator can be very easily ganged with the other tuning controls. The output from the oscillator is then impressed on the demodulator circuit to supplement therein the incoming 10 carrier. The operation of such a device requires no technical understanding of its principles or extraordinary skill, as it would be tuned for zero beat, an audible squeal being produced between the incoming signal modulated carrier and the local oscillations when the receiving circuits and the local oscillator circuits are not in resonance. Many operators of broadcast receivers now unwittingly tune receivers so that part of the signal spectrum falls outside the band of the receiver and they therefore obtain poor quality signals. The squeal mentioned above, which appears in tuning the present invention, facilitates such tuning and insures perfect tuning of the receiver which results in good quality signals.

A receiver including the novel features of my invention as outlined above will also increase the audio output of the receiver, at the same time decreasing the distortion. While the effective percentage of modulation is reduced the audio output of the demodulator is dependent on the product of the carrier wave and side band amplitudes and the effect of the local oscillator is to increase the amplitude of the apparent carrier.

The novel features of my invention have been pointed out with particularity in the claims appended hereto.

The nature of my invention and the manner in which the method thereof is carried out will be better understood from the following detailed description thereof and therefrom when read in connection with the drawing and in which,

Figure 1 shows for purposes of illustration only the essential elements of a receiver including a radio frequency amplifier and a demodulator arranged in accordance with my present invention; while,

Figure 2 shows a receiver of the heterodyne type which includes the novel features of my invention.

Referring to Figure l of the drawing, A indicates an aerial system which may pick up a signal modulated wave from a natural medium. The aerial system A may be replaced by an incoming line on which the modulated Wave has been impressed. The signal wave from A is impressed on the input circuit of a conventional radio frequency amplifier B. The signal modulated wave is amplified in B and impressed on a second radio frequency amplifier C and from the output of the radio frequency amplifier C is impressed on the input electrodes of a demodulator D. The amplified carrier wave is also impressed on the inductance I connected between the control grid and anode of the local oscillator O. The inductance I of the oscillator O is tuned by condenser K to a normal frequency substantially equal to the mean frequency of the received wave. The oscillator 0 produces oscillations which due to the controlling effect of the energy from C acting through the inductance I on O are in synchronism as to frequency and phase with the signal wave. These oscillations produced by O are then impressed in the desired amount on a circuit of the detector D. Preferably they are impressed on the control grid, cathode circuit of the detector D as shown. The radio frequency amplifiers and/or detector, and/or the oscillator may be tuned by a common control as shown. The detector output may supply the demodulated signal energy to any utilization circuit.

The energy impressed on the input of the detector should be of such a value as to not overload the detector in action and may be either fixed or may be adjustable in common with the manual volume control of the radio frequency amplifier. The second method may give a somewhat greater range in output volume.

In operation the oscillator circuit connected with 0 will act as a very sharp filter for the carrier frequency wave and at the same time as an amplifier and an amplitude limiter of the wave produced. The energy for entraining the oscillator 0 may be diverted from the wave received at any point but is preferably diverted from a point ahead of the detector since at this point the incoming carrier power will be at its greatest value and will be sufficient to insure locking of the oscillator in step.

It may be possible to accomplish in a manner similar results by a receiver of the autodyne type, thus eliminating the use of an extra oscillator. However, when the detector is also used as an oscillator as in the autodyne type of receiver the detector tube is automatically loaded up so that poor quality audio signal results. Moreover, due to this overloading of the tube the detecting action thereof is rendered less sensitive. Furthermore, the regeneration in the oscillation producing circuits introduces considerable frequency distortion in the detector by sharpening the tuning of the detector circuit and thereby over-emphasizing the lower frequencies.

My method of and means for eliminating distortion resulting from over-modulation of a carrier wave followed by selective fading or due to the effects of waxing and waning in carrier strength is applicable to receivers of the heterodyne type. For example, a receiver circuit as shown in Figure 2 may be used. In Figure 2, A is the aerial system. B is a radio frequency amplifier. D is a first detector. 0 is an oscillation generator which supplies oscillations to beat with the signal modulated wave to produce intermediate frequency. IFA is an intermediate fre quency amplifier and D is the second detector. The radio frequency amplifier, the first detector, the oscillator and the intermediate frequency amplifier may be conventional. In Figure 2, however, the oscillator O is normally tuned to a frequency equal to the difference frequency obtained by beating oscillations from O with the incoming signal. The intermediate frequency energy from the output of IFA is fed to the inductance I in the oscillations producing circuits of O and entrains O to produce oscillations which are synchronized as to phase and frequency with the oscillations of intermediate frequency in the output of IFA. Energy from the oscillator O of the de sired amplitude is impressed on the input of the second detector D. The operation of the demodulator of Figure 2 will be readily understood from the statement of operation of the receiver of Figure 1 and need not be repeated here. The signals from the output of the second detector tube may be utilized in any manner.

In both arrangements I find no difliculty in holding the oscillators O in step or in synchronism with the incoming wave or the intermediate frequency wave. In fact it is nearly impossible to keep the oscillators from pulling into step with a signal which is spaced from the frequency to which the oscillators are tuned by a few hundred cycles providing there is any appreciable coupling between the signal and the oscillator.

In some cases the coupling of the oscillator circuit too closely to the detector circuit may have the effect of narrowing too much the frequency band. In this event the coupling may be loosened in any manner. For example, the oscillator may be coupled to the detector by way of a coupling tube rather than inductively as shown or any satisfactory coupling arrangement may be utilized.

Having thus described my invention and the operation thereof, what I claim is:

l. The method of reducing the effect of attenuation of a signal modulated wave during the transmission thereof on the signal component which results from demodulation of said wave which includes the steps of producing local oscillations in synchronism as to phase and frequency with a received Wave, combining said oscillations with the received wave, and demodulating the combined energy.

2. Means for demodulating a signal modulated wave including means for eliminating the effect of over-modulation of the wave or of waxing or waning of the strength of the wave on the signal component resulting from said demodulation, comprising signal modulated wave receiving means, a local oscillator normally tuned to the frequency of the wave received, a coupling circuit interposed between the local oscillator and the output of the wave receiving means to feed received waves to the local oscillator to entrain the same, and a demodulator coupled at its input to the local oscillator and to the output of the wave receiving means.

3. Means for demodulating signal modulated waves including means for eliminating the effect of over-modulation of the wave or of waxing or waning of the strength of the wave on the signal component resulting from said demodulation, comprising signal modulated wave receiving means, a local oscillator comprising a thermionic tube having its electrodes connected in circuits normally tuned to the frequency of the wave received, and energized to tend to produce oscillations at said frequency, means for impressing oscillations from the output of the wave receiving means on the circuits of said oscillator to excite and entrain the same, and a demodulator coupled to the local oscillator and to the output of the wave receiving means.

4. Means for demodulating signal modulated waves including means for eliminating the effect of over-modulation of said waves or the effect of waxing and waning in the amplitude of said waves on the signal component resulting from said demodulation comprising a signal modulated carrier wave responsive means, a source of local oscillations, a demodulator coupled to said signal modulated carrier wave responsive means and to said source of local oscillation, an intermediate frequency amplifier coupled to said demodulating means, a local oscillator comprising a thermionic tube having its electrodes interconnected by circuits to produce oscillations, means for tuning said circuits to a frequency substantially equal to said intermediate frequency, a second demodulator, said second demodulator having its input electrodes coupled to said last named oscillator and to the output of said intermediate frequency amplifier, and means for impressing energy from said intermediate frequency amplifier on said last named oscillator to entrain the same to produce oscillations which are synchronized in frequency and in phase with the oscillations in the output of said intermediate frequency amplifier.

JAMES W. CONKLIN.

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