Single Sideband Am-fm Modulation System

Abstract

A sub-carrier signal is amplitude modulated. This sub-carrier AM modulated signal is then used to frequency modulate a radio frequency carrier. The radio frequency is then multiplied. One sideband in the modulated radio frequency signal is passed through a bandpass filter and then transmitted.

Parent Case Text

This application is a continuation-in-part of my prior copending application Ser. No. 825,485, filed May 19, 1969, now abandoned, of the same title.

Description

This invention relates to a single sideband AM-FM modulation system.

Due to the many growing uses of radio communication channels, there is a shortage of channels. In order to provide more channels,, it is necessary to make use of narrower bandwidths without sacrificing quality of transmission.

The present invention provides a modulation system using a very narrow bandwidth which occupies a very small portion of the radio frequency spectrum.

More particularly, the present invention provides a good quality single sideband modulation system in which the modulation is preformed at low power levels. The signal is then amplified and a single sideband is passed through a high Q bandpass filter and then transmitted.

The transmittal signal may be received and detected by a conventional radio type detector tuned to the center frequency of the single transmitted sideband.

Accordingly, a principal object of the invention is to provide a new and improved modulation system.

Another object of the invention is to provide a new and improved modulation system transmitting only a single sideband which would require a very small band width in the radio frequency spectrum.

Another object of the invention is to provide a new and improved combined AM-FM single sideband modulation system.

Another object of the invention is to provide a new and improved modulation means wherein the modulation takes place on low power levels.

Another object of the invention is to provide a new and improved single sideband AM-FM modulation means comprising a sub-carrier frequency signal generator, means connected to amplitude modulate said sub-carrier signal, a radio frequency signal generator, means to frequency modulate said radio frequency signal with said modulated sub-carrier signal, and bandpass filter means connected to pass one sideband of said modulated signal.

These and other objects of the invention will be apparent from the following specification and drawings, of which:

FIG. 1 is a schematic block diagram of an embodiment of the invention.

FIGS. 2, 3A, 3B and 3C diagrams illustrating the theory of operation of the embodiment of FIG. 1.

The modulating circuit shown in FIG. 1 is a new way to produce a radio signal that has single sideband characteristics occupying a very small space in the radio spectrum. This modulating circuit is made up of individual known circuits and the combination of these circuits produce a compatible single sideband signal that is only as wide as that required for a single frequency transmission.

Referring to FIG. 1, the intelligence which may be an audio signal is inserted in the microphone 1 which is connected to an audio amplifier 2, the output of which is connected to a diode modulator 3. The other input to modulator 3 is a sub-carrier signal from generator 4 which may be, for instance, 35 KHz. The output of the modulator 3 is then fed to sub-carrier amplifier 5, output of which is connected to reactance modulator 6, which is connected to modulate radio frequency oscillator 7.

An R.F. tripler 8 increases the frequency of the electron coupled oscillator 7 and some of this stage is fed into a mixer 12. A crystal oscillator 11 approximately 465 KHz away from the tripler frequency produces a beat at 465 KHz and this signal is fed into a discriminator 13 producing a DC voltage that is applied back into the electron coupled oscillator and is a means to further stabilize the electron coupled oscillator 7. A second output of the frequency tripler is coupled into additional frequency multipliers 9 and 10 which further increase the frequency to that desired. The linear amplifiers 14 and 15 are tuned to the sideband desired upper or lower. The output of these amplifiers is then fed into a high Q filter 16 having very narrow bandpass characteristics, for instance 5 KHz wide. The output of the filter is then amplified with linear amplifiers 17 and 18 to the power desired for transmission via the antenna.

The narrow high Q filter can be made with quartz crystals, resonant cavities or any other means for narrow bandpass.

The output of the radio frequency oscillator 7 has both amplitude and frequency modulation and this output is connected to frequency doublers 8,9,10. Therefore, all of the modulation takes place at low power level and the radio frequency is then multiplied by the multipliers 8, 9, 10. The multiplied modulated radio frequency is then amplified in linear power amplifiers 14, 15.

The output of the amplifier 15 is connected to a narrow bandpass filter 16, which passes for instance 5 KHz of the upper sidebands of amplifier 15, FIG. 2. The filter 16 determines the frequency to be amplified by amplifiers 17 and 18 and transmitted by the antenna 20.

The filter 16 is preferably of such design that it passes only a 5 KHz band S FIG. 2, of the spectrum generated by all the equipment before it, namely circuits 1 through 15, filter 16 may be composed of resonant cavities, lattice crystals or any other device with the proper bandpass characteristics.

FIG. 2 shows the output of the signal generated before the filter. The overall swing of the spectrum is shown with F1 as a reference. F2 is the portion of the spectrum that enters the filter band S and is on the high frequency side of F1. The deviation is adjusted so that energy will enter the filter as described below. The total deviation DF as shown is 350 KHz or KC and the 35 KHz with the audio impressed on it. The Filter S could be located for instance at a frequency of 28.140 KHz plus 1/2 DF or 28.315 KHz. This would permit energy to pass through the filter at 28.315 .+-. 2.5 KHZ. The spectrum when modulated by 35 KHZ can be moved by adjusting the discriminator 13 and positioned so that no 35 KHz side frequencies pass through the filter. Other methods for stabilizing the spectrum generated can be used to improve the system.

The output of filter 16 can be used for generating a signal at a lower frequency by hetrodyning or beating down an further filtering will permit transmission of a signal with very narrow bandwidth such as in the AM broadcast band.

The oscillator 7 when frequency modulated by the 35 KHz only produced by the oscillator 4, generates a series of carriers every 35 KHZ and one of these carriers could drift over and pass through the filter 16 and be amplified by 17 and 18, and transmitted by the antenna. Transmission of any of these unmodulated 35 KHz side frequencies is not desirable and adjustment of the DC voltage generated by the discriminator 13 can be made to position the 35 KHz side carriers so none fall into the filter frequency.

The overall stability of the 35 KHz side frequencies generated should be such that no one side carrier drifts into the filter.

Positioning of the side carriers is shown in FIG. 3. FIG. 3A shows a typical spectrum of a radio frequency when frequency modulated by for instance 35 KHZ. FIG. 3B is a detail of the side frequencies and FIG. 3C is a detail showing the positioning of frequencies when a 1250 KHz signal modulates the 35 KHz.

When a radio wave is frequency modulated, side frequencies are produced varying in amplitude as shown by Bessel's analysis. FIG. 3A shows a typical spectrum of a carrier frequency modulated by a 35 KHz signal. A 5 KHz filter is positioned between two of these unmodulated side frequencies an no signal will pass through the filter if the stability of the system is maintained. FIG. 3B shows the position of the filter in greater detail. When modulation at audio frequencies is applied to the 35 KHz, side frequencies are again produced in the Bessel's function arrangement and energy of any frequency below 5 KHz at the filter frequency will pass through and be amplified by amplifiers 17 and 18 and transmitted by antenna 20.

The signal transmitted by the antenna 20 can be detected by the majority of detection system as it varies both in amplitude and frequency.

The adjustment of the modulating system is not critical and using state of the art techniques and components, a stable transmitted signal can be acquired. With no modulation of the sub-carrier, the deviation is adjusted either below or above the filter aperture as shown in FIGS. 2, 3A, 3B and 3C. In the diagrams the filter is located on the upper sideband. Therefore, the sub-carrier is deviated to just below the filter frequency. This adjustment will insure good response of the low amplitude audio signals. Audio compression or speech clipping can be used in the audio circuit increasing the low amplitude signal transmission if required.

Preliminary tests of the transmitter with this type modulation showed good quality and very narrow band characteristics. A receiver with a ratio detector was used detecting leakage from a dummy load on the transmitter.

Advantages of the system are:

1. This modulating circuit permits transmission of a compatible radio signal having single sideband characteristics. 2. Transmission of unwanted signals are at a minimum as all R.F. generated before the narrow filter is at low level.

3. The space required in the frequency spectrum for transmission is very small and permits a greater number of transmitters in a given space.

4. High efficiency is attained due to the concentration of power on a given frequency.

5. It is very useful on the higher end of the radio spectrum where present state of the art single sideband transmitters with suppressed carrier are critical. Control of a re-injected carrier is difficult. With the above mentioned circuit, it is not necessary to inject the carrier.

6. This type of transmission will eliminate the beating of carriers experienced with many transmitters.

7. It can be used on all frequencies of the radio spectrum.

8. Stereo transmission is possible on say the AM broadcast band by placing two signals side by side producing a 10 KHz bandwidth. Two separate detectors in the receiver would give two separate audio outputs. This would permit stereo transmission on the AM bands occupying the same bandwidth now used.

Therefore, the present invention provides a new and improved system using a very small portion of the radio frequency spectrum and all of the modulation is done at lower power levels.

Claims

I claim:

1. Single sideband AM-FM modulation means comprising a sub-carrier frequency signal generator, means connected to amplitude modulate said sub-carrier signal,

a radio frequency signal generator, means to frequency modulate said radio frequency signal with said modulated sub-carrier signal, to create multiple sidebands,

frequency multiplying means connected to said frequency generator,

and narrow bandpass filter means tuned to a frequency between two of said multiple sidebands so as to pass one sideband of said modulated signal.

2. Apparatus as in claim 1 wherein said bandpass filter means is a narrow filter, of substantially 5 KHz.

3. Apparatus as in claim 1 having linear amplifier means connected between said frequency multiplying means and said bandpass filter.