WO1979000718A1 - Amplitude and frequency modulation system - Google Patents

Amplitude and frequency modulation system Download PDF

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Publication number
WO1979000718A1
WO1979000718A1 PCT/US1979/000101 US7900101W WO7900718A1 WO 1979000718 A1 WO1979000718 A1 WO 1979000718A1 US 7900101 W US7900101 W US 7900101W WO 7900718 A1 WO7900718 A1 WO 7900718A1
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Prior art keywords
amplitude
input signal
frequency
signal
waveform
Prior art date
Application number
PCT/US1979/000101
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French (fr)
Inventor
J Salz
J Werner
Original Assignee
Western Electric Co
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Publication of WO1979000718A1 publication Critical patent/WO1979000718A1/en

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/10—Frequency-modulated carrier systems, i.e. using frequency-shift keying
    • H04L27/12—Modulator circuits; Transmitter circuits
    • H04L27/122—Modulator circuits; Transmitter circuits using digital generation of carrier signals
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26

Definitions

  • This invention relates to method for decreasing the bandwidth of a waveform, frequency modulated by an input signal and a modulator for practicing the method having means for generating a periodic waveform having an angular-velocity that varies in response to an input signal amplitude.
  • Frequency-shift-keyed (FSK) modulation is used extensively in data communications in a primary channel of a low speed modem and in a secondary or reverse channel of a high speed modem.
  • Frequency-shift-keying is an abrupt modulation technique that creates a signal having theoretically infinite bandwidth.
  • the sidebands of an FSK signal can cause adjacent channel interference, therefore some filtering is generally required before the FSK signal can be transmitted. Filtering requirements can be' daunting, however, and efforts have been made to shape the- spectrum of the FSK signal prior to filtering to ease the filter design requirements and thereby reduce their cost.
  • U.S. Patent 3,054,073 there is disclosed a technique utilizing amplitude and angular-velocity modulation in which the angular-velocity of a carrier wave is modulated by a baseband signal and the amplitude of the angular-velocity modulated carrier is modulated by a signal that is an exponential transform of the base- band signal.
  • Such a technique results in a single side ⁇ band angular-velocity modulated signal but is complex and costly to implement. Also, the bandwidth of the resulting single sideband signal may be wider than the o i ginal signal .
  • U.S. Patent 3,895,316 the technique of combining frequency modulation and amplitude modulation without transformation of the baseband signal was utilized.
  • the steps of frequency modulation and suppressed carrier amplitude modulation are used to achieve a "chirping" quality in the resulting audible sound to thereby enhance the discernabil ty of the sound to a listener.
  • This suppressed carrier technique consumes more bandwidth than frequency modulation alone.
  • the foregoing problem is solved, according to the invention, by the method characterized by modulating the amplitude of the frequency modulated waveform in accordance with the amplitude of the input signal so that the amplitude and the frequency are individually and con ⁇ currently modulated by the same input signal, and setting the ratio of minimum waveform amplitude to maximum wave ⁇ form amplitude within a range between 0.6 and 0.9; and by apparatus characterized by means responsive to the input signal amplitude for regulating the amplitude of the envelope of the varied waveform within a range of ratios of minimum to maximum envelope amplitude sub ⁇ stantially between 0.6 and 0.9, the angular-vel oci ty and the envelope amplitude being individually regulated by the same input signal.
  • FIG. 1 discloses, in block form, an amplitude and frequency modulated generator in accordance with the principles of the invention.
  • FIG. 2 shows a graph illustrating the amount of power by which a pure FSK signal must be decreased to achieve the same sideband power reduction as an AM-FSK signal for various amounts of amplitude modulation.
  • FIG. 3 discloses, in schematic form, an embodiment of the invention utilizing analog circuits.
  • FIG. 4 illustrates time varying waveforms associated with FIG. 3 that are useful in understanding the invention.
  • FIG. 5 discloses, in block form, an all digital embodiment of the invention. Summary of the Invention
  • significant suppression of one side band of a carrier wave, frequency modulated by a baseband signal is achieved by simultaneous and direct amplitude modulation of the frequency modulated carrier wave with the baseband signal when a ratio (p) of minimum to maximum amplitudes of the envelope of the carrier wave is substantially constrained by the inequality 0.6 ⁇ p ⁇ 0.9.
  • the output of a prior art FSK modulator is ampl.itude modulated by the ⁇ same binary input signa.l used to control the FSK process.
  • Amplitude modulation is performed utilizing an analog technique whereby a pair of amplifiers, each having a predetermined gain, are selectively enabled by the binary signal to amplify the FSK signal to thereby achieve a combination of amplitude modulation and frequency-shift-keying hereinafter referred to as AM-FSK.
  • an all digital technique is used to synthesize the AM-FSK signal .
  • a periodic signal is generated by digitally accumulating the addition of predetermined phase increments. The period is set by the rate at which phase increments are added, the size of the phase increments and the capacity of the register used to accumulate the increments.
  • the phase is transformed into a d gital sine wave which is thereafter .multiplied with a digital representation of the amplitude level . Thereafter, digital to analog conversion completes the process.
  • FIG. 1 The method for combining amplitude and frequency modulation in accordance with this invention is depicted in FIG. 1.
  • An input signal having a time-varying amplitude is applied to voltage controlled oscillator 101 to control the frequency of the oscillator in accordance with the amplitude of the input baseband signal to thereby generate a frequency modulated (FM) signal .
  • the output of the voltage controlled oscillator (VCO) is a periodic waveform, such as a sine wave, having a constant envelope amplitude.
  • the envelope amplitude of the VCO output signal is regulated by envelope modul ator .103 in accordance with the amplitude- of the input baseband signal. It is noted that both the frequency and the amplitude of the output signal are varied.
  • Envelope range control 102 is that portion of envelope modulator 103 that sets the envelope range of the output signal.
  • the envelope range control is arranged to vary the amplitude of the output signal in direct proportion to its frequency Variation (i.e., decrease the amplitude when the frequency decreases)
  • the envelope modulator significantly suppresses the lower sideband of the FM signal.
  • the envelope range control is arranged to vary the amplitude of the output signal in inverse proportion to its frequency variation (i.e., decrease the amplitude when the frequency increases)
  • the envelope modulator significantly suppresses the upper sideband of the FM signal .
  • the amplitude of the resulting envelope is arranged to vary between a minimum value which for convenience is designated "A” and a maximum value designated “B".
  • a ratio ( ) of the minimum to the maximum value is useful in understanding the invention and is defined by the equa- n
  • envelope range control 102 scales the dynamic range of the input baseband signal into a range suitable to set the minimum level of the envelope amplitude of the output signal at 0.75 times the maximum level of the envelope amplitude of the output signal .
  • FIG. 2 demonstrates the equivalent attenuation of a pure FSK signal required to achieve the same reduction in the power of a suppressed sideband.
  • the optimum value of p is found by minimizing the residual power in an upper sideband, for example, where the power spectrum P of the AM-FSK signal given by:
  • FIG, 3 discloses a more detailed embodiment of the invention constructed with commercially available components and designed for the case of a binary input 5 signal.
  • FSK modulator 301 is a frequency shift transmitter such as the type taught in U.S. Patent 3,249,896.
  • Circuits 302-305 are parts of a programmable amplifier, 303 being a decode control and 305 being the output amplifier, which are available on a single chip such Q as the HA-2400 from Harris Semiconductor Corporation of U.S. of A.
  • the input signal to FSK modulator 301 is a binary signal such as showti in FIG. 4(A).
  • the FSK modulator generates a sine wave having a high frequency when the 5 input signal is a logic 1 and a low frequency when the input signal is a logic 0.
  • the output impedance of FSK modulator 301 is negligible in comparison with the value of resistor R Q .
  • the output of the modulator is a pure FSK signal such as the one shown in FIG. 4(B),
  • the 0 baseband signal is directly utilized to regulate the amplitude of the FSK signal.
  • Decode control circuit 303 responds to a logic 1 on input lead 311 by enabling amplifier 302 and disabling amplifier 304; it responds to a logic 0 input by enabling amplifier 304 and 5 disabling amplifier 302.
  • Amplifiers 302 and -304 are input stages of an overall amplifier whose output stage is designated amplifier 305.
  • the amplification of a signal that passes through amplifiers 302 and 305 is fixed by the magnitude of the 0 resistors R Q and R, and is equal to -R,/R Q .
  • the amplifi ⁇ cation of a signal that passes through amplifier 304 and 305 is similarly fixed and is equal to Ro/Rg.
  • amplitude ROM (Read Only Memory) 502 generates one set of eight binary digits, defining one amplitude level, when the binary data input signal is a logic 1 ; and another set of eight binary digits, defining another amplitude level, when the binary data input signal is a logic 0.
  • Binary out ⁇ put signals on leads (D Q ...D 7 ) are ⁇ lrt into " 8x8 multiplier 508 input leads (B Q ,..B 7 ).
  • Phase increment ROM 501 generates a phase increment represented by a set of four binary digits on outputs leads D Q , U-, , D 2> and D - Th.e total phase is expressed in eight binary digits and is stored in registers 505 and 506.
  • phase increment is added to the total phase by 4-bit adders 503 and 504 where the total phase is present on the "B" inputs to each adder and the phase increment is present on the "A” inputs to adder 504.
  • the adders perform an A + B operation; the sum is available on the "S” outputs.
  • C QU+ is an overflow of the addition operation to be fed to the next stage and is termed the carry output.
  • C. is the carry input and is added, to the A + B sum having a weight equivalent to the AQ binary digit (i.e., the least significant digit).
  • Phase increments are periodically added to the total phase by reading the sum into the storage registers at pre ⁇ determined transition times of a clock signal . The rate at which phase increments are added will determine the frequency of the output signal.
  • the phase increment to be added is selected by the state of the binary data input signal to ROM 501. In the logic 1 state one set of four binary digits is available on output leads
  • the incrementally varying phase is converted into a sinusoidal representation by Sine Look-Up Table 507 which is a circuit that converts eight binary input digits, representing a phase quantity, into eight binary output digits representing the sine of the phase quantity (i.e., the input digits specify an address in memory containing the sine transform of those ' digi ts) .
  • Multiplier 508 digitally performs the A x B operation in a conventional digital manner at a rate set by a high frequency clock. The digital product of the multiplication is then converted into an analog quantity by digital to analog converter 509.

Abstract

Significant suppression of one side band of a carrier wave, frequency modulated by a baseband signal, is achieved by simultaneous and direct amplitude modulation of the carrier wave with the baseband signal when a ratio (P, fig. 2) of minimum to maximum amplitudes of the carrier wave is substantially constrained by the inequality 0.6 P 0.9. A pair of amplifiers (302, 304, Fig. 3), each having a predetermined gain wherein the ratio of the gain of the amplifier is substantially between 0.6 and 0.9, are coupled to effect the desired amplitude modulation.

Description

AMPLITUDE AND FREQUENCY MODULATION SYSTEM Field of the Invention
This invention relates to method for decreasing the bandwidth of a waveform, frequency modulated by an input signal and a modulator for practicing the method having means for generating a periodic waveform having an angular-velocity that varies in response to an input signal amplitude. Background of the Invention
Frequency-shift-keyed (FSK) modulation is used extensively in data communications in a primary channel of a low speed modem and in a secondary or reverse channel of a high speed modem. Frequency-shift-keying is an abrupt modulation technique that creates a signal having theoretically infinite bandwidth. In frequency division multiplex transmission systems where each channel is allocated a fixed bandwidth the sidebands of an FSK signal can cause adjacent channel interference, therefore some filtering is generally required before the FSK signal can be transmitted. Filtering requirements can be' formidable, however, and efforts have been made to shape the- spectrum of the FSK signal prior to filtering to ease the filter design requirements and thereby reduce their cost.
In U.S. Patent 3,054,073, there is disclosed a technique utilizing amplitude and angular-velocity modulation in which the angular-velocity of a carrier wave is modulated by a baseband signal and the amplitude of the angular-velocity modulated carrier is modulated by a signal that is an exponential transform of the base- band signal. Such a technique results in a single side¬ band angular-velocity modulated signal but is complex and costly to implement. Also, the bandwidth of the resulting single sideband signal may be wider than the o i ginal signal . In U.S. Patent 3,895,316, the technique of combining frequency modulation and amplitude modulation without transformation of the baseband signal was utilized. The steps of frequency modulation and suppressed carrier amplitude modulation are used to achieve a "chirping" quality in the resulting audible sound to thereby enhance the discernabil ty of the sound to a listener. This suppressed carrier technique, however, consumes more bandwidth than frequency modulation alone.
The foregoing problem is solved, according to the invention, by the method characterized by modulating the amplitude of the frequency modulated waveform in accordance with the amplitude of the input signal so that the amplitude and the frequency are individually and con¬ currently modulated by the same input signal, and setting the ratio of minimum waveform amplitude to maximum wave¬ form amplitude within a range between 0.6 and 0.9; and by apparatus characterized by means responsive to the input signal amplitude for regulating the amplitude of the envelope of the varied waveform within a range of ratios of minimum to maximum envelope amplitude sub¬ stantially between 0.6 and 0.9, the angular-vel oci ty and the envelope amplitude being individually regulated by the same input signal. In the drawing:
FIG. 1 discloses, in block form, an amplitude and frequency modulated generator in accordance with the principles of the invention.
FIG. 2 shows a graph illustrating the amount of power by which a pure FSK signal must be decreased to achieve the same sideband power reduction as an AM-FSK signal for various amounts of amplitude modulation. FIG. 3 discloses, in schematic form, an embodiment of the invention utilizing analog circuits.
FIG. 4 illustrates time varying waveforms associated with FIG. 3 that are useful in understanding the invention. FIG. 5 discloses, in block form, an all digital embodiment of the invention. Summary of the Invention
In accordance with the invention, significant suppression of one side band of a carrier wave, frequency modulated by a baseband signal , is achieved by simultaneous and direct amplitude modulation of the frequency modulated carrier wave with the baseband signal when a ratio (p) of minimum to maximum amplitudes of the envelope of the carrier wave is substantially constrained by the inequality 0.6 < p < 0.9. In one illustrative embodiment the output of a prior art FSK modulator is ampl.itude modulated by the ■same binary input signa.l used to control the FSK process. Amplitude modulation is performed utilizing an analog technique whereby a pair of amplifiers, each having a predetermined gain, are selectively enabled by the binary signal to amplify the FSK signal to thereby achieve a combination of amplitude modulation and frequency-shift-keying hereinafter referred to as AM-FSK. In another illustrative embodiment an all digital technique is used to synthesize the AM-FSK signal . A periodic signal is generated by digitally accumulating the addition of predetermined phase increments. The period is set by the rate at which phase increments are added, the size of the phase increments and the capacity of the register used to accumulate the increments. The phase is transformed into a d gital sine wave which is thereafter .multiplied with a digital representation of the amplitude level . Thereafter, digital to analog conversion completes the process. Detailed Description
The method for combining amplitude and frequency modulation in accordance with this invention is depicted in FIG. 1. An input signal having a time-varying amplitude is applied to voltage controlled oscillator 101 to control the frequency of the oscillator in accordance with the amplitude of the input baseband signal to thereby generate a frequency modulated (FM) signal . The output of the voltage controlled oscillator (VCO) is a periodic waveform, such as a sine wave, having a constant envelope amplitude. The envelope amplitude of the VCO output signal is regulated by envelope modul ator .103 in accordance with the amplitude- of the input baseband signal. It is noted that both the frequency and the amplitude of the output signal are varied. Envelope range control 102 is that portion of envelope modulator 103 that sets the envelope range of the output signal. When the envelope range control is arranged to vary the amplitude of the output signal in direct proportion to its frequency Variation (i.e., decrease the amplitude when the frequency decreases), it has been discovered that the envelope modulator significantly suppresses the lower sideband of the FM signal. When the envelope range control is arranged to vary the amplitude of the output signal in inverse proportion to its frequency variation (i.e., decrease the amplitude when the frequency increases), it has been discovered that the envelope modulator significantly suppresses the upper sideband of the FM signal .
The amplitude of the resulting envelope is arranged to vary between a minimum value which for convenience is designated "A" and a maximum value designated "B". A ratio ( ) of the minimum to the maximum value is useful in understanding the invention and is defined by the equa- n
1 i ty P = p-. As will be shown later a range of values for p exists that provides substantial suppression of one of the sidebands of the FM signal. For the example case when P = 0.75, envelope range control 102 scales the dynamic range of the input baseband signal into a range suitable to set the minimum level of the envelope amplitude of the output signal at 0.75 times the maximum level of the envelope amplitude of the output signal . FIG. 2 illustrates that useful suppression of a sideband is possible over a range of values substantially defined by the inequality 0.6 < p < 0.9 in the case of binary frequency-shift-keying (FSK). Note that pure FSK is the case whereby p = 1 and on-off keying of a carrier wave is the case whereby p = 0. FIG. 2 demonstrates the equivalent attenuation of a pure FSK signal required to achieve the same reduction in the power of a suppressed sideband. For example, an FSK signal attenuated by approximately 9 dB. has the same power in either of its sidebands as the residual powerof the suppressed sideband of the same FSK signal amplitude modulated such that p = 0.75.
Due to. ampl itude modulation the performance of AM- FSK in the presence of Gaussian noise is somewhat decreased in comparison with pure FSK. In the case where p = 0.75 the degradation in signal-to-noise ratio is about 1.5 dB.
The optimum value of p is found by minimizing the residual power in an upper sideband, for example, where the power spectrum P of the AM-FSK signal given by:
Figure imgf000007_0001
where α = IT = 4γ and T is the period of each, trans^ mitted symbol. The functional P is minimized under the i power constraint:
P = A2 + B2 = Constant (2) This variational problem can be solved In a relatively straightforward manner by introducing Lagrangian multipliers for the evaluation with the ultimate result that the optimum value of p = 0.7623.
FIG, 3 discloses a more detailed embodiment of the invention constructed with commercially available components and designed for the case of a binary input 5 signal. FSK modulator 301 is a frequency shift transmitter such as the type taught in U.S. Patent 3,249,896. Circuits 302-305 are parts of a programmable amplifier, 303 being a decode control and 305 being the output amplifier, which are available on a single chip such Q as the HA-2400 from Harris Semiconductor Corporation of U.S. of A.
The input signal to FSK modulator 301 is a binary signal such as showti in FIG. 4(A). The FSK modulator generates a sine wave having a high frequency when the 5 input signal is a logic 1 and a low frequency when the input signal is a logic 0. The output impedance of FSK modulator 301 is negligible in comparison with the value of resistor RQ. The output of the modulator is a pure FSK signal such as the one shown in FIG. 4(B), The 0 baseband signal is directly utilized to regulate the amplitude of the FSK signal. Decode control circuit 303 responds to a logic 1 on input lead 311 by enabling amplifier 302 and disabling amplifier 304; it responds to a logic 0 input by enabling amplifier 304 and 5 disabling amplifier 302. Amplifiers 302 and -304 are input stages of an overall amplifier whose output stage is designated amplifier 305.
The amplification of a signal that passes through amplifiers 302 and 305 is fixed by the magnitude of the 0 resistors RQ and R, and is equal to -R,/RQ. The amplifi¬ cation of a signal that passes through amplifier 304 and 305 is similarly fixed and is equal to Ro/Rg. When, as in the example embodiment, it is desirable to significantly suppress the lower sideband of the FSK signal , the 5 following relation between the resistance values is appropriate: R = 0.75R-J, Such a selection would result in the output waveform of FIG. 4(C). If, on the other hand, the upper sideband is to be signif cantly suppressed, the following relation between resistors is appropriate: 1 = 0.75R2.
In many applications, however, the best mode of operation resides in the time-shared use of a digital processor which, while relatively complex in overall operation and unlikely to warrant serious consideration in the absence of special circumstances, may provide increased efficiency. Such an implementation is shown in FIG. 5, which' is all digital and advantageously yields to LSI (Large Scale Integration) construction.
In the circuit of FIG. 5 amplitude ROM (Read Only Memory) 502 generates one set of eight binary digits, defining one amplitude level, when the binary data input signal is a logic 1 ; and another set of eight binary digits, defining another amplitude level, when the binary data input signal is a logic 0. Binary out¬ put signals on leads (DQ...D7) are ¥lrt into" 8x8 multiplier 508 input leads (BQ,..B7). Phase increment ROM 501 generates a phase increment represented by a set of four binary digits on outputs leads DQ, U-, , D2> and D - Th.e total phase is expressed in eight binary digits and is stored in registers 505 and 506. The phase increment is added to the total phase by 4-bit adders 503 and 504 where the total phase is present on the "B" inputs to each adder and the phase increment is present on the "A" inputs to adder 504. The adders perform an A + B operation; the sum is available on the "S" outputs. C QU+ is an overflow of the addition operation to be fed to the next stage and is termed the carry output. C. is the carry input and is added, to the A + B sum having a weight equivalent to the AQ binary digit (i.e., the least significant digit). Phase increments are periodically added to the total phase by reading the sum into the storage registers at pre¬ determined transition times of a clock signal . The rate at which phase increments are added will determine the frequency of the output signal. The phase increment to be added, however, is selected by the state of the binary data input signal to ROM 501. In the logic 1 state one set of four binary digits is available on output leads
DQ...D 3 in the logic 0 state another set of four binary digits is available on output leads DQ,.,D3,
The incrementally varying phase is converted into a sinusoidal representation by Sine Look-Up Table 507 which is a circuit that converts eight binary input digits, representing a phase quantity, into eight binary output digits representing the sine of the phase quantity (i.e., the input digits specify an address in memory containing the sine transform of those'digi ts) . Multiplier 508 digitally performs the A x B operation in a conventional digital manner at a rate set by a high frequency clock. The digital product of the multiplication is then converted into an analog quantity by digital to analog converter 509.
It should be noted in retrospect that hybrid techniques can be advantageously utilized to combine analog and digital processing.

Claims

1. The method for decreasing the bandwidth of a waveform, frequency modulated by an input signal, CHARACTERIZED BY modulating the amplitude of the frequency modulated waveform in accordance with the amplitude of the input signal so that the amplitude and the frequency are individually and concurrently modulated by the same input signal ; and setting the ratio of minimum waveform amplitude to maximum waveform amplitude within a range between 0.6 and 0.9.
2. A modulator for practicing the method of claim
1 having means for generating a periodic waveform having an angular-velocity that varies in response to an input signal amplitude
CHARACTERIZED BY means responsive to the input signal amplitude for regulating the amplitude of the envelope of the varied waveform within a range of ratios of minimum to maximum envelope amplitude substantially between 0.6 and 0.9, the angular-velocity and the envelope amplitude being individually regulated by the same input signal.
3. A modulator according to claim 2 CHARACTERIZED IN THAT the angular velocity generating mean's comprises a frequency modulator.
4. A modulator according to claim 3 CHARACTERIZED IN THAT the input signal is in the form of a digital signal having two discrete states.
5. A modulator according to claims 3 and 4 CHARACTERIZED IN THAT the ratio of minimum to maximum envelope amplitude is substantially 0.75. 1. Apparatus for generating a waveform having amplitude and frequency concurrently and directly modulated by the same binary input signal including means responsive to the binary input signal for generating a frequency-shift-keyed output signal,
CHARACTERIZED BY amplification means, having a first and second gain, responsive to one state of the binary input signal for selecting the first gain to amplify the frequency-shift-keyed signal, and responsive to the other state of the binary input signal for selecting the second gain to amplify the frequency-shift-keyed signal wherein the ratio of said first and second gains is substantially between 0.6 and 0.9.
2. Apparatus for generating a waveform according to claim 1
CHARACTERIZED IN THAT the amplification means includes a pair of amplifiers each having a different gain and selected in accordance with the state of the binary input signal.
3. — Apparatus for generating a waveform according to claim 1
CHARACTERIZED IN THAT the frequency-shift-keyed signal is generated by periodically accumulating digital increments of phase whose amounts are regulated by the state of the binary input signal.
OMPI
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US4170764A (en) 1979-10-09
JPS55500117A (en) 1980-02-28
ES478337A1 (en) 1979-05-16
JPH0219662B2 (en) 1990-05-02
DE2963138D1 (en) 1982-08-12
EP0004046A1 (en) 1979-09-19
EP0004046B1 (en) 1982-06-23
CA1105575A (en) 1981-07-21

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