Abstract: A method and a transmitter system is provided for modulating binary information on a frequency shift keying carrier in a very narrow band. Phase reversal keying is initially used to create a carrier. The transmitter of the present invention generates a carrier signal by modulating a reference signal and a control signal having two frequencies representing the binary data to be transmitted. The resulting phase reversal carrier signal is processed by high order filtering such that the output of the transmitter provides a frequency shift keyed carrier at very close frequencies. The resulting carrier signal may, at the receiver, be processed by very narrow band filtering so as to create a communication system which will use only a small part of the available spectrum.

Abstract: A system and method are provided in which a phase modulation generator is used to modulate the output signal of a DDS with a modulation index, h/N. The phase-modulated. DDS output drives a PLL, which is an upconversion stage for a radio transmitter, with the modulated DDS signal as the PLL's reference. The output of the PLL will have a modulation index of h. Consequently, the frequency band of the DDS output is significantly narrower than the frequency band of upconversion stages used in conventional transmitters, and there are significantly less spurious emissions in the transmitted signal. However, a transmitter employing the present phase-modulated DDS-driven PLL topology can still transmit over a relatively wide frequency band.

Abstract: The output of a voltage-controlled oscillator (VCO), after being passed through a resonator having a single-peak characteristic, is detected by a detector. The output of the detector is sampled and held at a time instant that the control input voltage to the VCO reaches a maximum and also at a time instant that it reaches a minimum, and the difference between them is fed back to the control input of the VCO. In this way, the center frequency of the VCO is controlled so that it becomes equal to the center frequency of the resonator. In the case of a VCO as an FM modulator in an FM-CW radar, the sample-and-hold timing is derived from a clock signal based on which a triangle wave is generated. In the case of a VCO as an FSK modulator, the sample-and-hold timing is obtained by detecting a 0 and a 1 in input data.

Abstract: A method for generating minimum shift keying (MSK) modulation signals with relatively reduced energy sidelobes relative to known methods. In one embodiment, a second derivative of the phase waveform is taken and convolved with a selectable smoothing function and integrated twice to generate a smooth phase waveform S2D.sub.-- MSK (i)=where .o slashed. (i)=.pi. i d(i)/2T. In the alternate embodiment, the sidelobe energy can be varied by simply changing the smoothing function, which can be stored in a lookup table. In the embodiment, the MSK modulation signal has a constant amplitude envelope allowing it to be transmitted using class C amplifiers. Since the embodiment provides continuous phase derivatives, the frequency and thus the sidelobe energy is sufficiently reduced to minimize interference with cosite equipment and nearby communication channels bands.

Abstract: A Gaussian Minimum Shift Keying modulator that provides direct modulation of a carrier signal, produced by a single microwave high power voltage controlled oscillator. A continuous phase frequency shift keyed modulated signal with a modulation index of 0.5 is produced at the desired output frequency using a full 360 degree linear continuous phase modulator, controlled by a linear baseband signal that is the integral of the binary baseband information signal. This modulated signal is used as the reference signal for a phase locked high power voltage controlled oscillator. The phase locked loop provides frequency tracking and Gaussian spectral shaping to the modulated output signal.

Abstract: A telemetry transmitter having a dielectric resonator for frequency stabilization, a frequency or phase shift keying modulator and a high efficiency power amplifier. The transmitter can be implemented on gallium arsenide or other kinds of substrates The transmitter is of monolithic microwave integrated circuit (MMIC) technology. The transmitter may obtain its power from a power supply of a local or an associated system, thus not needing an integral power supply of its own.

Abstract: A multi-channel digital transceiver (400) receives uplink radio frequency signals and converts these signals to digital intermediate frequency signals. Digital signal processing, including a digital converter module (426), is employed to select digital intermediate frequency signals received at a plurality of antennas (412) and to convert these signals to baseband signals. The baseband signals are processed to recover a communication channel therefrom. Downlink baseband signals are also processed and digital signal processing within the digital converter module (426) up converters and modulates the downlink baseband signals to digital intermediate frequency signals. The digital intermediate frequency signals are converted to analog radio frequency signals, amplified and radiated from transmit antennas (420).

Abstract: Coherent modulation schemes for CPM signals. The disclosed schemes avoid the error probability increase due to the differential encoding by using a coherent encoder. A simple demodulator can be implemented by using a Nyquist-3 CPM scheme with coherent encoding. Such a scheme can be demodulated in substantially the same manner as signals modulated using M-PSK.

Abstract: A remote ground terminal transmitter for transmitting a modulated data signal having a constant envelope amplitude. The remote ground terminal transmitter comprises a source of data signals, a modulator for providing a carrier signal, receiving a data signal from the source of data signals, and modulating the carrier signal with the received data signal so as to produce a constant envelope minimum shift key modulation signal. The transmitter further comprises a power amplifier which operates in the saturation mode to amplify the modulated carrier signal produced by the modulator to the desired power level.

Abstract: A signal FSK-modulated with a binary data signal has first signal portions representing first logical level data contained in the binary data signal and second signal portions representing second logical level data contained in the binary data signal. Each of the first signal portions has a first frequency and lasts for a first time period and each of the second signal portions has a second frequency and lasts for a second time period. These first and second time periods are determined such that a number of cycles of the FSK-modulated signal appearing in each of the first time periods is equal to a number of cycles of the FSK-modulated signal appearing in each of the second time periods.

Abstract: A circuit for two-level/four-level frequency shift keying modulation, which may be used for a paging transmitter, is compatible with earlier two-level non-return to zero frequency shift keying modulation circuits and includes; a voltage leveling circuit for generating four voltage levels to selectively allow the two-level and four-level frequency shift keying modulation; a decoder for decoding the least significant bit and most significant bit data inputs to generate a decoded signal output; an analog switch for selecting the four voltage levels in accordance with the two-level or four-level frequency shift keying modulation, in response to the decoded signal; an adder for adding the selected voltage level to a reference voltage to generate a corresponding digital output voltage; an audio amplifier for amplifying an audio signal input; an input/filter selector for selecting the digital output voltage or the amplified audio signal in response to an input selection control signal and for selecting one of two fi

Abstract: A digital modulator capable of obtaining a high FM modulation index and reducing the quantization noise without increasing the sampling frequency of an A/D converter of input voice. A phase variation .phi.(Ts) of the digital data (Ts is the sampling period) produced on the basis of the output from the A/D converter is divided by a predetermined integer N (N>1) by a divider, and is fed to an integrator. The integrator integrates the divided phase variation .phi.(Ts)/N for every time interval Ts/N, thereby producing a phase component .phi.(t) of the complex envelope of the modulation signal. The phase component .phi.(t) is resolved into cosine and sine components at the time interval Ts/N. The cosine and sine signals are D/A converted into analog signals which are quadrature modulated by a quadrature modulator.

Abstract: A modulated clock MSK modulator that separately modulates a sinusoidal clock signal by square wave I and Q digital data signals prior to the I and Q data signals being modulated onto a carrier wave signal. Mixers or applicable biphase switches are utilized to impress the I and Q digital data signal information onto the clock signal by inverting or non-inverting the clock signal on separate I and Q data rails to create I and Q modulated clock signals separated in phase. The I channel data modulated clock signal and the Q channel data modulated clock signal are then separately applied to a conventional quadraphase modulator to be separately modulated onto a carrier wave signal, and the summed together to be transmitted.

Abstract: Device for continuous phase modulation, produced from a frequency synthesizer including a variable oscillator and a phase-locked loop with, in series, a frequency division circuit, a phase comparator and a low-pass filter. In order to reduce modulation in the loop, the modulation is applied not only to the input of the synthesizer but also, in compensation, in the loop. In order to eliminate the modulation residue in the loop, a control signal for the oscillator is derived by a circuit which performs a correlation between the residue of the modulation and the output signal from a filter producing the same filtering effect as the loop, and which receives the modulation signal from the input of the synthesizer.

Abstract: A FSK modulator which uses the combination of a state machine and a toggle flip-flop to generate the FSK signals. The state machine has a loop having a predetermined number of states. The number of clock cycles that the state machine takes to traverse the loop depends on the state or level of the signal at the input to the modulator. The flip-flop converts the state machine output signal to a signal having a rectangular waveform.

Abstract: A radio-frequency module detachably mounted to a transmitter in a radio-control apparatus to supply a high frequency signal to a transmitter. The module includes a phase-lock loop (PLL) circuit along with low pass filters and a voltage controlled oscillator to form a PLL oscillator loop. A rotary switch which is manually set provides an indication to a microprocessor of the desired frequency. The microprocessor supplies the PLL circuit with frequency data. An error voltage is generated by the low pass filters.

Abstract: A frequency-locked loop (100) employs a controllable oscillator (102) for generating an output signal having a frequency, optional sampler (104), coupled to oscillator (102), for sampling the frequency of the output signal, a divider (106), coupled to optional sampling circuit (104), for dividing the output signal frequency to generate a prescaled signal and a microprocessor (108), coupled between the divider 106 and oscillator (102), for comparing the prescaled signal to a reference signal and generating a control signal for correcting frequency shifts based upon the comparison. The control signal generated by microprocessor (108) is non-continuous. During that time when microprocessor (108) generates no control signals, power is removed from various frequency-locked loop circuitry.

Abstract: A method and modulator for modulating a digital signal to an analog intermediate-frequency signal. A baseband digital signal (I, Q) is taken to a digital-to-analog converter (71, 710) in which it is sampled at a certain sampling frequency (f.sub.in) and converted to an analog signal. The conversion produces a baseband signal (f.sub.sig) and signals (N*f.sub.in, f.sub.Ni) at multiples of the sampling frequency (f.sub.in). The sampling frequency is increased, advantageously according to a sigma-delta D/A conversion, and one of the signals (N*f.sub.in,f.sub.Ni) at multiples of the sampling frequency (f.sub.in), produced by the conversion, is selected as the output signal (f.sub.IF), advantageously a multiple (N*f.sub.in, f.sub.Ni) of the sampling frequency that is directly at the intermediate frequency or transmission frequency.

Abstract: A method and apparatus for encoding and convolutionally decoding a data word of a data word sequence. Each data word has k bits. An encoder selects, using a trellis encoding algorithm and a previous data word, a 2.sup.k subset of signal points from a square set of M by M signal points having X coordinates and Y coordinates. The square set of M by M signal points includes a constellation of 2.sup.k+1 signal points. The encoder selects, using the n.sup.th data word, a signal point from the selected subset of signal points. A modulator generates a first channel symbol and a second channel symbol from the selected signal point. A demodulator demodulates the first channel symbol as a received X coordinate and the second channel symbol as a received Y coordinate. A decoder estimates, using a second processor, a 2.sup.k received subset of signal points using the convolutionally decoding algorithm and a previously received data word. The decoder estimates, an n.sup.

Abstract: Continuous phase modulated (CPM) signals, such as quaternary CPFSK signals, are filtered to limit their bandwidth while simultaneously minimizing the amount of amplitude modulation and intersymbol interference introduced by this filtering process. By designing a modulation filter such as a finite impulse response (FIR) filter to add only a nominal, controlled amount of amplitude modulation, high efficiency, nonlinear power amplifiers may be employed to transmit the filtered CPFSK signals. Although the filter adds a small amount of amplitude modulation and intersymbol interference, digital signal processor-based receivers satisfactorily demodulate this type of signal using sequence estimation algorithms such as the Viterbi algorithm. Thus, the filtering of CPM signals permits narrowing of the transmitted signal spectrum while managing the intersymbol interference resulting from the controlled amount of amplitude modulation added by filtering.

Abstract: A circuit for processing related signal components, such as in-phase and quadrature signal components generated within a modulator, in which a first filter and a second filter are matched such that the gain of each such filter is nearly identical. A modulator is provided having matched filters in accordance with the present invention in which the gain "K" of each filter is equal to 1. By using a filter which has a gain of 1, the gain is removed from consideration when attempting to match the filter response of one filter to another. Furthermore, each filter is relatively simple to implement, having only four resistors, two capacitors, and two operational amplifiers.

Abstract: A microcomputer includes therein an operation control part which can be operated when a clock signal is applied thereto by an oscillation circuit. If the operation control part reads out an identification code from an EEPROM, then it switches a control signal to be given to a switching element in accordance with the levels of the respective bits. When the control signal is off, the oscillation frequency of the oscillation circuit is F1. On the other hand, if the control signal is turned on, then one terminal of a capacitor is grounded to thereby switch the oscillation frequency over to F2. The clock signal is divided by a frequency dividing circuit before it is output. The frequency of the output signal after being divided becomes f1 or f2 in accordance with the above switching and the output signal is transmitted in the form of a frequency modulation signal.

Abstract: A multiple-modulation communication system includes a transmitter that modulates and transmits communication signals modulated by a first modulation technique and communication signals modulated by a second modulation technique. The first modulation technique and the second modulation technique are different. The communication system also includes a receiver capable of receiving the communication signals modulated by the first modulation technique and the communication signals modulated by the second modulation technique and demodulating the communication signals.

Abstract: Data are transmitted using multiple carriers modulated by quadrature amplitude modulation. In general, the carriers carry different numbers of bits, to exploit the fact that the properties of a transmission path are more favorable to carriers at some frequencies than to those at others. The bit allocation process is made closer to optimum by permitting allocation of a non-integer number of bits to a carrier. Input bits are grouped for several carriers and this binary representation is converted to a number representation in which the base of each digit corresponds to the number of points in the QAM constellation for a respective carrier (or to this number divided by a power of two).

Abstract: A method and corresponding apparatus for error correction of MSK-coded data receives a signal (101) partitionable into adjacent frames (111,113) each subdividable into three tridents (103, 105, 107). A first sample quality metric (225) applicable to a p (123) of the signal (101) associated with a last trident (107) of the tridents of a first frame (111) of the adjacent frames (111, 113) is synthesized, and a second sample quality metric (227) applicable to a portion (125) of the signal (101) associated with a first trident (103') of the tridents of a second frame (113) following the first frame (111) is synthesized. Each of the first and second sample quality metrics (225, 227) have a magnitude dependent on a time that the associated portion (123) of the signal (101) has a magnitude exceeding an average magnitude (127) of the signal (101).

Abstract: A programmable apparatus is disclosed for generating a frequency modulated signal all a selected center frequency in accordance with digital data of at least first and second data levels. The modulating apparatus comprises a modulator having an input and an output and is responsive to an input modulation signal applied to its input for generating at its output the frequency modulated signal at a center frequency dependent on a quiescent voltage appearing at its input. A circuit is provided for sampling and storing a value of the quiescent voltage, An addressable memory stores a plurality of offsets. A programmable adding circuit adds a downloaded offset voltage to the stored value of the quiescent voltage to output a high modulation voltage. A programmable subtracting circuit subtracts a downloaded offset voltage from the stored value of the quiescent voltage to provide a low modulation voltage.

Abstract: Applicant's invention includes an apparatus and associated method for transmitting information at a high rate using an undermodulated frequency shift keyed signal. The transmission rate is independent of the data content and the system requires no zero crossing detectors. The demodulator combines non-linear processing circuitry with a conventional demodulator.

Abstract: The spectral encoding-decoding system and method are for encoding a sequence of N data bits where N is a predetermined integer, into an information signal to be transmitted on a communication medium, and for decoding the information signal transmitted on the communication signal into an estimated sequence of N data bits. The system comprises an encoder and a decoder. The encoder receives the sequence of N data bits, encodes the sequence into the information signal, and emits the information on the communication medium. The decoder receives the information signal transmitted on the communication medium, decodes the information signal and transmits the estimated sequence of N data bits. The method corresponds to the operations performed by the system.

Abstract: A modulator for digital modulation is described to produce a modulated output from a voltage controlled oscillator in a phase locked loop during transmission of a random modulating input. The input voltage is applied to a coupling capacitor and when transmission ceases, the state of charge on the capacitor will not change because of a biasing circuit which comprises two resistors fed by a tri-state buffer which holds the input terminal of the capacitor at the average level between logic zero and logic one when no modulation is applied.

Abstract: Binary and Quadrature Feher's (F)-Modulation/Amplification with significantly reduced envelope fluctuations and peak radiation, increased power efficiency and transmit power delivered by connected RF (radio frequency), Infrared (IR) and other devices. A subclass of these systems has a constant envelope. Extraordinary power advantages, robust BER performance and linearly or nonlinearly amplified narrow spectrum without the pitfalls of conventional BPSK and DBPSK is attained. The FBPSK improved efficiency transmitter is compatible with conventional BPSK receivers. FBPSK modems are based on using quadrature structure where Q channel data is inserted in quadrature with I channel data. The Q channel data is the same as the I channel but is offset by one half bit (Tb/2). Further improvement in the spectrum is attained using correlation between I and Q channels. FBPSK modem is shown to meet the IEEE 802.11 specified spectral mask (-30 dB point) for wireless LAN, and leads output power gain of 6.

Abstract: A modulation apparatus for a cellular radio system in which capacity is increased and power consumption is minimized, said system covering large and small area cells of different sizes. At least two different modulation schemes are used in the system. For large area cells, constant envelope modulation is used for minimizing power consumption, and for small area cells, linear modulation is used for minimizing the channel spacing, and in this manner, for increasing the cellular capacity.

Abstract: An apparatus (500) for generating first and second output signals having predetermined frequency shifts relative to a frequency provided by a reference signal is included in a system comprising a phase-locked loop (206) coupled to the reference signal for generating the first and second output signals. The apparatus (500) includes pulse deletion circuitry (204) coupled to the reference signal and the phase-locked loop (206) for deleting pulses from the reference signal at a first deletion rate to generate the first output signal and for deleting pulses from the reference signal at a second deletion rate to generate the second output signal.

Abstract: A surface acoustic wave (SAW) device is described that comprises a piezoelectric medium using input and output transducers for launching and detecting a surface acoustic wave on the piezoelectric medium. In particular, a device is described that uses a special combination of transducers for the direct generation of a coded minimum shift keyed (MSK) waveform from an input impulse waveform. The same device is used for the direct detection of an input MSK waveform in order to generate an impulse waveform with sidelobe characteristics similar to those expected from a specific code. The SAW device provides the advantages of MSK encoded waveforms for spread spectrum applications and the advantage of implementing such entirely on a SAW substrate.

Abstract: A system is described which achieves an increased rate of transmission by transmitting multiple symbols in one symbol time. The information in the overlapping symbols can be recovered if the symbol used for transmission is chosen so that a subset of the samples representing the symbol has an inverse. The construction of suitable symbols is illustrated. Effective operation of the system depends on equalizaton which will recover the the baseband signal shape. Methods of equalization are depicted which are suitable for use with this sytem including equalization realized by predistorting the signal at the transmitter so that it arrives at the receiver undistorted. The invention is applied to to baseband and passband systems and the application of predistortion equalization to the fading passband channel is described.

Abstract: A data transmission system includes a source of a data signal and a modulator, responsive to the data signal, producing a first modulated signal representing the data signal and a second modulated signal representing a signal 180 out-of-phase with the data signal. The first and second modulated signals are transported via a transmission channel. A first demodulator demodulates the transported first modulated signal and a second demodulator demodulates the transported second modulated signal. A subtractor, responsive to the first and second demodulators, produces a signal representative of the data signal.

Abstract: An FSK modulating apparatus includes a first phase locked loop arrangement having a first voltage controlled oscillator for generating a space frequency signal of a modulation signal and a second phase locked loop arrangement having a second voltage controlled oscillator for generating a mark frequency signal of the modulation signal. The apparatus also includes a third voltage controlled oscillator used to produce an FSK modulated output signal. After input data changes frequency to a space frequency or a mark frequency, a control voltage for the phase locked loop arrangement that is associated with the resulting frequency is routed through a low pass filter having a flat group delay characteristic to the third voltage controlled oscillator. After a set period of time has elapsed, the third voltage controlled oscillator replaces the voltage controlled oscillator of the particular phase locked loop arrangement associated with the current state of the modulator signal.

Abstract: A phase-locked loop frequency modulation circuit that compensates for the inability of a phase-locked loop to pass low-frequency content of an input modulation signal, and that may be utilized with existing communication apparatus, such as a cellular telephone voice radio, for accurate data transmission without having to modify such apparatus, includes a compensation circuit for processing an input modulation signal to provide a compensation signal that is added to a loop filter output signal by processing the input modulation signal to provide the same effect as adding the input modulation signal to an integrated input modulation signal that is filtered by a filter having the same transfer function as the loop filter to provide the compensation signal.

Abstract: A digital modulation technique is disclosed which is unique in that it does not utilize fixed time slots for varying the characteristics of an electromagnetic carrier signal, but actually uses variations of the time slots to transfer the digital information. The modulation is created by using direct digital synthesis techniques to produce a carrier waveform that closely approximates a sine wave carrier signal that can vary the time it takes for each peak to occur. The peaks of the carrier are tightly controlled to occur at exact discrete time slots that correspond to certain base band digital data. The time slot changes occur precisely at the sine wave peaks to minimize the bandwidth requirement and maximize the data rate.

Abstract: A communication system (10) includes a modulation section (12) and a demodulation section (14). The modulation section (12) performs frequency modulation in accordance with a frequency trajectory signal (28, 30, 32). An intersymbol interference (ISI) prediction filter (20) adjusts the amplitude of the frequency trajectory signal (28, 30, 32) in response to data code (16) sequences being conveyed over a plurality of symbols (18). More frequent data changes in the sequence of the data codes (16) lead to greater amplitudes in the frequency trajectory signal. The demodulation section (14) applies a distorted phase signal to a decision circuit (38). The distorted phase signal conveys a received phase (46) that includes ISI. Due to the equalization applied by the ISI prediction filter (20), the received phase (46) approximates a target phase (40, 42) in spite of the ISI.

Abstract: A modulator is adapted for use in a transmitter of an information transmission signal in which the information is in the form of a frequency shift keyed code. The signal is centered on a fictional carrier frequency fc, on either side of which are emitted two actual frequencies separated from the frequency fc by .DELTA.f, according to whether the binary information has a high value (+1) or a low value (-1). A first generator (1) creates carrier signals in the form of sin(2.pi.f.sub.c t) and cos(2.pi.f.sub.c t). A second generator (6) creates, as a function of the information, modulation signals of the form sin(2.pi..DELTA.ft) and cos(2.pi..DELTA.ft) which are mixed with the carrier signals. At each transition of the information, first phase shifting means (8,14) modify by k.pi. the phase of one of the modulation signals.

Abstract: A modulator capable of providing a fractional sample or symbol 995486 time employs a decimation counter (23) responsive to a clock having a frequency equal to M/N.times.(data symbol clock), where M is an interpolation factor and N is a decimation factor, for generating a data symbol clock to select FSK symbols from a sampled data array (21). A multiplier (26) receives the FSK symbols and multiplies the symbols by a weighting factor determined by the decimation counter. When the decimation counter wraps around due to a modulo M operation, a fractional weight is calculated for the current FSK symbol, then a new FSK symbol is selected, and then a fractional weight is calculated for the new FSK symbol. When the decimation counter has not wrapped, the full weighting, N, is output for the current FSK symbol.

Abstract: In a modulated signal transmission system operable in response to an input digital signal of a baseband to produce a modulated signal through a nonlinear amplifier, a nonlinear converter is controlled by a control signal generator to compensate for not only a nonlinear distortion but also a linear distortion. The control signal generator is given a difference signal between a linearly detected signal and the input digital signal to which an estimated linear distortion is added by a linear filter having filter coefficients controlled by a filter coefficient estimator. An additional nonlinear converter is located prior to the linear filter. Alternatively, the linear filter may be located after the nonlinear converter.

Abstract: A multilevel frequency-shift keying (FSK) modulator is disclosed in which the transient response for frequency transition can be easily set. The modulator is of a type in which a frequency generated by a voltage-controlled oscillator (VCO) is changed in accordance with multilevel code and is phase-locked by a phase-locked loop (PLL). The modulator is provided with a phase-lock feedback circuit that, combined with a VCO, makes up a PLL, and includes first and second VCOs and a filter having a desired transient response characteristic. At a transient time of frequency transition due to a change of the multilevel code, the first VCO and feedback circuit makes up the PLL, and a first control voltage for the first VCO produced in the PLL is inputted as a second control voltage to the second VCO through the filter.

Abstract: A data transmitter (100) operating at a transmit frequency provides a modulated data signal (114) by adjusting a reference frequency signal (108). The method of modulating an N-level data signal (102) inputted to the transmitter (100) provides for inputting, to a signal processor (201), a frequency deviation value. Using the frequency deviation value, the signal processor (201) then determines a maximum rate of enabling the frequency adjustment. Further, a deviation ratio corresponding to one of the N-levels is determined, and an increment value is then calculated (302) using this deviation ratio. Lastly, the frequency adjustment is enabled (312, 320) using the calculated increment value.

Abstract: One of a reference phase signal and an orthogonal phase signal is inverted to provide an inverted phase signal. when an arrangement of a local frequency is the same between transmitting and receiving sides, a carrier wave signal is FM-modulated by the reference and orthogonal phase signals. On the other hand, when the above arrangement is different between the transmitting and receiving sides, a carrier wave signal is FM-modulated by the inverted phase signal and the remaining one of the reference and orthogonal phase signals.

Abstract: A frequency agile homodyne full duplex FSK system having a receiver section using an image-cancellation mixer wherein sum and difference signals generated by a pair of double-balanced mixers are added together with proper phase to cancel out the image response normally associated with heterodyne receivers and directly down-convert the carrier frequency to the baseband frequency. The transmitter section of the FSK system consists of an FSK modulator whose peak deviation is controlled by an active derivative of a crystal clock. The FSK modulator functions in the manner of a phasing single sideband (SSB) mixer with upper or lower sideband signals generated by the proper phasing of quadrature components of the carrier frequency and the peak deviation signal derived from a crystal clock.

Abstract: A modem for use in a simulcast paging system includes a modulator (26) and a demodulator (30), both of which produce very low jitter, enabling the modem to be used at data rates well in excess of 1,200 baud. Both the modulator and the demodulator are implemented in software using a digital signal processor (DSP) (66). The modulator initially samples a non-return-to-zero (NRZ) input at a sample rate of 19.2 KHz, interpolates transitions between logic levels, and produces a frequency shift keyed (FSK) modulated signal at a center frequency different than that used for transmitting the modulated signal. Using an interpolation timer that responds to changes in logic level on the input, the modulator changes the frequency of the FSK modulated signal at the appropriate time with much greater accuracy than would be possible without interpolation. The FSK modulated signal is filtered to substantially attenuate frequencies outside a 3 KHz bandwidth, producing a filtered signal.

Abstract: A digital quadrature modulator having a first storage device for receiving an I phase component of a digital baseband signal and digital carrier phase information as a higher order address and a lower order address, respectively, and a second storage device for receiving a Q phase component of the digital baseband signal and the digital carrier phase information as a higher order address and a lower order address, respectively. The carrier phases of the carrier phase information applied to the first and the second storage devices are out of phase from each other by .pi./2. The first storage device outputs a multiplication result of the I phase component and a carrier signal and the second storage device outputs a multiplication result of the Q phase component and the carrier signal based on these addresses. These outputs are added to each other in a digital manner and then converted into an analog signal which is supplied as a modulated signal.

Abstract: The present invention provides a system and method of automatic frequency control (AFC) in a frequency-shift-key (FSK) data transmission system that allows a receiver to be used that has a bandwidth that approaches the requisite minimum bandwidth for a given data transmission rate and produces a substantial signal noise ratio (SNR) in the detected signal by the receiver. The invention includes a transmitter that outputs a SPACE signal and a MARK signal in a preamble that precedes the transmission data. The apparatus also includes a receiver that uses the SPACE and MARK signals to adjust the frequency of the signal output by a voltage-controlled-oscillator (VCO) to tune the receiver and thereby improve the signal-to-noise ratio (SNR) in the signals subsequently detected by the receiver.

Abstract: An apparatus is disclosed for generating an output signal in response to an input signal having a variable input level. The output signal has an output frequency which varies to represent variations in the input level. The apparatus comprises a processing circuit for implementing a digital signal processing algorithm to generate control signals in response to the input signal, which control signals are representative of the variable input level, and an oscillator circuit for generating the output signal in response to the control signals. The control signals include a first control signal representing a first output frequency, a second control signal representing a second output frequency, and at least one intermediate control signal representing output frequencies intermediate the first and second output frequencies.