Variable frequency sinusoidal signal generator, in particular for a modem

Claim 2

A sinusoidal signal generator comprising a clock signal generator, means for applying selection signals to the clock signal generator to produce a variable, frequency control signal; a switched impedance digital to anolog signal converter for generating from a fixed potential difference a step signal having a sinusoidal envelope, low pass filter means having an adjustable transfer function, and connected to receive the sinusoidal envelope step signal from said digital-to-analog signal converter means, and means for applying said control signals at the frequency determined by said selection signals (a) to said digital to anolog signal converter to effect impedance switching for varying the fundamental frequency of the sinusoidal envelope step signal, and (b) to the low pass filter means for adjusting the transfer function for rejecting harmonics contained in the sinusoidal envelope step signal, thereby to produce at the output of the low pass filter means a sinusoidal signal having reduced harmonic distortion.

Claim 3

A signal generator according to claim 1, wherein the digital to analog converter means and the low pass filter incorporate impedances and switch devices operable by sampling pulses to connect said impedances in a predetermined sequence to different fixed potentials.

Claim 4

A signal generator according to claim 2, wherein said sampling pulses have a repetition frequency which is multiple of the frequency of the sinusoidal signal to be generated.

Claim 5

A signal generator according to claim 3, wherein said multiple is one hundred and twenty eight, and in which each step of said step signal comprises sixteen samples.

Claim 8

A method according to claim 5, wherein the switched capacitances are switched at a rate equal to one hundred and twenty eight times the frequency of the sinusoidal signal to be generated; and wherein each cycle of said sinusoidal envelope comprises eight steps.

Claim 9

A sinusoidal signal generator comprising: clock signal generator means for generating a control signal the frequency of which can be varied; means for applying selection signals to the clock signal generator means to vary the frequency of the control signal; reference voltage means for providing voltages from different potential levels; digital-to-analog signal converter means coupled to said clock signal generator means and said reference voltage means for sampling said different potential levels under control of said control signals to produce a step signal having a sinusoidal envelope and a fundamental frequency which is a function of the control signal frequency and in which each step comprises a series of samples; and low pass filter means for receiving as an input the step signal from the digital to analog signal converter, and said control signals to control the transfer function of the low pass filter means as a function of said fundamental frequency of the sinusoidal envelope as determined by said control signals, for reducing harmonic distrotion in the output signal from the low pass filter means.

Claim 10

A sinusoidal signal generator comprising: clock signal generator means for generating a control signal the frequency of which can be varied; means for applying selection signals to the clock signal generator means to vary the frequency of the control signal; reference voltage means for providing voltages from different potential levels; digital-to-analog signal converter means coupled to said clock signal generator means and said reference voltage means for sampling said different potential levels under control of said control signals to produce a step signal having a sinusoidal envelope and a fundamental frequency which is a function of the control signal frequency and in which each step comprises a series of samples; and low pass filter means for receiving as an input the step signal from the digital to analog signal converter, and said control signals to control the transfer function of the low pass filter means as a function of said fundamental frequency of the sinusoidal envelope as determined by said control signals, for reducing harmonic distrotion in the output signal from the low pass filter means.

Claim 11

A modem including a generator according to claim 7, including means for varying the frequency of said clock signal generator means, on the one hand, by the value of a binary signal to be transmitted and, on the other hand, by said selection signals depending on a transmission standard for the binary signal.

Claim 12

A sinusoidal signal generator according to claim 7, wherein the reference voltage means is for providing two voltages of one polarity and two voltages of the opposite polarity, the relation R between two voltages of the same polarity being: ##EQU2##

Claim 13

A sinusoidal signal generator according to claim 9, wherein the reference voltage means comprises a plurality of capacitances and switch means for receiving signals from the clock signal generator means to connect the capacitances in different configurations with fixed potentials for providing said different voltage levels.

Claim 14

A sinusoidal signal generator according to claim 10, wherein the switch means comprise field effect transistors having gates for receiving said signals from the clock signal generator means.

Claim 15

A sinusoidal signal generator according to claim 7, wherein the digital-to-analog signal converter means includes CMOS transistors for effecting said sampling by connecting capacitances in different configurations such that the step levels of said step signal are determined by capacitance ratios.

Claim 16

A sinusoidal signal generator according to claim 7, wherein the low pass filter means includes first and second filter sections having different filter characteristics for filtering selected harmonics of said sinusoidal envelope step signal as a function of the fundamental frequency of the sinusoidal envelope of the step signal as determined by said control signals.

Claim 17

A sinusoidal signal generator according to claim 13, wherein each filter section includes a plurality of capacitances connected to switch means for receiving signals from said clock signal generator means to sample said capacitances at a sampling rate which is a function of the sampling frequency for said digital to analog converter.

Claim 18

A sinusoidal signal generator according to claim 14, wherein the said filter capacitance switch means comprise field effect transistors having gates for receiving said sampling signals.

Claim 19

A sinusoidal signal generator according to claim 14, wherein the sampling rate for the capacitances of the second filter section is a multiple of the fundamental frequency of the sinusoidal envelope of the step signal.

Claim 20

A sinusoidal signal generator according to claim 16, wherein the sampling rate for the capacitances of the second filter section is twice that for the capacitances of the first filter section.

Claim 21

A sinusoidal signal generator according to claim 17, wherein the sampling rate for the capacitances of the first and second filter sections are, respectively, 64 and 128 times the fundamental frequency of the sinusoidal envelope of the step signal.

Claim 22

A sinusoidal signal generator according to claim 13, wherein the low-pass filter means includes a Bessel type third filter section for smoothing the signal output from the second filter section.

Claim 23

A sinusoidal signal generator according to claim 13, wherein said multiple is sixty four.

Claim 24

A sinusoidal signal generator according to claim 20, wherein the first filter section characteristics include rejection of the seventh and ninth harmonics of the fundamental frequency of the sinusoidal envelope of the step signal.

Claim 25

A sinusoidal signal generator according to claim 21, wherein the second filter section characteristics include rejection of the 64th harmonic of the fundamental frequency of the sinusoidal envelope of the step signal.

Claim 26

A modem including a sinusoidal signal generator having a clock signal generator for generating control signals the frequency of which can be varied; reference voltage means for providing voltages from different potential levels; digital-to-analog signal converter means coupled to said clock signal generator means and said reference voltage means for sampling said different potential levels under control of said control signals at a selected frequency to produce a step signal having a sinusoidal envelope and fundamental frequency which is a function of the control signal frequency; and low pass filter means for receiving as an input the step signal from the digital to analog signal converter, and said control signals at said selected frequency to control the transfer function of the low pass filter means as a function of said fundamental frequency of the sinusoidal envelope as determined by said control signals at said selected frequency value, for reducing harmonic distortion in the output signal from the low pass filter means and means for applying selection signals to the clock signal generator means to vary the frequency of the control signal to selected values in accordance with (a) the binary value of a signal to be transmitted by the modem and (b) a transmission standard for said signal to be transmitted.

Claim 27

A modem including a sinusoidal signal generator having a clock signal generator for generating control signals the frequency of which can be varied; reference voltage means for providing voltages from different potential levels; digital-to-analog signal converter means coupled to said clock signal generator means and said reference voltage means for sampling said different potential levels under control of said control signals at a selected frequency to produce a step signal having a sinusoidal envelope and fundamental frequency which is a function of the control signal frequency; and low pass filter means for receiving as an input the step signal from the digital to analog signal converter, and said control signals at said selected frequency to control the transfer function of the low pass filter means as a function of said fundamental frequency of the sinusoidal envelope as determined by said control signals at said selected frequency value, for reducing harmonic distortion in the output signal from the low pass filter means and means for applying selection signals to the clock signal generator means to vary the frequency of the control signal to selected values in accordance with (a) the binary value of a signal to be transmitted by the modem and (b) a transmission standard for said signal to be transmitted.

Claim 28

A modem according to claim 23, wherein the means for applying selection signals applies signals for FSK modulation according to a selected one of a plurality of standards.