# US 11,309,838 B2 · Self-oscillating spread spectrum frequency control loop > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 11,309,838 B2 - Document ID: US11309838B2 - Status: Granted - Kind: B2 - Application number: 17/123,575 - Filed: 2020-12-16 - Published: 2021-07-01 - Granted: 2022-04-19 - Earliest claimed: 2019-12-31 - Assignees: Littelfuse, Inc. - Applicants: Littelfuse, Inc. - Inventors: Eric D. Blom - Web page: https://patentgrep.com/patent/US11309838B2 ## CPC - H03B 23/00: Generation of oscillations periodically swept over a predetermined frequency range (angle-modulating circuits in general H03C3/00) - H03L 7/02: using a frequency discriminator comprising a passive frequency-determining element - H03L 7/085: concerning mainly the frequency- or phase-detection arrangement including the filtering or amplification of its output signal (H03L7/10 takes precedence; circuits for comparing the phase or frequency of two mutually-independent oscillations H03D13/00) - H04L 25/0268: {with modulation and subsequent demodulation} - H04L 25/0272: {Arrangements for coupling to multiple lines, e.g. for differential transmission} - H03B: GENERATION OF OSCILLATIONS, DIRECTLY OR BY FREQUENCY-CHANGING, BY CIRCUITS EMPLOYING ACTIVE ELEMENTS WHICH OPERATE IN A NON-SWITCHING MANNER; GENERATION OF NOISE BY SUCH CIRCUITS (generators adapted for electrophonic musical instruments G10H; masers or lasers H01S; generation of oscillations in plasma H05H) - H03L: AUTOMATIC CONTROL, STARTING, SYNCHRONISATION OR STABILISATION OF GENERATORS OF ELECTRONIC OSCILLATIONS OR PULSES (generation of oscillations H03B) - H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M) ## Abstract A self-oscillating spread spectrum frequency control loop contains a gated voltage-controlled oscillator (VCO) which receives a digital signal that can start or stop its oscillation. The VCO generates a spread spectrum carrier by receiving a triangle wave signal from a delaying ramp generator in a loop, its ramp direction controlled by a frequency comparator. The loop generates a spectrum spread as wide as possible above a minimum frequency. RF isolators that utilize low-pass filters in the transmitter and high-pass filters in the receiver, where the F-3 dB cutoff frequencies of both filters vary in a correlated manner, are used to not produce spread spectrum frequencies below the minimum frequency. Die from a given wafer lot, when designed such that the low- and high-pass cutoff frequencies track, can be used to form RF digital isolators whose minimum spread spectrum frequency does not go below the minimum frequency required by that wafer lot. ## Claims ### Claim 1 A circuit to generate a gated spread spectrum carrier signal, the circuit comprising: a gated voltage-controlled oscillator (VCO) to receive a digital input signal and generate the gated spread spectrum carrier signal when gated on; a low-pass filter to receive first and second differential signals from the gated VCO and generate low-pass filtered differential signals; a demodulator to receive the low-pass filtered differential signals and to convert a presence of RF above a known amplitude into a binary frequency compared digital signal; and a delaying bidirectional ramp generator to receive the binary frequency compared digital signal, the binary frequency compared digital signal to ramp in either a positive or a negative direction and to generate a sawtooth waveform, wherein the sawtooth waveform is received into a voltage control input of the gated VCO. ### Claim 2 (depends on 1) The circuit of claim 1, wherein the sawtooth waveform received into a Vcontrol input of the gated VCO is a triangle waveform of substantially equal positive and negative slew rate. ### Claim 3 (depends on 1) The circuit of claim 1, further comprising: a radio frequency-(RF) passing isolation barrier, wherein the gated spread spectrum carrier signal comprises a carrier to be transmitted across the RF-passing isolation barrier. ### Claim 4 (depends on 1) The circuit of claim 1, further comprising an output circuit comprising a high-pass filter to receive third and fourth differential signals from the RF-passing isolation barrier. ### Claim 5 (depends on 4) The circuit of claim 4, wherein the RF-passing isolation barrier disposed between the input circuit and the output circuit protects the input circuit and the output circuit from common mode voltage differences and transients. ### Claim 6 (depends on 1) The circuit of claim 1, wherein a low-pass filter F-3 dB cutoff frequency tracks a high-pass filter F-3 dB cutoff frequency. ### Claim 7 (depends on 1) The circuit of claim 1, wherein the sawtooth waveform is a triangle waveform of substantially equal positive and negative slew rate. ### Claim 8 (depends on 6) The circuit of claim 6, wherein the low-pass filter comprises a first set of one or more resistors and a first set of one or more capacitors. ### Claim 9 (depends on 8) The circuit of claim 8, wherein the high-pass filter comprises a second set of one or more resistors and a second set of one or more capacitors. ### Claim 10 (depends on 9) The circuit of claim 9, wherein the first set of one or more resistors are fabricated on a first die and the second set of one or more resistors of tracking value are fabricated on a second die, wherein the first die and the second die are fabricated on the same wafer lot. ### Claim 11 (depends on 9) The circuit of claim 9, wherein the first set of one or more capacitors are fabricated on a first die and the second set of one or more capacitors of tracking value are fabricated on a second die, wherein the first die and the second die are fabricated on the same wafer lot. ### Claim 12 A system to generate a gated spread spectrum carrier signal in a radio frequency (RF) isolated circuit, the system comprising: a gated voltage-controlled oscillator (VCO) to receive a digital gate signal and, when gated on, to generate the gated spread spectrum carrier signal to be transmitted across an RF-passing isolation barrier; a demodulator to receive low-pass filtered differential signals and to convert a presence of RF above a known amplitude into a binary frequency compared digital signal; an integrator comprising an amplifier and a capacitor, wherein a first end of the capacitor is coupled to an output of the amplifier and a second end of the capacitor is coupled to an inverting input of the amplifier to form the integrator; a switch to be controlled by the binary frequency compared digital signal, the switch to control the inverting input of the integrator; wherein the inverting input is controlled by a switched input to form a bidirectional integrator. ### Claim 13 (depends on 12) The system of claim 12, wherein the switched input of the integrator comprises a digital-to-analog converter whose digital input is coupled to an output of the demodulator and whose analog output is applied to the inverting input. ### Claim 14 (depends on 13) The system of claim 13, wherein the RF-passing isolation barrier further comprises: a first capacitor with a first end coupled to the first differential signal and a second end coupled to the third differential signal; and a second capacitor with a third end coupled to the second differential signal and a fourth end coupled to the fourth differential signal. ### Claim 15 (depends on 14) The system of claim 14, further comprising an output circuit comprising a high-pass filter to receive the third and fourth differential signals from the RF-passing isolation barrier. ### Claim 16 (depends on 15) The system of claim 15, further comprising a low-pass filter to receive the first and second differential signals from the gated VCO and generate the low-pass filtered differential signals, wherein the gated VCO, the demodulator, and the low-pass filter are part of an input circuit. ### Claim 17 (depends on 16) The system of claim 16, wherein: the low-pass filter comprises a first set of one or more resistors and a first set of one or more capacitors; the high-pass filter comprises a second set of one or more resistors and a second set of one or more capacitors; and the first set of one or more resistors are fabricated on a first die and the second set of one or more resistors of tracking value are fabricated on a second die, wherein the first die and the second die are fabricated on the same wafer lot. ### Claim 18 (depends on 16) The system of claim 16, wherein: the low-pass filter comprises a first set of one or more resistors and a first set of one or more capacitors; the high-pass filter comprises a second set of one or more resistors and a second set of one or more capacitors; and the first set of one or more capacitors are fabricated on a first die and the second set of one or more capacitors of tracking value are fabricated on a second die, wherein the first die and the second die are fabricated on the same wafer lot. ### Claim 19 (depends on 12) The system of claim 12, further comprising a delaying bidirectional ramp generator to receive the binary frequency compared digital signal, the binary frequency compared digital signal to ramp in either a positive or a negative direction and to generate a sawtooth waveform to be received into a Vcontrol input of the gated VCO, wherein the sawtooth waveform is a triangle waveform of substantially equal positive and negative slew rate.