# US 10,767,995 B2 · Hyperbolic modulation offset error reducer for an RFOG

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- Number: US 10,767,995 B2
- Document ID: US10767995B2
- Status: Granted
- Kind: B2
- Application number: 16/114,991
- Filed: 2018-08-28
- Published: 2020-03-05
- Granted: 2020-09-08
- Earliest claimed: 2018-08-28
- Assignees: Honeywell International Inc.
- Applicants: Honeywell International Inc.
- Inventors: Lee K. Strandjord; Marc Smiciklas; Norman Gerard Tarleton; Glen A. Sanders
- Web page: https://patentgrep.com/patent/US10767995B2

## CPC

- G01C 19/721: {Details, e.g. optical or electronical details}
- G01C 19/723: {Heterodyning fibre optic gyrometers}
- G01C 19/726: {Phase nulling gyrometers, i.e. compensating the Sagnac phase shift in a closed loop system}
- G01C 19/727: {using a passive ring resonator}
- G01C: MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY (measuring liquid level G01F; radio navigation, determining distance or velocity by use of propagation effects, e.g. Doppler effects, propagation time, of radio waves, analogous arrangements using other waves G01S)

## Abstract

A hyperbolic modulation offset reducer circuit for a resonator fiber optic gyro (RFOG) is provided. The circuit includes a first demodulation circuit that is configured to demodulate a received transmission signal from a resonator at twice a sideband heterodyne detection modulation frequency to reject signals due to backscatter. A slave resonance tracking loop of the circuit is coupled to an output of the first demodulation circuit. The slave resonance tracking loop is configured to create an offset frequency signal from the transmission signal that is applied to an optical phase lock loop of a RFOG. A hyperbolic modulator offset control loop is also coupled to the output of the first demodulation circuit. The hyperbolic modulator offset control loop is configured to create a subharmonic common modulation signal from the transmission signal that is coupled to a common phase module in a silicon photonics chip of the RFOG.

## Claims

### Claim 1

A hyperbolic modulation offset reducer circuit for a resonator fiber optic gyro, the hyperbolic modulation offset reducer circuit comprising:
an input to receive a transmission signal from a resonator;
a first demodulation circuit configured to demodulate the received transmission signal at twice a sideband heterodyne detection modulation frequency to reject signals due to backscatter;
a slave resonance tracking loop coupled to an output of the first demodulation circuit, the slave resonance tracking loop configured to create an offset frequency signal from the transmission signal, the slave resonance tracking loop including,
a second demodulation circuit configured to demodulate the output of the first demodulation circuit at a select tracking demodulation frequency; and
a hyperbolic modulator offset control loop coupled to the output of the first demodulation circuit, the hyperbolic modulator offset control loop configured to create a subharmonic common modulation signal and a offset from the transmission signal that is coupled to a common phase module in a silicon photonics chip of the resonator fiber optic gyro, the a hyperbolic modulator offset control loop including,
a third demodulation circuit configured to demodulate the output of the demodulation circuit at a select harmonic frequency.

### Claim 2 (depends on 1)

The subharmonic modulation offset reducer circuit of claim 1, wherein the tracking demodulation frequency is two times a subharmonic common modulation frequency and the harmonic frequency is three times the subharmonic common modulation frequency.

### Claim 3 (depends on 2)

The subharmonic modulation offset reducer circuit of claim 2, wherein the subharmonic common modulation frequency is 3.5 kHz.

### Claim 4 (depends on 1)

The subharmonic modulation offset reducer circuit of claim 1, wherein the second demodulation circuit of the slave resonance tracking loop further comprises:
a first numerically controlled oscillator (NCO); and
a first mixer having a first input coupled to an output of the first demodulation circuit and a second input coupled to an output of the first NCO.

### Claim 5 (depends on 3)

The subharmonic modulation offset reducer circuit of claim 3, wherein the slave resonance tracking loop further comprises:
a first accumulator coupled to an output of the first mixer;
a first adder having a first input coupled to an output of the first accumulator and second input coupled to an output of a register;
a second adder having a first input coupled to an output of the first adder and a second input coupled to a second NCO; and
a direct digital synthesizer coupled to an output of the second adder, the direct digital synthesizer outputting the offset frequency.

### Claim 6 (depends on 1)

The subharmonic modulation offset reducer circuit of claim 1, wherein the third demodulation circuit of the hyperbolic modulator offset control loop further comprises:
a second NCO; and
a second mixer having a first input coupled to the output of the first demodulation circuit, the second mixer having a second input coupled to an output of the second NOC.

### Claim 7 (depends on 6)

The subharmonic modulation offset reducer circuit of claim 6, wherein the hyperbolic modulator offset control loop further comprises:
a third NCO including an output that outputs a subharmonic common modulation frequency;
an adder having a first input in communication with an output of the second mixer and a second input coupled to the output of the third NCO.

### Claim 8 (depends on 6)

The subharmonic modulation offset reducer circuit of claim 6, wherein the hyperbolic modulator offset control loop further comprises:
a gain circuit coupled to an output of the third mixer;
a second accumulator coupled to an output of the gain circuit;
a second adder having a first input coupled to an output of the second accumulator;
a third adder having a first input coupled to an output of the second accumulator and a second input coupled to an output of a fourth NCO; and
a digital to analog converter having an input coupled to an output of the third adder and an output that provides the subharmonic common modulation signal.

### Claim 9 (depends on 6)

The subharmonic modulation offset reducer circuit of claim 6, wherein the hyperbolic modulator offset control loop further comprises:
a gain circuit coupled to an output of the third mixer;
a second accumulator coupled to an output of the gain circuit;
a first digital to analog converter (DAC) having an input coupled to an output of the second accumulator;
a fourth NCO;
a second DAC having an output coupled to an output of the fourth NCO;
an analog adder having a first input coupled an output of the first DAC and a second input coupled to an output of the second DAC, an output of the analog adder providing the subharmonic common modulation signal.