# US 8,542,777 B2 · Amplitude modulation demodulating circuit and method thereof > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 8,542,777 B2 - Document ID: US8542777B2 - Status: Granted - Kind: B2 - Application number: 12/985,405 - Filed: 2011-01-06 - Published: 2011-07-07 - Granted: 2013-09-24 - Earliest claimed: 2010-01-07 - Assignees: MStar Semiconductor, Inc. - Applicants: Chien-Shan Chiang - Inventors: Chien-Shan Chiang - Web page: https://patentgrep.com/patent/US8542777B2 ## CPC - H03D 1/00: Demodulation of amplitude-modulated oscillations (H03D5/00, H03D9/00, H03D11/00 take precedence) - H03D: DEMODULATION OR TRANSFERENCE OF MODULATION FROM ONE CARRIER TO ANOTHER (masers, lasers H01S; circuits capable of acting both as modulator and demodulator H03C; details applicable to both modulators and frequency-changers H03C; demodulating pulses H03K9/00; transforming types of pulse modulation H03K11/00; coding, decoding or code conversion, in general H03M; repeater stations H04B7/14; demodulators adapted for AC systems of digital information transmission H04L27/00; synchronous demodulators adapted for colour television H04N9/66) ## Abstract An amplitude modulation (AM) demodulating circuit applied to a radio frequency identification (RFID) system attenuates carrier signals by a notch filter to increase a signal-to-carrier ratio and to reduce complexity and cost of circuit design. The AM demodulating circuit includes an envelope detector, a notch filter, a low-pass filter, and a comparing circuit. The envelope detector performs envelope detection on an AM signal modulated by a carrier frequency to generate an envelope signal. The notch filter filters the envelope signal to generate a first filtered signal, and a zero point of the notch filter corresponds to the carrier frequency of the AM signal. The low-pass filter filters the first filtered signal to generate a second filtered signal, and the carrier frequency of the AM signal corresponds to a stop band of the low-pass filter. The comparing circuit converts the second filtered signal to a digital signal according to a level value. ## Claims ### Claim 1 An amplitude modulation (AM) demodulating circuit, applied to a radio frequency identification (RFID) system, comprising: an envelope detector, for performing envelope detection on an AM signal modulated by a carrier frequency to generate an envelope signal; a notch filter, for filtering the envelope signal to suppress noise near the carrier frequency of the AM signal thereby generating a first filtered signal; a low-pass filter, for filtering the first filtered signal to generate a second filtered signal; a comparing circuit, for converting the second filtered signal to a digital signal according to a level value: and a bias-voltage circuit, for adjusting a bias voltage of the envelope signal to set the envelope signal within an operating voltage range of the notch filter. ### Claim 2 (depends on 1) The AM demodulating circuit as claimed in claim 1, wherein a zero point of the notch filter corresponds to the carrier frequency. ### Claim 3 (depends on 1) The AM demodulating circuit as claimed in claim 1, wherein the carrier frequency lies within a stop band of the low-pass filter. ### Claim 4 (depends on 1) The AM demodulating circuit as claimed in claim 1, wherein the AM signal is an amplitude shift keying (ASK) signal. ### Claim 5 (depends on 1) The AM demodulating circuit as claimed in claim 1, wherein the AM demodulating circuit is disposed in a Radio Frequency ID (RFID) receiver, and the AM signal is an RFID signal. ### Claim 6 (depends on 1) The AM demodulating circuit as claimed in claim 1, wherein a baseband frequency corresponding to the envelope signal lies within a pass band of the notch filter. ### Claim 7 (depends on 1) The AM demodulating circuit as claimed in claim 1, wherein the low-pass filter has a gain value to amplify baseband frequency components of the first filtered signal, wherein a cutoff frequency of the low-pass filter corresponds to a baseband frequency of the AM signal, and wherein the notch filter has a gain peaking corresponding to the baseband frequency. ### Claim 8 (depends on 1) The AM demodulating circuit as claimed in claim 1, wherein the bias-voltage circuit comprises an alternating current (AC) coupler, for removing DC components of the envelope signal. ### Claim 9 An AM demodulating method, applied to an RFID system, comprising: performing envelope detection on an AM signal modulated by a carrier frequency to generate an envelope signal; filtering the envelope signal in notch manner to suppress noise near the carrier frequency of the AM signal, thereby generating a first filtered signal; filtering the first filtered signal in low-pass manner to generate a second filtered signal; converting the second filtered signal to a digital signal according to a level value: and adjusting a bias voltage of the envelope signal before the notch filtering to set the envelope signal within an operating voltage range of the notch filter. ### Claim 10 (depends on 9) The method as claimed in claim 9, wherein the AM signal is an amplitude shift keying (ASK) signal. ### Claim 11 (depends on 9) The method as claimed in claim 9, wherein the AM signal is a Radio Frequency ID (RFID) signal. ### Claim 12 (depends on 9) The method as claimed in claim 9, wherein the carrier frequency of the envelope signal corresponds to a zero point of the notch filtering, and a baseband frequency corresponding to the envelope signal lies within a pass band of the notch filtering. ### Claim 13 (depends on 9) The method as claimed in claim 9, wherein the low-pass filtering process has a gain value to amplify baseband frequency components of the first filtered signal, wherein a cutoff frequency of the low-pass filtering process corresponds to a baseband frequency of the AM signal, and wherein the notch filtering has a gain peaking corresponding to the baseband frequency. ### Claim 14 (depends on 9) The method as claimed in claim 9, wherein the step of adjusting the bias voltage of the envelope signal comprises removing DC components of the envelope signal.