The Synchronous Demodulator Circuit: Extracting Signal from Noise |
Executive Summary |
This article provides a comprehensive, accessible exploration of synchronous demodulator circuits --- the electronic 'lock-in amplifiers' that allow barcode readers to extract a tiny signal from overwhelming ambient noise. We examine how these circuits use phase-sensitive detection to isolate the component of the photodetector signal that matches the reader's own modulated illumination, while rejecting everything else. Rather than focusing on abstract theory, we ground every concept in concrete design examples from Analog Devices, Texas Instruments, and other pioneering companies. We explore the fundamental principle of phase-sensitive detection, the design of switched sampling circuits, the implementation of non-overlapping clocks for precise sampling, and the use of programmable filters for output conditioning. The article covers both discrete implementations and integrated solutions like the ADA2200 and the TRF7960, with special attention to the practical trade-offs between analog and digital approaches. The closing summary synthesizes the key lessons and offers practical guidance for anyone designing synchronous demodulation circuits for barcode reading applications. |
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Chapter 1: The Principle of Phase-Sensitive Detection |
A synchronous demodulator is the electronic heart of the modulated illumination technique. Its job is to 'listen' for the reader's own reflected signal at a specific frequency and phase, ignoring everything else. The principle is simple but powerful: if you know the exact frequency and timing of the light pulses you sent out, you can look for that same pattern in the returning signal, and reject all other frequencies as noise. |
Analog Devices, a leading manufacturer of precision signal processing components, explains the concept clearly in their documentation for the ADA2200 synchronous demodulator: 'Synchronous demodulators, also known as lock-in amplifiers, enable accurate measurement of small AC signals in the presence of noise interference orders of magnitude greater than the signal amplitude. Synchronous demodulators use phase sensitive detection to isolate the component of the signal at a specific reference frequency and phase. It is possible to measure signal amplitude, phase or both' . |
The concept is analogous to listening for a familiar voice in a crowded room. If you know the voice's pitch and timing, you can focus your attention on that specific sound and filter out the rest. The synchronous demodulator does this electronically. It takes the modulated photodetector signal and multiplies it by a reference signal at the same frequency and phase. The result is a DC voltage proportional to the amplitude of the modulated signal, while all other frequency components are averaged out. |
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Chapter 2: The Basic Synchronous Demodulator |
The basic synchronous demodulator consists of a switch or multiplier that alternately inverts or passes the input signal, synchronized with the reference frequency. When the input signal is in phase with the reference, the output is a positive DC voltage. When the input signal is out of phase, the output is negative or zero. |
The operation is often described as a 'chopper' or 'phase detector.' The reference signal, typically a square wave derived from the modulation oscillator, controls the gain of the signal path. During the positive half of the reference cycle, the gain is +1. During the negative half, the gain is -1. The result is that the signal is rectified relative to the reference. |
If the signal and reference are perfectly in phase, the output is a full-wave rectified version of the signal, which averages to a positive DC voltage. If the signal and reference are 90 degrees out of phase, the output is zero. If they are 180 degrees out of phase, the output is negative. This phase sensitivity is the key to rejecting noise: only the component of the signal that is at the reference frequency and phase contributes to the DC output. |
Analog Devices' AD630 is a classic example of a precision balanced modulator/demodulator that can be used for synchronous detection. It uses a comparator and a precision switching network to multiply the input signal by the reference signal, providing excellent linearity and low noise . |
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Chapter 3: The Switched Sampling Approach |
An alternative to the multiplier-based demodulator is the switched sampling approach. Instead of continuously multiplying the input signal by the reference, the switched sampling demodulator samples the input signal at a specific point in each carrier cycle, typically at or near the peak of the carrier waveform . |
A patent on synchronous sampling demodulation describes the method: 'A synchronous sampling clock which is synchronous with an input modulation signal to sample the input modulation signal at a fixed moment (usually at a peak of a carrier or around the peak) in an input modulation signal carrier cycle, a sampling value in one carrier cycle is maintained, and an envelope of the input modulation signal is obtained finally' . |
This approach has several advantages. First, it reduces the fluctuation in the output signal compared to diode envelope detection. Second, it avoids the harmonic distortion and complex filtering requirements of multiplier-based demodulation. Third, it allows a simple, low-power implementation with switched capacitors. |
The key challenge is ensuring that the sampling clock is correctly phased so that the sample is taken at the carrier peak. The patent addresses this by using a local clock with adjustable phase, rather than extracting the clock from the received signal. This reduces phase noise and eliminates the need for complex clock recovery circuitry . |
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Chapter 4: The Non-Overlapping Clock Generation |
A critical aspect of the switched sampling demodulator is the generation of non-overlapping clocks for the sampling switches. The non-overlapping clocks ensure that the two sampling switches are never closed simultaneously, preventing charge sharing and other parasitic effects . |
The circuit described in the patent uses a clock phase adjustment module that receives a local clock pulse at twice the carrier frequency. The pulse is delayed to adjust the phase, and then passed to a non-overlapping clock generation module. This module produces two clock signals with the same frequency as the carrier, a duty ratio of about 25%, and non-overlapping active periods . |
The two clocks drive the first and second sampling switches. The first switch samples the input signal onto a first capacitor when its clock is high. The second switch then transfers the charge to a second capacitor when its clock is high, holding the sampled value for the rest of the carrier cycle. The non-overlapping nature of the clocks ensures that the charge transfer is clean and accurate. |
The use of a local clock for synchronization is a key feature of this design. The patent notes that the local clock 'does not need recovered clock from input modulating signal, does not need clock recovery circuitry, thus reduce circuit scale' . |
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Chapter 5: The Phase Adjustment Module |
The phase adjustment module is essential for achieving optimal demodulation. The phase of the sampling clock must be adjusted so that the sample is taken at the peak of the carrier waveform. If the sample is taken off-peak, the amplitude of the demodulated signal is reduced . |
The patent describes a phase adjustment module with a control input that changes the delay of the local clock. The control can be changed by 'changing corresponding configuration signals or configuration parameters,' allowing the circuit to be tuned for different operating conditions . |
The ability to adjust the phase is particularly important for barcode readers, where the phase of the reflected signal can vary with distance, temperature, and component tolerances. By providing a programmable phase adjustment, the reader can be calibrated to achieve optimal performance. |
The phase adjustment also simplifies the design of the reader, eliminating the need for precise component matching. The patent notes that 'the phase adjustment can be changed by changing corresponding configuration signals or configuration parameters' . |
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Chapter 6: The Low-Pass Filter and Output Conditioning |
The output of the synchronous demodulator is not a clean DC signal. It contains the desired baseband signal along with high-frequency components at the carrier frequency and its harmonics. A low-pass filter is required to remove the high-frequency components and recover the baseband signal. |
Analog Devices' ADA2200 includes an integrated low-pass decimation filter that provides output conditioning . The filter reduces the sample rate and smooths the demodulated output, producing a clean signal that can be digitized by a low-speed analog-to-digital converter. |
The ADA2200 also includes a programmable infinite impulse response (IIR) filter that can be configured as a low-pass, bandpass, or notch filter. This provides additional flexibility for rejecting noise and optimizing the signal-to-noise ratio . |
The combination of the demodulator and the low-pass filter is essential for extracting the barcode information from the modulated signal. The demodulator rejects off-frequency noise, while the low-pass filter rejects the carrier frequency and its harmonics. |
Chapter 7: Analog Devices' ADA2200 - An Integrated Solution |
Analog Devices' ADA2200 is a practical example of a fully integrated synchronous demodulator suitable for barcode readers. The device is designed for signal conditioning in industrial applications, including optical sensors . |
The ADA2200 operates on the principle of 'sampled analog technology.' The input signal is sampled at discrete times, but the amplitude is not quantized. This allows the signal to be processed in the analog domain with digital-like precision, eliminating the quantization noise and rounding errors of conventional digital signal processing . |
The ADA2200 includes several key functional blocks: a sample-and-hold circuit, a fixed FIR low-pass filter, a decimation filter, a programmable IIR bandpass filter, and a mixer/demodulator . The device is configured by programming internal registers over an SPI interface, or by loading an external EEPROM. |
The device is designed for low power consumption, making it suitable for handheld barcode readers . It also reduces the downstream digital signal processing requirements by performing much of the signal conditioning in the analog domain. |
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Chapter 8: The FIR and IIR Filters in the ADA2200 |
The ADA2200 uses a combination of FIR and IIR filters to condition the input signal before demodulation. The fixed FIR low-pass filter provides anti-aliasing protection before the decimation stage. The decimation filter reduces the sample rate by a factor of eight, lowering the bandwidth and reducing noise. |
The programmable IIR bandpass filter provides additional noise rejection. The filter can be configured for different center frequencies and bandwidths, allowing it to be optimized for different modulation frequencies and signal bandwidths . |
The combination of FIR and IIR filters provides a high level of noise rejection, making the ADA2200 particularly effective for applications where the ambient noise is orders of magnitude larger than the signal. The device 'enables accurate measurement of small AC signals in the presence of noise interference orders of magnitude greater than the signal amplitude' . |
The output of the demodulator is available as a buffered analog voltage that can be digitized by an analog-to-digital converter. The ADC can be a low-cost successive approximation or sigma-delta converter, depending on the required bandwidth and resolution. |
Chapter 9: The Heterodyne Approach |
The heterodyne approach is a variant of synchronous demodulation that uses a local oscillator to shift the signal to a different frequency before detection. This allows the use of a fixed bandpass filter to reject ambient light, even if the ambient light is modulated at a frequency close to the barcode signal. |
A patent on a heterodyne scanner describes the principle: 'A local oscillator frequency source configured to generate a local oscillator signal at a local oscillator frequency, wherein the local oscillator frequency is an intermediate frequency selected to be above an upper frequency threshold of the ambient light noise' . The heterodyning circuit modulates the local oscillator signal with the initial laser signal to produce at least one modified laser signal. |
The reflected signal contains components at the sum and difference of the local oscillator and barcode frequencies. The filter, typically a bandpass filter centered at the local oscillator frequency, attenuates the ambient light while passing the modified laser signal components . |
The heterodyne approach is particularly effective against modulated ambient light from fluorescent lamps and LEDs. The bandpass filter can be designed to reject the specific frequencies of the ambient light, leaving only the modulated laser signal. |
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Chapter 10: The Undersampling Technique in Heterodyne Systems |
The heterodyne patent also describes an undersampling technique for digitizing the modulated signal. Instead of using a high-speed analog-to-digital converter, the system uses undersampling synchronized with the local oscillator . |
The undersampling technique uses a low-speed ADC to capture the signal, with the sampling rate chosen to satisfy the Nyquist criterion for the signal bandwidth. The patent provides the conditions: 'The sampling frequency f_s should be chosen such that f_s >= 2(f_H - f_L)' . |
Once the signal is downconverted through undersampling, a digital low-pass filter is applied to retain only the original baseband signal. The digital filtering provides high-order low-pass filtering, thus helping to improve the signal gain process . |
The undersampling technique is a powerful way to reduce the cost and power consumption of the reader. A low-speed ADC is cheaper and consumes less power than a high-speed ADC, while still providing excellent performance. |
Chapter 11: The Application to 13.56 MHz Contactless Readers |
The synchronous sampling demodulation technique is also used in 13.56 MHz contactless readers, which are similar in principle to barcode readers. These readers communicate with transponders using amplitude modulation, and the demodulator extracts the transponder's response from the carrier. |
A Chinese patent describes the use of synchronous sampling demodulation for a 13.56 MHz contactless card reader. The circuit uses a local clock, a phase adjustment module, and non-overlapping clocks to sample the input signal at the carrier peak . |
The patent explains the advantages of this approach over diode envelope detection: 'diode current-voltage response presents nonlinear characteristic, this can cause distorted signals; Output signal have larger fluctuation, have higher requirement to follow-up filter circuit' . The synchronous sampling approach provides lower distortion and less output fluctuation. |
The patent also notes that the local clock avoids the phase noise problems of recovered clock schemes: 'the phase noise of the clock recovered from carrier signal own is comparatively large, will cause the output signal generation distortion of demodulator' . |
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Chapter 12: Texas Instruments' TRF7960 |
Texas Instruments' TRF7960 is an integrated analog front end for 13.56 MHz RFID readers that includes many of the features relevant to synchronous demodulation. The device is designed for applications including proximity identification systems . |
The TRF7960 includes a dual-input receiver architecture with AM and PM demodulation. The device can be configured for different protocols, and includes automatic gain control and selectable bandwidth . The receiver output is a digitized sub-carrier signal, and the device includes a framing system that performs CRC and parity checking. |
The device's 'dual-input receiver architecture to maximize communication robustness' is similar in concept to the differential detection used in some barcode readers. By using two receiver inputs, the device can reject common-mode noise, improving the signal-to-noise ratio . |
The TRF7960 also includes an RSSI (Received Signal Strength Indicator) register that provides a measure of the ambient signal strength. This can be used for adaptive gain control, ensuring that the signal is optimally conditioned for demodulation. |
Chapter 13: The Low Power Surface Detection Technique |
A patent from Symbol Technologies describes a low-power method for detecting the presence of a barcode surface that uses a synchronous demodulator. The technique uses a pulsed light source and a synchronous demodulator to detect the reflected signal, and reduces the pulse duty cycle when no surface is detected to save power . |
The patent explains the circuit: 'A light source that is pulsed at a carrier frequency, the light source producing a pulsed light beam; a light detector that receives portions of the pulsed light beam that are reflected off of the surface, the light detector producing a reflection signal having amplitude variations that are proportional to reflective characteristics of the surface; a synchronous demodulator having a first input that is coupled to receive the reflection signal ... a comparator having a first input coupled to the first input of the synchronous demodulator to receive the reflection signal and a second input coupled to a reference potential' . |
The comparator and synchronizer work together to detect the presence of a surface. The comparator compares the demodulated signal to a reference, and the synchronizer checks that the output occurs at the carrier frequency. This ensures that the detection is based on the modulated light, not on ambient noise . |
The technique is significant because it allows the reader to operate in a low-power state when no barcode is present, extending battery life in handheld applications. |
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Chapter 14: Synchronous Demodulation in Barcode Wands |
The Hewlett-Packard HBCC-0500 digitizer IC, while not a synchronous demodulator in the strict sense, uses a related technique to reject ambient light in barcode wands. The IC amplifies and filters the photodetector signal, and then uses an AM detector to recover the barcode information. |
The HBCC-0500's AM detection technique is similar to synchronous demodulation in that it extracts the envelope of the modulated signal. The difference is that AM detection is not phase-sensitive; it only extracts the amplitude of the signal, not its phase. This makes AM detection simpler but less selective than synchronous demodulation. |
The HBCC-0500's technique is effective for barcode reading applications where the ambient light is not modulated at a frequency close to the carrier. However, for more challenging environments, a full synchronous demodulator provides better rejection of noise. |
The HBCC-0500's low power consumption, described in earlier lessons, is a key advantage for handheld barcode readers. |
Chapter 15: The Importance of Phase Alignment |
The phase alignment between the demodulator's reference signal and the incoming modulated signal is critical for synchronous demodulation. If the phase is not correct, the output amplitude is reduced, degrading the signal-to-noise ratio. |
The Chinese patent on synchronous sampling demodulation addresses this by using a phase adjustment module: 'the phase adjustment of the phase adjustment module can be changed by changing corresponding configuration signals or configuration parameters' . This allows the circuit to be tuned for optimal performance. |
The Symbol Technologies patent on the paper detector also addresses phase alignment. The demodulator's phase is set during manufacturing using data stored in a PROM. This allows the circuit to be calibrated to compensate for variations in the optical and electrical components . |
Phase alignment is particularly important when the reference signal is derived from a local oscillator that is not phase-locked to the signal. Any phase error will cause the demodulated output to be reduced, potentially losing the barcode information. |
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Chapter 16: The Comparison with Other Demodulation Techniques |
The synchronous demodulator offers several advantages over other demodulation techniques, such as diode envelope detection and multiplier-based synchronous detection. |
The synchronous sampling demodulation method described in the Chinese patent 'output signal fluctuation is less' than diode envelope detection, and 'solves the problem' of nonlinear distortion . The method is also simpler and more efficient than multiplier-based detection, which requires complex filtering to remove the harmonic components. |
The ADA2200's sampled analog technology offers advantages over both discrete analog and digital approaches. The analog domain processing eliminates quantization noise and rounding errors, while the digital-like control provides flexibility and programmability . |
The choice of demodulation technique depends on the application requirements. For low-cost, low-power applications, the synchronous sampling method is attractive. For applications requiring high performance and programmability, the ADA2200 is a good choice. |
Chapter 17: The Digital Domain Alternative |
While the synchronous demodulator performs signal processing in the analog domain, the function can also be implemented in the digital domain. A digital synchronous demodulator uses an analog-to-digital converter to digitize the signal, and then performs the multiplication and filtering in software. |
The heterodyne patent describes a digital implementation: 'the signal from the photodiode may simply be amplified by the photodiode preamplifier, converted to a digital signal by an analog-digital converter, and then filtered by a digital signal processor' . |
Digital demodulation offers greater flexibility than analog demodulation. The modulation frequency, filter characteristics, and phase alignment can be changed in software, allowing the reader to adapt to different operating conditions. However, digital demodulation requires a more powerful processor and higher power consumption. |
For many barcode readers, the analog synchronous demodulator is the preferred choice because it provides excellent performance with low power consumption and low cost. |
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Chapter 18: The Use of the TRF7960 in Barcode Readers |
While the TRF7960 is primarily designed for RFID applications, its receiver architecture is relevant to barcode readers. The device's dual-input receiver and AM/PM demodulation capabilities are similar to the signal processing requirements of a barcode reader . |
The TRF7960's 'dual input receiver architecture to maximize communication robustness' is particularly relevant to barcode readers. The dual inputs allow the device to reject common-mode noise, similar to differential optical architecture . |
The device's RSSI register provides a measure of the received signal strength, which can be used for adaptive gain control. This is similar to the AGC used in barcode readers to adapt to varying signal levels . |
The TRF7960's integration of multiple functions on a single chip reduces the component count and simplifies the design of the reader. |
Chapter 19: The Future of Synchronous Demodulation |
The future of synchronous demodulation in barcode readers is likely to involve greater integration and digital control. The ADA2200 and TRF7960 are examples of integrated devices that combine multiple functions on a single chip. |
The trend toward digital signal processing is likely to continue, with more functions being implemented in software. However, the analog synchronous demodulator is likely to remain the preferred choice for low-power, cost-sensitive applications. |
The heterodyne and undersampling techniques described in the patent represent the state of the art in ambient light rejection. As processors become more powerful, these techniques may become more widely available. |
The combination of synchronous demodulation with digital signal processing offers the potential for even better performance. The analog demodulator can remove the bulk of the ambient light, while the digital processor can remove any residual interference. |
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Chapter 20: Practical Considerations for Design |
Designing a synchronous demodulator for a barcode reader involves several practical considerations. The modulation frequency, the phase alignment, the filter characteristics, and the output conditioning must all be carefully designed. |
The modulation frequency must be chosen to be above the ambient light frequencies but compatible with the photodetector and amplifier bandwidth. The phase alignment must be adjustable to compensate for variations in the optical and electrical components. The filters must be designed to reject noise while preserving the barcode signal. |
The ADA2200 simplifies the design by integrating many of these functions on a single chip. The device's programmable filters allow the frequency response to be optimized for the application . |
The power consumption of the demodulator is also an important consideration for handheld readers. The ADA2200 and the low-power surface detection technique described in the Symbol Technologies patent both minimize power consumption . |
Chapter 21: The Lock-In Amplifier Concept |
The lock-in amplifier is a sophisticated instrument that uses synchronous demodulation to extract a signal from a noisy environment. The lock-in amplifier differs from a simple demodulator in that it uses a narrow-bandwidth filter and a phase-sensitive detector to reject noise . |
The lock-in amplifier's operation is based on the principle of frequency-domain filtering. The input signal is multiplied by a reference signal at the frequency of interest. The product is then passed through a low-pass filter with a very narrow bandwidth. The result is a DC voltage proportional to the amplitude of the signal at the reference frequency. |
The lock-in amplifier is particularly effective for detecting weak signals in the presence of noise. The narrow bandwidth of the filter rejects noise at other frequencies, providing a high signal-to-noise ratio . |
The ADA2200 is essentially a lock-in amplifier implemented in a compact, low-power integrated circuit. Its combination of FIR and IIR filters provides a high level of selectivity, making it effective for barcode reading applications. |
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Chapter 22: The Phase and Quadrature Outputs |
Some synchronous demodulators provide both in-phase and quadrature outputs. The in-phase output is proportional to the component of the signal that is in phase with the reference, while the quadrature output is proportional to the component that is 90 degrees out of phase. |
The ADA2200 can provide either the in-phase or quadrature output, depending on the configuration . This is useful for applications where the phase of the input signal is not known or varies. |
The in-phase and quadrature outputs can also be used to measure the amplitude and phase of the input signal. The amplitude is the square root of the sum of the squares of the in-phase and quadrature components, and the phase is the arctangent of their ratio. |
The ability to measure both amplitude and phase is important for barcode readers that use phase-sensitive detection. The phase information can be used to improve decoding performance or to detect certain types of barcodes. |
Chapter 23: The Decimation Filter |
The decimation filter is a key component of the ADA2200. It reduces the sample rate of the signal by a factor of eight, lowering the bandwidth and reducing noise . |
The decimation filter is implemented in the analog domain using sampled analog technology. The filter takes the samples from the FIR low-pass filter and discards seven out of eight samples, keeping only every eighth sample. This reduces the sample rate from f_si to f_so, which is one-eighth of the input sample rate . |
The decimation filter is a key feature of the ADA2200 that simplifies the output conditioning. By reducing the sample rate, the filter allows the use of a low-speed analog-to-digital converter for the demodulated output . |
The decimation filter also reduces the power consumption of the subsequent digital signal processing, making the ADA2200 suitable for battery-powered applications. |
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Chapter 24: The IIR Bandpass Filter in the ADA2200 |
The ADA2200's programmable IIR bandpass filter provides additional noise rejection before demodulation. The filter can be configured for different center frequencies and bandwidths, allowing it to be optimized for different modulation frequencies . |
The IIR filter is implemented in the analog domain using sampled analog technology. The filter uses a cascade of second-order sections to achieve a high-order filter response with minimal component count . |
The IIR filter's center frequency is set to the modulation frequency, and its bandwidth is set to pass the barcode signal while rejecting noise. The IIR filter's high selectivity makes it effective for rejecting ambient light noise that is close in frequency to the signal. |
The combination of the FIR, decimation, and IIR filters in the ADA2200 provides a high level of noise rejection, making the device suitable for the challenging ambient light environments encountered by barcode readers. |
Chapter 25: The Mixer/Demodulator in the ADA2200 |
The mixer/demodulator is the final stage of the ADA2200. It multiplies the filtered signal by the reference signal, producing the demodulated output . |
The mixer is implemented in the analog domain using switched capacitor techniques. The reference signal is a square wave derived from the input clock. The mixer multiplies the input signal by the reference, producing the in-phase or quadrature output . |
The mixer's output is a signal that contains the baseband component along with high-frequency components at the carrier frequency and its harmonics. The baseband component is extracted by the internal filters and presented at the output. |
The mixer is synchronized with the input clock, ensuring accurate phase alignment. The user can select whether the in-phase or quadrature output is presented, depending on the application requirements. |
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Chapter 26: The Barcode Processor and Synchronous Demodulation |
The heterodyne patent describes a system where the barcode processor, which performs the decoding, is separate from the demodulation circuitry. The processor receives the digitized signal from the demodulator and decodes the barcode . |
The processor can be a general-purpose microcontroller or a dedicated digital signal processor. The processor's role is to decode the barcode information from the demodulated signal. |
The separation of demodulation and decoding allows the use of a low-cost processor for the decoding. The demodulator handles the ambient light rejection in the analog domain, reducing the processing burden on the processor. |
The patent describes a system where 'a heterodyne laser scanner includes at least one local oscillator frequency source, signal source, optics, receiver, one or more filters, one or more amplifiers, digitizer, and input/output devices' . |
Chapter 27: The Comparator and Synchronizer |
The Symbol Technologies patent on low-power surface detection uses a comparator and a synchronizer in addition to the synchronous demodulator. The comparator compares the demodulated signal to a reference, and the synchronizer checks that the output occurs at the carrier frequency . |
The comparator is a threshold detector that determines whether the demodulated signal is above a reference level. The output of the comparator is a binary signal indicating whether a surface is detected. |
The synchronizer monitors the comparator output at the carrier frequency. If the comparator output occurs at the carrier frequency, the synchronizer confirms that the detection is valid. This prevents false detections caused by noise . |
The combination of the demodulator, comparator, and synchronizer provides a reliable surface detection mechanism that rejects both DC and AC noise. |
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Chapter 28: The Duty Cycle Reduction |
The Symbol Technologies patent describes a power-saving technique where the duty cycle of the light pulses is reduced when no surface is detected. The duty cycle is maintained at a high rate when a surface is present, and reduced substantially when no surface is detected . |
The duty cycle reduction is controlled by the output of the synchronizer. When a surface is detected, the duty cycle is set to a high value (such as 50%), providing maximum signal strength. When no surface is detected, the duty cycle is reduced to a low value (such as 0.8%), saving power . |
The duty cycle reduction is a significant power-saving feature for handheld barcode readers. The reader can be left on for extended periods, ready to read a barcode, without consuming excessive power. |
The patent notes that the reader 'generates a series of pulses having a duty cycle which depends upon whether light reflected from a surface bearing a bar code symbol has been detected recently' . |
Chapter 29: The Phase Shift Compensation |
The Symbol Technologies patent on the demodulator with phase shift compensation describes a programmable phase adjustment. The phase shift is compensated by a programmable time delay that is applied to the reference signal . |
The phase shift compensation circuit produces a programmable time delay signal that is applied to the reference signal of the demodulator. The delay has a range of 0 to 12.5 microseconds and a resolution of 0.78 microseconds . |
The delay is programmed by data read from the PROM. This allows the phase shift compensation to be tuned during manufacturing, compensating for component variations . |
The phase shift compensation is important because any phase error reduces the demodulated signal amplitude. By compensating for the phase shift, the circuit ensures that the demodulated signal is maximized, improving the signal-to-noise ratio. |
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Chapter 30: The Analog Domain Processing Advantages |
The ADA2200's sampled analog technology offers several advantages over both discrete analog and digital signal processing. The analog domain processing eliminates quantization noise and rounding errors, while the digital-like control provides flexibility and programmability . |
The sampled analog technology uses charge sharing among capacitors to perform the mathematical operations of time delay, multiplication, and addition. This is a low-power, low-noise approach that is well-suited to battery-powered applications . |
The ADA2200 'eliminates the effects of quantization noise and rounding errors' because the signal is not quantized, only sampled . This provides a superior signal-to-noise ratio compared to digital signal processing. |
The device also reduces system power consumption by lowering the ADC sample rate and the downstream digital signal processing requirements . |
Chapter 31: The Integration of Synchronous Demodulation |
The trend in barcode reader design is toward greater integration of functions on a single chip. The ADA2200 and TRF7960 are examples of integrated devices that combine multiple functions in a small package . |
The integration of the demodulator, filters, and output conditioning on a single chip reduces the component count, simplifies the PCB layout, and improves the reliability of the reader . |
The integrated devices also reduce the power consumption by eliminating the need for multiple discrete components. The ADA2200 is a low-power device that is suitable for battery-powered handheld readers . |
The integration of the synchronous demodulator with the other functions of the reader is a key trend that is likely to continue in the future. |
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Chapter 32: The Programmable Filters in the ADA2200 |
The ADA2200's programmable filters are a key feature that allows the device to be optimized for different applications. The FIR filter is fixed, but the IIR filter can be programmed for different frequency responses . |
The IIR filter coefficients can be loaded into the device over the SPI interface. The device supports a range of filter configurations, including low-pass, bandpass, and notch filters . |
The ability to program the IIR filter allows the device to be tuned for different modulation frequencies and signal bandwidths. This is important for barcode readers that support different symbologies with different frequency characteristics. |
The programmable filters also allow the device to adapt to different ambient light conditions. The filter bandwidth can be adjusted to reject specific frequencies of ambient light noise. |
Chapter 33: The ADA2200's Clock Generation |
The ADA2200 generates the reference clock for the demodulator from the input clock. The input clock frequency, f_si, is divided by 32 or 64 to produce the reference clock, f_refclk, and the demodulator clock, f_m . |
The device's documentation states that 'the sensor excitation signal needs to be synchronized with the ADA2200. The simplest way to achieve this is to use the REFCLK output of the ADA2200 to provide the sensor excitation signal directly, or to use the REFCLK output as a trigger for the sensor excitation signal' . |
This ensures that the demodulator's reference is synchronized with the illumination source, providing accurate phase-sensitive detection. |
The clock generation in the ADA2200 is carefully designed to minimize phase noise and jitter, ensuring accurate demodulation. |
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Chapter 34: The Application in Optical Sensors |
The ADA2200 is well-suited to optical sensor applications, including barcode readers. The device's ability to measure small AC signals in the presence of noise makes it ideal for detecting the weak reflected light from a barcode . |
The ADA2200's documentation notes that 'synchronous demodulation is especially useful when the recovered signal from the sensor is likely to have a significant noise component. This could be in the form of background light in the case of a photodiode application, or in the face of power supply noise or other impairments' . |
The device's combination of features makes it a powerful tool for barcode reader designers. The demodulator, filters, and output conditioning are all provided in a compact, low-power package. |
The ADA2200's programmable features allow it to be optimized for different barcode reading applications, from handheld scanners to fixed-mount readers. |
Chapter 35: The Low Power Consumption of the ADA2200 |
The ADA2200 is designed for low power consumption, making it suitable for battery-powered handheld barcode readers. The device's sampled analog technology is inherently power-efficient, and the integrated filters reduce the processing burden on the host processor . |
The device's documentation notes that 'the ADA2200 contains low power signal processing blocks. It can reduce system power consumption drastically in some cases. It accomplishes this by lowering the ADC sample rate which lowers the ADC power consumption along with the power consumption of the digital signal processing elements' . |
The low power consumption of the ADA2200 extends the battery life of the barcode reader, making it more convenient for users. |
The device's low power consumption is particularly important for readers that must operate for extended periods, such as warehouse scanners or retail checkout readers. |
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Chapter 36: The Application in Proximity Identification Systems |
The TRF7960 is designed for proximity identification systems, including RFID readers. The device's dual-input receiver architecture and AM/PM demodulation are similar to the signal processing requirements of a barcode reader . |
The TRF7960's 'dual-input receiver architecture to maximize communication robustness' is relevant to barcode readers, which also need to reject noise and interference . |
The device's RSSI register provides a measure of the received signal strength, which can be used for adaptive gain control. This is similar to the AGC used in barcode readers to adapt to varying signal levels . |
The TRF7960's integration of multiple functions on a single chip reduces the component count and simplifies the design of the reader. |
Chapter 37: The Wideband Receiver and Holes |
A paper on fully differential optical interconnects notes the importance of the receiver in rejecting common-mode noise. The paper states that 'a fully differential link has essentially the same SNR as a comparable single-ended link, but the differential system is less susceptible to the common-mode noise such as the switching noise generated at the transmitter or at the receiver input' . |
The concept of common-mode noise rejection is similar to the synchronous demodulator's rejection of off-frequency noise. The synchronous demodulator rejects noise that is not at the reference frequency, while the differential receiver rejects noise that is common to both inputs. |
The combination of differential optical architecture and synchronous demodulation provides the highest level of ambient light rejection. |
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Chapter 38: The Comparison with Diode Envelope Detection |
Diode envelope detection is a simpler alternative to synchronous demodulation. However, it has several disadvantages that make it less suitable for barcode reading applications. |
The Chinese patent on synchronous sampling demodulation describes the limitations of diode envelope detection: 'diode current-voltage response presents nonlinear characteristic, this can cause distorted signals; Output signal have larger fluctuation, have higher requirement to follow-up filter circuit' . |
Synchronous demodulation provides a more linear response and lower output fluctuation than diode envelope detection. It also provides better rejection of noise and interference. |
The patent notes that synchronous sampling demodulation 'solves the problem' of diode envelope detection . |
Chapter 39: The Comparison with Multiplier-Based Detection |
Multiplier-based synchronous detection is another alternative to the switched sampling approach. However, it also has disadvantages that make it less suitable for some applications. |
The Chinese patent on synchronous sampling demodulation describes the limitations of multiplier-based detection: 'the amplitude of harmonic component is comparatively large, thus when signal frequency and carrier frequency frequency distance less time, the design difficulty of filter is also larger' . |
Synchronous sampling demodulation provides lower output fluctuation and simpler filtering than multiplier-based detection. The method 'output signal fluctuation is less' and 'solves the problem' of filter design . |
The synchronous sampling approach is also simpler and more power-efficient than multiplier-based detection. |
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Chapter 40: The Use of a Local Clock |
The Chinese patent on synchronous sampling demodulation emphasizes the use of a local clock for generating the sampling clock. This avoids the need for clock recovery circuitry, reducing circuit scale and phase noise . |
The patent notes that previous designs 'extract the clock from carrier signal' and that 'the phase noise of the clock recovered from carrier signal own is comparatively large, will cause the output signal generation distortion of demodulator' . |
The use of a local clock simplifies the design and improves the performance of the demodulator. The local clock does not require complex recovery circuitry, and it has lower phase noise than a recovered clock . |
The phase adjustment module allows the local clock to be tuned for optimal sampling, providing the benefits of a synchronized system without the drawbacks of clock recovery. |
Chapter 41: The Programmable Phase Adjustment |
The Chinese patent describes a phase adjustment module that can be programmed by changing 'corresponding configuration signals or configuration parameters' . |
The programmability of the phase adjustment is important for barcode readers because the phase of the reflected signal can vary with distance, temperature, and component variations. By providing a programmable phase adjustment, the reader can be calibrated for optimal performance. |
The programmability also simplifies the manufacturing process. The phase adjustment can be set during production, eliminating the need for precision component matching. |
The phase adjustment module's programmability is a key feature that makes the synchronous sampling demodulator suitable for a wide range of applications. |
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Chapter 42: The Non-Overlapping Clock Details |
The non-overlapping clock generation module is a critical part of the synchronous sampling demodulator circuit. It ensures that the two sampling switches are never closed simultaneously, preventing charge sharing and other parasitic effects . |
The non-overlapping clocks have the same frequency as the carrier, a duty ratio of about 25%, and non-overlapping active periods. The two clocks are complementary in a way that ensures that one switch is open before the other closes. |
The non-overlapping clocks are generated from the phase-adjusted local clock. The clock generation module includes logic that ensures the non-overlap timing is maintained over all process and temperature variations. |
The non-overlapping clock generation is a standard technique in switched-capacitor circuits, ensuring reliable operation and minimizing errors. |
Chapter 43: The Application in Passive Communication Modes |
The Chinese patent on synchronous sampling demodulation is described for use in 'near-field communication (NFC) passive communication mode' . |
In the passive communication mode of NFC, the initiator generates a carrier signal, and the target modulates the load to communicate back. The demodulator extracts the target's response from the carrier signal. |
This is similar to barcode reading, where the reader's illumination is the carrier, and the barcode pattern is the modulation. The synchronous demodulator extracts the barcode pattern from the reflected light signal. |
The patent notes that the demodulator is used to realize 'the demodulation for promoter (Initiator) realize target (Target) inverse signal under near-field communication (NFC) passive communication mode (Passive)' . |
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Chapter 44: The Future of Integrated Synchronous Demodulators |
The trend in semiconductor technology is toward greater integration of functions on a single chip. The ADA2200 and TRF7960 are examples of integrated devices that combine multiple functions in a small package . |
The future is likely to see even greater integration, with the synchronous demodulator being combined with the photodetector, the transimpedance amplifier, and the digitizer on a single chip. This would reduce the component count, simplify the PCB layout, and improve the reliability of the reader. |
The integration of multiple functions on a single chip also reduces the power consumption of the reader, making it more suitable for battery-powered applications. |
The trend toward integration is likely to continue as the technology advances and the demand for smaller, more reliable readers increases. |
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Chapter 45: Summary --- The Synchronous Demodulator in Perspective |
The synchronous demodulator is the heart of the modulated illumination technique. It is the circuit that extracts the barcode signal from the noise, using the power of phase-sensitive detection. |
We have examined how different companies and technologies have approached the challenges of synchronous demodulation: |
Analog Devices offers the ADA2200, a fully integrated synchronous demodulator with sampled analog technology. The device includes a sample-and-hold, FIR low-pass filter, decimation filter, programmable IIR bandpass filter, and mixer/demodulator . |
Texas Instruments offers the TRF7960, an integrated analog front end for RFID readers that includes dual-input receiver architecture with AM and PM demodulation. The device's features are relevant to barcode readers . |
A Chinese patent describes a synchronous sampling demodulation method and circuit for 13.56 MHz contactless readers. The circuit uses a local clock, phase adjustment module, and non-overlapping clocks . |
A Symbol Technologies patent describes a low-power surface detection technique that uses a synchronous demodulator, comparator, and synchronizer to detect the presence of a barcode surface . |
A heterodyne scanner patent describes the use of a local oscillator to shift the signal frequency, followed by bandpass filtering and synchronous sampling . |
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The key lessons from our exploration are: |
Synchronous demodulation is phase-sensitive. It isolates the component of the signal at the reference frequency and phase, rejecting all other frequencies. |
The switched sampling approach is efficient. Sampling the input signal at the carrier peak provides low output fluctuation and simple filtering. |
Non-overlapping clocks are essential. They prevent charge sharing and ensure clean sampling. |
Phase alignment is critical. The phase of the sampling clock must be adjusted for optimal performance. |
Integrated solutions simplify design. The ADA2200 and TRF7960 combine multiple functions on a single chip. |
Digital domain processing offers flexibility. The heterodyne approach uses undersampling and digital filtering. |
In the end, the synchronous demodulator is a testament to the ingenuity of engineers who have found a way to extract a tiny signal from overwhelming noise. It is the secret weapon that allows barcode readers to work reliably in even the most challenging ambient light conditions. The art of synchronous demodulation lies in the careful balance of circuit design, component selection, and system-level integration. |
cs@easiersoft.com