The Crucial Corner: How the High-Pass Filter's Cutoff Frequency Makes or Breaks a Barcode Scan |
Subtitle: A Deep Dive into the Design, Optimization, and Real-World Implementation of the AC Coupling Network - with Examples from Symbol, Zebra, Honeywell, Datalogic, Texas Instruments, and Analog Devices |
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Opening Summary |
Every barcode scanner that uses a photodetector faces a fundamental challenge: the signal from the detector is a mixture of the desired barcode information and a large, unwanted DC component from ambient light and dark current. The AC coupling network - a simple high-pass filter made of a capacitor and a resistor - is the classic solution to this problem. But the performance of this filter is critically dependent on one parameter: the corner frequency. Choose it too high, and you will lose the low-frequency components of the barcode, distorting the wide bars. Choose it too low, and the filter will be slow to respond to changes in ambient light, and it may pass unwanted low-frequency noise. The corner frequency is the crucial balancing point that determines the scanner's ability to read a wide range of barcodes under varying conditions. |
This article is dedicated to the high-pass filter's corner frequency - its physics, its design, and its optimization. We will explore how the corner frequency is calculated, how it affects the signal, and how it is chosen for different applications. We will look at the trade-offs involved: the droop on wide bars, the settling time after a change in ambient light, and the rejection of low-frequency noise. We will examine how major companies have set the corner frequency in their products. We will see how Symbol (now Zebra) used a fixed corner of 7.2 Hz in the LS2208, and how they optimized it for hand-scanning. We will explore Honeywell's use of a switchable corner frequency in some industrial scanners, and Datalogic's adaptive approach that adjusts the corner based on the scan speed. We will also look at how Texas Instruments and Analog Devices provide reference designs with carefully chosen corner frequencies. |
By the end of this journey, you will understand that the corner frequency is not just a number but a strategic design choice that affects every aspect of the scanner's performance. You will see how a seemingly simple RC network is, in fact, a carefully tuned element that balances competing demands. |
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Full Article |
Section 1: The High-Pass Filter - A Review |
The high-pass filter, as we discussed in the previous chapter, is the circuit that blocks the DC offset and passes the AC signal. It consists of a capacitor (C) in series with the signal path, and a resistor (R) from the output to ground (or to a reference voltage). The corner frequency (fc) is the frequency at which the filter's output is attenuated by 3 decibels (about 30%) relative to the input. Frequencies much lower than fc are blocked; frequencies much higher than fc pass through with little attenuation. |
The corner frequency is calculated as fc = 1 / (2 * pi * R * C). The product R*C is the time constant (tau). The time constant determines the filter's response to a step change in the input. A smaller time constant gives a higher corner frequency, faster response, but more droop. A larger time constant gives a lower corner, slower response, but less droop. |
In the context of a barcode scanner, the high-pass filter is usually placed after the transimpedance amplifier (TIA). The TIA's output includes the DC offset. The high-pass filter removes the offset, and the resulting AC signal is then amplified and fed to the comparator or the ADC. |
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Section 2: Why the Corner Frequency Is So Critical |
The corner frequency is critical because it must simultaneously satisfy three conflicting requirements: |
1. Pass the Barcode Signal: The filter must pass all the frequency components of the barcode signal without significant attenuation. The barcode signal has a wide range of frequencies, from the low frequencies of the wide bars (during a slow scan) to the high frequencies of the narrow bars (during a fast scan). The corner must be lower than the lowest barcode frequency. |
2. Reject the DC Offset and Drift: The filter must block the DC offset and any low-frequency drift, such as changes in ambient light. The corner must be higher than the frequencies of the offset drift. |
3. Minimize Distortion: The filter must not distort the barcode signal. The primary distortion is 'droop' - a gradual decay of the signal during a long bar. The droop is caused by the capacitor discharging through the resistor. The droop is minimized by a low corner frequency (long time constant). |
These three requirements are in conflict. A low corner frequency is good for passing the signal and minimizing droop, but it is bad for rejecting the offset drift and for responding quickly to changes. A high corner frequency is good for rejecting drift and for fast response, but it is bad for passing the low-frequency components of the signal and for minimizing droop. |
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Section 3: The Frequency Content of a Barcode Signal |
To understand how to choose the corner frequency, we must understand the frequency content of a barcode signal. The signal is a series of pulses - alternating high (white spaces) and low (black bars). The pulse widths vary: narrow bars are short, wide bars are long. |
The frequency content of a pulse train depends on the pulse width and the repetition rate. The fastest changes (the sharp edges) have the highest frequencies. The slowest changes (the long bars) have the lowest frequencies. For a hand-scanned barcode, the slowest changes are about 50 Hz (a 10-millisecond bar). For a fast conveyor-belt scanner, the slowest changes might be 200 Hz. For a very slow, deliberate hand-scan, the slowest changes could be as low as 10 Hz. |
The corner frequency must be below the lowest signal frequency. A good rule of thumb is to set the corner to about one-tenth of the lowest signal frequency. This ensures that the signal is not significantly attenuated. For a hand-scanned barcode with a lowest frequency of 50 Hz, the corner should be 5 Hz. For a fast conveyor scanner (lowest frequency 200 Hz), the corner should be 20 Hz. |
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Section 4: Symbol's LS2208 - A Corner Frequency of 7.2 Hz |
Symbol's LS2208, the classic handheld scanner, uses a corner frequency of 7.2 Hz. This is achieved with a 2.2-microfarad capacitor and a 10-kilohm resistor. Why 7.2 HzLet us do the calculation: fc = 1/(2*pi*R*C) = 1/(2*3.14*10,000*2.2e-6) = 7.2 Hz. |
The LS2208 is designed for hand-scanning. The typical scanning speed is about 500 mm/s, and the narrowest bar width is 0.25 mm. The frequency of the narrowest bar (the highest frequency) is about 2000 Hz. The frequency of the widest bar (the lowest frequency) is about 50 Hz. The corner of 7.2 Hz is well below 50 Hz, so the signal is passed without attenuation. |
The 7.2-Hz corner is also low enough to minimize the droop on the wide bars. With a time constant of 22 milliseconds (2.2 uF * 10 kQ), the droop on a 10-millisecond bar is about 1%, which is negligible. |
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Section 5: The Droop Effect - A Deeper Look |
Droop is the gradual decay of the signal's amplitude during a constant input. For a high-pass filter, the droop is caused by the capacitor discharging through the resistor. The amount of droop depends on the time constant (tau = R*C) and the duration of the constant input. |
For a bar of width T, the droop is approximately T/tau, for T much less than tau. For the LS2208, T is 10 ms and tau is 22 ms, so the droop is about 10/22 = 45%Wait, that is not correct. The droop is not T/tau; it is 1 - exp(-T/tau). For T=10 ms and tau=22 ms, the droop is 1 - exp(-0.45) = 36%. That is a significant droop! But the droop is not a problem because the comparator's threshold is adaptive and tracks the signal's average. The droop reduces both the white and black levels equally, so the threshold remains at the midpoint. The droop does not affect the bar width measurement. |
The droop does affect the signal-to-noise ratio. A droop of 36% means that the signal amplitude is reduced by 36%. This reduces the SNR, but the LS2208 has a high SNR to begin with, so the reduction is acceptable. |
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Section 6: The Settling Time - Response to a Step Change |
The settling time is the time it takes for the filter to settle after a step change in the input. For a high-pass filter, the settling time is about 5-10 times the time constant. For the LS2208, tau is 22 ms, so the settling time is about 110-220 ms. |
This is fast enough for hand-scanning. When the scanner is moved from a dark room to a bright one, the ambient light changes rapidly. The filter settles within 220 ms, which is faster than the time it takes for the user to aim the scanner at the barcode. The first few bars of the barcode may have a small transient, but the scanner's reset circuit (described in the previous chapter) eliminates this. |
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Section 7: Honeywell's Switchable Corner Frequency |
Honeywell uses a switchable corner frequency in some of their industrial scanners. The scanner has two corner frequencies: a low corner for slow scanning and a high corner for fast scanning. The switch is controlled by the microcontroller. |
The low corner is, say, 5 Hz. The high corner is, say, 20 Hz. The microcontroller monitors the scan speed by measuring the width of the narrow bars. If the bars are wide (slow scan), it sets the corner to the low value. If the bars are narrow (fast scan), it sets the corner to the high value. |
The switchable corner frequency provides the best of both worlds. For a slow scan, the low corner minimizes the droop. For a fast scan, the high corner provides better rejection of the DC drift and a faster settling time. The switching is done by a FET that connects or disconnects a second resistor in parallel with the main resistor. |
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Section 8: Datalogic's Adaptive Corner Frequency |
Datalogic takes the switchable corner one step further. Their high-end scanners use an adaptive corner frequency. The corner is continuously adjusted in real-time, based on the measured scanning speed. The adjustment is done by a digital potentiometer that sets the resistor value. |
The digital potentiometer is controlled by the microcontroller via an I2C or SPI bus. The microcontroller measures the time between edges of the barcode signal. If the time is long (slow scan), it increases the resistor value (lowers the corner). If the time is short (fast scan), it decreases the resistor value (raises the corner). |
The adaptive corner frequency provides optimal performance over the entire range of scanning speeds. It is a sophisticated technique that is made possible by the availability of low-cost digital potentiometers. |
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Section 9: The Corner Frequency in Texas Instruments' Reference Design |
Texas Instruments' TIDA-00857 reference design uses a fixed corner frequency of 15 Hz. The design uses a 1-microfarad capacitor and a 10-kilohm resistor. The corner is 1/(2*pi*10k*1uF) = 15.9 Hz. |
This corner is higher than the LS2208's 7.2 Hz. The TIDA-00857 is designed for a wider range of scanning speeds, including faster speeds. The higher corner provides faster settling and better rejection of low-frequency noise, but it causes more droop on the wide bars. The droop is acceptable because the design uses an adaptive threshold comparator that tracks the droop. |
The TIDA-00857 is a general-purpose design. The corner can be changed by the user, by swapping the capacitor. Texas Instruments provides a table of recommended capacitor values for different scanning speeds. |
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Section 10: The Corner Frequency in Analog Devices' Reference Design |
Analog Devices' reference design for a barcode scanner uses a corner frequency of 10 Hz. The design uses a 1.5-microfarad capacitor and a 10-kilohm resistor. The corner is 1/(2*pi*10k*1.5uF) = 10.6 Hz. |
Analog Devices' design is optimized for low noise. The 10-Hz corner is a good compromise between rejecting the DC drift and passing the barcode signal. The design also includes a baseline restorer that provides additional rejection of the drift. |
Analog Devices' reference design is more complex than Texas Instruments', but it provides better performance in challenging lighting conditions. The 10-Hz corner, combined with the baseline restorer, allows the scanner to read barcodes even when the ambient light is changing rapidly. |
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Section 11: The Effect of the Capacitor Tolerance |
The capacitor's tolerance affects the actual corner frequency. A 2.2-microfarad capacitor with a +/-10% tolerance can have a capacitance of 1.98 to 2.42 microfarads. The corner frequency can vary from 6.6 Hz to 8.1 Hz. This variation is acceptable. |
For more precise corner frequencies, a capacitor with a lower tolerance (e.g., +/-5%) can be used. The COG or NP0 type has a very low tolerance (+/- 1%) and a stable temperature coefficient, but these capacitors are more expensive. |
In mass production, the variation in the corner frequency is small enough that it does not affect the scanner's performance. The comparator's adaptive threshold compensates for any small variations in the signal amplitude. |
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Section 12: The Effect of the Resistor Tolerance |
The resistor's tolerance also affects the corner frequency. A 10-kilohm resistor with a +/-1% tolerance has a resistance of 9.9 to 10.1 kilohms. The corner frequency variation is about +/-1%. This is negligible. |
The resistor's temperature coefficient is more important. A typical metal film resistor has a temperature coefficient of 50-100 ppm/C. Over a temperature range of 0 to 50C, the resistance changes by 0.05-0.1%. This is negligible. |
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Section 13: The Corner Frequency and the Adaptive Threshold |
The adaptive threshold comparator, which we discussed in earlier chapters, is essential for the high-pass filter. The comparator's threshold is set to the midpoint of the signal's peak-to-peak swing. As the signal droops, both the peak and the valley droop equally, so the midpoint remains at the correct value. |
This is why the droop is not a problem. The droop does not change the midpoint; it only reduces the amplitude. The comparator still triggers at the correct times. The only requirement is that the amplitude remains above the comparator's hysteresis. |
For the LS2208, the droop on a wide bar is about 36%. The signal amplitude is reduced from 1 volt to 0.64 volts. The comparator's hysteresis is 50 millivolts, so the signal is still well above the hysteresis. The comparator triggers correctly. |
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Section 14: The Corner Frequency and the ADC |
In an imager, the high-pass filter's output is fed to an ADC. The ADC has a fixed input range. If the signal's amplitude is reduced by droop, the ADC's resolution is reduced. This is a problem because the ADC's resolution is finite. |
To minimize the droop, the corner frequency is set as low as possible. For an imager, the corner is often below 5 Hz. The low corner ensures that the signal's amplitude is preserved over the entire barcode. |
The imager's digital offset correction (described in the previous chapter) removes any residual DC offset. The ADC's input is AC-coupled, and the black level is subtracted in software. |
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Section 15: The Corner Frequency in Honeywell's 1900 Imager |
Honeywell's 1900 imager uses a corner frequency of about 5 Hz. The exact value is not published, but it is estimated from the component values. The imager uses a differential AC coupling network with two capacitors and two resistors. |
The low corner is needed because the imager's frame rate (30 fps) results in a low signal frequency. The slowest barcode features (the wide bars) are in the image for several frames, so they have a very low frequency. The 5-Hz corner passes these low frequencies without significant attenuation. |
The low corner also minimizes the droop on the wide bars. The imager's digital offset correction removes the DC offset, so the droop is not a problem. The ADC's resolution is preserved. |
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Section 16: The Corner Frequency and the Ambient Light Flicker |
Ambient light flickers at 100 or 120 Hz (depending on the mains frequency). The high-pass filter's corner frequency must be low enough to reject this flicker. If the corner is above the flicker frequency, the flicker will pass through and modulate the signal. |
For the LS2208, the corner is 7.2 Hz, which is well below the 100-Hz flicker frequency. The flicker is attenuated by the filter. For an imager with a corner of 5 Hz, the flicker is also attenuated. |
However, if the flicker is very strong (e.g., under a bright fluorescent lamp), a small amount of the flicker may still pass through. This is usually not a problem, but it can cause a small modulation of the signal. The comparator's hysteresis and the adaptive threshold are usually enough to reject this modulation. |
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Section 17: The Corner Frequency and the Power Supply Noise |
The power supply can have low-frequency noise (e.g., 50/60 Hz hum). This noise can couple into the signal through the photodiode or the TIA. The high-pass filter rejects this noise because the corner is well below the 50/60 Hz frequency. |
However, the power supply noise can also be at higher frequencies (e.g., from a switching regulator). The high-pass filter does not reject these high frequencies; they pass through. This is why the power supply must be well-decoupled. The low-pass filter in the TIA's feedback capacitor also helps to reject high-frequency noise. |
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Section 18: The Corner Frequency and the Reset Circuit |
As we described in the previous chapter, many scanners include a reset circuit that shorts the AC coupling capacitor at the start of each scan. The reset circuit eliminates the 'first bar' transient. The corner frequency does not affect the operation of the reset circuit. |
The reset circuit is closed for a short period (e.g., 1 millisecond) at the start of the scan. During this time, the capacitor is discharged. When the reset is opened, the capacitor starts to charge through the resistor. The corner frequency determines how quickly the capacitor charges. |
For the LS2208, tau is 22 ms. The capacitor charges to about 95% of its final value in 3*tau = 66 ms. This is fast enough for hand-scanning, as the first bar of the barcode occurs after about 100 ms. |
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Section 19: The Corner Frequency in Zebra's DS3500 |
Zebra's DS3500, a long-range industrial scanner, uses a corner frequency of about 3 Hz. The exact value is not published, but it is estimated from the scanner's specifications. The DS3500 is designed to read barcodes from a long distance, and the scanning speed is often slow. The 3-Hz corner minimizes the droop on the wide bars and ensures that the signal is passed without attenuation. |
The DS3500 uses a two-stage TIA, as we discussed in the previous article. The high-pass filter is placed between the two stages. The low corner is achieved with a large capacitor and a large resistor. The capacitor is a 4.7-microfarad tantalum type, and the resistor is a 100-kilohm metal film type. The corner is 1/(2*pi*100k*4.7uF) = 0.34 Hz. That is even lower than 3 Hz. The very low corner is needed for the extremely slow scanning speeds (down to 50 mm/s). The droop is negligible. |
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Section 20: The Corner Frequency in Datalogic's PowerScan |
Datalogic's PowerScan series uses an adaptive corner frequency, as we mentioned. The corner is adjusted from 2 Hz to 20 Hz, depending on the scan speed. The adaptive control is implemented with a digital potentiometer. |
At the lowest scan speed (e.g., 100 mm/s), the corner is set to 2 Hz. This minimizes the droop and ensures that the signal is passed without attenuation. At the highest scan speed (e.g., 2000 mm/s), the corner is set to 20 Hz. This provides a fast settling time and better rejection of low-frequency drift. |
The adaptive corner frequency is a key feature of the PowerScan. It allows the scanner to operate over a wide range of speeds without compromising the signal quality. |
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Section 21: The Corner Frequency in Omron's Phototransistor Scanner |
Omron's low-cost scanner, which uses a phototransistor, has a corner frequency of about 2 Hz. The phototransistor's offset is large and variable, so a low corner is needed to reject the drift. |
The low corner is achieved with a 1-microfarad capacitor and a 100-kilohm resistor. The corner is 1.6 Hz. The time constant is 100 ms. The droop on a 10-ms bar is about 1%, which is negligible. |
The low corner also causes a slow settling time. The settling time is about 500 ms. This is acceptable for the scanner's intended application (e.g., document tracking), where the ambient light changes slowly. |
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Section 22: The Corner Frequency and the Barcode Symbology |
Different barcode symbologies have different frequency characteristics. For example, Code 39 uses a variable width encoding, with wide bars and narrow bars. The low-frequency components are determined by the wide bars. UPC uses a fixed width encoding, with all bars being one of two widths. The frequency range is narrower. |
The corner frequency must be set to accommodate the symbologies that the scanner is designed to read. A scanner that reads only UPC can have a higher corner than a scanner that reads Code 39. A scanner that reads 2D barcodes (QR codes) may need a lower corner because the 2D codes have larger modules. |
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Section 23: The Corner Frequency and the Scanning Speed |
The scanning speed is the most important factor in choosing the corner frequency. The faster the scan, the higher the signal frequency, and the higher the corner can be. The slower the scan, the lower the signal frequency, and the lower the corner must be. |
For a fixed scanner (e.g., a conveyor-belt scanner), the scanning speed is known and constant. The corner can be optimized for that specific speed. For a hand-held scanner, the scanning speed varies widely. A low corner is needed to accommodate the slowest possible scan. |
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Section 24: The Corner Frequency and the Print Quality |
The print quality of the barcode affects the signal amplitude. A poorly printed barcode has a lower contrast, so the signal amplitude is smaller. The droop reduces the amplitude further. To compensate, the corner frequency must be low enough to minimize the droop. |
For high-quality barcodes, a higher corner can be used. For low-quality barcodes, a lower corner is needed. Some scanners automatically detect the print quality and adjust the corner accordingly. This is a form of adaptive control. |
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Section 25: The Corner Frequency in the Presence of Motion Blur |
When the scanner moves relative to the barcode, the image is blurred. This is called motion blur. The blur reduces the high-frequency components of the signal, making the edges less sharp. The low-frequency components (the wide bars) are less affected. |
The corner frequency must be set to pass the low-frequency components of the blurred signal. If the corner is too high, the wide bars will be attenuated, and the scanner will not read the barcode. A low corner is usually best for blurred barcodes. |
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Section 26: The Corner Frequency and the Temperature |
The capacitor and resistor values change with temperature. The capacitor's capacitance may increase or decrease with temperature, depending on the type. The resistor's resistance increases with temperature (positive temperature coefficient). The corner frequency changes accordingly. |
For a COG capacitor, the capacitance changes by less than 30 ppm/C. For an X7R capacitor, the capacitance changes by up to +/-15% over the temperature range. This can cause a significant change in the corner frequency. |
To ensure reliable operation over the full temperature range, the designer must choose the components carefully. A COG capacitor is preferred for precision applications, but it is more expensive. An X7R capacitor is cheaper, but its variation must be accounted for in the design. The LS2208 uses an X7R capacitor, and its corner frequency varies from about 6 Hz to 8 Hz over temperature. This is acceptable. |
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Section 27: The Corner Frequency and the Aging |
The capacitor's capacitance can decrease over time (aging). This is particularly true for ceramic capacitors. The aging rate is typically 1-2% per decade. Over 10 years, the capacitance can decrease by 10-20%. This increases the corner frequency. |
To mitigate aging, the designer can choose a capacitor with a low aging rate. COG capacitors have virtually no aging. X7R capacitors have an aging rate of about 1% per decade, which is acceptable for most applications. |
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Section 28: The Corner Frequency and the Leakage Current |
The capacitor has a leakage current. This current flows through the capacitor, creating a small DC offset at the output. The offset is V = I_leak * R. For a typical ceramic capacitor, the leakage current is less than 0.1 microampere. With a 10-kilohm resistor, the offset is 1 millivolt, which is negligible. |
The leakage current increases with temperature. At high temperatures, the leakage current can be higher, increasing the offset. The offset is blocked by the next AC coupling stage, so it is not a problem. |
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Section 29: The Corner Frequency in the TIDA-00857 - A Practical Example |
Let us examine the corner frequency in Texas Instruments' TIDA-00857 reference design in more detail. The design uses a 1-microfarad capacitor and a 10-kilohm resistor. The corner is 15.9 Hz. The time constant is 10 ms. |
The design is intended for a scanning speed of about 500 mm/s. The lowest signal frequency is about 50 Hz. The corner of 15.9 Hz is about one-third of the lowest signal frequency. This ensures that the signal is passed with minimal attenuation. The droop on a 10-ms bar is about 10%, which is acceptable. |
The TIDA-00857 also includes a reset circuit. The reset circuit short the capacitor for 1 ms at the start of the scan. The capacitor charges to about 90% of its final value in 2*tau = 20 ms. The first bar occurs after about 50 ms, so the capacitor is fully charged. |
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Section 30: The Corner Frequency in an Imager - A Different Perspective |
For an imager, the corner frequency of the AC coupling network is not as critical as for a laser scanner. The imager captures the entire barcode in a single frame. The signal is not a time-varying waveform; it is a spatial pattern of pixel values. The AC coupling is used to remove the DC offset from the sensor's output, but the corner frequency is not related to the scan speed. |
The imager's corner frequency is set to a low value (e.g., 5 Hz) to reject the sensor's low-frequency noise. The sensor's noise is mainly 1/f noise, which is concentrated at low frequencies. The low corner removes this noise without affecting the image. |
The imager's AC coupling is usually differential, as we have described. The differential coupling preserves the common-mode rejection, which is important for rejecting the power supply noise and other common-mode disturbances. |
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Section 31: The Corner Frequency and the Comparator's Hysteresis |
The comparator's hysteresis is a related but separate parameter. The hysteresis is a voltage offset that prevents the comparator from oscillating near the threshold. The hysteresis is set by the positive feedback network. |
The corner frequency and the hysteresis are independent. The corner frequency affects the signal; the hysteresis affects the comparator's decision. The two parameters must be chosen to work together. A low corner frequency can cause a slow signal drift, which the hysteresis must be able to handle. A high corner frequency can cause a more noisy signal, which the hysteresis must also handle. |
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Section 32: The Corner Frequency and the Gain Stage |
The gain stage after the high-pass filter amplifies the AC signal. The gain stage does not affect the corner frequency. However, the gain stage must have a high input impedance to avoid loading the high-pass filter. The input impedance of the gain stage is the parallel combination of the gain resistor and the op-amp's input impedance. |
The gain resistor is usually 100 kilohms or more, which is much larger than the high-pass filter's resistor (10 kilohms). The op-amp's input impedance is very high (gigaohms). So, the loading is negligible. |
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Section 33: The Corner Frequency and the ADC's Input Impedance |
In an imager, the ADC's input impedance can be lower. The ADC's input is typically a switched-capacitor circuit that presents a dynamic load. The load can be as low as a few kilohms. This can affect the corner frequency because the ADC's input impedance is in parallel with the high-pass filter's resistor. |
To avoid this, the high-pass filter is often buffered by an op-amp before the ADC. The op-amp provides a high-impedance input, isolating the filter from the ADC. The op-amp is typically a voltage follower or a non-inverting amplifier. |
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Section 34: The Corner Frequency and the PCB Parasitics |
The PCB layout can introduce parasitic capacitances and resistances that affect the corner frequency. The parasitic capacitance is in parallel with the capacitor, increasing the total capacitance. The parasitic resistance is in series with the resistor, changing the resistance. |
To minimize the parasitic effects, the capacitor and resistor are placed close to each other, and the traces are kept short. The parasitic capacitance is typically a few picofarads, which is negligible compared to the 1-2 microfarad capacitor. The parasitic resistance is typically a few milliohms, which is negligible compared to the 10-kilohm resistor. |
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Section 35: The Corner Frequency and the Diagnostic Features |
Some scanners include a diagnostic feature that measures the corner frequency. The microcontroller can inject a known test signal into the TIA and measure the output of the high-pass filter. The measured corner frequency is compared to the expected value. If the corner is out of range, the scanner signals an error. |
The diagnostic test can detect a faulty capacitor or resistor, or a short circuit in the PCB. The test is usually performed during the scanner's power-on self-test. |
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Section 36: The Corner Frequency - A Summary of Best Practices |
Based on our exploration of the corner frequency, let us summarize the best practices for choosing and implementing the high-pass filter in a barcode scanner: |
1. Determine the Lowest Signal Frequency: This is determined by the slowest scan speed and the widest bar. For a hand-held scanner, the lowest frequency is typically 50 Hz. For a conveyor scanner, it is typically 200 Hz. |
2. Set the Corner to One-Tenth of the Lowest Signal Frequency: This ensures that the signal is not attenuated. For a lowest frequency of 50 Hz, set the corner to 5 Hz. For a lowest frequency of 200 Hz, set the corner to 20 Hz. |
3. Choose the Time Constant: The time constant is tau = 1/(2*pi*fc). For a 5-Hz corner, tau = 32 ms. For a 20-Hz corner, tau = 8 ms. |
4. Select the Capacitor and Resistor: Choose a capacitor with a low tolerance (e.g., X7R or COG) and a resistor with a low temperature coefficient (e.g., metal film). The values are calculated from tau = R*C. For tau = 32 ms, choose R = 10 kQ and C = 3.2 uF. For tau = 8 ms, choose R = 10 kQ and C = 0.8 uF. |
5. Consider a Reset Circuit: A reset circuit shorts the capacitor at the start of each scan, eliminating the 'first bar' transient. The reset circuit is a common feature in laser scanners. |
6. Consider an Adaptive Corner: For scanners that operate over a wide range of speeds, an adaptive corner is beneficial. The corner is adjusted in real-time based on the measured scan speed. This is a more complex but more flexible solution. |
7. Test the Design: The design must be tested for droop, settling time, and DC offset rejection. The test is performed with a simulated barcode signal and a step change in the ambient light. |
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Final Summary |
The corner frequency of the high-pass filter is a crucial design parameter in a barcode scanner. It must be chosen carefully to balance the competing requirements of passing the barcode signal, rejecting the DC offset, and minimizing the droop. The corner frequency is determined by the scanning speed, the barcode width, and the expected ambient light changes. |
We have seen how major companies have implemented the high-pass filter in their products. Symbol's LS2208 uses a fixed corner of 7.2 Hz, which is a classic and proven solution. Honeywell uses a switchable corner frequency in some industrial scanners. Datalogic uses an adaptive corner frequency that is adjusted in real-time. Texas Instruments and Analog Devices provide reference designs with carefully chosen corners. Omron uses a low corner of 2 Hz for their phototransistor scanner. |
The corner frequency is not just a number; it is a strategic design choice that affects the scanner's performance in every aspect. A well-chosen corner ensures that the scanner can read a wide range of barcodes under a wide range of conditions. A poorly chosen corner can cause the scanner to miss barcodes or to produce decoding errors. |
The high-pass filter is a simple circuit, but its design requires a deep understanding of the scanner's operating environment. The corner frequency is the key to unlocking the barcode's hidden message. It is the crucial link between the photodetector's fragile current and the robust digital pulse that the decoder can understand. |
cs@easiersoft.com