Make magnetic card readers more reliable in noisy environments
Optimizing card reading
With a method now to resolve the discrepancies in MRH specifications, we can improve card reading performance. Our focus will be on reducing the effects of noise, which mostly affect the zero crossings (ZX). After characterizing the MRH model for the entire frequency range (note: the characterization must include lead wires and the PCB routing), we follow these steps:
Step 1. Choose an Ro value to get an appropriate damping ratio and limit the gain peaking.
- In general, the target should be critically damped to slightly overdamped. As an exception, if for some cases the gain at the 3rd harmonic drops below half, we can equalize the gain by a slightly underdamped system.
- An underdamped system can introduce noise from ringing of the input signal. Ringing noise adversely affects the zero crossing detector, but may also result in false peaks due to gain peaking.
- Keep Ro ≥ 5Rh with the maximum Ro set by ζ =1.
Step 2. On noisier printed circuit boards (PCBs) it helps to make the system overdamped, especially Track 2 (T2).
- T2 has 40 numeric digits, as opposed to 79 alphanumeric characters for T1/T3.
- On T2 longer gaps exist between the peaks where noise can affect ZX.
- Overdamping integrates the T2 signal. The signal approaches a saw-tooth waveform, as shown in Figure 13. Overdampening helps the ZX by filtering out high-frequency glitches.
- Keep Ro ≥ 5Rh so that the head attenuation stays under 20%.
- A note of caution: an excessively overdamped system can lead to errors due to slow settling and peak shifts.
Step 3. If using less expensive but noisier read heads, overcome the noise by reducing the input signal without affecting the damping ratio.
- Choose an appropriate Ro.
- Divide Ro into smaller segments so that the total Ro remains the same as in Step 1.
- Use the appropriate tap to get the required signal division.
- Several ways to do this are described under Practical Examples below.
Step 4. When the read head output on the MAXQ1740 exceeds 300mVP-P, internal clipping of the signal occurs. This clipping can also cause reading errors.
- Use the method described in Step 3 to reduce the signal.
Input signal and noise reduction
Suppose the optimized output resistor value is Ro.
Goal: achieve a 25% reduction in the signal.
- Use one 0.25 Ro and one 0.75 Ro in series across the head. Then 0.75 Ro is tied to the head common-pin side. Tie the midpoint to the input.
- Use four 0.25 Ro in series across the head. Tie the midpoint to the input.
Goal: achieve a 75% reduction in the signal.
- Use one 0.25 Ro and one 0.75 Ro in series across the head. Then 0.25 Ro is tied to the head common-pin side. Tie the midpoint to the input.
- Use four 0.25 Ro in series across the head. Tie one tap above midpoint to the input.
Effects of damping factors
We next consider various damping factors and their effects on the actual signal behavior when test magnetic cards are swiped using an MCR based on the MAXQ1740. MRH 2 was used in the tests. There are two important things to note about the test cards used. First, cards are commercially available from Q-Card and follow ISO/IEC 7811 through 7816 standards. Second, the card signal amplitudes are specified as a percentage of the nominal level. Thus, a 40% card implies a maximum output level that is 40% of the nominal ISO level.
Before we had the MRH model available, it took time-consuming and frustrating guesswork to determine the right value of external resistors. With the model available here, we solved the noise issues by using a 13.5kΩ external resistor that matched our model prediction. Figure 13 shows the overdamped behavior, while Figure 14 shows the critically damped behavior. Comparing Figures 11 and 12, we note the slow settling and peak shifts for the overdamped case compared to the critically damped behavior. Both slow settling and peak shifts can cause timing errors resulting in reading errors as described earlier.
Take out da noise
Using methods presented in this article, one can predict and prevent potential problems early in the design phase or when deciding which MRH to pick. For example, designers can now anticipate that in an underdamped system, reading errors can occur due to false peaks and false zero crossings. Both ringing and excessive gain peaking (around the 3rd and 5th harmonics of the swipe speed) can produce false peaks and zero crossing. Conversely, if the system is drastically overdamped, timing errors can occur because of peak shifts.
The methods presented here are also useful for improving the performance of an existing card-reader system that uses a specific read head. For example, in a noisy system one can first use several series external resistors to make the system critically damped and then tap the MRH output from an appropriate node to divide down the MRH output level. Finally, the methods were verified in an actual card-reader system based on the MAXQ1740 microcontroller.
Irfan A. Chaudhry was a principal member of the technical staff for IC design at Maxim Integrated Products at the time this article was written but is no longer with the company. He joined Maxim in 2009 with over 16 years of mixed-mode IC design experience in data converters, hard disk drive controllers, nuclear weapon testers, and power management. He has more than two dozen designs in production and holds four U.S. patents. He attended the University of Idaho and Washington State University for his undergraduate and graduate studies, respectively.
Our methods are the result of rediscovering the elegant methods developed by A. S. Hoagland in the late 1950s and published in his 1963 book.3
Many thanks are due to Maxim engineers: Steve Grider who provided the most help; Bryan Taylor, Stephen Umfleet, and Lonnie Hornsby for their help in the lab; Kevin Kwak, Don Pearce, Mark Weldele, Gary Zanders, Brandon Priddy, Nadeem Mirza, Mark Lovell, Kathy Vehorn, Aaron Minor, and Jeff Owens for their design, test, and system-level contributions and suggestions.
- ISO/IEC 7810, ISO/IEC 7811, ISO/IEC 7812, ISO/IEC 7813. www.iso.org/iso/search.htm?qt=identification+cards&searchSubmit=Search&sort=rel&type=simple&published=on.
- Cuccia, C. L. Harmonics, Sidebands and Transients in Communication Engineering. McGraw-Hill, New York, 1952.
- Hoagland, Albert. Digital Magnetic Recording, Wiley, New York, 1963.
- Chu, W. W. "Computer Simulations of Waveform Distortions in Digital Magnetic Recordings," IEEE Transactions on Electronic Computers, Vol. 15, pp. 328–336, Jun. 1966.
- Chu, W. W. "A Computer Simulation of Electrical Loss and Loading Effect in Magnetic Recording," IEEE Transactions on Electronic Computers, Vol. EC-16, No. 4, pp. 430–434, Aug. 1967.
- Nilsson, J.W. Electric Circuits, 3rd ed., (Reading, MA, Addison-Wesley Publishing Co.), 1990.
- VanValkenger, M.E. Network Analysis, 3rd ed., (Englewood Cliffs, NJ Prentice-Hall), 1974.
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This material was first printed in April 2012 Embedded Systems Design magazine.
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