Category Archives: Electronics

Steinhart-Hart Thermistor Calibration in Excel Using Matrix Formulas

I had a job this week that required that I use the Steinhart-Hart equation for modeling the thermistor resistance versus temperature relationship. The requirement was driven by the customer's need for high accuracy. Most thermistor applications do not demand high accuracy, but this application can tolerate no more than ±0.2 °C of error.  This means that I cannot use the β-based thermistor model, which in this application would have an error of more than ±2 °C. This page will show how how to perform an efficient 3-point calibration using Excel and a bit of matrix math. As a side benefit, I am using this workbook as an example of matrix math in my Excel tutoring at a local library. Continue reading

 
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BER Test Time Spreadsheet

I test high-speed serial channels every day. The most common test parameter that I need to measure is the Bit Error Rate (BER). Figure 1 shows the most common test configuration used for measuring BER. Because bit errors occur randomly, there is a certain amount of error involved in measuring the parameter. So when you state a BER measurement, you also give a confidence interval to express your level of uncertainty. Continue reading

 
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GPS and Y2K Deja Vu

I have been working since May 2018 as a contractor for various companies on resolving specific issues – I am a troubleshooter. This role has provided me with a number of interesting challenges. One of my recent challenges is dealing with the GPS Week Number Rollover (WNRO) issue that will occur on 7-April-2019, which involves a 10-bit counter that has been counting weeks since 21-August-1999, which is when the counter was last 0.  A 10-bit counter can only count to 1023 and then it will rollover to 0 on the next count. This issue shares many similarities with the Y2K problem. Continue reading

 
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MTBF Predictions Often Misused

erforming an MTBF prediction is to designing HW as putting a license plate on your car is to driving the car. You need the license to legally drive the car, but it adds no value to your driving experience. Similarly, every company I have worked for demands a predicted MTBF for every HW product, but it adds no value to the design process. In fact, I would argue that generating the MTBF predictions actually adds negative value to the product deployment because it generates a number that is often misused by customers to estimate spare requirements and field support costs. Since no one has told customers otherwise, they think the MTBF value accurately reflects the real failure rate of a product. In fact, MTBF predictions provide a gross estimate of the rate of random parts failure at product maturity. Continue reading

 
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Calculating the Pointing Angle for My Television Antenna

I spend quite a bit of time at a cabin I have built in northern Minnesota. Technically, I spend most of my time in the garage on the site and I have decided that I need to be able to watch the local television stations in Duluth. These stations are ~75 miles away and I need to determine the bearing along which to point my antenna. This seemed like a good Excel exercise that I can also use as an example for those I tutor at the Hennepin County Library. There are web calculators available that perform this calculation (example), but it is more fun doing it myself. Continue reading

 
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Test Time vs BER and Confidence Level Using Excel

I am currently working as a project manager on a serial data channel qualification task. During this work, I need to estimate the time required to perform dozens of Bit Error Rate (BER) measurements under different scenarios (see Figure 1). In these measurements, we are working to ensure that our BER is always 1E-10 or less. I have performed this calculation many times and have derived all the associated formulas here. BER is a statistical parameter and its measurement is a function of the confidence level you want in your answer.  In this case, we want the highest confidence level possible consistent with a two-week test effort. Continue reading

 
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Switch-Controlled Circuit To Isolate Battery from Load

I recently was asked if it was possible to design a circuit that will isolate a battery from a circuit until a momentary switch is closed (Figure 1). Once the momentary switch is closed, the battery is connected to the rest of the circuit and it cannot be disconnected by further mechanical switch closures. This was just a proof-of-feasibility exercise and NOT a final implementation. However, it was a good example of how to use LTSpice to verify the first design concept and is worthwhile documenting here. Continue reading

 
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Accelerometer Power Calculation Example

I am working on a product that uses a Bosch BMA253 accelerometer as a motion sensor. This family of products has become a defacto standard for inexpensive motion detection. In this post, I will provide a simple power calculation example along with some empirical data that Bosch provided me. Normally, I would not consider a simple power calculation worth writing about, but the datasheet did not provide a worked example. I also provide an Excel workbook that parameterizes the critical variables. Continue reading

 
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Estimating Component Junction Temperature Using Psi-JT

An engineer stopped by my cube today and asked a question about how to estimate the junction temperature of a part on a circuit card that may have an over-temperature problem. Using the common thermal resistances (θJA and θJC), he was obtaining nonsensical results. This problem was a good illustration of the difficulties present in estimating Integrated Circuit (IC) junction temperatures using the commonly supplied thermal resistances. Continue reading

 
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Thermoelectric Cooler Calculation Example

My team is work hard on Dense Wavelength Division Multiplexing (DWDM) optical systems for communications applications. These systems put multiple wavelengths on a single fiber, and they provide our customers the opportunity to deliver significantly more bandwidth without needing to additional fiber optic cables. Continue reading

 
Posted in Electronics, Excel, optics | 1 Comment