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Low-cost, high-resolution time-measurement application.

Time Domain Reflectometry (TDR) measures the impedance of a transmission line using the reflected energy of a pulse sent down that transmission line at the speed of light for the media it is in {typically, 60 to 80 percent of the speed of light in free space). As the pulse arrives at an impedance, mismatch energy is reflected back to the pulse source and arrives at a time that is two times the electrical distance from the pulse source to the impedance mismatch. Figure 1 shows the typical waveform at the pulse source of an unterminated transmission line.




Goal of Design

A great deal of information can be determined using a TDR, such as the consistency of the impedance along the transmission line, quality of connectors, location of connectors, length of transmission lines, faults due to shorts, or faults due to opens. But the cost of the test equipment involved is not cheap--typically many thousands of dollars. A low-cost solution can be implemented using one of three pieces of information--the length of the transmission line, faults due to shorts or faults due to opens.

The goals for this design are a worst-case time resolution of 1 ns (better then 0.5 ft); overall accuracy of time measurement on the order of [+ or -]1 percent; a maximum length of measurement with 0.5 ft. resolution <200 ft; and the cost to be <$10, for component costs associated with the TDR

Time Measurement with CTMU Peripheral

The heart of the low-cost TDR system is a peripheral called the Charge Time Measurement Unit (CTMU). A CTMU peripheral on a microcontroller (MCU) can be used to measure time with a high degree of precision and resolution [typical resolution is <1 ns].

A simplified CTMU block diagram is shown in Figure 2 and consists of a constant current source and high-speed switch, an ADC, a discharge switch, and an analog multiplexer. All of these blocks are integrated onto a single MCU.



The CTMU is a constant current source that can be turned on and off in <1 ns. This current can be converted to a time-dependent voltage with the addition of a capacitor, using the following standard equation:

I is the output current of the CTMU current source; C is the input capacitance of the ADC plus any stray capacitance; and V is measured by the ADC. This results in the ability to calculate T. The current output of the CTMU is connected to the on-chip 10-bit ADC, which uses a capacitive digital-to-analog converter (DAC) for an input.

So, we have a current source that can be turned on and off in <1 ns and an ADC] with a fixed capacitance for an Input The analog-to-digital input is charged for an unknown amount of time, after which a voltage measurement is made using the ADC. Knowing C and I, and measuring V, we can calculate T. Figure 2 shows a typical waveform generated by the CTMU. The first pulse closes the switch, starts the charging of the analog-to-digital input capacitor and creates a linear voltage ramp (Figure 1). The second pulse opens the switch and stops charging the analog-to-digital input capacitor. The voltage can now be measured and the time calculated using Eq. 2. Software calibration can be accomplished by measuring two known times and calculating a value for C/I.

Implementation of TDR Design

Figure 1 shows a schematic of the TDR circuitry and the TDR waveform. Not shown is a 16-bit MCU with the CTMU peripheral on-chip (a PIC24FJ32GA102). The MCU provides a pulse that goes to a high-speed buffer, which drives a 50 [ohm] coax through a 50[ohm] resistor. This results in the waveform shown in Figure 1.

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By Jim Bartling, Microchip Technology Inc.
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Title Annotation:Logic ICs
Author:Bartling, Jim
Publication:ECN-Electronic Component News
Date:Aug 1, 2009
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