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A line driver is that circuit or portion of a well logging tool responsible for imposing pulses or other telemetry on the logging line or cable. Many line driver designs have been proposed and utilized in logging tools, and a few are discussed below. In simple pulse tools, the line driver is often combined with the pulse former or shaper circuit, but below only the line driver is addressed. On this page "logging line", "logging cable", and simply "line" are used interchangeably. Many SIE tools, Bell tools, and older GO / GOI tools use a transformer to couple the line driver to the logging line. These designs are less sensitive to long logging lines or changes in logging line length than other approaches. Transformer designs are probably a superior solution, but they cost more than the transformerless designs discussed below. The downside, aside from higher initial cost, is that these line drivers require a proprietary transformer that may not be readily available if replacement is required. While transformer based line drivers are still used by a few manufacturers, transformerless designs have become more common. These capacitively coupled line drivers can be implemented with discrete transistors or with integrated circuit (IC) buffer chips. The
GO / GOI / MLS COSMOS Tool Discrete Component Line Driver is a good example of a discrete transistor line driver. This tried and true design was used in COSMOS tools from their introduction in 1969 (the nuvistor tools replaced by the COSMOS tools used a transformer based line driver). The line coupling capacitors and added zener diode pulse clamping diodes are a modification of the original design to make it more versatile. The original design relied on a 50 ohm resistor in the low voltage power supply to control pulse height and ran on only 40 volts or so (larger capacitance, lower votage rated capacitors handled the coupling to the logging line). Both positive and negative pulses are inputted as positive in this and similar circuits. An interesting consequence of this design is that the negative pulses will have slightly higher amplitude than positive pulses imposed on the line. The LH0002 and later the HA-5002 buffer amplifier integrated circuits have become favored components in well logging tool line driver circuits. The LH0002 chip was actually a hybrid circuit while the HA-5002 is a true monolithic integrated circuit. The LH0002 hybrid chip was obsoleted by National Semiconductor a few years back, but they are still available, and second sourcing is also possible. The HA-5002 has become the choice for new designs. It should be noted that the eight pin metal can version of the LH0002 (H, H-MIL, H/883, and CH suffixes) and the HA-5002 (HA2-5002-2 and HA2-5002-5) are interchangeable in most line driver circuits. Unfortunately the LH0002 (CN suffix pdip) ten pin dip package is not pin compatible with the HA-5002 (HA3-5002-5 pdip, HA3-5002-5Z pdip, and HA7-5002-5 cerdip) eight pin dip package. There have been many other line driver schemes over the years including one based on the 555 timer chip where pulse processing and line driving was done with this old workhorse (sometimes referred to as the "time machine"). The use of cmos buffers or inverters has been suggested, but there do not seem to be any commercial designs using those devices. 11-27-09 |
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