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Frequently Asked Questions


Could you explain to me what the "PULSE MON" test point is and how it might be used? Is it true that the "PULSE MON" test point is available on each of the PDT110A, PDT120A and PDT100A modules?

PDT110A Top ViewThe "PULSE MON" test point gives users the ability to observe a signal resulting from a neutron or gamma event after it has been amplified and appropriate wave shaping applied. The amplified pulse, as measured from the detector, provides users with a method to observe and monitor the signal without affecting the performance of the system. The output impedance of the signal at the "PULSE MON" test point is 50 ohms (about 10kHz), but capacitively coupled. Applications requiring analog signal monitoring rather than the TTL OUT signal will work well with this test point. The signal at the "PULSE MON" can be substituted for the "TTL OUT" if that suits your applications needs better. Yes, the "PULSE MON" test point is available on each of the PDT100A, PDT110A, and PDT120A modules.

Can the TTL OUT signal on your products drive long cables lengths,
and if so, how long?

Each of the PDT100A, PDT110A, PDT120A and PDT210 products has a TTL OUT signal designed with an internal 50-ohm transmission line driver. Instruments are factory tested using a 100 meter cable. In some applications, customers have used these products with cable lengths in excess of 300 meters with good results.

Have the PDT110A and your other products been tested
in high radiation fields?

Our product line has been designed to operate in high radiation fields. The PDT110A has been tested in radiation fields in excess of 1Mrad without failure.

I really do not understand the difference between
the PDT110A and the PDT120A. Which one should be
used for my application?

PDT120A/PDT122A Top View

The PDT120A module performs all of the functions of a PDT110A but also contains an integral cable that is used only to monitor additional detectors by using the PDT122A support module. The integral attached cable on the PDT120 connects directly to the PULSE MON connector of the PDT122A. The PDT120A/PDT122A combination exists only to provide users with a viable lower cost solution for applications that require the monitoring of many detectors.

As an example, suppose you have an application that requires monitoring of thirty-six detectors. Using PDT110A modules, thirty-six modules are required. However, for each PDT120A that is used, generally two PDT122A support modules are used. (Note: Depending on your application, three or four PDT122A may be used for each PDT120A.) For a system requiring thirty-six detectors, twelve PDT122A modules and twenty-four PDT122A's support modules would be used. This has the advantage of simplifying the cable setup, including the cable routing and power supply current requirements. The PDT122A contains only passive components, therefore a power cable is not required.

The PDT122A is substantially lower in cost than a PDT110A. Another advantage, since the PDT122A contains only passive components, is improved resistance to radiation effects.

The disadvantage of the PDT120A/PDT122A combination is its effect on amplifier sensitivity. The PDT122A capacitively loads the PDT120A preamplifier. As PDT122A modules are added, the sensitivity of the PDT120A can be increased by changing the "THRES ADJ" trimpot. For applications requiring the highest sensitivity, the PDT120A/PDT122A could be a disadvantage.

To summarize, for application requiring many detectors, the PDT120A/PDT122A configuration is a cost-effective solution that simplifies your system cabling demands. However, for applications where only a few detectors are used and the highest sensitivity is required, the PDT110A is the answer.

I understand that the PDT100A contains all of the features and performance of a PDT110A plus a high voltage bias supply. What are the maximum voltage and maximum current that this bias supply provides? Obviously, the bias supply will bias the detector attached to the PDT100A via the HN connector. But can this internal bias supply also be used for other applications, and for biasing additional detectors while I'm monitoring the gas-filled detector attached to the PDT110A?

PDT100A Top ViewThe bias supply is designed to be adjusted via an internal 12-turn trimpot or by application of an external dc voltage for use with remote applications. The voltage range is specified at 0 volts to 2200 volts. The output current should not exceed 100ľA. The bias supply is designed to bias detectors only and is not designed to support currents in excess of 50 ľA.

The PDT100A contains a "HV OUT" SHV connector for use as an external bias supply for interfacing to PDT110A, PDT120A/PDT122A modules, and other detector bias needs.

When I use several PDT110 instruments in a system, they seem to have low sensitivity and excess noise. When I use only one PDT110A, everything works great! What am I doing wrong?

This problem could very likely be a power supply problem. Remember, with no counts, a PDT110 requires about 30mA from the power supply. At 100k counts, the supply requirement is about 40mA. For counts in excess of 100k, the supply load requirement remains at about 42mA. RG176 coaxial cable has a resistance of about .32 ohms per meter. If you are using ten PDT110 instruments running at a high count level, your supply current for the system will be about .4 amps. If your power supply cable is 20 meters, the cable resistance will be 6.4 ohms. A voltage drop of 2.56 volts [(6.4ohms X .4 amps)=2.56 volts] will occur from the supply source to the PDT instruments. If you are using PDT110A's that operate on +5volts, the voltage at the amplifier will be 2.34 volts, which is below the power supply specification. To alleviate this problem, you can use PDT110A instruments that are designed for +12V operation. Twelve-volt PDT110A's require less supply current, and operate over a supply range of 9 volts to 15 volts. If you are using PDT110A's that require a +5 volt supply, you must either reduce or eliminate the cable resistance, or increase the power supply voltage at the source until the PDT110A instruments receive the correct voltage.

When I use a 50 meter cable on the TTL OUT, the pulse is not very good.

The PDT110A instruments are shipped from the factory with a 50nS "TTL OUT" pulse setting. The 50nS pulse width can be increased by changing the trimpot setting. A single-turn trimpot is located just beneath the hole labeled "PW ADJ." A 50nS pulse width is not recommended for long cable length. The "TTL OUT" signal is produced using a 50-ohm driver that is expecting to see a 50-ohm terminator. Although it is not necessary, if the 50-ohm coaxial cable is properly terminated, reflections will be prevented and the possibility of double pulse counting will be eliminated. A terminated cable will also help prevent phase delay problems. If the cable length is less than 15 meters, a cable termination will likely not be required. When the cable is terminated, the magnitude of the pulse will be reduced. However, the spectral purity will be greatly improved, and the pulse will still look like a pulse, with good rise and fall times.

What bias voltage should I use with the PDT110A instruments?

PDT110A instruments have been tested with several detector types. Since the bias supply and sensitivity are indirectly related, a lower bias supply will require a higher amplifier sensitivity. The best procedure is to set the sensitivity to about 0.04pC (the factory setting) and adjust the bias voltage until a good plateau is achieved. For .05pC, a good plateau is typically achieved at a bias voltage of 1800-1850 volts. To increase the sensitivity of PDT instruments, turn the "THRES ADJ" trimpot (a 12-turn PDT110A Top Viewtrimpot) clockwise. To optimize the sensitivity, set the sensitivity about midscale. If you're not sure, set it at approximately six turns below maximum (maximum is turned all the way clockwise). Then sweep the bias voltage, looking for a plateau. Increase the sensitivity, and look again for an improved plateau. Continue doing so until an improved plateau is found. Typically, the factory setting is quite good for a bias voltage of about 1800 volts using He3 detectors. This will vary greatly, however, depending on the detector gain and type of detector.

I was tightening one of the SMA connectors with a wrench when the connector itself became very loose.

If you tighten any of the connectors on a PDT110A, PDT120A or PDT100A instrument with a wrench, you will most likely damage the connector solder connections. These connectors will operate quite satisfactorily with hand tightening. Excessive torque on the connector will break the connector leads soldered into the printed circuit board. The connector leads are gold plated brass and are easily damaged with only a little torque applied.

 

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