Thursday, November 24, 2011

Simple Pulse Generator

A Pulse generator usually allows control of the pulse repetition rate, pulse width, pulse delay and pulse amplitude. More sophisticated pulse generators may allow control over the rise time and fall time of the pulses. A pulse generator’s delay is measured with respect to an internal or external trigger. The pulse generator’s rate may be determined by a frequency or period adjust (rep rate). 

Pulse generators may use digital techniques, analog techniques, or a combination of both techniques to form the output pulses. For example, the pulse repetition rate and duration may be digitally controlled but the pulse amplitude and rise and fall times may be determined by analog circuitry in the output stage of the pulse generator. With correct adjustment, a pulse generator can also produce a 50% duty cycle square wave. Pulse generators are generally single-channel providing one frequency, delay, width and output. To produce multiple pulses, these simple pulse generators would have to be ganged in series or in parallel. 

Some Pulse Generators like the BNC Model DB-2 Random pulse generator simulate actual operating conditions without requiring a live source and detector combination. Such parameters as frequency response, linearity, and discrimination levels may easily be measured without the inconvenience of dim oscilloscope display or long accumulation times by a pulse generator. Proper operation of baseline restorer circuits may be quickly verified. Scalers and ratemeters may be checked for satisfactory pulse recognition under random pulse (each pulse generator may be equipped with different capabilities and features).

The negligible amplitude shift with frequency of the pulser (pulse generator) makes the standard frequency test using a live source and a low rate precision pulse generator unnecessary. 

Although most test applications will find the pulser connected to the test input of a charge sensitive preamplifier, it is possible to simulate the preamp itself with the pulse generator. The pulser is connected directly to the main amplifier and the preamp decay time constant is matched by proper selection of the pulser fall time. Set up of a system containing an inaccessible preamp can then be accomplished with ease. 

For accurate simulation of detector pulse shapes, the rise time control should be adjusted to match 2.2 times the detector decay time constant. For example, if a pulse shape analyzer working with CsI-NaI phoswich is to be tested, the pulse generator rise time should be set to 0.5 µsec rise time for the NaI signal, and 2 µsec for the CsI signal. Intermediate signals are best obtained by mixing the outputs from two synchronized generators, 2 µsec rise time. By varying the amplitude ratio of the two generators, intermediate values of rise time are generated. 

Solid state and plastic detectors have decay constants far shorter than the adjustment range of this generator. However, the shaping time constants used in virtually all systems are greater than the 100 nsec minimum rise time. The ballistic deficit formula predicts the reduction in amplitude, B. D., for a shaping system containing identical time constants for all shaping. 

The external reference allows remote programming of the amplitude of the pulser, and the external trigger permits control of the output pulse rate. The latter provision is especially convenient if the average random rate needs to be controlled and an external random clock is unavailable. By placing the pulser in the random mode, a periodic waveform at the external trigger input will control the average random rate. 

A new family of pulse generators can produce multiple-channels of independent widths and delays and independent outputs and polarities. Often called digital delay/pulse generators, the newest designs even offer differing repetition rates with each channel, differing delays and differing widths. They can be producing timing signals and operate in output modes independent of the other channels. These digital delay/pulse generator are useful in synchronizing, delaying, gating and triggering multiple devices usually with respect to one event. One is also able to multiplex the timing of several channels onto one channel in order to trigger or gate the same device multiple times. 

These pulses can then be injected into a device under test and used as a stimulus or clock signal, or analyzed as they progress through the device, confirming the proper operation of the device or pinpointing a fault in the device. Pulse generators are also used to drive devices such as switches, lasers and optical components, modulators, intensifiers and resistive loads. The output of a pulse generator may also be used as the modulation signal for a signal generator.

Pulse Generator Multiplexing

A common request among Pulse Generator users is the ability to sum outputs on a single channel to create a more diverse pulse train. The BNC Model 575 Pulse Generator successfully achieves this function with the MUX feature described below.

Using the Output Multiplexer

Multiplexing allows for the combination of any or all channel settings to be output to any of the outputs. Channel multiplexing only combines timing events of the channels and not the actual output voltages or currents.

The natural choice for generating a pulse of variable frequency and duty cycle is a good bench-top pulse generator. If you don't have a generator or must build one into your system, however, you can do it with a few op amps and some external components (Figure 1).

Figure 1. This simple and versatile pulse generator includes only 15 components.
Figure 1. This simple and versatile pulse generator includes only 15 components. 

U1 and Q1 form a voltage-controlled oscillator (VCO) with a square-wave output and a triangle-wave output. The square wave feeds back to control the charge and discharge of integrating capacitor C1, and is also useful as a sync-signal output. The triangle wave drives the non-inverting input of duty-cycle controller U2-2. R2 fully adjusts the output duty cycle (from 0% to 100%) by controlling U2-2's inverting input. U2-2 generates the output signal directly.

U2-1 buffers R1's wiper voltage, which controls the VCO frequency. A change in supply voltage (V+) varies the output swing without effecting the output frequency or duty cycle, and the circuit operates equally well with a single supply or with dual supplies. The frequency range shown is 20Hz to 13kHz, but you can modify that range by changing the value of C1. The dual op amps (designed for controlling power amplifiers) are chosen for their high output drive, rail-to-rail input and output capabilities, single-supply operation, and exceptional open-loop behavior.

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