Vladimir Rentyuk
EDN
This Design Idea is a solution for the following problem: I needed to provide a pulse of a specified duration from “dirty” pulse bursts that can vary over a 60 dB range. Minimal delay time between input and output was also required, so I could not use an integrator or other simple design. The circuit is used in a distance meter, which is why little and constant delay between input and output pulses is so important. Testing has demonstrated excellent operation.
Figure 1 provides a well specified output from dirty input pulses, such as relaxation, harmonic oscillations, pulse bursts, pulses with undefined durations, etc., ranging from 10 mV to 5 V. The output is a positive logic-level pulse of controlled duration.
 |
|
| Figure 1. | A pulse discriminator. |
Input pulses come to a positive input of comparator IC1 through a differentiating circuit C4 & R5, which should be set to the minimum permissible for your application). Pot R6 establishes the threshold of the comparator, in this case from 13 mV to 60 mV. The threshold should be set to the minimum allowable level of the input signal.
Monostable multivibrator IC2-1 generates the output pulses. A second monostable, IC2-2, generates a protective or blanking interval. This interval (established by R10, C7) should be longer than the output pulses, and shorter than the input pulse burst period.
When the level of the input drops below the threshold of comparator IC1, its output will change from high to low. (Note: the initial condition of IC2-2 is high on pin 12.) Monostable IC2-1 generates output pulses by its negative-edge triggered input (pin 1). The positive edge of its output triggers (by its positive edge-triggered input pin 10) IC2-2, which generates the blanking interval. During this time interval, the open-collector output of the comparator will not be able to go high. A timing diagram of this process is presented in Figure 2.
 |
|
| Figure 2. | Timing diagram of the pulse discriminator, 5 µs/div Traces:
by choosing faster parts. The dynamic range is more than 60 dB. |
R3 & C10 provide a power-on reset. R7 & C6 provide stability of trigger of both monostable multivibrators. Otherwise, a starting pulse has a glitch of a few nanoseconds only. The diode V1 provides rapid discharge of all parasitic capacitances connected to the output of comparator IC1. Hence, any additional input impulses will not retrigger IC2-1. R13 & V2 are optional – they protect the comparator input against signals with amplitudes beyond ±5 V. Naturally, this Design Idea can be used with positive polarity inputs by changing the connections of the comparator inputs.
