Pulse Immunity Against Noise
At a times pulses are narrow enough to be mixed with noise and spikes. This circuit described below will help you separate the desired pulses from the undesired. In the radar system application it is vital for the detection pulse to be as narrow as possible but at the same time any mixed, foreign signals, or pulse stretching may lead to loss of vital resolution and eventually losing the target. It is also very easy for the intended pulses to lose sync in a hostile environment. In TV application the horizontal scanning problems may cause data loss and images may get lost.
Qa is the amplifier transistor and is used to amplify the incoming and probably noisy pulses. Qa can be used as inverting or non inverting amplifier depending on where the signal is taken from. In the circuit above it is used as an inverting amplifier. If output is taken from emitter of Qa then it would be non inverting or as emitter follower configuration. Rc and Re are selected to provide the proper collector to emitter current and so to set the gain.
Qs is the separator transistor and is used as a switch transistor in which a process to isolate noises and spikes from intended pulses. Bear in mind that if filtering is used instead then the pulse shape may suffer and lose its original shape, also the pulse edges may smoothen and transition may then suffer.
As mentioned above the configuration is arranged in a way to accept a negative going pulses. Ci is selected to shunt higher frequencies to ground and therefore block them from appearing at the output. C1 is selected to provide a proper voltage to forward CR within an appropriate period of time and is discharged via Rb. When C1 is charged CR1 is forward biased causing Qs to switch ON. Rf, Cf1, and Cf2 provide zeros for filtering while C1 provides a pole and this is dependant on the polarity of Qs (NPN or PNP) and its power supply configuration. At lower rates of pulses C1 charges fully and Rf dominates Cf1 and also Cf2 seems to be open. Therefore bringing Qs to full saturation. Please bear in mind that if C1 is selected to be large (pulse rate dependant) then Rb needs to be reduced accordingly depending on the desired ON-STAY period.
You may start by setting Rf to be equal to XCf1 (Impedance of Cf1 cap) at the desired operating pulse rate. Then select Cf2 to cause certain desired signal shunting at the undesired higher pulse rate so to rid the output from the higher rates of pulses. The selection of these components are really dependant on your application. An application of this circuit could be the separation of the radar equipment pulses or the TV sync separator. Select Ro and Rv based on your power supply requirements and the desired level of signal at the output. If needed the output could be further buffered and/or amplified for further conditioning.
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By Engineer: Firas Faham