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Fast low duty cycle pulse oscillator

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neazoi

neazoi

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I have found this circuit on the net. I wonder

1. What the sync does and with what signal to drive it (eg. a negative pulse in the sync produces a single pule on the outputs? or what?)

2. What frequency should I expect with 100ns pulse width? I am not talking about the differential pulses repetition rate, I mean a single 100ns pulse, assuming it is repeated every 100ns (which is not in that case).

3. Starting values for C?
 

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Possibly the sync begins a one-shot cycle. The sync is an upward going edge, which produces a brief spike through the capacitor. It turns on the NPN, sending a 100nS pulse, then ending the timing cycle. Quickly the diode discharges the capacitor, and also pulls the NPN bias low (or lower) to prevent oscillations. If the sync circuit is omitted then oscillations happen.

Your schematic resembles a complementary multivibrator found at the webpage below (a collection of different multivibrator circuits). The caption states that the classic astable (usually drawn with NPN's) can be rearranged to use PNP's, or a combination of both types.

https://www.4qdtec.com/mvibs.html
 
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    neazoi

    neazoi

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BradtheRad said:
Possibly the sync begins a one-shot cycle. The sync is an upward going edge, which produces a brief spike through the capacitor. It turns on the NPN, sending a 100nS pulse, then ending the timing cycle. Quickly the diode discharges the capacitor, and also pulls the NPN bias low (or lower) to prevent oscillations. If the sync circuit is omitted then oscillations happen.

Your schematic resembles a complementary multivibrator found at the webpage below (a collection of different multivibrator circuits). The caption states that the classic astable (usually drawn with NPN's) can be rearranged to use PNP's, or a combination of both types.

https://www.4qdtec.com/mvibs.html
Click to expand...

So If I keep the sync input low (gnd) the circuit is inactive and when I send a positive pulse there then just on pulse is output on one side of the multivibrator and then the multivibrator stops and waits for another pulse in the sync?
 

Its a complimentary multivibrator as Brad stated. Being astable it produces a constant oscillating output. The two waveforms are the complimentary outputs and the sync is an input which disables the oscillator when it is held at 0V. Note that the diode must have a lower Vf than Vbe of the NPN transistor (1N64 is germanium, 2N2369 is silicon) and the sync must go to, or very close to ground and be high impedance to let it run.

To work in monostable mode, one of the capacitors must be changed to a resistor.

Brian.
 
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    neazoi

    neazoi

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From a project i did a long time ago, i recall microwave oscillators taking a good microsecond or two to fully stabilize after start up. That is because the transistors go from being off, to ON, and therefore their internal temperatures are increasing. Changing transistor temperature often means the oscillating frequency is drifting pretty far.

So 100 nS? I would worry about the frequency drifting, and the power changing, during that 100 ns pulse.
 
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    neazoi

    neazoi

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