S-53

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The Tektronix S-53 'Trigger Recognizer is essentially the entire trigger circuit of an oscilloscope. It is rated to 1 GHz and has a BNC input with 50 Ω impedance. It uses tunnel diode triggering with an amplifier before the tunnel diode. The input signal to trigger out jitter is rated at 15 ps or less. The trigger-out signal is 1 V into 50 Ω with 600 mV/ns slew rate.

The signal first passes through a compensated NPN differential amplifier, Q10 and Q12, which provides an adjustable trigger offset voltage and trigger polarity selection. Switching diodes CR6 and CR8 are used for selecting the trigger polarity. The differential amplifier (i.e., phase splitter) generates both phases, inverted and noninverted. Depending on the position of the trigger polarity switch, one or the other of the phases is shunted by 200 ohm resistor. The two signals are buffered by common-base amplifiers Q20 and Q22, whose collector currents are summed. R25 sets up a bias current on trigger tunnel diode CR25. The extra current from Q20 or Q22 pushes CR25 over its peak current, and it switches to the high voltage state.