Internal Trigger Pickoff Amplifier

The Internal Trigger Pickoff Amplifier supplies trigger signals to the Internal Trigger Amplifier in the Trigger circuitry (Diagram 4). Internal trigger signals are provided by the vertical preamplifiers and are applied to the bases of U170B and U170C (for Channel 1) and U270B and U270C (for Channel 2). These transistor pairs are biased on, either individually or together, from the Internal Trigger Switching Logic circuit (Diagram 3).

When Channel 1 is the selected internal trigger source, Q173 and U170A (CH 1) will be biased on and Q273 (CH 2) biased off. Current flowing through R173, R183, and R197 will bias on U197A to keep U197E cut off. Emitter current is supplied to U170A by U197D. In turn, U170A then supplies emitter current to U170B and U170C to enable the Channel 1 internal trigger signals to pass to the Internal Trigger Amplifier.

When Channel 2 is selected as the internal trigger source, Q273 and U270A will be biased on and Q173 biased off. Transistor U197A will remain on, and current supplied by U197D will supply emitter current to U270A. Then U270A in turn supplies the emitter current to U270B and U270C and enables the Channel 2 internal trigger signals to pass to the Internal Trigger Amplifier.

When the TRIGGER INT switch is set to VERT MODE, the actual signal source selected depends on the setting of the VERTICAL MODE switches. If either CH 1 or CH 2 VERTICAL MODE is selected, the preceding discussion on Channel 1 or Channel 2 internal trigger signals applies. When the VERTICAL MODE switch is set to BOTH, the VERTICAL MODE ADD-ALT-CHOP switch setting determines the switching action for selecting the internal trigger source.

Selecting ADD VERTICAL MODE causes both internal trigger-select signals (CH 1 Trig and CH 2 Trig) to be LO, and both Q173 and Q273 are biased off. Transistor U197A then becomes biased off causing U197E to saturate. With U197E saturated, emitter current is supplied to both

Channel 1 and Channel 2 Trigger Pickoff Amplifiers (U170C and U170B for Channel 1 and U270B and U270C for Channel 2) via R196-CR196 and R296-CR296 respectively. When both pickoff amplifiers are enabled, the resulting trigger signal is the sum of the Channel 1 and Channel 2 internal trigger signals. The sum of the current supplied by U197E to both pickoff amplifiers is the same magnitude as the current from U197D when either CH 1 or CH 2 is selected individually. Therefore, the dc output to the Internal Trigger Amplifier will be the same for CH 1, CH 2, and ADD VERTICAL MODE trigger signals.

When ALT VERTICAL MODE is selected with the previously established settings (VERTICAL MODE to BOTH, INT to VERT MODE, and SOURCE to INT), the internal trigger-select signals alternate between channels. On one sweep the Channel 1 internal trigger will be selected as previously described. On the alternate sweep, Channel 2 internal trigger will be selected, again as previously described.

Under the same switch-setting conditions, selecting CHOP VERTICAL MODE produces the same trigger-selection conditions as described for ADD VERTICAL MODE. The sum of the Channel 1 and Channel 2 internal trigger signals will be passed to the Internal Trigger Amplifier. See the "Internal Trigger Switching Logic" discussion for a description of how the internal trigger selection signals are generated.

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