3B5

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The Tektronix 3B5 is a timebase plug-in for 560-series scopes.

It can be remotely controlled and also has an automatic "seek" mode. It is typically used with a 3A5 vertical plug-in.

Seek Mode

When the 3B5 is put into seek mode, it automatically finds a good sweep speed. The criterion for a sweep speed being good is that it is the slowest sweep such that a second trigger event does not occur before the sweep has progressed beyond a set time window. The time window begins at the start of the sweep and typically extends for three divisions. The "CYCLES/SWEEP" potentiometer controls when the time window ends.

The sweep seek state machine starts with the sweep reset and the sweep speed at its slowest. The trigger is armed and if a signal is present, the sweep begins. If a second trigger event occurs during the time window, the sweep is considered too slow. In that case, the sweep speed is incremented to the next faster setting. If the sweep gets beyond the window without a second trigger event occurring, the sweep is considered good, and the sweep seek state machine stops. When the sweep seek state machine is stopped, the scope operates in normal mode, i.e., waiting for trigger, sweeping, resetting, and waiting for trigger in an endless loop.

For example, say the 3B5 window is three divisions and the input signal is a 1 kHz square wave, and rising edge triggering is selected. The user engages SEEK mode.

1. The sweep seek state machine sets the sweep speed to 5 sec/div. Three divisions at 5 sec/div is 15 seconds, so the time window begins at the rising edge event that caused the sweep to start, and ends 15 seconds after the that event. The second rising of the 1 kHz occurs after 1 ms, so it occurs within the time window. Therefore, the sweep speed is advanced to 2 sec/div.
2. At 2 sec/div, the window is 6 seconds. 1 ms < 6 seconds; sweep speed is advanced to 1 sec/div.
3. At 1 sec/div, the window is 3 seconds. 1 ms < 3 seconds; sweep speed is advanced to 0.5 sec/div.
4. At 0.5 sec/div, the window is 1.5 seconds. 1 ms < 1.5 seconds; sweep speed is advanced to 0.2 sec/div.
5. At 0.2 sec/div, the window is 0.6 seconds. 1 ms < 0.6 seconds; sweep speed is advanced to 0.1 sec/div.
6. At 0.1 sec/div, the window is 0.3 seconds. 1 ms < 0.3 seconds; sweep speed is advanced to 50 ms/div.
7. At 50 ms/div, the window is 150 ms. 1 ms < 150 ms; sweep speed is advanced to 20 ms/div.
8. At 20 ms/div, the window is 60 ms. 1 ms < 60 ms; sweep speed is advanced to 10 ms/div.
9. At 10 ms/div, the window is 30 ms. 1 ms < 30 ms; sweep speed is advanced to 5 ms/div.
10. At 5 ms/div, the window is 15 ms. 1 ms < 15 ms; sweep speed is advanced to 2 ms/div.
11. At 2 ms/div, the window is 6 ms. 1 ms < 6 ms; sweep speed is advanced to 1 ms/div.
12. At 1 ms/div, the window is 3 ms. 1 ms < 3 ms; sweep speed is advanced to 0.5 ms/div.
13. At 0.5 ms/div, the window is 1.5 ms. 1 ms < 1.5 ms; sweep speed is advanced to 0.2 ms/div.
14. At 0.2 ms/div, the window is 0.6 ms. 1 ms > 0.6 ms; sweep speed stays at 0.2 ms/div.

Key Specifications

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Links

• Tektronix 3A5 and 3B5 Automatic Seeking Mode @ YouTube
• Automatic / Programmable Plug-ins @ vintagetek.org

Pictures

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  Logic Board

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  Counter Board

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  Panel Wiring

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  3B5 description in 1969 catalog

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  Programming port pinout.