3B4

The Tektronix Type 3B4 is a timebase plug-in for 560-series scopes. It was introduced in 1964. It was intended primarily for use with the 561A, but works fine in other 560-series scopes. The 3B4 is unusual in having an explicit "FREE RUN" mode. In the triggered modes, the tunnel diode-based triggering is rated to work to 20MHz. The vertical bandwidth of typical 560-series (non-sampling) scopes is about 10MHz, so 20MHz is adequate. Triggering is done with a TD3A 4.7mA tunnel diode. The trigger input is amplified and buffered by a 6DJ8-based differential amplifier driving a common-emitter 2N964 germanium PNP transistor whose collector current switches the tunnel diode.

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