4S1

The Tektronix Type 4S1 is a dual-trace sampling unit for the 661. The 4S1 was developed in 1961 and introduced in 1962. Each channel has a trigger pickoff, a delay line, and a sampler. Specified rise time is 350ps. The sampling pulses for each channel are generated by a step recovery diode.

Sampling Bridge

The sampling pulses are fed to a sampling bridge made of four GaAs diodes. These diodes are fragile and exact replacements are hard to find. People have improvised replacements using modern integrated Schottky diodes such as those in HP/Agilent's HSMS-28x series.

Dual-trace and X-Y Modes

The 4S1 has a dual-trace mode, where the A waveform and the B waveform are both displayed in the time domain at the same time with a single trigger source and a single equivalent-time sweep rate. Another mode is called "A vert/B horiz". This mode is used in conjunction with an X-Y mode switch on the panel of the 661. The signals enter the 4S1 through GR-874 connectors on the front panel.

Trigger Amplifier

The 4S1 uses an unusual trigger amplifier with two trigger signal paths. One path is for low frequencies, the other for high frequencies. (A similar approach was also used in the main vertical signal path of the 7A29 several years later.) From serial number 101 to 1349, the low frequency path starts with a 25K resistor that DC-couples the input signal to a 2N1429 PNP emitter-follower stage. After that is a 151-103 NPN common-emitter. The high frequency path starts with a trigger pickoff transformer on the input. The pickoff signal feeds a common-base amplifier that uses a 2N700 PNP transistor. The low frequency and high frequency trigger paths meet at the trigger output of the 4S1. From there, the trigger signal passes through the 661, and into a 5T1 or 5T3. 4S1 units with serial number 1350 and up use a slightly less complex trigger amplifier circuit. The later circuit uses a TA2333 silicon NPN transistor to amplify the entire frequency range.

Comparison with S1 Sampling Heads

The sampler board in the 4S1 is roughly equivalent to a pair of S1 sampling heads. A short piece of RG/174 coax connects the 661 timing pulse coaxial connector to the SMB timing pulse input on the 4S1 sampler board. This signal triggers a blocking oscillator which drives a snap-off diode, D2004, which generates the fast falling edge for the channel A and channel B sampling bridges. The 4S1 uses 7586 triodes as high-impedance buffers between the sampling bridge and the amplification that follows.

The 4S1 has a main chassis and sub-chassis that plug into it. There is a sub-chassis extension, part number 012-069, that allows maintenance of the sub-chassis.

• 4S1 manual without schematics
• 4S1 schematics
• Late Issue Tektronix 4S1 Manual (PDF)
• Tek 661 4S1 5T1 Preliminary Manual (PDF)

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  top view of 4S1

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  Coaxial interconnect from timing plug-in goes through the mainframe, into the 4S1, and ends here, at the sampler.

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  This is the sampler. The GaAs sampling diodes are arranged in a diamond shape and are directly connected to the socket from the delay line.

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  The delay line is a coil of coax going from the trigger pickoff to the sampler.

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  LF and HF trigger takeoff

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  Bottom wiring

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  Dual trace card

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  Dual trace card

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  Dual trace card

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  Inverter card

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  Inverter card

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  Inverter card

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  Memory card

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  Memory card

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  Memory card

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  Amp card

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  Amp card

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  Amp card

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  Sampler card

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  Sampler card

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  Sampler card