S-51

The Tektronix Type S-51 is a synchronizing head for sampling oscilloscope systems such as the 3S2 which use S-series sampling heads. From a periodic input signal in the 1GHz to 18GHz range, the S-51 generates trigger pulses which have a fixed time relationship with the input signal. The input is a 50-ohm SMA connection. The input impedance is 50 ohms real in parallel with 1pF. The minimum input sensitivity is 100mVpp and the maximum kickout is 400mV. The maximum jitter of the generated trigger signal is specified to be 15ps for input signals less than 5GHz and 10ps for input signals above 5Ghz.

Equivalent-time sampling equipment generally requires the input signal to be repetitive. The S-51 is more restrictive; it is specifically designed for periodic signals. This is true of all oscilloscopes that use synchronization instead of triggering.

The S-51 contains a tunnel diode sync oscillator that oscillates at about 200 MHz (165 MHz to 285 MHz). The input signal is coupled with this oscillator, resulting in injection locking. When locked, f_osc is a subharmonic of f_in.

• f_osc = f_in / n, where n is an integer between 5 and 90, inclusive
• f_osc is between 165 MHz and 285 MHz.

The approximate range of f_osc is adjustable.

Trigger pulses are produced at a the beginning of a cycle of the sync oscillator. After each trigger pulse is produced, the sync oscillator is turned off for about 15 microseconds. As a result, the S-51 produces trigger pulses at a repetition rate of approximately 67 kHz. Even without any input, the S-51 will produce trigger pulses at approximately 67 kHz.

The S-51 uses a BSM connector for its trigger output on the front panel. The trigger output signal is, of course, also available at the rear interface connector.

• Tektronix S-51 Manual (PDF)

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