S-2

Template:Plugin Sidebar 2 The Tektronix S-2 Sampling Head plug-in provides a single GR-874 50 Ω input with a 75 ps risetime.

The S-2 can be used in the 7000 series and 3S series of sampling plug-ins. It provides an internal trigger pickoff signal for internal triggering. Noise is specified as less than 10 mV.

When the S-2 was first introduced, the rise-time specification was 50 ps, but that was not achieved in practice, so the rise-time specification was revised to be 75 ps.

The interface between the GR-874 connector and the printed circuit board is described in US Patent 3,426,311.

Key Specifications

Rise time	75 ps
Bandwidth	4.66 GHz eq.
Operating input voltage range	−1 V to +1 V with ≤1 V_p-p signals
Maximum single sample step	200 mV
Maximum input voltage	±5 V_DC or sine of 10 V_p-p up to 10 MHz
Input impedance	50 Ω ±1% (GR-874)
Noise	< 10 mV
Features	trigger pickoff for internal triggering

Similarity to S-1

The S-1 and S-2 samplers are constructed very similarly. The mechanical construction including internal connector locations is identical, and the preamp boards are almost identical. The main difference in these boards is the value of the gain adjust trimpot R46, which is 5k for the S-1 and 30k for the S-2, to allow higher gain settings to compensate for the shorter sampling pulses of the S-2. Bipolar transistors Q41 and Q45 are different types (why?). Preamp boards can be swapped between S-1s and S-2s for testing without issues.

The strobe boards use identical active components (avalanche transistor Q69 153-0556-00 and Snap-off diode D61 152-0335-00), but the board layout is different to accommodate longer strobe clipping lines in the S-1.

The sampling circuit itself is almost identical, except that the dampening resistors R7B and R8B of the S-1 are missing in the S-2. The sampling diodes are different, the S-2 uses a 152-0361-00 GaAs matched pair.

Repair issues

The input should always be terminated when not in use because the sampling diodes are easily damaged by excessive inputs or static discharge. See that article for possible replacements.
The ground contact of input termination resistor R4 can become intermittent, causing the input impedance to go high or fluctuating. The cure is to disassemble the sampler incuding the sampler board covers, and then carefully bend the ground tabs of the input termination resistor outward.