Sampling diodes

There are at least two kinds of sampling diodes used in classic Tek sampling gear, GaAs junction diodes and Schottky diodes. It is easy for these diodes to get destroyed by ESD. They are designed to have low capacitance, which means that a small amount of charge can produce a potentially damaging voltage across the diode.

If the output of a sampler has a large DC shift even with no input to the sampler (an external triggering), and does not respond to the DC offset control, it is possible that one or more sampling diodes could be damaged.

The original diodes are no longer available. When sampling diodes need replacement, there are two options. They can be scavenged from other gear or replacements can be improvised using modern parts. HP/Avago and now Broadcom makes integrated pairs and quads of high-speed Schottky diodes (e.g. HSMS-28xx series). These are a decent replacement for the original diodes.

Possible modern substitutes

Type of Diode	Tek sampler	Measured rise time	Remarks
HSMS2822	4S1	320 ps	better than specification and very smooth trace
HSMS2822	S-2	120 ps	didn't match original specification
MMDL301	S-2	185 ps	high blow-by, not recommended
BAT1704	S-2	110 ps	didn't match original specification, but very smooth trace

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  4S1 Sampling diode replaced by Avago HSMS2822