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.

modern diode as substitute

Type of Diode	Unit	reached risetime	note
HSMS2822	4S1	320 ps	better than specification and very smooth trace
HSMS2822	S-2	120 ps	don't reached specification
MMDL301	S-2	185 ps	much blowby, not recommend
BAT1704	S-2	110 ps	don't reached specification. very smooth trace.

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  4S1 Sampling diode replacing by Avago HSMS2822 Diode.