Re: 1S1 sampling bridge GaAs diodes: What alternatives?


Dave Wise
 

The four bridge diodes were identical in my instrument. (Serial Number 002993.)

Since the front and back pairs don't need to be matched as closely as the diodes in a pair, it would not have been unreasonable to make up two pairs instead of a quad.

Tek-made Schottky diodes were a troublesome, low-volume product. D11, the 1S1 sampling bridge, changed several times during its production life.

*152-0014-00 101-?
*152-0314-00 ?-2539
*152-0755-00 2540-2830
*152-0433-00 2831-up

Less forward drop -> better efficiency and loop gain
050-0422-02 mod kit

Here are the characteristics of 152-0433-00 from "Tektronix Schottky Diodes Memo From 1979 (PDF)" at TekWiki.

Reverse breakdown >= 8V at 1uA
Forward resistance <= 25 ohms at 10mA
Capacitance <= 0.6pF at 0V and 1MHz
Leakage <= 30nA at 3V and 55C
Forward voltage matching, each pair <= 50mV 1mA to 10mA
Forward voltage matching, pair to pair <= 200mV 1mA to 10mA

The 1S1 had quite a few changes overall, not just D11. Q324 was a sore spot. The Mod Summary is good reading.

Dave Wise

I'm currently using four HSMS-8202, with two diodes in series replacing one original diode, but they are delicate.
If I were to do it over again I'd try HSMS-2822/HSMS-2825/BAT68-04, again putting two diodes in series, this time to reduce capacitance not increase breakdown voltage.

________________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of snapdiode via groups.io <snapdiode=yahoo.com@groups.io>
Sent: Friday, December 11, 2020 5:58 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 1S1 sampling bridge GaAs diodes: What alternatives?

One thing that springs to mind is the four diodes are not identical in the bridge. The two facing the input are physically a bit smaller and have a yellow dot painted on, and diodes facing the memory circuit are bigger and have an orange dot.
Who knows what that means...

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