Attenuators and Preamps - Oscilloscope Front End Design (part 3)

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What's behind your oscilloscope's BNC? Analog IC designer Mike Lujan lays it out - part 2 of "Oscilloscope Front End Design" talk Click to subscribe! ► http://bit.ly/Scopes_Sub ◄

Part 1: What is an Oscilloscope? https://youtu.be/o-JvRlKCOaM

Part 2: Hardware Layout - What's Behind the BNC? https://youtu.be/EaBSN6dpqP8

Part 3: Attenuators and Preamps https://youtu.be/gPQRSWcRaFg

Part 4: Signal Offsets and Filters https://youtu.be/1sVEWzMjkBo

Part 5: Probing https://youtu.be/pdbzIwelCL4

What technologies are used for oscilloscope attenuators and oscilloscope preamps?

Attenuators

There are two main things that influence attenuator technologies. The main limiter is device parasitic capacitance and inductance, either in the attenuator itself or in it's support circuitry.

The most basic attenuator is a relay with a resistive divider network. The bandwidth is limited because there is a long signal path required.

A solid state attenuator is often good up to several GHz. They use GaAs MESFET with stepped attenuation. They are similar to a MOSFET, and if the device is on it provides near zero impedance, and when on it switches to high impedance. The bandwidth can be limited by capacitance. At higher frequencies, the higher frequency components can "blow through" the capacitors and the attenuator becomes less effective.

The other limitation is that it has a voltage limit. With too high of a voltage, the system can be non-linear or even damaged.

For the highest bandwidth oscilloscopes, a coaxial stepped attenuator is used. It uses an RF input and gives the appearance of 50 ohms up to very high frequencies.

Preamps

SiGe BiCMOS bandwidths go up to 30 GHz depending on technology node and packaging. A standard QFN can go up to 8 GHz, but with custom IC packaging you can get much higher using custom launches and bypass circuitry. The impedances are also much easier to control.

For very high performance, InP HBT preamps can be used beyond 50 GHz. Indium Phosphide processes allow you to design very fast transistors that are faster than germanium doped silicon.