ACVS

Tektronix 11000-series plug-ins and mainframes contain a subsystem called the Analog Control Voltage System (ACVS). The ACVS produces several independently controllable DC voltages. The voltages are fed to the analog signal path circuitry, e.g., the gain and offset control pins of the M377, or the OFFSET pin on the TekProbe input connector. In plug-ins, the number of voltages produced by the ACVS depends on how many channels the plug-in has. For example, the 11A34 has four channels and has an ACVS that produces sixteen control voltages.

AVCS in 11000-Series Plug-ins

The ACVS comprises three parts:

• AD667 12-bit DAC (single output)
• 74HC4051 analog switches and holding capacitors, implementing a set of sample and hold circuits
• TL074 JFET-input opamps, providing a set of unity-gain output buffers

The output from the DAC is sampled and held, and the buffered voltages are fed to the rest of the plug-in. The voltages in the sample and hold are periodically refreshed. The sample and hold and output buffers are on a daughterboard. (See photo below)

The ACVS is controlled by the M382 chip, which also implements Serial Data Interface (SDI) and sequences the amplifier output enable signals.

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  ACVS sample-and-hold circuits on daughterboard on 11A32

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  ACVS sample and hold schematic in the 11A52

ACVS in 11000-Series Mainframes

In 11300 mainframes, the ACVS is controlled by an 8051-family microcontroller. It employs a DAC312 12-bit DAC, whose output is fed to an array of 32 sample-and-hold circuits similar to the ones in the plug-ins. A refresh cycle sequentially refreshes the voltage on each of the sample-and-hold capacitors. The time to complete the entire refresh cycle is 900 μs. The precision of each sample-and-hold voltage 12 bits.

26 of the sample-and-hold voltages are used independently. The other six voltages are combined in groups of three, using resistor summing networks, to produce two outputs that each have 14.6 bit precision. These two precision voltages are DLYREF0 and DLYREF1. DLYREF0 is the comparator voltage for the SWEEP A delay. DLYREF1 is the comparator voltage for the SWEEP B delay. See schematic below. Historical note: In earlier analog scopes (e.g., the 565), the basic architecture of the sweep delay is the same, but precision DC control voltage for the delay ramp comparator was produced by a multi-turn potentiometer on the front panel.

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  ACVS schematic in 11300 mainframes

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  The weighted sum of three ACVS voltages is fed to the delay ramp comparator