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 also has a loopback function using one of the 74HC4051 analog multiplexers. The output voltages can be routed to a TLC540 8-bit ADC in the plug-in (U750 in the 11A52) which allows the microprocessor to verify that the control voltages are approximately correct.
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.
Twenty six 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: Earlier analog scopes (e.g., the 565) used the same basic architecture for sweep delay: the instantaneous sweep ramp voltage is compared with a user-controllable DC voltage that represents the desired amount of delay. In 11300 scopes, the DC voltage is produced by the ACVS; In the 565 it is produced by a multi-turn potentiometer on the front panel.
The ACVS in DSA600 mainframes generates 48 control voltages.