Tektronix TDS 340A, Technical Reference

Page: 52 / 288

Theory of Operation
TDS 340A, TDS 360 & TDS 380 Technical Reference
3–9
Time Base Controller (TBC). The time base controller (TBC) provides the
horizontal acquisition control for the oscilloscope. It counts pretrigger and
posttrigger samples and writes data points into acquisition memory. Program-
ming and control of the sampler is through the TBC. CPU access to acquisition
memory is also through the TBC.
The TBC operates in three basic acquisition modes.
In Fast mode the sampler acquires and stores the complete record internally.
When stopped, the analog data can be read out, digitized, and moved into
acquisition memory. This process is based on the 60.6 MHz oscillator
(Y401).
In Slow mode, the sampler acts as a sample and hold device. The data points
are transferred point by point to be digitized and stored in acquisition RAM
as they are acquired. This process is based on the 40.0 MHz oscillator
(Y402).
In Peak Detect mode the sampler holds the minimum and maximum values
over a sample interval. The data points are transferred point by point to be
digitized and stored in the acquisition RAM as they are acquired.
The processor initiates the acquisition. Once ACQINIT is released and the
pretrigger count is satisfied, EPTHO (end of pretrigger holdoff) is asserted to the
trigger logic. Once the trigger logic receives the EPTHO, it will accept triggers.
A trigger from SYNTRIG A will start the posttrigger counter in the TBC. Once
the posttrigger count is finished, the sampler will be stopped.
Acquisition Memory.
The acquisition memory consists of an 8K-by-8K SRAM.
The CPU reads this memory through the time base controller.
The time interpolator counter in the TBC counts for the duration of the slow
ramp and terminates the count when it receives COUNTSTOP from the time
interpolator.
The holdoff counter holds off trigger from being accepted for a programmable
period of time. It is asynchronous to the FAST system clock. HOLDOFF begins
on MAT (main accepted trigger).
Time Interpolator. The Time Interpolator is a dual-ramp timing circuit that detects
and measures the time difference between a trigger event and the sample clock.
The CPU uses this time to correctly place the data points obtained on different
trigger events. The TBC contains the ramp counters.
The dual ramp consists of a short-duration, positive-going ramp and a long-dura-
tion, negative-going ramp. The ramps are the result of charging or discharging
integrating capacitors C307 and C305 from constant current sources. The