Executive Block - HP 5334B Service Manual

proximately
+
50 mV. The U22A comparator then sets the sensitivity for the instrument to approximately
8 mV rms sine wave.
8-151. When the Counter is operating normally, switches U26, U27, and U28 are used to select the proper trig-
ger level or hysteresis voltage for the Input Amplifier. If the Counter is in the Trigger Level mode, the amplifier
is set to the predetermined sensitivity level and the Front Panel Control or a recalled voltage level from the
DAC sets the amplifier trigger level. This allows the user to adjust the trigger level for small signals with dc
offset. When the counter is in the Sensitivity mode, the amplifier trigger level is preset to 0 volts and the Front
Panel Control or a recalled voltage level from the DAC sets the hysteresis of the Input Amplifier.
8-152. The one area where the DAC B circuitry differs from the DAC A circuitry is following the Trigger Level
Control Switch. DAC B has an extra analog switch, U28D. The operation is identical to the other switches such
as U27C. During the READ LEVELS-Trigger Levels mode, a 'lTL HIGH at U28D pin
1 1
closes the contacts
between pins 13 and 14. The Front Panel Control voltage is then present at pin 3 of comparator U36A.
8-153. The DAC operates in three modes. The first mode is the READ LEVELS- Trigger Levels mode. In this
mode, the voltage setting of the Front Panel Control is determined by using the Measurement MCU, the DAC,
and the comparator U36B. To discover the Front Panel Control setting, the MCU executes binary search, step-
ping the DAC voltage through its range and monitoring the output of the comparator. As the comparator output
toggles, the MCU responds by shifting the DAC voltage closer to the Front Panel Control setting. At the end of
the search routine, the DAC voltage is equal to the Front Panel Control setting and this value is sent to the Ex-
ecutive MCU to be displayed.
8-154. The second mode is READ LEVELS-Peak Levels. In this case, the Input Amplifier is used as the com-
parator instead of U36B. The DAC voltage is directed to the amplifier (at U30 pin 3) to be used as a trigger
level control. During the DAC binary search the comparator U22A triggers on the positive and negative peaks of
the input waveform, and the trigger points are sensed by the Measurement MCU and displayed as the voltage
peaks of the input signal. This method is used when Auto Trigger is enabled and for Pulse Width, and RiseFall
measurements as well.
8-155. In Auto Trigger, the DAC voltage is first used for performing the READS LEVELS-Peak Levels opera-
tion. The Measurement MCU then sets the Counter trigger level by setting the DAC voltage to the 50% point be-
tween the peaks. A Risepall time measurement is similar except that the MCU set the trigger points to the 10%
and 90% points of the peak values.
8-156. The third mode of DAC operation is to program the amplifier trigger level or the sensitivity that has
been stored in CMOS RAM or programmed through HP-IB. The instrument powers up in the Trigger Level
mode and enables front panel control of the trigger level or the sensitivity level. Under HP-IB control, the DAC
is active and the Front Panel Control is disabled.
8-1 57. EXECUTIVE BLOCK
8-158. Overall operation of the HP 5334B is controlled by the Executive Microcomputer (MCU). It receives in-
structions from the front panel keyboard or the HP-IB MCU, then sends instructions to and receives data from
the Measurement MCU. The Executive MCU continuously scans the keyboard for any change and updates the
display annunciators and front panel key lamps accordingly. When a measurement is complete, the Executive
MCU accepts the data from the Measurement MCU, then sends the measurement data to the front panel dis-
play and the HP-IB MCU.
8-159. The Executive Block contains three major components: The Executive MCU (U19), a 4-to-16 line demul-
tiplexer (U32), and a 4 MHz oscillator (U18).
H P 5334B
- Service
Manual
8-22