HP 5334B Service Manual page 139

8-118. In the low frequency path, operational amplifier U30, transistors Q6 and Q10 operate together as a dif-
ferential feedback amplifier. The signal is DC coupled to the op-amp
(U30)
input at pin 2. A voltage from either
the Digital-to-Analog (DAC) Block or the front panel trigger level control goes to the op-amp at pin 3.
Capacitor C79 provides external frequency compensation. On the trigger level control voltage line, R112 and
C84 form a low-pass fdter to keep out digital noise from the DAC Block. At the output of U30, resistor R79
limits the base drive to transistors Q6 and combines with R61 and C58 to filter out noise on the
-
5.2 volt supply
line. R61 performs the additional function of s t a b i i n g Q6 at high temperatures.
8-119. Common-emitter transistor Q6 operates simultaneously as a low frequency amplifier and as a current
source for the Q8 FET in the High Frequency circuit. The current to the FET is determined by emitter resistor
R59. The inverted signal at the collector of common-emitter Q6 is summed with the high frequency signal at the
source of Q8 and sent through emitter-follower Q10.
8-120. The signal gain of the Low Frequency circuit is determined by the feedback resistor R96 and op-amp
input resistors R111, R110, and R95. The values of these resistors are chosen to provide a gain equal to the high
frequency gain of approximately 0.9.
8-121. Resistors R%, R111, R110, and R95 also determine the 1.8V dc gain for the trigger level control voltage.
This control voltage from the DAC Block or the Front Panel Control is variable from
-
2.5 volts to +2.5 volts.
The corresponding level shift at the emitter of Q10 ranges from
-
4 to
+
4 volts. This level shift is used to cancel
out any dc component of the input signal.
8-122. It should be noted that the trigger level control voltage to the Low Frequency circuit does not equal the
trigger level voltage range of the instrument. The trigger level range of the HP 5334B is approximately twice the
voltage setting of DAC Block or Front Panel Control. This higher range is due to the attenuation in the Signal
Conditioning stage and the dc gain in Low Frequency circuit.
8-123. SCHMITT STAGE. The Schmitt stage receives the analog signal from the High and Low Frequency
amplifier and converts it to a square wave output. It also provides variable control of the Counter's sensitivity.
8-124. The major component in this circuit is high speed comparator U22A. At the comparator input, resistors
R55 and R75 form a .74X divider to keep the input voltage within a range of -3 to + 3 volts. Capacitor C70
provides frequency compensation for this divider. The comparator offset adjustment is set by trimmer poten-
tiometer R58, and resistors R56 and R57. The adjustment range is
-
20 to +20 millivolts around the zero volt
trigger point.
8-125. It should be noted that the Schmitt stage always triggers at zero volts. Setting the Counter's trigger level
control does not change the trigger point of the Schmitt. The trigger level control along with the High-Low
Frequency amplifier stage compensates for the dc level of the input signal and shifts it to zero volts.
8-126. The input hysteresis is controlled through the comparator's Latch Enable line at pin 4. The control volt-
age from the DAC Block or Front Panel Control has a range of approximately
-
50 to
+
50 millivolts dc to U22.
This result is in an input sensitivity range of approximately 8 millivolts rms to greater than 120 millivolts rms.
8-127. In the Sensitivity Mode, the voltage at the Latch Enable pin is varied through the Front Panel Control
thereby affecting the sensitivity of the Counter. The trigger level is automatically set to zero volts.
8-128. In the Trigger Level Mode, the voltage at the Latch Enable pin is automatically set to approximately
+
50 millivolts to give maximum sensitivity while the Front Panel Control or the DAC block sets the trigger level.
8-129. The comparator's complementary ECL outputs are buffered by resistors R36E and R36F before the Buf-
fer stage. Resistor pack R37 provides a current sink for the output lines to the
-
5.2 volt supply.
HP 5334B
-
Service Manual
8-19