Power Supplies - Tektronix 11A52 Service Manual

Theory of Operation
g. A variable gain control input which linearly adjusts the overall gain (pin 9). The gain is zero at -1 V and
maximum at+ 1 V.
h.
An input current null used to cancel input base current and the protection bridge offset current (pin 6).
The input current null signal can correct for up to ±280 µA of offset current caused by both the main amplifier
input current and mismatches in the bridge protection network. During Enhanced Accuracy calibration, the
11AS2 operates the lOX attenuators with the Cal/Norm relay in Cal and K960 in the open position.
This is effectively the same as first placing a SO
Q
resistor across the input, then replacing that SO
Q
resistor
with an open circuit. The input current is detected by measuring the resulting change in input voltage; it is
corrected by adjusting the input current null voltage at pins 19 and 20 on Sample/Hold module A1A2. US40
inverts the input current null signals and aids in ±lS V power supply rejection of noise picked up in the
protection bridge biasing network in the main amplifiers. R6SO, R6Sl, R6S2, and R6S3 prnvide additional
power supply noise rejection.
The outputs of the signal amplifiers are reverse-terminated in 100
Q
impedance matching the output
transmission lines leaving the hybrids. The Display and Trigger signal paths for both channels separately sum
before reaching the edge connector; at the summation points the line impedance becomes SO
Q.
The auxiliary
signals, which are not summed, require external resistive loads to match the SO
Q
line to the edge connector .
Power Supplies
All power for the 11AS2 is furnished by the host oscilloscope and referenced to chassis ground. Each supply is
protected by a reverse-biased Schottky diode to help prevent damage to the circuitry in the event of a power
supply fault.
The decoupled power supplies in the 11AS2 are:
+ lSA, +SA, -SA, - lSA Standard supplies for analog control circuits, on diagrams 1, 2, and 3.
+lSB, +SB, -§B, -lSB Power for U310, U33S, U410 and U540, on diagram 3.
+SD Digital power, used on diagrams 2, 3, and 4.
+lSP, +SP,-SP, -lSP Probe power, fused, from +lSA, +SA, -SA, and-lSA supplies; diagram L
The B supplies are separately filtered to reject noise from digital circuits.
11A52 Extended Service Manual
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