System Board Assembly; System Board Assembly Theory - HP 54501A Service Manual

HP 54501A SERVICE
6-9 . SYSTEM BOARD ASSEMBLY
THEORY OF OPERATION
The System Board Assembly is built around
the 68000 microprocessor and acquisition cir-
cuitry that provide probing, shaping, storing,
and analyzing of data from a target system .
The system board block diagram has been
divided into two sections : acquisition and
system control.
Figure 6-2 is the acquisition
block diagram and figure 6-3 is the block
diagram for system control .
6-10 . Acquisition
The main blocks of the HP 54501A System
Board Assembly acquisition consists of four
input attenuators, four impedance converters
and preamplifiers, two channel select circuits,
two track and hold circuits, two post
amplifiers, two A/D converters, two trigger
comparators, logic trigger, timebase IC, fine in-
terpolator, D/A converter, and acquisition
RAM.
Attenuators. Channels 1 and 4 attenuators are
divide by 1, 10, and 100 inputs and are fully
programmable through 1/2/5 sequence levels
over 2 mV/5 V/div (1 mV/div and 5 mV/div are
software expands of the 10 mV/div setting) .
Channels 2 and 3 attenuators are divide by 10,
non-programmable and display external trig-
gers at 100 mV/div, 200 mV/div, and 500
mV/div .
The attenuators divide the incoming signal to a
level the impedance converter and preamplifier
can handle . Variable capacitors on the at-
tenuators allow adjustment for stray
capacitance.
Impedance Converters and Preamplifiers. The
four impedance converters and preamplifiers
of the HP 54501A are identical. When the sig-
nal reaches the impedance converter from the
attenuator, it is split into its ac and do com-
ponents. This allows implementation of AC
coupling to block the do component from
passing through . The impedance converter
has an ac adjustment for pulse response .
The low-frequency component is applied to a
low-frequency amplifier and the
high-frequency component to a high-pass
6- 4
filter . DC offset from the DAC is incorporated
in the low-frequency amplifier. A variable
resistor in the low-frequency amplifier is
provided for compensation adjustments for
each channel.
The high- and low-frequency components of
the signal are recombined and applied to the
input FET of the preamplifier providing a high
input impedance for the preamplifier . The
preamplifier changes the incoming signal into
a differential signal (2 complementary signals)
and adds a gain of X2 .5 to one and X5 to the
other . The edges of the outgoing signals
represent the selected trigger point of the input
signal . One of the signals goes to channel
select, to be displayed on screen, and the
other to the trigger comparator .
Channel Select . The channel select deter-
mines one of two channels to be digitized and
displayed by the oscilloscope . There are two
channel select circuits: one for channels 1
and 2 and one for channels 3 and 4. Thus,
only two A/D converters are necessary for the
four-channel oscilloscope . When both chan-
nels are selected for display, the two channels
are sampled alternately so all four channels
are displayed simultaneously .
Track and Hold . Track and hold limits the slew
rate of the signal coming from channel select
for the A/D converter. The adjustment in
track and hold eliminates offset in the output
of the circuit due to the switching between the
two modes.
Post Amplifier. The post amplifier receives the
signal from track and hold and divides it by 1,
2, or 5 . These dividers are what differentiate
the signals from the 10, 20, and 50 mV/div so
that all signals are the same at the A/D con-
verter . (The same is true for the 100, 200, and
500 mV/div and 1, 2, and 5 V/div ranges .) The
signal is then amplified by 12 for an input of 2
V at the A/D converter for full scale on all
ranges .
A/D Converter . The analog-to-digital converters
digitize the signal into an 8-bit binary code that
is then stored in the acquisition RAM . One
A/D converter digitizes the signal from either
channel 1 or 2 and the other the signal from
channel 3 or 4, depending on Channel Select .