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

HP 54501A SERVICE
6-9 .
SYSTEM BOARD ASSEMBLY
THEORY OF OPERATION
The System BoardAssembly 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 54501ASystem
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, twoA/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, and100
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 10mV/div setting) .
Channels2 and 3 attenuators are divide by 10,
non-programmable and display
external trig-
gers at 100 mV/div, 200
mV/div,
and500
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
passingthrough
. 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 andone for channels 3 and 4 . Thus,
only twoA/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, dependingon
Channel Select .