Tektronix 492, 492P, Руководство по обслуживанию

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Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
Video Blanking Circuits
The Video Blanking circuits allow for selective blanking of
the lower and upper ends of the local oscillator range. This
is required because the local oscillator sweeps full span
mode regardless of the prescribed band limits. Thus, the
video system is designed to effectively open a display win­
dow only during the time for display. Data bits 5, 6, and 7,
under control of the microcomputer, select the appropriate
amount of display for each band.
Since the video filtering is on the Video Processor board,
and the PRESELECTOR DRIVE signal (which provides fre­
quency information, in voltge form) is also available, this
board is a logical place for video blanking. Switch U3063
incorporates a disable function that, when provided a low
input, opens all switch sections regardless of individual sec­
tion input. Using this feature, the Video Filter Out signal may
easily be blanked at will.
Control for this disable function is from a combination of
outputs from two comparators, U3015A and U3015B. In­
puts to these comparators are from the PRESELECTOR
DRIVE signal and a combination of voltage dividers that are
switch selected under control of data bits 5, 6, and 7. The
PRESELECTOR DRIVE signal is applied from edge connec­
tor pin 54 to the minus input side of U3015A through divider
resistors R4013 and R4012, and to the plus input side of
U3015B through divider resistors R4014 and R4011. These
dividers reduce the +10 V to — 10 V excursion of the drive
signal to +2.5 to -2 .5 V, the maximum input level to the
comparators.
Input to the plus side of U3015A is from a divider that
consists of resistors R3011, R3012, R4024, and R4015.
Note that the excursion of R4024 is controlled by data bit 5
through pins 15 and 14 of switch U3025, and that the inclu­
sion of R4015 is controlled by data bit 7 through pins 2 and
3 of the same switch. Thus the junction of divider resistors
R3011 and R3012 may be connected to - 1 0 V through
R4024 or to ground through R4015. Refer to the VIDEO
BLANKING table on Diagram 23 for data bit states for dif­
ferent bands.
Input to the minus side of U3015B is from a divider that
consists of resistors R4018, R4017, and R3028. Note the
inclusion of R3028 is controlled by data bit 6 through pins
10 and 11 of switch U3025. Adding resistor R3028 connects
the junction of R4018 and R4017 to +10 V through R3028.
This arrangement of switching negative and positive levels
for comparison with the reduced PRESELECTOR DRIVE
signal, enables the top and bottom extremes of the frequen­
cy excursion to be blanked by activating the disable function
of switch U3063. This blanking is under the control of the
microcomputer.
DIGITAL S T O R A G E < ^ > < ^ >
The addition of Option 02 to the basic 492/492P provides
the operator with the capability of selecting the method for
displaying and processing information contained in the digi­
tal storage memories. This allows operations such as deter­
mining the highest amplitude that occurred during a selected
period (MAX HOLD mode), storing a signal for later exami­
nation (SAVE A mode), subtracting one signal from another
(B-SAVE A mode), averaging signals (AVERAGING mode),
and comparing signals (VIEW A, VIEW B modes). Two
memories are used independently in these operations to
store two complete signals that are each digitized at 512
points across the sweep. Thus, two signals may be ob­
served simultaneously or processed in various ways.
In MAX HOLD mode, the highest amplitude at each of
the 1024 points in successive sweeps is stored and dis­
played. In SAVE A mode, a signal is stored in one memory
for later examination, and is not updated. In the B —SAVE A
mode, the A signal is stored and not updated, then
arithmetically subtracted from the B signal, which is stored
and continually updated. In the AVERAGING mode, the dis­
play area is divided by a horizontal cursor. Above the
cursor, signals are peak detected and displayed; below the
cursor signals are averaged. In the VIEW A and VIEW B
modes, the contents of the selected memory or memories
are displayed.
Graphical presentation of mathematic functions or ex­
perimental data is common today. One class of such graphs
is those that have a single Y value for each X value. An
alternate presentation of the data in this graph would be a
table in which the X coordinate values were simply listed
along with a corresponding Y value for each X value. In
further simplification, if the first X value and the spacing be­
tween X values (assuming that all spacings are equal) were
known, the two column table could be reduced to a single
column with the X value implied by the position of the Y
value in the column. This then is the essence of digital stor­
age: to convert a vertical analog voltge (Y coordinate value)
to a binary number and insert that number in a stored table.
The location of the Y value in the table is determined by
converting to binary the analog sweep voltage (X coordinate
value). Once the table is created by storing a set of binary
numbers representing values across a waveform, the
waveform can be recreated at any time by converting the
table values (Y) and positions (X) back to analog voltages
representing amplitude and sweep position.
The digital storage system used in the 492/492P uses
two tables: A and B. Table B is always updated on every
sweep. Table A is changed unless SAVE A mode is select­
ed. There are 512 A values and 512 B values. The spacing
between values is the same throughout both tables, but the
5-38 REV AUG 1981