Auto Cal And Auto Setup; Timebase Generator - Tektronix 2212 Service Manual

Tnnebase
Generator
2212 Service Manual
Horizontal System and Z-Axis Circuit
PAL U501 contains the horizontal logic. It is a state machine that controls the
horizontal magnifier (diagram 6) to switch between lx gain, 10x gain and 50x
gain. The hold off pulse is the clock signal for this state machine. Some gates
inside U501 are used to decode TIME6 and TIME?. These bits are part of the
control byte that sets the sweep speed. The decode logic selects the proper
capacitor for the hold off generator to obtain the correct hold off time for each
sweep speed.
Auto Cal and AUTO Setup
In calibration mode, the sweep length is set to exactly 10 divisions and appro-
priate time markers are applied to the scope. The first time marker triggers a
sweep. A second time marker is detected by U503B. After EOS, U503A samples
the status of U503B and generates the AUTOTIMOK signal. The system proces-
sor reads AUTOTIMOK and controls the sweep speed (VARTIM in diagram 5B)
The sweep speed is adjusted so a 10 division sweep fits exactly within two time
markers. In auto set up the same circuit is used with the normal sweep length
>10 divisions to make an estimation of the input signal frequency. In this way
the proper sweep speed can be selected for that signal.
The timebase generator consists basically of a capacitor ( C554, C555, C556,
C557) and a current source (Q551 and U552A) that charges that capacitor.
Q552 and Q553 switch on the desired capacitor in a 1 to 100 sequence. Multi-
plexer U550 controls the current source in a 1 to 10 sequence, U551 controls the
current source in a 1, 2, 5 sequence. So there is a 3 dimensional switching
matrix to obtain all the possible sweep speeds. The matrix is controlled by the
code byte from the serial data chain TIMCTR[0 .. 5). TIMCTR[6 .. 7) are used as
TIME6 and TIME? for-the hold off time setting. (See Sweep Logic diagram 5A).
There is some redundancy in the matrix, some sweep speeds can be obtained in
more then one way. The system processor is programmed to select the most
accurate ones. With the VARTIM signal, the current source and as a result the
sweep speed can be controlled continuously.
This signal is used for sweep calibration and for variable sweep. When SWPGT
goes low, U554A, -Band-Dare on. The timing capacitor is discharged to a level
of about -2.5 V. When SWPGT goes high, U554C turns on and U554A, -Band -D
turn off. The current from Q551 charges the timing capacitor and the sweep
runs.
SWPSTRT is a signal from the daculator and is programmed according to the
diode curve of Q554B and the selected current. This compensates for the voltage
across U554B to achieve a sweep that always starts at the same level. The
buffered sweep is passed on to the end of sweep comparator and the horizontal
magnifier.
In X-Y mode the sweep does not run. Q561 switches XAXIS from the CH1
storage pick off to the horizontal magnifier.
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