Tektronix DM 5010, Инструкция по эксплуатации

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Theory of Operation —DM 5010
new function and range information a new measurement
may require.
To set the registers, a new string of serial data must be
clocked into the registers. Since the registers have been
previously reset, any data clocked into the registers from
the IRD line also results in 32 consecutive lows being ap­
plied to U1530. This keeps the registers enabled while the
new data is clocked into the registers. The first bit of func­
tion and range information is always a high signal bit, indi­
cating the beginning of the new data. As the data string is
shifted into the registers, this high is eventually clocked into
the 32nd position. This, in itself, does nothing; but, another
high is applied to the IRD line, signalling the end of the func­
tion range information. This causes U1530 to generate a
strobe that latches the shifted data into the output registers
of U1430, U1330, U1500, and U1300. To change the func­
tion and range settings to any new set of conditions, the
reset-set sequence must be followed again.
With the exception of U1400, the remainder of the Func­
tion and Range Control circuitry consists of current
buffering transistors. On the ADC board, U1400, U1500,
and the shifted I M/2 signal generate the correct timing for
the Function Switch FETs Q1319, Q1217, Q1315, and
Q1317.
OHMS CONVERTER
<3>
The Ohms Converter stage supplies a known current
that flows through a range-dependent reference resistor,
out the front-panel input connectors, and through an un­
known resistance. The voltage drop across the unknown
resistance Vex/ is measured, and then the drop across the
unknown and reference resistance V,, is measured. The
ratio Vex, /(VT-Vexf ), multiplied by , is calculated by the
microprocessor, and the value of the unknown resistance is
determined. Figure 4-10 illustrates signal routing for each of
the phases of an ohms measurement. Table 4-2 gives the
range-dependent switching information.
Operational amplifier U1120 is configured as a voltage
source. The noninverting input is referenced at 0.65 V by
VR1 123 and its associated components. Feedback from the
output at the cathode of CR1225 is through the voltage
divider R1229, R1321, and R1225. In this configuration, the
output voltage is approximately 1.2 V. Depending on the
measurement range, R1223 may be switched into the
feedback voltage divider, increasing the stages output to
approximately 10.2 V.
Transistor Q1021 and its associated components pre­
vent the open circuit output voltage of the DCV Converter
from exceeding +5 V.
Table 4-2
OHMS CONVERTER PARAMETERS
RANGE V^OUT /Rx AT FS
200 10.2 V 1 mA 0.2 V
10 kΩ
2k 1.2 V 0.1 mA 0.2 V
10 kΩ
20 k 10.2 V 0.01 mA 0.2 V 1 MΩ
200 k 1.2 V 1.0 µA
0.2 V
1 MΩ
2 M 1.2 V 0.1 µA 0.2 V
10 MΩ
20 M 1.2 V 40 nA 0.8 V
10 MΩ
RMS CONVERTER
<3>
The RMS Converter stage consists of two selectable
gain stages and a true rms-to-dc converter IC. Operational
amplifier U1500 is configured as a feedback amplifier with a
selectable gain (actually
an attenuation) factor. The ampifier
is referenced to the rms LOW input applied to its
noninverting input. The HIGH input from the front-panel
passes through relay contact K1633S to the amplifiers input
when an rms measurement is to be taken. The ac signal
passes through C1621 to a frequency compensation net­
work and then to the operational amplifiers inverting input. If
the de component of the signal is also to be taken into ac­
count, relay K1621 is turned on and C1621 is bypassed.
The amplifiers attenuation is set by the various feedback
networks. With switches K1405S and K1503S open, the
input amplifiers gain is unity. Closing switch K1503S re­
duces its gain by a factor of 10. Closing switch K1405S by
itself results in an attenuation factor of 1000.
The output of U1500 is then applied to R1307, the input
of a selectable gain amplifier, also referenced to rms LOW.
With relay switch K1201S open, gain of the amplifier is 10
as determined by R1201 and R1307. Closing switch
K1201S sets the gain factor to 1. Table 4-3 illustrates the
various gain switch configurations for the RMS Converter.
Also refer to Tables 10-2 and 10-5 in the pullout pages.
The output of the gain stage, U1200, is applied to a rms
converter IC. This converter, referenced to rms LOW, com­
putes the root-mean-square value of the applied ac or
ac + dc input signal, and outputs at pin 8 a representative de
level to the Function Switch FET Q1217. Both operational
amplifier gain stages have offset adjustments to establish
their quiesecent operating points.
GROUNDED POWER SUPPLIES
<4>
The Grounded Power Supplies regulate the +8 volts
supply from the TM 5000-Series power module down to +5
volts for the bulk of the digital circuitry in the Grounded
Section.
4-14 ADD JAN 1982