Tektronix 2236, Instruction Manual

Page: 79 / 350

Theory of Operation— 2236 Service
Input signals to the V/F Converter from the Multiplexer
are applied to unity-gain buffer U1900D. The divider at the
buffer output composed of R1927 and R1928 attenuates
the signal by a factor of five, and both the unattenuated and
attenuated signals are connected as inputs to CMOS switch
U1907A. Diodes CR1906, CR1907, and resistor R1935 de­
code control signals from Serial Interface 1C U1906 pins 4
and 7 such that a HI on either line selects the attenuated
signal for output.
The output of U1907A drives unity-gain buffer/limiter
U1900A. This amplifier limits the input signal at its inverting
input to plus and minus 700 mV. The noninverting input of
U1900B is biased by resistors R1933 and R1934 to a volt­
age of about 750 mV, and the op amp will maintain this
voltage on its inverting input. Therefore the difference be­
tween the 750-mV bias voltage and the voltage to be mea­
sured (at pin 2 of U1900A) is impressed across R1931. This
voltage develops the input excitation current for the V/F
convertor stage. The integrator comprised of U1900B and
C l917 integrates this current, and output at pin 7 swings
positive. The same 750 mV bias is applied to pin 6 of U1908
and is compared against the rising integrator voltage ap­
plied to input pin 7 through R1942. When the integrator out­
put reaches 750 mV, U1908 generates an 18
current
pulse at pin 1 and sources it into the summing junction at
the noninverting input of U1900B. This current level is set by
R1943 at about 500 mA and always exceeds the input ex­
citation current. Therefore, the integrator output reverses
direction for 18 ns and integrates toward ground. The 18 fis
pulse width is set by R1930 and Cl 914, while R1929 and
Cl 913 ensure short term output frequency stability by filter­
ing the 12-V supply.
Since input excitation current never goes to zero, the V/F
convertor circuit always oscillates. All the output direct cur­
rent from U1908 pin 1 must eventually be sunk by R1931.
Current pulse width and amplitude are constant, so the rep­
etition rate (frequency) of current pulses will be exactly pro­
portional to the input excitation current. The output drive
current of U1908 at pin 3 flows in 18 pulses and is con­
nected through R1941 to the LED portion of optical-isolator
U1806.
The limiting action of U1900A is accomplished by estab­
lishing standing diode bias currents such that for positive
input swings, CR1905 carries less and less current until it
disconnects at about +700 mV. In the negative direction,
CR1984 is biased to disconnect at -7 0 0 mV. Since the
feedback loop is open during limiting, U1900A’s output
swings to the positive or negative supply level dependent
upon the direction of the input signal.
Limiting is provided for positive input swings by CR1905
being biased off. Only insignificantly small amounts of cur­
rent flows into the inverting input of U1900A, and the volt­
age at that pin is established by the divider network
composed of R1931 and R1932. The voltage to this divider
is the same voltage that is present at the inverting input of
U1900B. Since this voltage is maintained by circuit opera­
tion to be the same as that at its noninverting input, pin 2 of
U1900A cannot go more positive than that set by divider
(about 700 mV). The divider voltage therefore sets the posi­
tive limit point.
For negative input swings, the output of U1900A goes
negative and allows input pin 2 to also go negative. Conse­
quently R1931 and R1932 both source current into the an­
ode of CR1905 which is then sunk by R1984. The output
voltage of U1900A maintains regulation by sourcing the dif­
ference between what is being sunk through R1984 and
what is available from R1931 and R1932. As the input volt­
age nears -7 0 0 mV, the latter two currents approach
equality, current through CR1984 drops to zero and it dis­
connects. This opens the loop and limits negative output
swings.
The output frequency increases from 6 KHz for an input
voltage of 0 V to 11 KHz with an input voltage of —700 mV.
When the input voltage is +700 mV, the frequency drops to
1 kHz. The V/F Out signal is coupled through opto-isolator
U1806 and transistor Q1808 to the Input Multiplexer in the
Counter circuitry.
Ohms Reference
The Ohms Reference circuitry supplies excitation to the
bottom of the input divider when making resistance
measurements.
1C U1904 is configured as an astable multivibrator and
has a frequency of about 400 Hz and a duty factor of about
60% set by R1924, R1925, and C1916. Output pin 3 drives
the select input of switches U1907B and U1907C, and di­
odes CR1908 and CR1909 prevent oscillation by forcing pin
3 HI if either pin 6 or pin 7 of U1906 is LO.
When U1904 is disabled (pin 3 is HI), U1907C connects
Cl 911 to the 2.5-V reference. The noninverting input of
unity-gain buffer amplifier U1950 is also biased to the 2.5-V
reference by R1912, and the amplifier acts through R1960
and RT1915 to maintain the Ohms Drive line at the same
voltage. This signal level is fed back to the inverting input of
U1950 by R1983 and is also applied to pin 5 of U1905 by
R1913. The Ohms Drive signal drives the bottom of the in­
put divider network when the Ohms function is selected.
Diodes CR1901 and CR1902 are used to clamp U1950 pin 2
during overload conditions, and Cl 910 stabilizes the pulse
response of U1950.
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