Stanford Research Systems SR620, Руководство пользователя

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84 Circuit Description
SR620 Universal Time Interval Counter
The peak input level is detected by Q801, and
held on C832. The minimum input level is detected
by Q802 and held by C831. Q801 and Q802 are
biased by the 10 MOhm resistors R834 and R831
respectively. The detected voltages are averaged
and filtered by R832, R833 and C833 and buffered
by the FET input op amp U809A.
DIGITAL TO ANALOG CONVERTER
(Sheet 12 of 16)
A 12 bit D/A converter, U806, is used to provide
analog voltages to the system. The D/A converter
is referenced to the 5.00 vdc reference from U906.
The current output is converted to a voltage by the
op amp U807B. The voltage is amplified and offset
by U807C to cover the range of +/-5.12 vdc. The
D/A output is used to refresh several S/H
amplifiers to set input threshold, conversion gain,
and control the rear panel chart recorder outputs.
The D/A output is multiplexed to the S/H amps by
the 1:8 analog multiplexer, U808 which is
controlled by four latched processor bits Dac_Mpx
and Dac_Inh. The outputs from the analog
multiplexer are passed through 10 KOhm resistors
to reduce switching noise.
UNREGULATED POWER SUPPLIES
(Sheet 13 of 16)
CAUTION: Unregulated voltages are present
on the PCB whenever the instrument is
attached to an ac power source--whether or
not the front panel power switch is "on". The
front panel "power" switch is used to enable
most of the voltage regulators in the unit. This
approach is used to provide power to the
timebase oscillator even when the instrument
is not in use.
A power entry module, with RF line filter, is used
to configure the unit for 100,120,220 or 240 Vac.
The line filter reduces noise from the instrument,
and reduces the unit's susceptibility to line voltage
noise. A 130 Vac Metal Oxide Varistor (MOV)
stunts the line voltage to reduce spikes and to
protect the unit from 220 vac when configured for
110 vac.
Full wave bridge rectifiers are used to provide
unregulated dc at +/- 7V, +/- 9V, +/- 20V and at -
4V. A Schottky diode bridge is used for the high
c/- 7V supplies in order to reduce rectifier
losses.
The comparator, U901B, provides a squarewave
to synchronize the scope display refresh to the line
frequency.
POWER SUPPLY REGULATORS
(Sheet 14 of 16)
The voltage regulators provide outputs at +15, +5,
-2.0, -5.2, and -15 Vdc from the UNREGULATED
power. The high current regulators (+5, -5.2 and -2
V) are designed to operate with a very low drop-
out voltage. All of the regulated outputs are current
limited for short circuit protection.
All three high current regulators are essentially the
same: circuit references in the following
description refer to the +5.0 Vdc supply.
The main pass transistor is Q901: the base of this
transistor is controlled so that the emitter will
provide a low impedance source of 5 Vdc. The
current gain of Q901 remains large until the
collector-emitter voltage drops to about 0.4 vdc,
hence the low drop-out voltage for the regulator.
The base of Q901 is driven by the emitter of Q902,
which is driven by the output of the op amp,
U902A. By comparing the output of the regulator
to the +5.00 vdc reference, the op amp maintains
the regulator's output at 5.00 vdc. The current
output from the regulator is measured by R901, a
0.1 Ohm resistor. If the current exceeds about 2.7
Amps, then the comparator, U901A, turns on,
pulling the reference input below ground, thereby
turning off the regulator's output.
U909 and U910 are regulators for +/-15 vdc. If the
power switch, SW901, is opened, then the output
of these regulators drops to about 1.25 volts,
turning off the instrument. The unregulated power
is not turned off so that the regulators U907 and
U908 can p/-15 vdc to the timebase.
Two status bits are also generated: Drop_Out tells
the processor that the +5.0 VDC supply has
dropped below 4.6 vdc, and Reset is asserted if
the +5 vdc supply is below 4.4 vdc, or has been
below 4.4 Vdc in the previous 1 second.
The 24 Vdc brushless fan speed is controlled by
the temperature in the box: the warmer the box,
the faster the fan turns. When cool, the 6 mA
drawn by R908 through R907, a 50 Ohm 40° C
transition thermistor, is not sufficient to turn on
Q907. As the box warms above 40°C, R907
becomes a high resistance, and some portion of
the full 6 mA can be drawn from the base of Q907,
which turns on Q908. This proportional