Models 78100A, 78101A Section IV
Principles of Operation
the output voltage of U3, thus causing Q2 to conduct less. of U9 is plus, it is held plus by the positive voltage fed to its
The positive supply for U3 is derived from U4, Pin 9 via a non-inverting input (pin 3) by R82 and R83. It changes
breakdown diode in U4, between pin 9 and the plus supply, state when C35 is charged sufficiently through R81, so that
C5 and C28 assure frequency stability of U4 and U3. C3 the inverting input (pin 2) is driven more positive than the
and C4 assure low output impedance of the supplies at high non-inverting input. The multivibrator functions in a
frequencies. complementary way in its negative state.
487. AFC Amplifier. 4-92. When U9 switches plus, it injects current through
R85 and C34 into the summing junction of integrator U5.
4-88. During normal operation (not inoperative mode), the
If U9 switches minus, it pulls current out of the summing
local oscillator frequency is automatically adjusted to keep
junction. The output of U5 compensates for this minus
the dc output voltage of the discriminator at a constant
switching by moving plus .so an equal amount of current
level. This voltage is sensed, amplified, and fed back to the
moves into the summing junction through C29 and Q27.
local oscillator by the AFC amplifier, changing its fre-
The bias voltage on the base of Q27 is reduced simul-
quency by means of varactor diode CRl. This adjustment
taneously by current flowing from ground through CR23,
compensates for drift in the crystal oscillators of both the
R88, and C33 into U9. The output of U5 compensates for
transmitter and receiver, and keeps the intermediate fre-
positive switching in an analogous way. Thus, the output of
quency at a constant 21.82 MHz (paragraph 4-36).
U5 does not travel all the way to its normal limits. When
the transmitter is captured, the rangelbattery inoperative
4-89. During the inoperative rangelbattery mode, the local
light turns off and AFC multivibrator U9 is prevented from
oscillator frequency is varied first to one side of its range
switching again by Q29.
and then to the other to give it a better chance to capture a
weak transmitter signal. The automatic frequency control
(AFC) amplifier sweeps the local oscillator to search for the
493. Status Line.
signal.
4-90. At the amplifier input, integrated circuit U5 is con- 4-94. INOP is discriminated in this circuit to indicate
)
nected as an integrator of the discriminator output. R77, RANCElBATTERY INOP or LEADS OFF INOP or normal
R78, and R79 shift the level of the discriminator output to operation at jumper pin ?0?. Then when jumper pin ?0? is
zero volts and also act as the input resistor of the inte- connected to the ECG OUTPUT phone jack (J4), the ring
grator. C29 is the integrating capacitor, and R93, CR26, carries STATUS information in tristate form regarding
and R38 (along with AFC input resistor R16 on the high INOP. In normal operation, the voltage divider R109 and
frequency board) form an attenuator of the AFC output. R110 supplies 3V dc to jumper pin ?0? (and 54). If a
To compensate for the non-linearity of varactor diode CRl leads-off condition occurs, transistor Q30 saturates and
at the local oscillator input, this attenuator reduces positive R107 pulls the output down to 1.5V dc. If the battery
AFC output more than the negative output, due to action becomes discharged or an out-of-range condition occurs,
of CR26. Limiters Q27 and Q28 limit the voltage range of transistor Q31 saturates and causes the output to jumper
the integrator by adding a feedback path in parallel with pin ?0? to drop to OV dc. Diodes CR30 and CR31 are
C29 whenever the output voltage varies far enough to turn voltage limiters which protect voltage sensitive circuits
on either transistor. These limiting transistors are biased by should panel lamps DS2 or DS3 (ELECTRODE or RANGE/
resistors R86 through R9 1. BATTERY indicators) open or be removed while the
Receiver is in operation. In addition, these diodes limit the
4-91. The sweeping of the local oscillator is accomplished reverse current that flows through resistors R104 and
by U9, a free-running multivibrator. When the range/ RlO5. Capacitor C37 and resistor R1 11 provide high fre-
battery inoperative light is on, Q29 is turned off and the quency feedback to stabilize U2 when connection to a
multivibrator changes state every few seconds. If the output remotely located instrument is made.
4-9
078 100-2 |
ACEC
