Models 78100A, 78101A Section IV
Principles of Operation
SECTION IV
PRINCIPLES OF OPERATION
4-1. INTRODUCTION. current for U1A. UlB is connected as an integrator of the
UlA output. The input resistor is R49 and the feedback
4-2. The telemetry transmitter is carried by a patient.
capacitor is C5. R16 conducts bias current to the amplifier.
Electrodes on the patient pick up the ECG signal, which the
R15 and R17 bias its non-inverting input. R43 introduces a
transmitter uses to modulate the signal it sends to the
high frequency roll off. C41 reduces the effect of noise
receiver. The receiver translates the radio signal back into
current pulses from the UlB non-inverting input. R14
an ECG waveform, which appears at rear panel connectors
furnishes feedback around the entire ECG gain stage. C4
as a 1 mV, 50 mV or 1V output. If the patient goes out of
and C42 provide frequency.stability.
range, if one or more electrodes become detached, or if the
battery needs replacement, an automatic signal lights a
4-1 1. Slew Rate Limiter.
front panel indicator to show that the channel is in-
operative.
4-12. The slew rate of the ECG amplifier output is limited
4-3. TRANSMITTER. since the voltage into the integrator is limited by the power
supply. The highest slew rate is determined by the
44. The ECG signal from the patient is routed to the
maximum voltage out of U1A divided by R49 times C5.
transmitter from the electrodes through shielded cables. A
two-stage FM/FM modulation process then creates a modu-
lated carrier: After buffering and amplification (Figure
4-13. Subcarrier Oscillator.
4-l), the ECG signal frequency modulates the audio
frequency subcarrier oscillator. The fdtered subcarrier
signal, in turn, varies the frequency of a radio-frequency
4-14. MULTIVIBRATOR CONTROL CIRCUIT. UlC,
oscillator. The RF oscillator operates at one-fourth of the
Q12, and Q13 are connected as a non-inverting feedback
'
transmitted frequency. Two frequencydoubling circuits in
amplifier coupled to the ECG amplifier through C6, which
series quadruple the frequency and apply the broadcast
,) determines the system low-frequency roll off. As far as this
signal to the right-leg (reference) electrode lead, which also
amplifier is concerned, Q12 and Q13 are emitter followers
serves as an antenna.
connected in parallel. R25 and R26 assure that equal
currents will flow in Q12 and Q13. R18 and R19 bias both
4-5. Auxiliary circuits in the transmitter reduce the sub-
inputs through other resistors (R20 and R58 in series to the
carrier frequency to about half its normal value if an
inverting input and R21 to the non-inverting input). R3
electrode comes off the patient. This condition can be
supplies bias current to the amplifier, R41 furnishes
detected in the receiver. A voltage sensing circuit shuts off
feedback around the amplifier, and C7 provides frequency
the RF oscillator if the battery voltage falls below
stability. As the output voltage of this amplifier varies, the
approximately 6.2 to 6.4 volts.
collector currents of Q 12 and Q 13 vary in proportion. The
4-6. ECG Amplifier.
value of these currents is controlled by potentiometer R23
4-7. The input, Q7, consists of two emitter follower in series with R24.
buffers (schematic, Figure 6-3). R44, R45, C2, C3, C43,
C44 and two feed-through capacitors isolate Q7 from RF 4- 1 5. VOLTAGE-CONTROLLED MULTIVIBRATOR.
energy which may be picked up from the antenna. R8, R9, The currents produced by Q12 and Q13 are used in the
R10, R12, and R22 shift the voltage level out of the buffers subcarrier oscillator (multivibrator Q14, Ql5, Q16 and
to drive the next stage. Q17) to charge C11 and C8. Only two of the above
transistors are on during each quasi-stable state of the
4-8. The voltage gain of the ECG amplifier is supplied by a
multivibrator (first Q14 and Q16, then Q15 and Q17). If
feedback amplifier consisting of Q8, UlA, and UlB. UlA
Q 15 suddenly turns on, then Q 17 is turned on through R27
and U1B are identical transconductance amplifiers. In
and C9. This charges C11 through Q17, R30, and the base
addition, U1 contains a third transconductance amplifier
of Ql5 and also turns off Q16 through R29 and C10 and
(UlC) and a pair of PNP transistors used in the regulator
turns off Q14 through C8. The voltage on the base of Q14
(UlD).
drops to about -2 times the supply voltage. The voltage
rises due to C8 being charged from Q13. When the voltage
'1 4-9. ECG Gain Stage.
rises enough to turn on Q14, each transistor changes state
4-10. Q8 is a differential amplifier with common emitter to the inverse pattern of that described above. C8 is charged
resistor R46 and collector resistors R47 and R48. The further by Q16 through the base of Q14 at a rapid rate
outputs connect to the inputs of UlA. R13 supplies bias limited by R30. The nominal 1000 Hz frequency of the
078 100-2 4-1 |
ACEC
