Modulated frequency detector and 1mv calibration signal.
(a) The frequency modulated signal from the preamplifier crosses over to
the grounded section through the opto-coupler K7. The emitter output of its
phototransistor drives a PLL U100.
(b) U101 is connected as a voltage follower which drives a 100Hz Lo
pass filter. Its output can be switched directly to the manual gain control or replaced by a
circuit generating an equivalent 1mv pulse of fixed duration and amplitude each time the
knob corresponding to the beeper volume is pushed. The timing of the pulse is obtained
by the combination of C107 and R109. The amplitude is preadjusted by R111.
(3) Paddle pick-up filter. The amplified and filtered paddle pick-up signal from
the preamplifier, located on the defibrillator assembly, is filtered by a Lo pass 30Hz filter
with a 4dB enhancement at approximately 25Hz. This, together with the filters mentioned
earlier, presents 3dB at 30Hz and a very fast roll off thereafter.
Patient cable or paddle pick-up selection.
(a) As long as regular lead positions are in use at the patient cable
preamplifier, the light emitting diode in opto-coupler K6 is not energized. Consequently, its
phototransistor is in cut-off, and the collector of Q100 is pulled high by resistor R116. The
analog gate in U112 is closed since pin 5 is at a logic high.
(b) When the paddle pick-up is selected, the emitter of the
phototransistor in K6 goes high and so does pin 13 of U112. This enables the signal from
the 30Hz filter to proceed to the size control. At the same time, the collector of Q100 goes
low, and the corresponding gate disconnects the signal coming from the 100Hz filter.
Size control and notch filter.
(a) A linear 50k ohm control acts as the size or sensitivity adjustment.
Note that the patients signal and the 1mv calibration pulse are attenuated by the same
control. This maintains the correct relationship of both no matter the amplitude setting.
(b) The wiper of this control feeds the input of a high to low impedance
converter U101 (section C1) from its output. The signal is split in two ways. R122
together with R128 defines the "Q" of the filter between 4 and 5, and through R123 goes to
one arm of a modified Wien bridge composed of R124, R125, C111, and C112. Jumpers
x and y, when closed, lower the bridge frequency to 50Hz by adding C113 and C114