(c) The bridge signal output is one third of the input value (disregarding
phase rotation) and is applied to pin 5 of U102 whose output is fed back to the bridge input
in the right amplitude, and phased to cancel the 60Hz or 50Hz signal. The attenuation at
the bridge frequency is typically over 40dB. Any other frequency signal is attenuated
approximately five times by divider composed of R119, R120, and R121 and enters U102
through pin 6. The fine-tuning of the bridge frequency is accomplished by R123 and the
depth of the notch by R119. Input and output signal amplitudes (outside the notch) are the
same, but inverted in phase.
(d) If the filter action is not desired, it can be eliminated by grounding pin
5 of U102. This occurs when the test switch is thrown to the right.
(e) The upper two switches select 60Hz (left) or 5OHz notch when both
face towards the right. The filter is followed by an inverting gain stage which leads to an
amplification of six times. The real time chart recorder signal and the 1v per mv output is
derived from this stage, having diagnostic frequency characteristics.
0.5Hz to 40Hz filter.
(a) The low frequency cut-off is obtained by the time constant of C117
and R132 in parallel with R133. The high frequency cut-off is accomplished with a three
pole filter similar to the 100Hz low pass filter but with different time constants.
(b) R132 and R133 form a voltage divider so as to provide a center point
of the 5v supply which permits an equal number of bits for positive or negative signals at
the A/D converter.
Quasi-Random Signal recognizer and synchronizer stages.
(a) The input signal for the QRS recognizer is taken from the output of
the first amplifier stage. This signal is shorted to the ground by the analog gate U113 each
time the 1mv calibration pulse is generated avoiding the counting of this signal as a QRS
(b) After adjustable attenuation and additional high frequency filtering,
the signal enters U102. This is due to the double T bridge in its negative feedback branch.
This enhances the amplitude of signals around 13Hz presenting a gain of one for all other
(c) This 13Hz sine wave drives Q101 if the excursion is positive or Q102
if it is negative. Each of these transistors, in turn, drives a complimentary transistor Q111
or Q103. In either case, a negative-going pulse is triggering the monostable oxillator