GE Corometrics 170 Series Service Manual page 89

Theory of Operation: Main Board Theory of Operation
U43 is a quad-switched capacitor integrated circuit configured as a balanced ring
demodulator. Its analog inputs are connected to the differential outputs of the pre-
amplifier at the secondary of T1. The switches within the device are actuated by the
signal DET_BURST1.
The two analog outputs of U43 drive the difference amplifier U44 and its associated
components. Integrated circuit U45 switches the output of the difference amplifier
U44 (pin 14) to the track-and-hold circuit of either Channel 1 or Channel 2.
The track-and-hold circuit for Channel 1 is comprised of components R408, C225,
and buffer amplifier U46 (pin 7). The track-and-hold circuit for Channel 2 is
comprised of components R409, C226 and buffer amplifier U46 (pin1).
The track-and-hold circuits retain the amplitude of the last value from the previous
demodulator activation. If this method were not employed, the system gain would
have to be twice as high since it is a time sampled system. The samples must be
integrated over time (averaged) to produce the desired output. In summary, this
method improves the signal-to-noise ratio of the system.
Since the DET signal is driven at the same frequency as the transmitted signal, any
return signal of the same frequency will be averaged and produce either 0 V or a dc
offset at the output of the demodulator; this is not seen by the main filter driven by
the demodulator since it is AC coupled by capacitors C253 and C262. Therefore, a
difference frequency (Doppler shift frequency) must be present to produce an output
from the demodulator. The output of the track-and-hold circuit of Channel 1 at U46
(pin 7) is switched to the Channel 1 main filter through U39 (pin 11) to U39 (pins 6
and 10).
Telemetry (if present) is switched to the Channel 1 main filter through U39 (pin 7)
to U39 (pins 6 and 10). Telemetry FECG (if present) is switched by U39 (pin 3) to
U39 (pins 2 and 15) to the analog-to-digital converter. The main filter is a band-pass
amplifier. Its 3 dB points are 120 Hz and 300 Hz, respectively. The bandwidth
slopes are greater than 40 dB/octave. The maximum gain of the filter is 46 dB with
peaking of approximately 48 dB at 250 Hz. The nominal gain in the system is
approximately 36 dB and is controlled by R454. The Channel 1 main filter is
comprised of U51 and associated components.
Revision C 170 Series Monitor 5-29
2000947-004