GE Medical Systems Corometrics 340, Service Manual, Page: 43 / 82

Revision A Model 340 Telemetry System 4-3
2006920-001
Theory of Operation: Transmitter Board (No. 2003708-001)
Following the pre-amp is a buffer amplifier and transformer
combination. The buffer stage is unity gain and is used to establish a
high impedance for the pre-amp output. The transformer converts the
single ended pre-amp output to differential for the demodulator that
follows.
The demodulator circuit is composed of a quad FET switch, a difference
amplifier, and a sample-and-hold circuit. The switches are arranged in a
doubly balanced ring detector configuration. When the CPLD generates
the detector burst, the demodulator produces an output that corresponds
to the Doppler shift of the input signal from the transducer. The
differential output of the ring demodulator is then converted to single
ended by the difference amplifier stage. Due to the gating of the detector
stage, a sample-and-hold circuit is used to retain the last output level
while the detector is inactive.
After the detector are four stages of main filters consisting of two high-
pass and two low-pass filters, all active second order. Each filter stage
has a gain of approximately 20 dB (80 dB total gain), and the composite
band-pass filter is 100 to 270 Hz. A gain adjustment in the filters is used
to set the gain for the entire receiver circuit (pre-amp input to filter
output).
Following the main filters is the frequency doubler. This stage is
necessary in order to bring the low frequency content of the Doppler
shifted signals from the transducer up to a more suitable band for the
human ear as well as to prevent disturbance of control signals
transmitted along with the ultrasound audio in the composite
modulation. The doubler consists of a precision full-wave rectifier, an
active band-pass filter, and an audio amplifier. The full-wave rectifier
produces two output peaks for every one peak applied to the input,
effectively doubling the input frequency. However, the rectifier output is
not purely sinusoidal. All undesired frequency components produced by
the rectification process are removed in the next stage—the active band-
pass filter. This filter has a gain of approximately 3 and a band-pass
range of 200 to 500 Hz. The output from this filter is used as a component
in the composite modulation for the transmitter module as well as to
provide the input source for the audio amplifier. The audio amplifier is a
unity-gain current amplifier that provides sufficient current output to
drive a 40
Ω
headphone.