GE MEDICAL SYSTEMS PROPRIETARY TO GE
D
IRECTION 2294854-100, REVISION 3 LOGIQ™ 9 PROPRIETARY MANUAL
5-16 Section 5-3 - Front End Processor
5-3-4-8 Signal Processing
The signal path includes TGC (time gain compensation) followed by TFC (time frequency
compensation). All reductions in data precision are done via rounding. Overflows are prevented by
providing clamping when necessary.
5-3-4-9 Time Gain Compensation (TGC)
The TGC block provides analog gain control to the frontend analog signal path and applies digital gain
to beamformer output signal. The maximum signal of the TGC is limited only by the 16-bit output and
the 24-bit input. There are not any internal reduction to less than what can be supported by the input
and output data widths. The EQ TGC shall include three gain functions:
- Range Dependent Base TGC. The TGC gain shall include a range dependent base TGC to
compensate for transmit and receive diffraction and round trip attenuation.
- Vector Gain Offset. The vector gain offset provides compensation for intensity variations as
a result of steering and elemental directivity.
- Vector and Range Dependent Dynamic Apodization Compensation Gain. A range and
vector dependent function shall adjust the digital TGC to compensate for dynamic receive
apodization. Adjusting gain inversely proportional to the aperture size maximizes dynamic
range by fixing the relationship between maximum beamformer output and maximum EQ
signal across all vectors, and during reception while analog gain is ramping up.
After TGC, a frequency rotator (numerically controlled oscillator mixer) will shift the I and Q data desired
frequency band to baseband. The frequency shift will vary with range to track the band shift due to
attenuation. It will also account for non-destructive aliasing which has already taken place in the
beamformer.
A real coefficient lowpass FIR will provide increased SNR, fundamental rejection for harmonics, and
decoding for coded excitation. Since it is a real filter, I and Q will be filtered identically.
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