LG MG300d, Service Manual

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3. TECHNICAL BRIEF
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(1) Receiver Part
A. LNA and Quadrature Demodulator
Four separate LNAs are integrated in the SKY74400 to address different bands of operation. These LNAs have
separate singleended 50Ω inputs. The LNA gain is switchable between high and low settings using the three-wire
bus. The LNA outputs feed into a quadrature demodulator that downconverts the RF signals directly to baseband.
The baseband I and Q paths consist of cascaded amplifiers and low pass filter sections. The baseband section
provides eight programmable bandwidth settings ranging between 90 kHz and 160 kHz to allow for added
flexibility when interfacing to any mixed signal baseband device.
No external capacitors are required for baseband filtering. The filter chain consists of two fixed real poles, two
fixed conjugate pole pairs, and one programmable conjugate pole pair. The result is a flat passband with minimal
group delay distortion at any bandwidth setting.
B. DC Offset Correction
Five DC offset correction loops ensure that DC offsets generated in the SKY74400 do not overload the
baseband chain at any point. After correction, the corrected voltages are held digitally for the duration of the
receive slot(s). The positive edge of the RXENA signal starts the digital DC offset correction. Since the correction
is digital, a system clock is required.
To generate the clock, the reference frequency is divided down internally. A special, fast DC offset correction is
carried out every time the receiver gain is programmed while RXENA is high. This ensures that a DC offset
correction is complete in the time available, even if the gain is changed between slots in multislot mode.
C. AM Suppression and IP2 Calibration
For direct conversion GSM applications, it is imperative to have extremely low second-order distortion.
Mathematically, secondorder distortion of a constant tone generates a DC term proportional to the square of the
amplitude. In general, a strong interfering Amplitude Modulated (AM) signal is, therefore, demodulated by second-
order distortion, which generates an Acinterfering baseband signal. The SKY74400 can effectively handle such
AM-modulated interferers. A commonly used measure for receiver second-order distortion is the second-order
intercept point, IP2. For example, to ensure that the unwanted baseband signals are 9 dB below the wanted signal
required under the AM suppression test for type approval (see 3GPP TS 51.010-1), an input IP2 of 43 dBm is
required. The SKY74400 RF transceiver includes a circuit that minimizes second-order distortion. This IP2
calibration circuit effectively compensates for any second-order distortion in the receive chain that would
otherwise generate unwanted baseband signals in the presence of strong interfering signals. When calibrated
correctly, the SKY74400 IP2 meets the GSM AM suppression test requirements in all bands with good margin.
A one-time factory calibration procedure produces a set of I/Q compensation coefficients that are programmed in
the device to minimize the DC voltage shift resulting from the second-order distortion. The IP2 performance is
optimized when the DC due to the interfering signal is minimized. The determined coefficients are transmitted to
the serial interface, stored in nonvolatile memory, and programmed to the SKY74400 upon each power-up as part
of device initialization. The optimization process is performed internal to the SKY74400.
D. Flexible Receiver Bandwidth Control
The receive baseband filters have programmable bandwidths with eight possible settings.
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