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RF troubleshooting
Receiver
Receiver troubleshooting
Figure 110 Receiver signal paths
Each receiver path is a direct conversion linear receiver. From the antenna, the received RF signal is fed to a
front end module where a diplexer first divides the signal to two separate paths according to the band of
operation: either lower, EGSM900 or upper, GSM1800/1900 path.
At each of the paths follows a pin-diode switch, which is used to select either a receive- or transmit mode. At
the upper band in the receive mode either GSM1800 or 1900 path is further selected by another pin-diode
switch. The selections are controlled by Helgo, which obtains the mode/band and timing information through
the RFBus.
After the switches an external bandpass filter follows each receiver paths. Thereafter, the signal is fed to the
LNA's. EGSM900 and GSM1800 LNA's are integrated in Helgo, while the GSM1900 LNA is a discrete component
placed between SAW filter and balun. In GSM1900, the amplified signal is fed to a pre-gain stage of the mixer.
EGSM900 and GSM1800 LNA's are connected directly to the pregain stages. The pregain stages as well as all the
following receiver blocks are integrated in Helgo. The LNA's have three gain levels. The first one is the maximum
gain, the second one is about 30 dB below the maximum, and the last one is the off state.
After the pregain stages there are demodulator mixers at each signal path to convert the RF signal directly down
to baseband I and Q signals. Local oscillator signals for the mixers are generated by an external VCO. The
frequency is divided by two in GSM1800 and GSM1900 and by four in EGSM900. Those frequency dividers are
integrated in Helgo and in addition to the division they also provide accurate phase shifting by 90 degrees
which is needed for the demodulator mixers.
The demodulator output signals are all differential. After the demodulators there are amplifiers called DtoS
(differential to single ended) which convert the differential signals to single ended. Before that, they combine
the signals from the three demodulators to a single path which means that from the output of the demodulators
to the baseband interface are just two signal paths (I and Q), which are common to all the frequency bands of
operation. In addition, the DtoS amplifiers perform the first part of the channel filtering and AGC (automatic
gain control). They have two gain stages, the first one with a constant gain of 12 dB and -3 dB bandwidth of
85 kHz and the second one with a switchable gain of ±6 dB. The filters in the DtoS blocks are active RC filters.
The rest of the analog channel filtering is provided by blocks called BIQUAD which include modified Sallen-Key
biquad filters.
Page 7–8
Company Confidential
9235618 (Issue 2)
Copyright ©2005 Nokia. All Rights Reserved.
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