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BRU3 MANUAL
BRU3 MANUAL, Doc.no: 155 16-ANNA 805 08 Uen, Rev H, 2003-02-07
By using this modulation method, it is possible to obtain a fast settling time for
the transmitter synthesizer and maintain a low cut-off frequency for the GMSK
modulated signal.
6.2.2
Modulation Synthesizer
A modulated reference signal to the transmitter synthesizer reference is
generated using an extra reference synthesizer. The modulation signal is added
to the control voltage of the Voltage controlled crystal oscillator (VCXO). In
order to keep the high frequence accuracy of the reference oscillator, the VCXO
is phase-locked to this signal. This loop is designed to be very slow in order to
get a low cut-off frequency for the modulated signal.
6.2.3
Receiver
The input signal from the antenna is filtered by a duplex filter, then amplified in
a Low Noise Amplifier (LNA). Further filtering is obtained in a ceramic filter
after the LNA.
The signal is down-converted to 77.25 MHz in a double balanced high-level
mixer. A Local Oscillator (LO) signal to the mixer is generated by the receiver
synthesizer. The LO signal is filtered before it is fed into the mixer.
After the mixer, the signal is amplified and filtered in a 77.25 MHz crystal filter.
An IF-integrated circuit with two mixers and limiting amplifiers, are used to get
the 42 kHz signal to the discriminator. Also, there is an output signal with a
voltage proportional to the input signal strength (RSSI), from this circuit. The
oscillator signal to the IF-section is generated from the overtones and divided
signals from the reference oscillator.
6.2.4
Reference Oscillator
The reference oscillator is a oven-controlled high-performance crystal
oscillator. The output frequency is adjusted with an input control voltage. For
information on how to adjust the reference oscillator, please refer to Software
Commissioning Procedure in the Commissioning Procedure section of the
BRU3 - General module.
