Benefon EXION TDP80DN, Service Manual

Page: 39 / 63

9.2.2001
4Z2700GB.fm 12
Product: TDP80DN / OC2720 Processor/Audio/RF
5.4 TX-SYNTHESIZER
5.4.1 General
The TX synthesizer generates the final modulated transmission frequency, which is
fed to the transmitter.
5.4.2 Function Description
The TX frequency is generated with a phase locked loop (PLL). The VCO produces
the injection frequency determined by the control voltage, and the VCO also has a
modulation input. After the VCO stage come two amplifier stages Q470 and Q480.
These ensure separation between the VCO and transmitter, and also boost the
injection level sufficiently. Part of the signal is fed from between the amplifier stages
to the synthesizer circuit I400 pre-scaler input Fin2. The synthesizer circuit contains
a pre-scaler, programmable divider, reference frequency divider, and a phase detec-
tor.
The synthesizer circuit produces current pulses at output DO2 as a result of the
phase detection. The current pulses either charge or discharge the loop filter con-
densators. The VCO control voltage is derived from the loop filter output.
The synthesizer 12.5kHz reference frequency is made by dividing the 13.00 MHz
signal from the temperature compensated oscillator (TCXO). The series-form signal
which controls the channel frequency comes from the processor. SDATA and SCLK
signals are common to both TX- and RX-synthesizers. A positive pulse in the SLE-
line loads the division ratio (corresponding to the TX frequency of the channel) fed to
the SDATA line, to the synthesizer circuit I400.
Modulation is fed to the TX synthesizer TX-AUDIO line. The frequency response is
corrected by the components R455 - R458, C454 and C455. Modulation sensitivity
is set by the resistor R459.
Operating voltage 3.0 VDC
Current consumption <30 mA
Output level to the transmitter +6 dBm
Frequency range 452.500...457.475 MHz