Motorola GM1200E, Руководство по обслуживанию

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Frequency Synthesis
Introduction/Theory of Operation 3.1-31
In TX mode AUX3 is about 5V DC and Q3742 is switched off. Q3752 is switched on and enables a
current flow from the source of FET Q3741 while Q3751 is switched off. When switched on the FETs
draw a drain current of 8 mA from the FRAC-N IC super filter output. The frequency of the receive
oscillator is mainly determined by L3752, C3752, C3754 - C3756 and varactor diodes D3751 /
D3752. Diode D3754 controls the amplitude of the oscillator. The frequency of the transmit oscillator
is mainly determined by L3734, C3736 - C3740 and varactor diodes D3732 / D3733. Diode D3739
controls the amplitude of the oscillator. With a steering voltage from 3V to 10V at the varactor diodes
the RX frequency range from 181.1MHz to 219.1MHz and the TX frequency range from 136MHz to
174MHz are covered. In TX mode the modulation signal coming from the FRAC-N synthesizer IC
(U3701 pin 28) modulates the TX VCO via varactor diode D3731.
Both oscillator outputs are combined and buffered by the VCO Buffer Q3760. Q3760 draws a
collector current of 13 mA from the stabilized 5V (U3801) and drives the Mixer Buffer Q3770. Q3770
draws a collector current of 17 mA from the 9V3 voltage and drives the PA Buffer Q3780 (Pout =
13dBm) and the Pre-scaler Buffer Q3790. Q3790 draws a collector current of 8 mA from the
stabilized 5V (U3801) and drives the pre-scaler internal to the FRAC-N IC. In transmit mode Q3780
is switched on by the K9V1 signal and draws a collector current of 19 mA from the K9V1 voltage.
The injection signal VCO MIXER with a level of 10dBm feeds the mixer through R3774. The buffer
stages Q3760, Q3770, Q3780 and the feedback amplifier Q3790 provide the necessary gain and
isolation for the synthesizer loop.
13.4 Synthesizer Operation
The complete synthesizer subsystem works as follows. The combined output signal of the RX VCO
(Q3751) and TX VCO (Q3741) is buffered by VCO Buffer Q3760, Mixer Buffer Q3770 and Pre-scaler
Buffer Q3790. To close the synthesizer loop, the collector of Q3790 is connected to the PREIN port
of synthesizer U3701 (pin 20). The output of the VCO Buffer (Q3770) also provides signals for the
mixer (via VCO MIXER) and the PA Buffer (Q3780).
The pre-scaler in the synthesizer (U3701) is basically a dual modulus pre-scaler with selectable
divider ratios. This divider ratio of the pre-scaler is controlled by the loop divider, which in turn
receives its inputs via the SRL. The output of the pre-scaler is applied to the loop divider. The output
of the loop divider is connected to the phase detector, which compares the loop divider´s output
signal with the reference signal.The reference signal is generated by dividing down the signal of the
reference oscillator (Y3701 or Y3702).
The output signal of the phase detector is a pulsed DC signal which is routed to the charge pump.
The charge pump outputs a current at pin 29 (I OUT of U3701). The loop filter (which consists of
R3715 - R3718, C3723 - C3725, C3727, C3735) transforms this current into a voltage that is applied
to the varactor diodes D3732, D3733 (TX), D3751, D3752 (RX) and alters the output frequency of
the TX VCO (Q3741) and RX VCO (Q3751). The current can be set to a value fixed in the FRAC-N
IC or to a value determined by the current flowing into CPBIAS 1 (U3701-27). The current is set by
the value of R3723 and R3724. The selection of the two different bias sources is done by software
programming.
To reduce synthesizer lock time when new frequency data has been loaded into the synthesizer the
magnitude of the loop current is increased by enabling the IADAPT line (U3701-31) for a certain
software programmable time (Adapt Mode). The adapt mode timer is started by a low to high
transient of the FRACN CE line. When the synthesizer is within the lock range the current is
determined only by the resistors connected to CPBIAS 1 or the internal current source.
A settled synthesizer loop is indicated by a high level of signal LOCK DET (U3701-2).