June 15, 2005
6815854H01-A
3-46
Theory of Operation:
Frequency Generation Unit (FGU)
Figure 3-34. Frequency Generation Unit Block Diagram (VHF)
3.13.1.1 Reference Oscillator
The radio's frequency stability and accuracy is derived from the 16.8 MHz reference oscillator
(Y3750). The 16.8 MHz reference oscillator circuitry provides a 16.8 MHz reference to the LV Frac-N
(U3751), receiver backend IC (U3000), and the controller section of the XTL 1500 radio. The
reference oscillator circuitry consists of the reference oscillator Y3750 and the inverter/buffer circuitry
containing the active device U3752. Y3750 is a voltage-controlled, temperature-compensated crystal
oscillator (VCTCXO). Circuitry internal to Y3750 compensates for frequency error over temperature.
Warping of the oscillator on frequency is accomplished via a programmable DAC in the LV Frac-N.
The warp voltage is present at pin 25 (WARP) of U3751 and is applied to pin 1 of Y3750. The
16.8 MHz output frequency of Y3750 is capacitor-coupled to pin 23 of the LV Frac-N (U3751) and
also to the inverter/buffer stage U3752. L3754and C3824 at the output of U3752 filter the 16.8 MHz
signal, and R3768 along with C3830 set the appropriate amplitude of the signal for the receiver back-
end and controller sections.
3.13.1.2 LV Frac-N Synthesizer IC
The LV Frac-N IC (U3751) functions include frequency synthesis, modulation control, voltage
multiplication and filtering, and auxiliary logic outputs for VCO selection.
Bias 1
3.0V
3.0V
3.0V
3.0V
8.2V
8.2V
Q3765
Q3763
Q3766
Q3764
Lock Detect
to Controller
Prescaler Input
Q3757
RF
Feedback
RX
Buffers
TX
Buffers
RX
Injection
TX
Injection
Keyed
9.1V
Q3753
Q3751
8.2V
C3809
LOOP
FILTER
Bias 2
9.3V
9.3V
SFIN
SFBASE
SFOUT
Lock
PREIN
AUX4
AUX3
Mod Out
IADAPT
IOUT
AUX2
AUX1
VMULT4
VMULT3
5.0V
16.8MHz Reference
to Controller and
Receiver Back End
VOLTAGE
MULTIPLIER
D3758 and
D3757
16.8MHz OSC
8.2V
Super Filter
Q3756
8.2V SW
8.2V SW
8.2V SW
8.2V SW
U3752
U6751
3.0V
Regulator
U3750
PHASE
DETECTORBIAS
Q3755 RX and
Q3750 TX
CLOCK
SYN_SEL
DATA
MOD_IN
From
Controller
MAEPF-27804-O_VHF
Q3754
Tx VCOs
Q3752
Rx VCOs
Содержание XTL 1500
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