Reference Oscillator; Lv Frac-N Synthesizer Ic; Figure 3-34. Frequency Generation Unit Block Diagram (Vhf) - Motorola ASTRO XTL 1500 Detailed Service Manual

Digtal, vhf/uhf range 1/uhf range 2/700–800 mhz
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3-46
5.0V
16.8MHz OSC
3.0V
Regulator
U3750
CLOCK
SYN_SEL
From
Controller
DATA
MOD_IN
PHASE
DETECTOR IAS
Q3755 RX and
Q3750 TX
9.3V
8.2V
Super Filter
Q3756

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.
June 15, 2005
3.0V
3.0V
16.8MHz Reference
Receiver ack End
U3752
VOLTAGE
MULTIPLIER
3.0V
D3758 and
D3757
3.0V
VMULT3
VMULT4
AUX1
AUX2
U6751
IOUT
IADAPT
Mod Out
C3809
ias 1
ias 2
AUX3
AUX4
SFIN
SF ASE
PREIN
Lock
SFOUT
8.2V
Theory of Operation: Frequency Generation Unit (FGU)
to Controller and
8.2V
8.2V SW
8.2V SW
Tx VCOs
Q3753
Q3765
Q3763
Q3754
LOOP
FILTER
Q3751
Q3766
Q3764
Q3752
Rx VCOs
Prescaler Input
Q3757
Lock Detect
to Controller
Keyed
9.1V
TX
TX
uffers
Injection
8.2V SW
8.2V SW
9.3V
RX
RX
uffers
Injection
8.2V
RF
Feedback
MAEPF-27804-O_VHF
6815854H01-A

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