Voltage Control Oscillators (Vco); Receive Vco; Transmit Vco; Buffer - Motorola PR860 Detailed Service Manual

2-32
Both the normal and adapt charge pumps get their supply from the capacitive voltage multiplier
made up of C247, C283, C284, C285, C286, and triple diodes D210 and D211. This circuit provides
13.3V at U205, pin 47.
DATA (U409 Pin 100)
CLOCK (U409 Pin 1)
CSX (U409 Pin 2)
MOD IN (U404 Pin 40)
+5V (U204 Pin 4)
5,20,34,36
(U400 Pin 1)
Reference
Oscillator
Voltage
Multiplier

2.19 Voltage Control Oscillators (VCO)

2.19.1 Receive VCO

The receive VCO is a Colpitts type design and using two active devices in parallel, Q202 and Q204.
The oscillator is powered off of the 4.3 volt super filter supply when the AUX3 line goes low. The
oscillator operates from 139 to 152 MHz for range 1 and 145 to 160 MHz for range 2. The frequency
is tuned by varactor diodes CR201 and CR202.

2.19.2 Transmit VCO

The transmit VCO is a Hartley-type design with active devices Q203. The oscillator is powered off of
the 4.3 volt super filter supply when the AUX2 line goes low. The oscillator operates from 29.7 to
42 MHz for Range 1 and 35 to 50 MHz for Range 2. The frequency is tuned by varactor diodes in
U203. Note that the values of the inductive tap, L208 and L209, and the capacitor C215 which
couples the varactor to the oscillator tank vary between the ranges.

2.19.3 Buffer

Both the receive and transmit VCO are fed to a buffer amplifier Q201. This is a BJT amplifier that
boosts the signal levels to +4 dBm and provides reverse isolation to the oscillators. The amplifier is
powered off the 4.3 volt super filter supply and the feed network is combined with the transmit filter.
7
DATA
8
CLK
9
CEX
10
MODIN
13,30
V , DC5V
CC U205
V , 3.3V Low Voltage
DD
Fractional-N
23
XTAL1
Synthesizer
24
XTAL2
25
WARP
32
PREIN
47
VCP
VMULT2
14

Figure 2-21. Low Band Synthesizer Block Diagram

Theory of Operation: Voltage Control Oscillators (VCO)
4
LOCK
19
FREFOUT
6,17,22,29,31,33,44
GND
43
IOUT
45
IADAPT
41
MODOUT
1
AUX2
2
AUX3
28
SFOUT
40
BIAS1
39
BIAS2
VMULT1
15
Prescaler In
LOCK (U409 Pin 56)
FREF (U303 Pin 21 & U404 Pin 34)
Steering
2-Pole
Line
Loop Filter
Switching
Network
Voltage
Controlled
Filtered 4.3V
Oscillators
(First Stage of PA)
LO RF
Injection
TX RF
Injection