Sailor RE2100, Technical Manual

Page: 117 / 186

5 CIRCUIT DESCRIPTION AND SCHEMATIC DIAGRAMS RE2100
PAGE 5-17 9406
BLOCK DIAGRAM OF USB/LSB SYNTHESIZER
R
DATA
CLOCK
STROBE
TCXO 2
25743A
MC145158
Divider
N
PH 131
+
F(S )
1
vco
Control
Modulus
R
P
P+1
2 Modulus
Prescaler
<
VCO1
VCO2 F(S )
2
>
F
LO1 to
TX
LO2 to
>
RX
From the block diagram it can be shown that the output frequency f
VCO
has the following function of f
TCXO
:
f
VCO
= f
TCXO
· 1/R · (N · P + A), P chosen to 32.
f
TCXO
= 10.73152 MHz
When upper sideband is chosen the following counts are read into the divider U35. R = 2 x 131, N = 45,
A = 7, this gives N · P + A = 1447 and the lock frequency is f
VCO
= 59.269120 MHz.
When lower sideband is chosen the following counts are read into the divider U35. R = 2 x 131, N = 61,
A = 19, this gives N · P + A = 1971 and the lock frequency is f
VCO
= 80.732160 MHz.
VOLTAGE CONTROLLED OSCILLATOR
The circuit contains two similar VCO’s. One is active in LSB mode and the other is active in USB mode.
The oscillator which is active in the LSB mode is built around the transistor Q30. Coil L31, parallelled with
C130 and C129 in combination with the variable capacitor D19 form the main part of the frequency
determining elements.
D20 and R120 which are connected to the gate of the oscillator transistor prevent the gate source voltage
from becoming positive which will cause the oscillator noise to increase.
To activate the oscillator U01 - pin Qg is “LOW” which means that transistor Q28 is off and the transistors
Q29, Q31 are on supplying the oscillator transistor with DC voltage.
The oscillator which is active in the USB mode is built around the transistor Q33. Coil L37 parallel with
C150 and C149 in combination with the variable capacitor D22 form the main part of the frequency
determining elements.
D23 and R133 which are connected to the gate of the oscillator transistor prevent the gate source voltage
from becoming positive which will cause the oscillator noise to increase.
To activate the oscillator U01. Pin Qg is “high” which means that the transistors Q32 and Q35 are on
supplying the oscillator transistor with DC voltage.
When one oscillator is supplied with DC voltage the other is off.
The DC supply to the oscillator switches the diode D21 or D24 on and thereby supplies DC voltage to the
oscillator buffer transistor Q36. From this transistor the local oscillator signal is fed to the prescaler buffer
and to the output local oscillator buffer.