Maintenance Manual
7
Voltage Controlled Oscillator (VCO)
There are two Voltage Controlled Oscillator in the synthesizer circuit. One is the TX VCO which
generates the actual transmit frequency of 400 ~ 470
㎒
during transmit and the other is RX VCO which
generates the 1
st
LO frequency of 354.7 ~ 414.7
㎒
during the receive. These two VCO is selected by the
VCO_TX and VCO_RX signal from the microprocessor.
When VCO_TX signal is High, it turns transistor Q66 on and the TX5V1 (5Vdc) is supplied to the
TX_VCO. TX VCO oscillates transmit frequency by C463, C415, C418, C419, L85, D76, and Q62. D77
(variable capacitor diode for modulation) and C412, C413, R452 play a role to maintain regular audio
modulation within the using frequency range. The TX VCO generates the actual transmit frequency of
400 ~ 470
㎒
for UHF model and 136 ~174
㎒
for VHF model.
When VCO_RX signal is High, it turns transistor Q67 on and the RX5V1 (5Vdc) is supplied to the
RX_VCO. RX VCO oscillates transmit frequency by C458, C402, C404, C405, L81, D74. The RX VCO
generates the actual 1
st
local frequency of 354.7 ~ 414.7
㎒
for UHF model and 114.7 ~152.7
㎒
for
VHF model.
Q63 operates as an amplifier which amplifies the output signal of the VCO and operates as an buffer
amplifier which helps the stable operation of VCO. The output level from this amplifier is about –
10dBm.
The output signal of the VCO fed through the local amplifier and TX pre amplifier. A control voltage
from the Loop Filter is applied to L78 and L79 and is used to control VCO frequency. The feedback for
the synthesizer circuit is taken from the C429. This signal connects though a band-pass Filter consisting
VCO
(TX / RX)
Synthesizer
IC(U302)
Pre Amplifier
& Local
Amplifier
Loop
filter
Charge
pump
VCTCXO
(U303)
PLL_EN, PLL_DARA, PLL_CLK
Voltage
Converter
TX5V1, RX5V1
RF+3.3
DC+5V
Audio/Tone
modulation
