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CIRCUIT DESCRIPTION
phase becomes the control signal for the VCO operat-
ing frequency after going through a charge pump circuit
(Q2 and Q3) and a passive low-pass filter.
The VCO frequency is controlled by the voltage ap-
plied to varactor diodes (D1 and D2) in the VCO sub-unit.
The operating frequency is set according to the con-
tents (programmed frequency) of the EEPROMs
(Elec-
tronically Erasable Programmable Read Only Memory,
iC7 and IC8 in the control unit) and the channel select-
ed by the channel selector switch. The channel selector
position is recognized by the microcomputer (IC1 in the
control unit) and whenever the receiver circuit is turned
on {initial power on and/or revert to the receive mode
from the transmit mode), the microcomputer retrieves
the frequency information (16-bit serial data) from the
EEPROMS. This frequency information is sent to IC1 in
the PLL unit, by the microcomputer, to set the program-
mable counter.
$u@
UNIT
(VCO)
(X58-3570-
Xx): A4
* Transmitter PLL (TX PLL)
The transmitter PLL (TX PLL) shares major compo-
nents with the RX PLL. However,
Q2 in the VCO sub-
unit is used for generating the TX RF signal. The prograrn-
mable counter in iC1 of the PLL unit is re-programmed,
every time the transmitter is activated, for the proper
operating frequency according to the channel selector
position.
In the transmit mode, the oscillator (Q2) output sig-
nal is buffer amplified by Q3 and Q4 in the same man-
ner as in the RX PLL operation. The signal is then steered
to the driver sub-unit by D4. Other loop operations are
the same as for the RX PLL.
cu
4
Ql2
2
3
SUB
UNIT (PLL) (X58-3550-11P2 AS
Co
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;
Ra
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Joe
it
| °
I
8
7
(* ie)
ma
Lz
!
Rn
|
{5
an a
3
j
e2
UL
nie
ria
| COMPARATER
Re
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ce
Fig. 2 PLL Circuit
25
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