9
FT-250R Technical Supplement
This pulse train is filtered to DC and returned to the var-
actor
D1011
(
HVC350B
). Changes in the level of the DC
voltage applied to the varactor affect the reference in the
tank circuit of the VCO according to the phase difference
between the signals derived from the VCO and the crys-
tal reference oscillator.
The VCO is thus phase-locked to the crystal reference os-
cillator. The output of the VCO Q1023, after buffering by
Q1022 and amplification by Q1018, is applied to the first
mixer as described previously.
For transmission, the VCO Q1023 oscillates between 144
and 148 MHz according to the model version and pro-
grammed transmit frequency. The remainder of the PLL
circuitry is shared with the receiver. However, the divid-
ing data from the microprocessor is such that the VCO
frequency is at the actual transmit frequency (rather than
offset for IFs, as in the receiving case). Also, the VCO is
modulated by the speech audio applied to
D1010
(
HSC277
), as described previously.
Receive and transmit buses select which VCO is made
active by
Q1020
(
DTA144TE
). When the power saving
f e a t u r e i s a c t i v e , t h e m i c r o p r o c e s s o r
Q 1 0 3 5
(
M3826AEFGP
) periodically signals switches the
Q1033
(
DTA114TE
) and
Q1034
(
2SC4081
) for the PLL subsystem
IC Q1030 to conserve power and shortens the lock-up time.
Miscellaneous Circuits
Push-To-Talk Transmit Activation
The PTT switch is connected to pin 28 of microprocessor
Q1035
(
M3826AEFGP
), so that when the PTT switch is
closed, pin 6 of Q1035 goes “High.” This signals the mi-
croprocessor to activate the TX/RX controller
Q1028
(
UMW1
), which then disables the receiver by disabling
the 5 V supply bus at
Q1026
(
DTA143XE
) to the front-
end, FM IF subsystem IC
Q1017
(
TA31136FN
).
At the same time,
Q1027
(
2SA1586Y
) activates the trans-
mit 5V supply line to enable the transmitter.
Circuit Description
Summary of Contents for FT-250R
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