User Manual

CIRCUIT DESCRIPTION
4-4
June 1997
Part No. 001-3412-002
The -2.0V applied to the VCO is derived from
the 17.5 MHz TCXO frequency that is amplified by
Q701, rectified by CR701 and filtered by C706, C707,
C708 and C709 on the RF board.
The VCO frequency is modulated using a similar
method. The transmit audio/data signal from J201,
pin 6 is applied across varactor diode CR852 which
varies the VCO frequency at an audio rate. Series
capacitors C856/C870 couple the VCO to CR852.
R854 provides a DC ground on the anodes of CR852/
CR853, and isolation is provided by R852 and C855.
C858 is an RF bypass. C853 provides isolation.
The DC voltage across CR853 provides compen-
sation to keep modulation relatively flat over the
entire bandwidth of the VCO. This compensation is
required because modulation tends to increase as the
VCO frequency gets higher (capacitance of CR854/
CR855/CR856/CR851 gets lower). CR853 also bal-
ances the modulation signals applied to the VCO and
TCXO. The DIAG on J201, pin 14 can also adjust the
modulation.
The DC voltage applied across CR853 comes
from the modulation adjust control R827 on the RF
board. R826 applies a DC biasing voltage to CR852;
C821 provides DC blocking. RF isolation is provided
by C858, R853 and R847.
4.2.2 VCO AND REFERENCE OSCILLATOR
MODULATION
Both the VCO and reference oscillator (TCXO)
are modulated in order to achieve the required fre-
quency response. If only the VCO were modulated,
the phase detector in U811 would sense the frequency
change and increase or decrease the VCO control volt-
age to counteract the change (especially at the lower
audio frequencies).
If only the reference oscillator frequency is mod-
ulated, the VCO frequency would not change fast
enough (especially at the higher audio frequencies).
Modulating both VCO and reference oscillators pro-
duces a flat audio response. Potentiometers R825,
R826 and R827 set the VCO modulation sensitivity so
that it is equal to the reference oscillator modulation
sensitivity.
4.2.3 CASCODE AMPLIFIERS/VCO (Q851/Q852)
Q851/Q852 form a cascode amplifier to provide
reverse isolation for the VCO. Q851 is configured as
a common emitter and Q852 as a common base. The
output signal is taken from the collector of Q851 and
coupled to the base of amplifier Q853 through cou-
pling capacitors C868, C871 and a PI-attenuator made
up of R859 and R875.
4.2.4 AMPLIFIER (Q853)
Amplifier Q853 provides final amplification of
the VCO signal. Bias for Q853 is provided by R871,
R872 and R874. L856/C874 provide a match to the
transmitter and first injection frequency. The T-pad
attenuator made up of R892, R893 and R894 provides
6 dB of isolation between the transmitter and first
injection frequency.
4.2.5 VOLTAGE FILTER (Q901)
Q901 on the RF board is a capacitance multiplier
to provide filtering of the 4.6V supply to the VCO.
R901 provides transistor bias and C901 provides the
capacitance that is multiplied. If a noise pulse or other
voltage change appears on the collector, the base volt-
age does not change significantly because of C901.
Therefore, base current does not change and transistor
current remains constant. CR901 decreases the charge
time of C901 when power is turned on. This shortens
the start-up time of the VCO. C902 and C903 are RF
decoupling capacitors.
4.2.6 VCO FREQUENCY SHIFT (Q841)
The VCO must be capable of producing frequen-
cies from 403-564.95 MHz to produce the required
receive injection and transmit frequencies. If this
large of a shift was achieved by varying the VCO con-
trol voltage, the VCO gain would be undesirably high.
Therefore, capacitance is switched in and out of the
tank circuit to provide a coarse shift in frequency.
This switching is controlled by the T/R pin shift
(RX_EN) on J201, pin 4, Q841/Q842 and pin diode
CR850. When a pin diode is forward biased, it pre-
sents a vary low impedance to RF; and when it is
reverse biased, it presents a very high impedance. The
capacitive leg is switched in when in transmit and out
when in receive.