CIRCUIT DESCRIPTION
6-6
September 1994
Part No. 001-2008-300
6.2.2 VCO AND TCXO FREQUENCY MODULA-
TION
Both the VCO and TCXO are modulated in order
to achieve the required frequency response. If only the
VCO was modulated, the phase detector in U403
would sense the frequency change and increase or
decrease the VCO control voltage to counteract the
change (at the lower audio frequencies inside the
closed loop bandwidth of the Synthesizer). If only the
TCXO frequency was modulated, the VCO would not
track the higher audio frequencies (those beyond the
closed loop bandwidth of the Synthesizer). However,
by modulating both the VCO and TCXO, the two
phase detector inputs remain in phase and no fre-
quency shift is sensed. This produces a flat audio
response. Potentiometers R425 and R446 balance the
modulating signals.
There are two 3.5V sources on the Exciter board;
one is a reference for the modulation amplifier to the
VCO, the other is for the modulation amplifier to the
TCXO. Maintaining the voltage reference to the
TCXO is critical to maintain correct frequency over
temperature.
The reference voltage on U402B, pin 5 is sent to
buffer U407B, J401, pin 9 to RFIB connector J102,
pin 9 and out on J101, pin 14 to J2, pin 27 on the back-
plane, to the bottom connectors via pin 7 and finally to
the TPI on P100, pin 7.
With reference to the ground on the Exciter, the
3.5V reference stability is maintained by U206A/B.
The 3.5V DC passes through summing amplifier
U208B to P100, pin 29 (Tx MOD). P100, pin 29 is
connected to backplane connector J2, pin 8 and RFIB
connector J101, pin 22 to J102, pin 13. The transmit
modulation and 3.5V reference enter the Exciter on
J401, pin 13 and is routed to U402B pin 6. R425 sets
the output modulation level and along with the 3.5V
DC is applied to U402A, pin 2. U402A, pin 3 is set to
3.5V to provide a very stable output on pin 1 to the 1
PPM TCXO Y401.
6.2.3 ACTIVE FILTER
Q801 functions as a capacitance multiplier to pro-
vide filtering of the 12V supply to Q802. R801 and
R802 provide transistor bias, and C809 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 C809.
Therefore, the base current does not change and tran-
sistor current remains constant. R803 is the low
impedance collector load for Q802 and decouples the
output from AC ground. L803 is an RF choke and
C807, C808, C810 and C811 provide RF bypass.
6.2.4 BUFFER (Q406, Q407)
A cascode amplifier formed by Q406 and Q407
provides amplification and also isolation between the
VCO and synthesizer. A cascode amplifier is used
because it provides high reverse isolation. The input
signal to this amplifier is tapped from the VCO RF
output. DC blocking to the VCO is provided by C441
and to the buffer by C433. Bias for the amplifier is
provided by R451, R453, R454 and R455. Q407 is a
common-emitter amplifier and Q406 is a common-
base with C432 providing RF bypass. L403 decouples
the output from AC ground and R452 lowers the Q of
L403. The output of the amplifier is coupled by C429
and C499 to U403, pin 11.
6.2.5 SYNTHESIZER (U403)
The synthesizer inputs/outputs are shown in Fig-
ure 6-3. The synthesizer output signal is the transmit
frequency. This signal is produced by a VCO (volt-
age-controller oscillator) that is frequency controlled
by a DC voltage produced by synthesizer chip U403.
This DC voltage is filtered by a loop filter made up of
C805, C806 and R804 in the VCO circuitry.
Frequencies are selected by programming
counters in U403 to divide by a certain number. This
programming is provided through J401, pins 12, 19
and 20. The frequency stability is established by the
±
1.0 PPM stability of TCXO Y401. The oscillator is
stable from -30
°
C to +60
°
C.
The VCO frequency of A007 is controlled by a
DC voltage produced by the phase detector in U403.
The phase detector senses the phase and frequency of
the two input signals and causes the VCO control volt-
age to increase or decrease if they are not the same.
When the frequencies are the same, the VCO is then
"locked" on frequency.