Page 6-8
Vector-LP Radio Beacon Transmitter Technical Instruction Manual
Issue 1.1
Section 6 Theory of Operation
6.4.2.7 BALANCED DRIVE
The balanced drive buffer is a switching
circuit that ensures the rise and fall times of
its output square wave are minimal. The
g
C
signal is a 15 V peak-to-peak square wave
that is the low level RF drive signal for the
RF power stage of the transmitter.
6.4.2.7.1 Balanced Drive Matching
Different values are possible on the
balanced drive output, depending on the
transmitter’s requirements. The position of
shorting shunt post E5 on 7-position header
XE5 can be changed to choose between
different capacitor and resistor values. E5 is
factory installed in the
D
position (shorting
pins 7 and 8) for Vector transmitters.
6.4.2.8 N DIVIDER
The N divider circuit is a cascade counter
made up of U6, U7, and inverters U8:A and
U8:F. The 4
g
C
/
B
signal is divided by N to
provide a
2
g
PDM
frequency, which is between
126 kHz and 134 kHz (when E3 is set in
LOW PDM
mode) or between 245 kHz and
276 kHz (when E3 is set in
HIGH PDM
mode). The value of N is supplied by
microprocessor U4 (see 6.5.2.1.2). The
2
g
PDM
output, which is nominal 5 V pk-to-pk
pulses, is applied to the transmitter's PDM
generation circuit, and ultimately determines
the PDM frequency.
6.4.2.9 RF DRIVE SOURCE SELECTION
The RF drive (
g
c) source is provided by the
integral numerically controlled oscillator or
an external RF generator. The position of
shorting shunt posts E1, E2 and E4
determine which source is selected. Vector
transmitters use the internal source.
6.4.2.9.1 Internal
g
c Source
To use the integral numerically controlled
oscillator’s output as the RF drive source,
set the shorting shunt posts as follows:
x
E1 in
INT
position (pins 2 and 3 shorted)
x
E2 in
INT
position (pins 2 and 3 shorted)
x
E4 in
INT
position (pins 2 and 3 shorted)
6.4.3 Interphase PDM Driver PWBs
See Figure SD-18. The interphase PDM
driver PWBs (A6 and, if installed, A9)
produce a pulse train of variable width as
their PDM output. The PDM repetition rate
(
g
PDM
) is a fixed frequency normally between
130 kHz and 133 kHz. The frequency is
determined by the 2
g
PDM
input, which is
produced by the active exciter monitor/
generator PWB. The PDM drive signal, which
determines the transmitter output power level,
is applied to the modulator assemblies in RF
power modules A or B (see Figure SD-22).
6.4.3.1 CARRIER LEVEL CONTROL
The carrier level control circuit consists of U6,
U7, U2A, U2D, U3B, U3D, U13A and their
associated components. U6 and U7 are
analog multipliers connected as a variable
gain, wide-band, linear amplifier. The
modulation reference (U6-X1) is multiplied by
a factor determined by the
carrier ref
(U6-Y1)
and
B+ sample
(U7-X1) signals to determine
the gain and, in turn, the status of U7-Y1.
When there is no
Unbalanced Audio
input at
P1-15, U2D's output (TP3), which is the
modulation reference, will be a nominal 1.4 V.
This voltage is applied as U6's X1 input.
The
Carrier Ref
input at P1-2 is a dc voltage
directly proportional to the square of the
expected RF carrier level. This voltage is
applied to U6-Y1.
