Nautel Vector-LP VR125 - 6.6.2 RF Filter PWB; 6.6.3 RF Power Probe PWB

Vector-LP Radio Beacon Transmitter Technical Instruction Manual Page 6-29
Section 6 Theory of Operation Issue 1.1
6.6.2 RF Filter PWB
See Figure SD-4. The RF filter PWB (A14) is
a band pass filter that attenuates the
harmonics of the square wave output applied
from the selected RF power module before it
is applied to the RF combiner/probe. The filter
has a flat response characteristic over the
operating bandwidth [two PWB options;
standard frequency band (190-535 kHz) and
extended frequency band (536-1250 kHz and
1600-1800 kHz)]. It has a nominal input and
output impedance of 50 : and a loaded Q of
2. The band-pass is selected using frequency
dependent link connections (E1 through E36;
see Tables 2-7a and 2-7b).
6.6.3 RF Power Probe PWB
See Figure SD-4. The RF power probe PWB
(A15) contains various circuits that monitor
the RF output and provide RF voltage, RF
current, forward power and reflected power
monitoring samples to the control/monitor
stage.
6.6.3.1 FORWARD/REFLECTED POWER
PROBE
The forward/reflected power probe circuit
consists of 40:1 RF current transformer T1,
80:4 RF voltage transformer T3 and
associated components. These transformers
form the current and voltage arms of a
forward/reflected power bridge, which
samples the RF output.
The current flowing into the RF output passes
through transformer T1’s primary. The current
in T1’s secondary develops a voltage across
resistors R33, R34, R41, R42, R43 that is
proportional to the RF output current. The
anti-phase voltage across the secondary of
RF voltage transformer T3 is applied
(summed) to the centre-tap of T1’s
secondary. When the RF output impedance is
precisely 50 :, the RF current waveform is in-
phase and of equal amplitude to the RF
voltage waveform on one half of T1’s center-
tapped secondary and equal amplitude, but
180q out-of-phase on the other half.
The in-phase voltages are summed, rectified
by CR35, low-pass filtered by L9/C53/L11,
resulting in a dc voltage being applied to the
Fwd Pwr Sample output (J1-1). This voltage
is proportional to the RF output's forward
power level.
The out-of-phase voltages are summed,
rectified by CR36, low-pass filtered by
L8/C54/L10, resulting in a dc voltage being
applied to the Refld Pwr Sample output
(J1-5). This voltage is proportional to the RF
output's reflected power level.
T3’s secondary voltage, which is a true
sample of the RF output’s voltage waveform,
is also applied to the RF Volts Monitor output
(J1-9). This output is intended for monitoring
by a modulation monitor and for test
equipment during maintenance.
6.6.3.2 RF CURRENT PROBE
The RF current probe circuit consists of 40:1
RF current transformer T2 and associated
components. The current flowing into the RF
output passes through the primary of
transformer T2. The current in T2’s
secondary winding develops a voltage across
resistors R37 and R40 that is proportional to
the primary (RF output) current. This voltage
is applied to the RF Current Sample output
(J1-17), which is used by the control/display
PWB's high RF current detector and
metering circuit. This high RF current
detector produces a high RF current alarm
and – via the control/display PWB - causes
the transmitter's RF output to shut back (turn
off) when the RF current exceeds a
threshold that represents the maximum
stress current for the RF power modules.
The current sample voltage is also applied to
the RF Current Monitor output (J1-13), which
is used for external monitoring purposes.