Continental Electronics 816R - Page 18

4. The reflected power signal from the directional
coupler is compared against an internal limit to
smoothly fold the forward power level back
when a slowly rising VSWR level is detected.
Forward power is reduced to keep the reflected
power at 5 to 6% of the customer ’ s specified
TPO.
5. A reflected power signal from the directional
coupler can remove power from the transminer
and light the VSWR tally LED when a rapidly
rising reflected power level greater than 10%
of the customer ’ s specified TPO is detected.
Theory of Operation-
The forward power signal from the directional
coupler, DC1, is amplified and buffered by U4.
Resistor R9 is an offset null adjusted for zero
output at TP1 when no input signal is present.
Resistor RIO is adjusted to provide a 100% forward
power indication on the panel power meter (A1M4)
at the customer ’ s specified TPO.
The output of U1 is also present on terminal board
A10TB2-13, terminal 34, through a 2.2K ohm
isolation resistor, R13, to provide remote metering
of forward power.
The positive output of U4 is coupled through diode
CR1 1 and compared at the inverting input of U3
against the negative voltage from either resistor R8
(Normal Power) or resistor R7 (Second Power) in
the automatic mode. The output of U3 is used to
raise or lower the transtniner plate voltage, as
necessary, to maintain the selected power level.
The input to U3 is switched from resistor R8 to
resistor R7 by relay KI which is activated by
applying +28 V de to relay KI coil through
terminal board A10TB2-1. Normally, resistor R8
sets the normal operating TPO reference while
resistor R7 is adjusted for some lower value,
perhaps necessary during emergency operation with
a generator unable to supply the full power load.
The reflected power signal from directional coupler
DC1 is amplified and buffered by Ul. Resistor R1
is an offset null adjustment for zero output at TP2
when no input signal is present. Resistor R2 is
adjusted to cause a 10% reflected power indication
on the panel power meter A1M4 when the reflected
power reaches 10% of the customer ’ s specified
TPO. Resistor R3 is adjusted to simulate that 10%
reflected power level when the TEST switch SI is
depressed. This allows testing of the VSWR
protection and metering circuits. The output of Ul
is fed through resistor R43 to the gate of the VSWR
overload SCR A2A5Q4 when the VSWR protect is
in the enable position. Resistor R40 is adjusted to
fire SCR A2A5Q4 when the reflected power
reaches 10% of the customer ’ s specified TPO.
When SCR Q4 fires, VSWR overload relay A11K9
activates, removing power from the transmitter and
illuminating the VSWR OVERLOAD LED.
The output of U2 is also present on terminal board
A10TB2-12 through 2.2K ohm resistor R23 to
provide remote metering of reflected power.
Devices U6 and U7 form the VSWR foldback
circuit. A sample of the buffered reflected power
signal from Ul is fed to U2 through resistor R5.
Resistor R6 is the offset null adjustment for U2 and
is adjusted to give zero output at U2-6 when no
input signal is present. The output of U2 is coupled
through diode CR6 to the automatic power
comparator, U3. Resistor R5 sets the gain so that
the output voltage of U2 will exceed that of U4,
causing the power to be reduced, when the
reflected power exceeds 5-6% of the normal TPO.
The VSWR foldback circuit is defeated by the
circuitry of the ten second timer U5. The timer is
triggered by sampling the anode voltage of VSWR
overload SCR A2A5Q4. The response time of the
VSWR foldback circuit is relatively slow. A sudden
significant increase in VSWR, as in an arc, would
cause the VSWR overload SCR to fire. Power to
the transmitter is removed, the VSWR
OVERLOAD LED is illuminated and the VSWR
foldback circuit is disabled for ten seconds. The
VSWR foldback circuit is disabled to allow the
VSWR overload circuit to sample the VSWR at full
power thereby preventing operation into a
dangerously deficient load.
Overload Protection.
Relays A11K6, A11K7, and A11K9 are adjusted to
energize and remove power from the transminer
when an overload occurs in the plate or screen
supply or when the VSWR exceeds a preset level.
Screen current through resistor A5R8 produces a
voltage that is applied to relay A11K7 through
resistor A11R65. The plate current through resistor
14