Crown Low Power Transmitters - Crown Broadcast

To get a direct reading of SWR, the reference input of the digital panel meter is fed
from a voltage proportional to the forward-minus-reflected RF voltage, while
forward-plus-reflected is fed to the digital panel meter input. The panel meter
provides the divide function.
U408 and U409 function as data selectors for the digital panel meter input and
reference voltages. Binary select data for U408 and U409 comes from the display
board.
The output voltage of U403D goes positive when the temperature exceeds about 35
degrees C (set by R426) providing proportional fan control (FM100 and FM250).
When the Carrier switch is off or the RF power is less than about 5 watts, the SWR
automatically switches to a calibrate-check mode. U406C provides a voltage that
simulates forward power, while Q403 shunts any residual DC from the reflectedpower
source. The result is a simulation of a 1.0 to 1 SWR. (See section 5.4.)
4.6 Motherboard
The motherboard is the large board in the upper chassis interconnecting the audio
processor, stereo generator, RF exciter, and metering boards. The motherboard
provides the interconnections for these boards, eliminating the need for a wiring
harness, and provides input/output filtering.
It also contains the +5.00 volt reference and the composite drive Op amp and its
associated circuitry.
This board has configuration jumpers associated with diffeent options that can be
added at the time of order or at a later time as an upgrade. Options include
FMX-DMS, FMX-RMS, Crown/Omnia DP3, and other standard options.
Principles of Operation
4–9