General Dynamics C4 Systems URC-200 (V2)
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Figure 5 - Transceiver Simplified Functional Block Diagram
In the receive mode, the RF signal arrives at the antenna and passes through the coupler/detector
which is used only in the transmit mode. A solid state Transmit/Receive switch connects the RF
signal path to the receiver circuitry. In the receiver, the RF signal is first switched to the appropriate
VHF or UHF preselector which uses a self-tuned bandpass filter to reject all out-of-band signals
and provides some low noise signal amplification. The received signal then mixes with a Local
Oscillator (LO) signal provided by the synthesizer to produce a 45.0 MHz Intermediate Frequency
(IF) signal. This first IF signal s then filters and applies to the 2nd IF/Baseband circuit, which
produces a 10.7 MHz IF using a 2nd LO of 55.7 MHz. An AM detector and FM limiter and
discriminator then demodulate the signal and provide a baseband signal to the audio circuitry. The
receiver uses a delayed Automatic Gain Control (AGC) scheme to adjust gain along the RF receive
chain in order to maintain noise figure at low to medium signal levels. The audio circuitry provides
appropriate filtering and amplification of the baseband signal that is then applied to the speaker
and/or front panel handset and remote connectors, as determined by the operating mode.
In the transmit mode, the synthesizer is programmed to generate an RF signal at the desired
transmit frequency. This RF signal is amplified and applied to a Voltage Controlled Attenuator
(VCA) that is used to regulate the signal strength applied to the RF amplifier. The RF signal is
amplified to the desired output power by the power amplifier and then filtered by a bandpass filter
to reduce harmonic and spurious transmit signals. The RF signal then passes through a
coupler/detector that senses the signal strength of the transmitted RF signal. The coupler/detector
provides a control voltage to the Automatic Level Control (ALC) circuitry that then sets the
appropriate attenuation level of the VCA. This closed loop approach allows the RF output power to
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