General Dynamics C4 Systems URC-200 (V2)
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be controlled within 2 dB of nominal settings. The coupler/detector also senses reverse power to
detect a high VSWR condition. When a high VSWR is detected, the coupler/detector provides a
control voltage to the ALC that reduces the output power so that the RF amplifier is not damaged.
For FM operation, the modulation signal is applied directly to the synthesizer and produces an FM
modulated RF signal. For AM operation, the RF signal from the synthesizer is unmodulated and the
modulation signal is applied to the VCA control voltage that produces an AM modulated signal.
All radio functions are controlled by the processor circuitry that accepts inputs either from the front
panel keyboard or from a remote control unit connected to the remote connector. The processor
uses a control bus to set the internal transceiver circuitry for appropriate operating modes as
determined by the selection of VHF/UHF frequencies, AM/FM modulation, power settings, and
receive or transmit operation. An internal power supply is used to accept an input power of +22 to
+34 VDC and convert this input power to a filtered internal power source which provides +22 to
+34 VDC (depending on the input power), plus regulated ±5 VDC, ±12 VDC, +20 VDC, +70 VDC
voltages, and an external DC output limited to +30 VDC and 500mA for URC accessories and
other external devices. The radio is capable of self-calibration. This is accomplished by routing the
synthesizer to the preselector , where it is detected and calibrated.
3.2 Receiver Functional Description
Figure 6 shows a block diagram of the receiver. The received signal arrives at the antenna and is
applied to the preselector that contains RF limiters to protect the receiver from high signal levels
(for short durations to prevent damage) and provides RF filtering and low noise amplification for
the RF signal. The filtering in the preselector is performed by varactor tuned bandpass filters tuned
to the desired receive frequency by the Processor. The received RF signal is mixed with an LO
signal from the Synthesizer to produce an IF signal at 45.0 MHz. The IF signal is amplified, wide
or narrow bandpass filtered (25 kHz or 8.33 kHz channel), then amplified again before being
applied to the 2nd IF/Baseband circuitry. The 2nd IF/Baseband circuitry translates the 45.0 MHz IF
signal to 10.7 MHz IF using a 2nd LO frequency of 55.7 MHz and 10.7 MHz bandpass filter
(wideband or AM PT VHF narrowband). For FM modulation, the circuit uses a limiter and
discriminator to complete the demodulation of the signal to produce a baseband signal that is
applied to the audio circuitry. For AM modulation, the 10.7 MHz IF is applied to an AM detector to
demodulate the AM signal. The demodulated baseband signal is then applied 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. There are four total
AGC-controlled points in the receive chain: (1) the voltage controlled attenuators (RF limiters) in
the preselector; (2) an attenuator prior to the first 1st IF amplifier; (3) an attenuator prior to the
second 1st IF amplifier; and, (4) the 2nd IF amplifier within the 2nd IF/Baseband circuitry. At low
signal levels, maximum end-to-end receiver gain is desired. As signal levels increase, the AGC first
decreases gain at the 2nd through 4th control points (which have minimal impact on total noise
figure), and only decreases gain at the 1st control point (prior to the first low noise amplifier) when
strong signals are present (since increased noise figure does not matter as much).
The baseband signal is amplified and applied to either the CT Data Filter or PT (Audio) Filter
depending on the operating mode of the transceiver. The CT Filter is a 30 Hz to 10.24 kHz
bandpass filter and the PT is a 300 Hz to 3 kHz bandpass filter. For CT operation, the signal is then
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