General Dynamics C4 Systems URC-200 (V2)
30
All 30-90 functions are controlled by the APPS 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 30-90 circuitry for appropriate operating modes, as
determined by the selection of power settings and receive or transmit operation. The radio’s
internal power supply on the APPS board is used to provide filtered +24 VDC (nominal
unregulated) and regulated +5 VDC, ±12 VDC, +20 VDC, and +70 VDC voltages to the 30-90
board.
Some limitations of the 30-90 are worth noting. The front panel display signal strength meter and
power meter do not operate when the radio is in the 30-90 MHz band. Also, the 30-90 option
requires use of a different remote control unit than the main radio. (The remote control units are
optional accessories.)
Finally, it should be mentioned that most of the 30-90 filters and power amplifier are implemented
at 12.5 or 25 ohms characteristic impedance, versus the usual 50 ohms for such designs.
3.4.1 30-90 MHz Receiver Functional Des cription
Figure 10 shows a block diagram of the 30-90 MHz receiver. The received signal arrives at the 30-
90 antenna and is applied to the preselector that contains high-level RF limiting amplifiers to
protect the receiver from high signal levels to prevent damage and provides RF filtering and
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 10.7 MHz. The IF signal
is filtered before being applied to the integrated 2nd IF/Baseband chip. The 2nd IF/Baseband chip
translates the 10.7 MHz IF signal to 455 kHz IF using a 2nd LO frequency of 10.245 MHz
generated from a crystal external to the chip. The chip includes a mixer, limiting amplifier, and
quadrature discriminator to complete the demodulation of the IF signal to produce a baseband
signal that is applied to the APPS audio circuitry.
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
amplified and routed to the remote connector on the front panel. For PT operation, the audio signal
is amplified and routed to the remote connector, the speaker, and the handset.
Normally the URC-200 in the 30-90 band will use a 150 Hz Tone Squelch (vs. carrier-to-noise
squelch), which is compatible with other radios in this band of frequencies. This fixed threshold
tone-operated squelch is turned ON/OFF from the keypad. If desired, the tone may be turned OFF;
however, “white-noise” will come from the speaker and/or handset with the Squelch Control knob
in the full CCW position. Even though the Squelch Control knob may be adjusted CW to squelch
this noise, this may result in the incoming signal not being able to overcome the squelch.
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