6815854H01-A June 15, 2005
Theory of Operation: Frequency Generation Unit (FGU) 3-53
3.13.2.11 Transmitter VCOs
Transmitter frequencies are generated from two VCOs, Q5825 and Q5826.
• Q5825 supplies frequencies in the range 380 MHz up to (but not including) 425 MHz.
• Q5826 supplies frequencies in the range from 425 MHz to 470 MHz.
3.13.2.12 Receiver VCOs
Receiver first local-oscillator frequencies are generated from three VCOs, Q5901, Q5903 and
Q5905.
• Q5901 supplies frequencies in the range 489.65 MHz up to (but not including) 519.65 MHz.
• Q5903 supplies frequencies in the range 519.65 MHz up to (but not including) 549.65 MHz.
• Q5905 supplies frequencies in the range 549.65 MHz up to 579.65 MHz.
The RX VCOs operate at frequencies which are 109.65 MHz higher than the radio channel selected
frequency since the receiver is high side injected, and the first IF frequency is 109.65 MHz.
The five VCOs are selected by the following pattern of logic levels on the AUX pins from the
synthesizer chip U5752 (Table 3-9):
3.13.2.13 Prescaler Feedback
RF feedback for the synthesizer loop is provided by prescaler amplifier Q5755. Feedback from both
the transmitter and receiver injection strings are coupled to this amplifier through capacitors C5863
and C5957. The output of Q5755 is coupled to U5752 at pin 32 (PREIN), which is the prescaler input
for the synthesizer.
Table 3-9. VCO AUX Pin Logic UHF Range 1
VCO AUX1 AUX2 AUX3
RX VCO Q5901 0 1 0
RX VCO Q5903 1 1 0
RX VCO Q5905 0 0 1
TX VCO Q5825 0 0 0
TX VCO Q5826 1 0 0
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