Lmx - Dual Synthesizer; Second Lo Vco; Offset Vco - Motorola MTH500 Service Manual

LMX - Dual Synthesizer

Second LO VCO

Offset VCO

68P02963C70-O
The LMX233XL is an integrated dual frequency synthesizer that includes prescal-
ers. It is used with the second LO at the RF section and with the Offset VCO at the
IF section.
The LMX uses the 16.8 MHz from the TCXO for both synthesizers and divides
the TCXO frequency as follows:
• The reference frequency for the second LO is 100 KHz.
• The reference frequency for the Offset VCO is 50 KHz
The Second LO VCO is a discrete VCO that is controlled by the LMX Dual Syn-
thesizer RF section. The VCO produces a frequency of 219.3 MHz.
The internal phase detector is of the charged pump type.
The output signal of the of the VCO is routed via the phase detector at U808,
pin 5 (FINRF).
The output of the phase detector is present at U808, pin3 (DORF) and than via the
loop filter R915, C903, C948.
The Offset VCO is a discrete VCO that is controlled by the LMX Dual Synthesizer
(U808) IF section. The Offset VCO produces frequency of 113.65 MHz.
The internal phase detector is of a charged pump type. The output signal of the
VCO is routed via the phase detector at U808, pin 18 (FINIF).
The output of the phase detector is present at U808, pin 20 (DOIF) and than via
the loop filter R829, C895, C860.
Theory of Operation
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