Motorola M3097 Service Manual download pdf (Page 91)

TDMA M3090/M3097: VCO

Description

U301 is the local oscillator module used to add frequency selectivity to the transceiver. The frequency of oscillation is dependent on the channel and band that the trans-

ceiver will be operating in. The frequency is controlled by U401. U401 will receive channel information via the SPI bus and adjust the frequency of U301 by varying the

voltage level to the input (VCO_STEER) of U301.

The output of U301 is split into three paths. One path (VCO_FDBK) is used to feed back the produced signal to U401. The signal is prescaled and sent through a phase

lock loop circuit for frequency locking. The second path (MAIN_LO) is sent to the receive circuit for carrier signal removal. The third path (DBLR_IN) goes to the

Down Mixer U101 for PCS down mixing. U101 uses the same DBLR_IN to provide a local osc. to the TX circuit (refer to B8).

When the transceiver is operating in the 800MHz band, U301 will oscillate at a frequency of 112.32+RX frequency. In the 1900MHz band, U301 will oscillate at a fre-

quency of (112.32+RX)/2.

RX_SF (Super Filter) comes from U401 and provides a clean supply voltage to the VCO module.

LP_SWITCH control comes from U401. In Analog mode, the input to Q301 becomes high causing it to close so the filter ciruitry becomes active which in turn is used

to improve phase noise due to stringent adjacent channels in Analog mode.

U301

VCO_STEER

RX_SF

VCO_FDBK

MAIN_LO

DBLR_IN

LP_SWITCH

Q301B

Filter

Ckt.

Q301A

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Motorola M3097 Service Manual Page 91

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