Motorola P8190 Service Manual download pdf (Page 90)

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TDMA Timeport™ P8190General Description

78

VCO

Merlin IC

VCO

The VCO is a 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 con-

trolled by the Custom IC. The Custom IC will

receive channel information via the SPI bus

and adjust the frequency of the VCO by vary-

ing the voltage level to the input of the VCO.

The output of the VCO is split into two paths.

One path is used to feed back the produced

signal to the Custom IC. Then the signal is

prescaled and sent through a phase lock loop

circuit for frequency locking. The second path

is sent to the receive circuit for carrier sig-

nal removal.

TX Offset Oscillator

The offset oscillator frequency is controlled

by the Custom IC via TXCP_OUT. The oper-

ating frequency will depend on the dc bias-

ing of CR300. The offset oscillator frequency

is 157.32MHz in analog mode, and

112.32MHz in digital mode.

The LP_Switch, which comes from the Cus-

tom IC pin 40, changes the offset frequency.

The LP_Switch is on in analog mode and off

digital mode. The LP-Switch is currently not

supported in the TDMA StarTAC, therefore,

the frequency of the offset oscillator will be

157.32MHz regardl ess of whether it is in a

nalog or digital mode.

In analog mode the FM signal comes from

the DCI pin 39 and is modulated directly in

the offset oscillator. The offset oscillator with

the FM then enters QMOD. The Quadmod

will feed back the offset frequency to the

Custom IC for proper PLL operation.

The Merlin IC is a TX modulator. It takes

the TX information and modulates it on a

carrier for RF transmission. In analog mode

the offset oscillator with the FM enters the

Merlin IC and gets mixed with the main

VCO. The resulting signal is a differential

carrier with the modulated information. The

carrier is then passed through an IQ modu-

lator.

The A_D line controls the state of the IQ

modulator. In analog mode, the IQ modula-

tor simply feeds the carrier through to a volt-

age controlled amplifier(VCA). The gain of

the VCA is controlled via the AOC_CNTL

line, therefore, controlling the TX power

steps of the transceiver. Once the carrier

passes through the VCA, it passes through

the cellular final power amplifier which is

still internal to the Merlin IC

.

In digital mode the offset oscillator doesnt

contain the modulated information from the

FM line. The offset oscillator is mixed with

the main VCO and a differential out-put sig-

nal is sent through the IQ modulator. In digi-

tal mode, the A_D line configures the IQ

modulator to allow IQ modulation to the car-

rier. The result carrier signal is then passed

through the Voltage Controlled Amplifier

(VCA). The gain of the VCA is adjusted by

the AOC_CNTL line, thus controlling the TX

digital power steps of the transceiver. The

carrier is then routed through the cellular

final power amplifier and out to the trans-

mit circuit.

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