Motorola LB3 (42.0 - 50.0MHz) 서비스 정보 - 페이지 16

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2-10

THEORY OF OPERATION

The VCO output is taken from the source and applied to the first buffer transistor (Q1304 receive,

Q1307 transmit). The first buffer output is further amplified by the second buffer transistor (Q1305

Rx, Q1308 Tx) before being applied to the receiver first mixer or transmitter first stage input.

In TX mode the modulation signal coming from the LVFRAC-N synthesizer IC (MODOUT, U1201 pin

41) is superimposed on the DC steering line voltage by capacitive divider C1215, C1208 and

C1212, causing modulation of the TX VCO using the same varactor as used for frequency control.

4.3

Synthesizer Operation

The complete synthesizer subsystem comprises mainly of low voltage LVFRAC-N synthesizer IC,

Reference Oscillator (crystal oscillator with temperature compensation), charge pump circuitry, loop

filter circuitry, and voltage-controlled oscillators and buffers. A sample of the VCO operating signal

PRE_IN is amplified by feedback buffer Q1202, low-pass filtered by L1205, C1222 and C1224, and

fed to U1201 pin 32 (PREIN).

The pre-scaler in the synthesizer (U1201) is basically a dual modulus pre-scaler with selectable

divider ratios. This divider ratio of the pre-scaler is controlled by the loop divider, which in turn

receives its inputs via the serial interface to the microprocessor. The output of the pre-scaler is

applied to the loop divider. The output of the loop divider is connected to the phase detector, which

compares the loop divider´s output signal with the reference signal. The reference signal is

generated by dividing down the signal of the reference oscillator, whose frequency is controlled by

Y1201.

The output signal of the phase detector is a pulsed DC signal which is routed to the charge pump.

The charge pump outputs a current at pin 43 of U1201 (I OUT). The loop filter (which consists of

R1205-6, R1208, C1212-14) transforms this current into a voltage that is applied to the varactor

diodes (CR1310 for transmit, CR1302 for receive) and alters the output frequency of the appropriate

VCO. The current can be set to a value fixed in the LVFRAC-N IC or to a value determined by the

currents flowing into BIAS 1 (U1201-40) or BIAS 2 (U1201-39). The currents are set by the value of

R1211 or R1207 respectively. The selection of the three different bias sources is done by software

programming.

To reduce synthesizer lock time when new frequency data has been loaded into the synthesizer the

magnitude of the loop current is increased by enabling the IADAPT (U1201-45) for a certain

software programmable time (Adapt Mode). The adapt mode timer is started by a low to high

transient of the CSX line. When the synthesizer is within the lock range the current is determined

only by the resistors connected to BIAS 1, BIAS 2, or the internal current source. A settled

synthesizer loop is indicated by a high level of signal LOCK (U1201-4).

In order to modulate the PLL the two spot modulation method is utilized. Via pin 10 (MODIN) on

U1201, the audio signal is applied to both the A/D converter (low frequency path) and the balanced

attenuator (high frequency path). The A/D converter converts the low frequency analog modulating

signal into a digital code that is applied to the loop divider, thereby causing the carrier to deviate.

The balance attenuator is used to adjust the VCO's deviation sensitivity to high frequency

modulating signals. The output of the balance attenuator is present at the MODOUT port (U1201-

41) and superimposed on the VCO steering line voltage by a divider consisting of C1215, C1208

and C1212.