Philips Semiconductors

Preliminary specification

SA900

I/Q transmit modulator

1997 Sept 16

11

control voltage from the VCO tank DC bias voltages. The VCO

tuning voltage is generated from the output of the PLL loop filter.

The VCO tank configuration is shown in Figure 6.

Crystal Oscillator (XO) Operation

For cellular radio applications, the SA900 will most likely utilize an

external reference TCXO in order to provide the frequency stability

necessary to operate to system requirements. The output of the

system TCXO can be AC coupled to the XTAL_1 input. However,

for applications that do not require such accuracy the XO circuit can

be configured as a Colpitts type oscillator with the addition of two

external capacitors along with the reference crystal and a trim

capacitor as shown in Figure 7.

Programmable Clock Outputs

The SA900 generates three buffered XO outputs used for external

reference signals. The XO feeds three sets of programmable

prescalers, the prescaler outputs are buffered to provide the CLK1,

CLK2 and MCLK signals. The CLK1 signal is a selectable divide 3/1

(X=1 divide 3, X=0 divide 1), 50% duty cycle, of the XO reference

signal. The CLK2 signal is a selectable divide 2/1 (Y=1 divide 2,

Y=0 divide 1), 50% duty cycle, of the XO reference signal. The

cycle, of the XO reference signal. MCLK is externally set by means

of the tri-level CLKSET input to provide a default master system

clock prior to programming the SA900.

Programming Operation

The SA900 is configured by means of a 3-wire input (CLOCK,

STROBE, DATA) to program the AMPS and DUAL modes, in

addition there are two power saving modes of operation, SLEEP and

STANDBY. The control logic section of the SA900 is designed using

low power CMOS logic. During SLEEP mode only the circuitry

required to provide a master clock (MCLK) to the digital portion of

the system is enabled. During the STANDBY mode of operation

MCLK, CLK1 and the TXLO and buffered LO outputs are powered

on, which may be the case when the system is in the receive only

mode. In the AMPS or DUAL operational modes all functions of the

SA900 are powered on to support receive, transmit and system

clock functions. The programming of the SA900 is identical to the

programming format of the SA7025 low-voltage 1GHz fractional-N

synthesizer, that can be used in conjunction with the SA900 to

provide the cellular radio channel selection.

The programming data is structured as a 24 bit long serial data

word; the word includes 4 address bits (dedicated 1 0 1 1) for chip

select. Data bits are shifted in on the leading edge of the clock, with

the least significant bit (LSB) first and the most significant bit (MSB)

last. Table 2 shows data word format, the 15th and last 5 bits are

not used. Figure 8 shows the chip timing diagram.

Address

A0

A1

A2

10

1

A3

1

Divide By N

N0

N1

Divide

00

6

10

7

01

8

11

9

AMPS/DUAL Mode

The A/D mode select enables or disables that portion of the circuitry

used for either the AMPS or DUAL mode of operation.

AD

Mode

0

AMPS

1

DUAL

Synthesizer Enable

The SE bit turns on and off the synthesizer circuitry.

SE

Operation

0

Disabled

1

Enabled

Sleep Mode 1

The SM1 bit is used to power down the TXLO buffer, the divide 3/1

prescaler and the CLK1 output buffer.

SM1

Operation

0

Power down

1

Power up (STANDBY)

Sleep Mode 2

The SM2 bit is used to power down the divide 2/1 prescaler and the

CLK2.

SM2

Operation

0

Power down

1

Power up (with

SM1=1 normal

operation)

Divide 3

X

Operation

0

Divide 1

1

Divide 3

Divide 2

Y

Operation

0

Divide 1

1

Divide 2