Philips Semiconductors

Preliminary specification

SA900

I/Q transmit modulator

1997 Sept 16

10

Table 1.

VGA Power Control Limits

VGA

Min.

Typ.

Max.

Relative VGA

0

0

0

0

0

1

-1

-.63

-.2

0

2

-1

-.63

-.2

1

3

-1

-.63

-.2

2

4

-1

-.63

-.2

3

5

-1

-.63

-.2

4

6

-1

-.63

-.2

5

7

-1

-.63

-.2

6

15

-6.6

-5

-3

7

23

-6.6

-5

-3

15

31

-6.6

-5

-3

23

39

-6.6

-5

-3

31

47

-6.6

-5

-3

39

55

-6.6

-5

-3

47

63

-6.6

-5

-3

55

63

-43.2

-40.4

-37.2

0

1. Guaranteed to be monotonic.

AMPS Mode Operation

The SA900 can be configured to operate in the AMPS mode, where

FM modulation is applied to the SA900’s VCO. For the AMPS

mode, the VCO is configured with the proper synthesizer bandwidth

to allow the application of the AMPS modulation to the VCO varactor

tuned tank circuit. The modulated VCO signal is input into an image

reject mixer along with the TXLO signal, where the upper sideband

is rejected. This single sideband modulated signal then drives the

AMPS output power amplifier. The PA provides +2dBm power level

into 50

Ω, with no external matching components required. The AD

(AMPS/DUAL) and the SE (synthesizer enable) bit control the power

up/down of the AMPS mode function. The transition of the TXEN

signal from low to high turns on the modulator. The falling edge of

TXEN signal disables the synthesizer and the modulator.

Synthesizer Operation

The SA900 synthesizer is comprised of the differential VCO circuit,

with external tank components, the Gilbert cell multiplier phase

detector with programmable charge pump current, crystal oscillator

and programmable prescalers. The charge pump output drives an

external second order loop filter. The output of the loop filter is used

to provide the control voltage to the VCO tuning varactor to

complete the PLL synthesizer. The synthesized VCO output

frequency is mixed with the TXLO signal to generate the transmit LO

from the lower sideband (the difference of the VCO and TXLO

frequencies). The output of VCO is fed to a programmable /N

prescaler with user selectable divides of 6, 7, 8 and 9 (all divides

configured to provide 50% duty cycle). The output of the /N divider

drives the A8/1 prescaler. The A8/1 divide is selected by the AD

control bit (AD=1 for /1, and AD=0 for /8). The output of the divide

A8/1 is fed into one input of the phase detector. The reference input

for the phase comparator is generated from the crystal oscillator

(XO) output from the B8/1 prescaler. The B8/1 divide is selected by

the AD control bit (AD=0 for /8, and AD=1 for /1). The phase

detector compares the prescaled XO reference phase to the VCO

prescaled phase, to generate a charge pump output current

proportional to the phase error. The phase detector, a Gilbert cell

multiplier type, having a linear output from 0 to

π (π/2 ± π/2). The

charge pump peak output current is programmable from 100

µA for

the AMPS mode (AD=0) to a maximum of 6.4mA for the DUAL

mode (AD=1) by way of an external current setting resistor placed

in Figure 5. The charge pump current output is programmed by

AD

+ 0I

1.25V

R

SET

AD

+ 1I

1.25V

R

SET

The PLL frequency is determined by

VCO

+ XO @ N @

(

A8

1

)

(

B8

1

)

where N=6, 7, 8, 9 and A8/1 and B8/1 are controlled by the AD bit

(AD=1 A8/1 and B8/1 are divide by 1, AD=0 A8/1 and B8/1 are

divide 8).

Table 2.

Data Word Format

Mnemonics

Bits

Function

A0

1 (MSB)

Address bit 0 (1)

A1

2

Address bit 1 (0)

A2

3

Address bit 2 (1)

A3

4

Address bit 4 (1)

PC0

5

Power control bit 0

PC1

6

Power control bit 1

PC2

7

Power control bit 2

PC3

8

Power control bit 3

PC4

9

Power control bit 4

PC5

10

Power control bit 5

N0

11

Divide N bit 0

N1

12

Divide N bit 1

AD

13

AMPS/DUAL mode select bit

SE

14

Synthesizer enable bit

NA

15

NA

SM1

16

Sleep mode 1 control bit

SM2

17

Sleep mode 2 control bit

X

18

Divide 3/1 control bit

Y

19

Divide 2/1 control bit

NA

20

NA

NA

21

NA

NA

22

NA

NA

23

NA

NA

24 (LSB)

NA

VCO Operation

The VCO is designed to operate from 90MHz to 140MHz. The VCO

tank is configured using a parallel inductor and a dual common

cathode tuning varactor diodes. DC blocking capacitors are used to

isolate the varactor