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RF2483

LOW NOISE DUAL-BAND QUADRATURE MODULATOR WITH AGC

RF Micro Devices 

RF2483

Pin Function Description

Interface Schematic

13

QSIG P

Quadrature Q channel positive baseband input port.

Best performance is achieved when the ISIGP and ISIGN are driven differ-

entially. The recommended CW differential drive level (VQSIGP-VQSIGN) is

800 mVP-P.

This input should be DC-biased at 1.2V±0.05V. The common-mode DC

voltage on the QSIGP and QSIGN input signals is used to bias the modula-

tor. In sleep mode an internal FET switch is opened, the input goes high

impedance and the modulator is de-biased. The input impedance is typi-

V CC2

cally 5.5k at low frequencies and at higher frequencies can be modeled

as 50 in series with 12pF to ground.

50 

Phase or amplitude errors between the QSIGP and QSIGN signals which

may result in an increase in the even order distortion of the modulation in

the output spectrum.

12 pF

DC offsets between the QSIGP and QSIGN signals will result in an

increased carrier leakage. Small DC offsets may be deliberately applied

between the ISIGP/ISIGN and QSIGP/QSIGN inputs to cancel out the LO

leakage. The optimum corrective DC offsets will change with mode, fre-

quency and gain control.

Common-mode noise on the QSIGP and QSIGN should be kept low as it

may degrade the noise performance of the modulator.

Phase offsets may be applied between the I and Q channels to improve the

sideband suppression performance.

14

VREF

Voltage reference decouple with an external 10nF capacitor to ground.

The voltage on this pin is typically 1.67V when the chip is enabled. The

4 k

VCC2

voltage is 0V when the chip is powered down.

The purpose of this decoupling capacitor is to filter out low frequency noise

(20MHz) on the gain control lines.

-

+

Poor positioning of the VREF decoupling capacitor can cause a degrada-

tion in LO leakage.

A voltage of around 2.5V on this pin indicates that the die flag under the

chip is not grounded and the chip is not biased correctly.

15

GC DEC

Voltage reference decouple with an external 1nF decoupling capacitor to

ground.

The voltage on this pin is a function of gain control (GC) voltage when the

chip is enabled. The voltage is 0V when the chip is powered down.

4 k

VCC2

The purpose of this decoupling capacitor is to filter out low frequency noise

(20MHz) on the gain control lines. The size of the capacitor on the GC DEC

line will effect the settling time response to a change in gain control volt-

+

age. A 1nF capacitor equates to around 200ns settling time and a 0.5nF

-

capacitor equates to a 100ns settling time. There is a trade-off between

settling time and noise contributions by the gain control circuitry as gain

control is applied.

Poor positioning of the VREF decoupling capacitor can cause a degrada-

tion in LO leakage.

16

GC

Gain control voltage. Maximum output power at 2.0V. Minimum output

power at 0V. When the chip is enabled the input impedance is 10k refer- VCC2

enced to 1.7VDC. When the chip is powered down a FET switch is opened

4 k

and the input goes high impedance.

10 k

-

1.7 V +

17

RF OUT LB RF low band output. Open collector output.

The output should be biased at VCC through an inductor that can be used

to form part of an output matching circuit.

In our proposed applications circuit some power is dissipated in R6 (130)

which appears as a de-Qing resistor in parallel with the output inductor L4.

If R6 is eliminated and the RFOUT LB pin is re-matched to 50 it is possi-

ble to get approximately 5dB extra power out of the device in low band

mode.

18

GND2

Ground for RF output sections.

DS110505

7628 Thorndike Road, Greensboro, NC 27409-9421 · For sales or technical

support, contact RFMD at (+1) 336-678-5570 or sales-support@rfmd.com.

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