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RFMD2081

DS140110

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

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

On cold start, or if REFSTBY is programmed low, the reference circuits will need a warm-up period. This is set by the SU_WAIT

bits. This will allow the clock to be stable and immediately available when the ENBL bit is asserted high, allowing the PLL to

assume normal operation.

If the current consumption of the reference circuits in standby mode, typically 2mA, is not critical, then the REFSTBY bit can be

set high. This allows the fastest startup and lock time after ENBL is taken high.

IQ Modulator

The IQ modulator core of the RFMD2081 is wideband covering from 45MHz to 2700MHz. It has been designed to achieve

exceptional linearity for the amount of DC power consumed.

The modulator mixer cores have four coarse gain/current settings. Each setting steps the gain and linearity by 6dB and can be

used to optimize performance or reduce power consumption.

The differential I and Q baseband inputs have 3dB bandwidth of 100MHz, and their input impedance is dominated by 10K



pull down resistors on each pin, as shown in the diagram below, so presenting a 20K

 differential impedance. A common

mode DC bias voltage of around +1.3V is required to set the current through the mixer cores for optimal performance. The off-

set between the common mode voltages on the differential pins can be adjusted to minimize LO leakage at the modulator out-

put. The baseband input signals will be typically of the order of 1Vp-p differential. If required the phase and amplitude of the I

and Q signals can be adjusted to reduce the level of the unwanted sideband signal at the modulator output.

The modulator output is differential and requires a balun and simple matching circuit optimized to the specific application fre-

quencies. The modulator output pins are also used to source current for the modulator mixer circuits, about 20mA on each pin.

This is usually via a center-tapped balun or by RF chokes in the external matching circuitry to the supply.

The modulator output is high impedance, consisting of approximately 2K

 resistance in parallel with some capacitance,

approximately 1pF. The modulator output does not require a conjugate matching network. It is a constant current output which

will drive a real differential load of typically 200

. Since the mixer output is a constant current source, a higher resistance load

will give higher output voltage and gain. A shunt inductor can be used to resonate with the mixer output capacitance at the fre-

quency of interest. This inductor may not be required at lower frequencies where the impedance of the output capacitance is

10K



10K



Qp

Qn

Q mixer core

(FET gates)

10K



10K



Ip

In

I mixer core

(FET gates)