RFMD2080 Datasheet, PDF (7/19 Pages) RF Micro Devices – 45MHz TO 2700MHz IQ MODULATOR WITH SYNTHESIZER/VCO AND BASEBAND INTERFACE`

RFMD2080
External Reference
The RFMD2080 has been designed to use an external reference such as a TCXO. The typical input will be a 0.8Vp-p clipped
sine wave, which should be AC-coupled into the reference input. When the PLL is not in use, it may be desirable to turn off the
internal reference circuits, by setting the REFSTBY bit low, to minimize current draw while in standby mode.
On cold start, or if REFSTBY is programmed low, the TCXO 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 RFMD2080 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 best linearity is achieved using the modulator bias setting
MODDC=4. This setting adjusts bias mixer current and can be used to trade off linearity and current consumption.
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 10mA 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 differ-
ential load of typically 200. Since the mixer output is a constant current source, a higher resistance load will give higher out-
put power and gain. A shunt inductor can be used to resonate with the mixer output capacitance at the frequency of interest.
This inductor may not be required at lower frequencies where the impedance of the output capacitance is less significant. At
higher output frequencies the inductance of the bond wires (about 0.5nH on each pin) becomes more significant.
The following diagram is a simple model of the modulator output:
0.5nH
1K
1K
1pF
0.5nH
RFMD2080
RF Output
It is recommended to use a 4:1 balun on the modulator output, presenting 200 to the output in a single ended 50 system.
The RFMD2080 evaluation board has an RFXF8553 wideband transmission line transformer.
Baseband Section
The RFMD2080 features a baseband section that consists of an active low pass filter, variable attenuator and DC offset con-
trol circuitry. DC offset calibration is performed using digital-to-analog converters (DACs) that apply an offset voltage to various
parts of the circuit to compensate for DC offsets introduced by the internal buffers and the mixer core. This can be done to opti-
mize LO suppression by setting registers to program the DAC offset voltages.
DS140110
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