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ADL5811

Rev. 0 | Page 21 of 28

LO SUBSYSTEM

The LO amplifier is designed to provide a large signal level to

the mixer to obtain optimum intermodulation and compression

performance. The resulting LO amplifier provides very high

performance over a wide range of LO input frequencies.

The ideal waveshape for switching the passive mixer is a square

wave at the LO frequency to cause the mixer to switch through

its resistive region (from on to off and off to on) as rapidly as

possible. While it has always been possible to generate such a

square wave, the amount of dc current required to generate a

large amplitude square wave at high frequencies has made it

impractical to create such a mixer. Novel circuitry within the

ADL5811 permits the generation of a near-square wave output

at frequencies of up to 2800 MHz with dc current that compares

favorably with that employed by narrow-band passive mixers.

The input stages of the LO amplifier provide common-mode

rejection, permitting the LO input to be driven either single ended

or balanced. For a single-ended input, either LOIP or LOIN can

be grounded. It is desirable to dc block the LO inputs to avoid

damaging the part by the accidental application of a large dc

voltage to the part. In addition, the LO inputs are internally dc

blocked.

Because the LO amplifier is inherently wideband, the ADL5811

can be driven with either high-side or low-side LO by simply

setting the optimum RF balun and LPF inputs to the SPI.

The LO amplifier converts a variable level, single or balanced input

signal (−6 dBm to +10 dBm) to a hard voltage limited, balanced

signal internally to drive the mixer. Excellent performance can be

obtained with a 0 dBm input level; however, the circuit continues to

function at considerably lower levels of LO input power.

The performance of this amplifier is critical in achieving a high

intercept passive mixer without degrading the noise floor of the

system. This is a critical requirement in an interferer rich

environment, such as cellular infrastructure, where blocking

interferers can limit mixer performance. Blocking dynamic

range can benefit from a higher level of LO drive, which pushes

the LO amplifier stages harder into compression and causes them

to switch harder and to limit the small signal gain of the chain.

Both of these conditions are beneficial to low noise figure under

blocking. NF under blocking can be improved several decibels

for LO input power levels above 0 dBm.

The LO amplifier topology inherently minimizes the dc current

based on the LO operating voltage and the LO operating frequency.

It is permissible to reduce the LO supply voltage down as low as

3.6 V, which drops the dc current rapidly. The mixer dynamic

range varies accordingly with the LO supply voltage. No external

biasing resistor is required for optimizing the LO amplifier.

In addition, the ADL5811 has a power-down mode that can be

used with any supply voltage applied to the part.

All of the SPI inputs are designed to work with any logic family that

provides a Logic 0 input level of less than 0.4 V and a Logic 1 input

level that exceeds 1.4 V.

All pins, including the RF pins, are ESD protected and have been

tested up to a level of 2000 V HBM and 1250 V CDM.

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