LINEAR Fast TDD Transmission Using an Upconverting Mixer with a High Side Switch – Design Note 480

Fast Time Division Duplex (TDD) Transmission Using an Upconverting Mixer with a High Side Switch – Design Note 480 Vladimir Dvorkin

Introduction adding an external high side power supply switch (note Many wireless infrastructure time division duplex (TDD) that this technique is equally effective for the lower fre- transmit applications require fast on/off switching of the quency upconverting mixer, LT5578).

transmitter, typically within one to fi ve microseconds.

High Side VCC Switch for a Burst Mode Transmitter

There are several different ways to implement fast Tx

Using the LT5579 Mixer

on/off switching, including the use of RF switches in the

The high side VCC switch circuit in Figure 1 uses a P-chan-

signal path, or on/off switching of the supply voltage for

nel MOSFET (IRLML6401) with an RDS(ON) of less than

different stages of the transmitter chain. The advantages

0.1Ω. An N-channel enhancement mode FET (2N7002),

of the latter method are low cost, very good performance

connected from the drain of IRLML6401 to ground, further

and power saving during the Tx off-time. In particular, a

improves fall time. The 2N7002’s RDS(ON) is less than 4Ω,

good place to apply supply switching is at the transmit

which is suffi cient for this application.

upconverting mixer because this removes both the transmit signal and all other mixing products from the The input driver for the high side VCC switch is a high mixer RF output. speed CMOS inverter (MC74HC1G04) capable of driving

capacitive loads. The IRLML6401 input capacitance is

The LT®5579 high performance upconverting mixer fi ts

typically 830p F and the 2N7002 input capacitance is

various TDD and Burst Mode® transmitter applications

L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered

with output frequencies up to 3.8GHz. Fast on/off supply

trademarks of Linear Technology Corporation. All other trademarks are the property of

voltage (VCC) switching for the LT5579 is as simple as their respective owners.

1.0μF 270p FON STATE VCC = 3.29V

VCC = 3.3V

HIGH SIDE

VCC SWITCH

Tx IF

IRLML6401 Tr-r IF+ RFVCC

RF OUTPUT

INPUT

2140MHz

240MHz

LT5579

MC74HC1G04

GVCC 11Ω LO+ LO–

ON/OFF 2N7002 CONTROL

DN4VD F01

LO INPUT

Figure 1. Upconverting Mixer with High Side VCC Switch