LINEAR LT5579 User Guide and Manual

design ideas

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

Many wireless infrastructure time division duplex (TDD) The 2N7002’s RDS(ON) is less than 4Ω, which is sufficient for this application.

transmit applications require fast on/off switching of the transmitter, typically within one to five microseconds. The input driver for the high side

There are several different ways to implement fast Tx on/ VCC switch is a high speed CMOS inverter off switching, including the use of RF switches in the signal (MC74HC1G04) capable of driving capaci- tive loads. The IRLML6401 input capaci-

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

tance is typically 830p F and the 2N7002

stages of the transmitter chain. The advantages of the latter

input capacitance is under 50p F. For faster

method are low cost, very good performance and power

rise times, two high speed CMOS drivers can

saving during the Tx off-time. In particular, a good place

be used in parallel. Likewise, for faster fall

to apply supply switching is at the transmit upconverting times, a different N-channel MOSFET with

mixer because this removes both the transmit signal and lower on-resistance can be used.

all other mixing products from the mixer RF output.

With the LT5579 supply current of 220m A, HIGH SIDE VCC SWITCH FOR A the power supply voltage drop across the

The LT5579 high performance upconvert-

BURST MODE TRANSMITTER USING MOSFET is only 11m V. The response time

ing mixer fits various TDD and Burst Mode

THE LT5579 MIXER of the high side VCC switch is shown in

transmitter applications with output

The high side VCC switch circuit in Figure 1 Figure 2. Total turn-on time is only 650ns

frequencies up to 3.8GHz. Fast on/off sup-

uses a P-channel MOSFET (IRLML6401) with and total turn-off time is 500ns. These

ply voltage (VCC) switching for the LT5579

an RDS(ON) of less than 0.1Ω. An N-channel measurements were performed using two

is as simple as adding an external high

enhancement mode FET (2N7002), con- RF bypass capacitors at the mixer VCC pin

side power supply switch (note that this

nected from the drain of IRLML6401 (33p F and 270p F). Higher value RF bypass

technique is equally effective for the lower

to ground, further improves fall time.

frequency upconverting mixer, LT5578). (continued on page 27)

1.0µF 270p FON STATE VCC = 3.29V

VCC = 3.3V

HIGH SIDE

VCC SWITCH

4:1 33p F INPUT CONTROL SIGNAL

IRLML6401 Tx IF FOR HIGH SIDE VCC SWITCH

Tr-r IF+ RFVCC RF OUTPUT

INPUT

2140MHz

240MHz

IF– HIGH SIDE VCC SWITCH OUTPUT

LT5579

MC74HC1G04

GVCC 11Ω LO+ LO– LT5579 RF OUTPUT

ON/OFF 2N7002 CONTROL

1µs/DIV

LO INPUT

Figure 1. Upconverting mixer with high side VCC switch Figure 2. VCC turn-on and turn-off waveforms

July 2010 : LT Journal of Analog Innovation | 25