LINEAR TECHNOLOGY LTC5532 Technical data

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Wideband RF ICs for Power Detection and Control – Design Note 335

James Wong and Vladimir Dvorkin

Introduction

Radio frequency devices are being deployed in ever
increasing numbers, not just in cell phones and cordless
telephones. Other applications include 802.11 wireless
LAN, RFID (Radio Frequency Identification) tags, inven­tory monitors, satellite transceivers, fixed wireless
access and wireless communications infrastructure. All
RF devices must carefully monitor and control their RF
power transmission to comply with government regula­tion and minimize RF interference with other radio
devices. For this reason, accurate RF power detection is
important in both RF receivers and transmitters.

This article presents some solutions using Linear
Technology’s versatile family of high frequency Schottky
diode detectors. Table 1 summarizes the features of this
family and lists more applications.

A Dual-Band Mobile Phone Transmitter Power
Control Application

Figure 1 is a simplified block diagram illustrating transmit
power control for a dual band mobile phone (the receiver
is not shown here). In this example, a 324Ω, 1% tolerance
resistor (R1) followed by a 2.2pF capacitor (C1) form a
coupling circuit with 18dB to 20dB coupling factor at
850MHz to 1850MHz, referenced to the LTC

®

5509 RF
input pin. C1 is also a DC blocking capacitor. R1 should
have a tolerance of 1% while C1 should be 2% to 5%. The
coupling circuit (R1 and C1) introduces about 0.15dB to

0.2dB losses into the main signal line. R1 should be

placed as close as possible to the antenna without forming
a “T” connection on the microstrip line and immediately
followed by capacitor C1 and the LTC5509. Ideally, C1, R1
and LTC5509 should be placed on the same side of the
PCB as the Tx output microstrip line to the antenna. The
component values shown here should be used as a
reference. In the actual product implementation, compo­nent values may differ slightly depending on the output
impedance of the Tx PAs, antenna impedance, compo­nent placement and PC board parasitics.

An RFID Reader Application

RFID (Radio Frequency Identification) is a promising tech­nology for many monitoring and tracking applications,

,LTC and LT are registered trademarks and ThinSOT is a trademark of
Linear Technology Corporation.

C1

R1

2.2pF

324Ω

Tx PA MODULE

CELL BAND

PCS BAND

Li-Ion

VPC

1
2
3

LTC5509

SHDN

RF

GND

GND

V

V

OUT

MOBILE PHONE BB/DSP

6

IN

5
4

CC

0.1µF

+

Figure 1. A Dual Band Mobile Phone Transmit Power
Control Using a Resistive Tap

ANTENNA

DIPLEXER

DN335 F01

Table␣ 1. Summary of RF Detector Specifications and Applications

DEVICE FREQUENCY RANGE PACKAGE DYNAMIC RANGE/FEATURES APPLICATIONS

LTC5505-1 300MHz to 3GHz ThinSOT
LTC5505-2 300MHz to 3GHz ThinSOT –32dBm to 12dBm* General Purpose, Phones, ISM
LTC5507 100 KHz to 1GHz ThinSOT –34dBm to 14dBm* General Purpose LF & Broadband Detection
LTC5508 300MHz to 7GHz SC–70
LTC5509 300MHz to 3GHz SC–70
LTC5532 300MHz to 7GHz ThinSOT Adjustable Gain & Starting Voltage Precision RSSI & Envelope Detection
*Gain compression extends the dynamic range with a tradeoff of reduced linearity in the transfer characteristic.

**No gain compression. †Smallest package.

04/04/335

TM

†

†

–28dBm to 18dBm* General Purpose, Phones, ISM

–32dBm to 12dBm* General Purpose, WLAN, Microwave
–30dBm to 6dBm** Mobile Phones Tx Power Control

including retail store check-out registers, inventory man­agement, vehicle tracking, tire-pressure monitoring and
live-stock/agricultural tracking. Common to all of them are
the need for well-controlled RF power and a cost-effective
means of reliably detecting the received data. Well-regu­lated RF power allows maximum power transmission to
the ID tags while staying within regulatory emission lim­its. A well-controlled transmitter is possible if an RF
detector is used in a closed-loop feedback circuit, similar
to the example shown in Figure␣ 1. The choice of RF power
detector is determined by the RF frequency, as well as by
other constraints such as the required dynamic range and
sensitivity.

To form a complete RFID reader receiver, an RF Schottky
peak detector can also make an excellent low cost data
receiver to demodulate ASK or AM modulated signals
with data rates up to 3MHz. Because RF detectors such as
the LTC5507 can detect RF signals over a wide frequency
range, filtering can improve the sensitivity of the receiver.
Figure 2 shows a data receiver with an input LNA (Low
Noise Amplifier) and an input BPF (Bandpass Filter). The
LNA can be a general-purpose, low cost gain block that
provides fixed gain at the operating frequency of interest.
The added gain increases sensitivity and extends the
detection range. A lowpass or bandpass filter at the
detector output provides additional receiver selectivity, if
needed. The RSSI (Receive Signal Strength Indicator)
DC-coupled output provides signal strength information
using a lowpass filter (R2 and C5) to filter out the
modulation components.

Application of RF Power Detectors at Frequencies
above 7GHz

Although the LTC5532 is optimized for an operating
frequency range from 300MHz to 7GHz, it can offer
useful performance well above this frequency range. The
performance at higher frequencies does fall off but
gracefully. Figure 3 shows a plot of the LTC5532’s output
voltage versus RF input power characteristics at 12GHz.
Figure 4 shows the LTC5532’s input S11 Smith Chart,
extending to 12GHz. Coupling to the LTC5532 at these
high frequencies is in principle very similar to lower
frequency operation.

2.6

2.4

2.2

2.0

1.8

1.6

1.4

(V)

1.2

OUT

V

1.0

0.8

0.6

0.4

0.2
0

–16

–20

Figure 3. LTC5532 Typical Detector Transfer
Characteristics at 12GHz Frequency

–8

–4

–12

RF INPUT POWER (dBm)

04

8

DN335 F03

V

CC

ANTENNA

RF BPF

R1

SHDN
GND
V

LNA

LTC5507

OUT

RF

PCAP

V

C1

V

CC

FILTER

RSSI OUTPUT

C5

C2

DATA
OUTPUT

C4

IN

CC

C3

R2

DN335 F02

Figure 2. An RFID Reader ASK Receiver with Output Filter

Data Sheet Download

http://www.linear.com

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone] ● www.linear.com

5.0-12.0GHz

Figure 4. LTC5532 Input S11 Smith Chart

For applications help,

call [phone], Ext. 2482

dn335f LT/TP 0404 305K • PRINTED IN THE USA

 LINEAR TE CHNOLOGY CORPORATION 2004