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©2008 by RF Monolithics, Inc.

TR1004 - 4/4/08

ASH Receiver Block Diagram & Timing Cycle

Antenna

Pulse

Generator

SAW

Delay Line

SAW Filter

RFA1

RFA2

Data

Out

Detector &

Low-Pass

Filter

RF Data Pulse

P1

P2

RFA1 Out

RF Input

P1

Delay Line

Out

P2

t

PW2

t

PW 1

t

PRI

t

PRC

Figure 1

ASH Transceiver Theory of Operation

Introduction

RFM’s amplifier-sequenced hybrid (ASH) transceiver is specifically

designed for short-range wireless data communication

applications. The transceiver provides robust operation, very small

size, low power consumption and low implementation cost. All

critical RF functions are contained in the hybrid, simplifying and

speeding design-in. The ASH transceiver can be readily

configured to support a wide range of data rates and protocol

requirements. The transceiver features excellent suppression of

transmitter harmonics and virtually no RF emissions when

receiving, making it easy to certify to short- range (unlicensed)

radio regulations.

Amplifier-Sequenced Receiver Operation

The ASH transceiver’s unique feature set is made possible by its

system architecture. The heart of the transceiver is the amplifier-

sequenced receiver section, which provides more than 100 dB of

stable RF and detector gain without any special shielding or

decoupling provisions. Stability is achieved by distributing the total

RF gain over time. This is in contrast to a superheterodyne

receiver, which achieves stability by distributing total RF gain over

multiple frequencies.

Figure 1 shows the basic block diagram and timing cycle for an

amplifier-sequenced receiver. Note that the bias to RF amplifiers

RFA1 and RFA2 are independently controlled by a pulse

generator, and that the two amplifiers are coupled by a surface

acoustic wave (SAW) delay line, which has a typical delay of

0.5 µs.

An incoming RF signal is first filtered by a narrow-band SAW filter,

and is then applied to RFA1. The pulse generator turns RFA1 ON

for 0.5 µs. The amplified signal from RFA1 emerges from the SAW

delay line at the input to RFA2. RFA1 is now switched OFF and

RFA2 is switched ON for 0.55 µs, amplifying the RF signal further.

The ON time for RFA2 is usually set at 1.1 times the ON time for

RFA1, as the filtering effect of the SAW delay line stretches the

signal pulse from RFA1 somewhat. As shown in the timing

diagram, RFA1 and RFA2 are never on at the same time, assuring

excellent receiver stability. Note that the narrow-band SAW filter

eliminates sampling sideband responses outside of the receiver

passband, and the SAW filter and delay line act together to provide

very high receiver ultimate rejection.

Amplifier-sequenced receiver operation has several interesting

characteristics that can be exploited in system design. The RF

amplifiers in an amplifier-sequenced receiver can be turned on and

off almost instantly, allowing for very quick power-down (sleep)

and wake-up times. Also, both RF amplifiers can be off between

ON sequences to trade-off receiver noise figure for lower average

current consumption. The effect on noise figure can be modeled as

if RFA1 is on continuously, with an attenuator placed in front of it

duty factor is the average amount of time RFA1 is ON (up to 50%).