ASH Transceiver Theory of Operation
Introduction
RFM’s amplifier-sequenced hybrid (ASH) transceiver is specifically de-
signed for short-range wireless data communication applications. The
transceiver provides robust operation, very small size, low power con-
sumption and low implementation cost. All critical RF functions are con-
tained 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 re-
ceiver 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 ampli-
fiers 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 di-
agram, 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 character-
istics that can be exploited in system design. The RF amplifiers in an am-
plifier-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 with a loss equivalent to 10*log
10
(RFA1 duty factor), where the
duty factor is the average amount of time RFA1 is ON (up to 50%). Since
an amplifier-sequenced receiver is inherently a sampling receiver, the
overall cycle time between the start of one RFA1 ON sequence and the
start of the next RFA1 ON sequence should be set to sample the narrowest
RF data pulse at least 10 times. Otherwise, significant edge jitter will be
added to the detected data pulse.
ASH Receiver Block Diagram & Timing Cycle
Antenna
SAW Filter
RFA1
P1
SAW
Delay Line
RFA2
P2
Detector &
Low-Pass
Filter
Data
Out
Pulse
Generator
RF Input
t
PW1
P1
RF Data Pulse
t
PRI
t
PRC
RFA1 Out
Delay Line
Out
t
PW2
P2
Figure 1
RF Monolithics, Inc.
Phone: (972) 233-2903
Fax: (972) 387-8148
RFM Europe
Phone: 44 1963 251383
Fax: 44 1963 251510
©1999 by RF Monolithics, Inc. The stylized RFM logo are registered trademarks of RF Monolithics, Inc.
E-mail: info@rfm.com
http://www.rfm.com
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