ASH Receiver Theory of Operation
Introduction
RFM鈥檚 RX5501 amplifier-sequenced hybrid (ASH) receivers are
specifically designed for short-range wireless control applications.
The receivers provide 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 receiver can be readily configured to support a wide range of
data rates and protocol requirements. The receiver features virtually
no RF emissions, making it easy to certify to short-range (unli-
censed) radio regulations.
Amplifier-Sequenced Receiver Operation
The ASH receiver鈥檚 unique feature set is made possible by its sys-
tem architecture. The heart of the receiver is the amplifier-
sequenced receiver section, which provides more than 100 dB of
stable RF and detector gain without any special shielding or de-
coupling 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 am-
plifier-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 碌s.
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 碌s. 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 碌s, 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 sig-
nal 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 re-
ceiver ultimate rejection.
Amplifier-sequenced receiver operation has several interesting char-
acteristics 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 consump-
tion. 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
A S H R e c e iv e r B lo c k D ia g r a m
A n te n n a
& T im in g C y c le
S A W
F ilte r
R F A 1
P 1
S A W
D e la y L in e
R F A 2
P 2
D e te c to r &
L o w -P a s s
F ilte r
D a ta
O u t
P u ls e
G e n e ra to r
R F In p u t
t
P
P 1
R F A 1 O u t
W 1
R F D a ta P u ls e
t
P
t
P
R C
R I
D e la y L in e
O u t
t
P
P 2
W 2
Figure 1
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