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TA0028
TA0028
RF2905/2915/2925: Silicon Transceiver Family for Low-Power Wireless
Communications
          

A family of low-power RF transceivers has been devel-
oped for wireless data communications devices oper-
ating in the European 433MHz to 869MHz ISM bands
or the U.S. 902MHz to 928MHz ISM band. All three
IC's have been implemented in a 15GHz silicon bipolar
process technology that allows low-power FSK/ASK
transceiver operation in commercial wireless products.
Typical applications include wireless security systems,
wireless meter reading equipment, or virtually any
wireless data link application operating up to 1Mb/s. All
three devices include an inherent transmit/receive
switch, a complete receiver/demodulator, a low-power
voltage-controlled oscillator (VCO), and a complete
modulator/transmitter. Additionally, the RF2905/2925
include an on-chip PLL frequency synthesizer that can
be used to support up to 4-channel operation. For
applications requiring more channels, the RF2915 is
designed to interface directly to commercially available
programmable PLL frequency synthesizers. The
RF2905 provides for two-channel transceiver opera-
tion, while the RF2925 is optimized for single channel
operation in systems using Time Division Duplexing
(TDD). The typical DC power consumption of these
devices is less than [email protected] in the sleep mode,
[email protected] in the receive mode, and [email protected]
in the transmit mode. At 3.0V operation, the receiver
achieves a typical cascaded noise figure of 10dB and
the transmitter produces +4dBm (@ 915MHz) typically
into a 50load. The chip also provides for modulation
flexibility and can be configured as an FSK/FM, or
ASK/AM data transceiver. The RF2905/2925 are pack-
aged in a low-cost 48-pin LQFP package. The RF2915
is packaged in a 32-pin LQFP package.
This paper also discusses operation of the device as
an unlicensed FSK transceiver in the U.S. 902MHz to
928MHz ISM band with regulatory compliance to FCC
Part 15.249. In addition, a variety of application consid-
erations are provided to assist system designers in
implementing the device as a one-way transmitter/
receiver, or as a full two-way RF transceiver using FSK
or OOK AM data modulation.
        
 
Until recently, low-cost wireless data or voice products
for a wide range of consumer and industrial applica-
tions have predominately utilized lower VHF frequency
bands for unlicensed operation of low-power RF
devices. Operation in these frequency bands, such as
the U.S. 49.82MHz to 49.90MHz band, has been an
obvious choice for designers in these cost sensitive
applications due to the abundant supply of low-cost
highly integrated components. The success and wide
market acceptance of wireless products, such as
49MHz analog cordless telephones, have resulted in
the demand for next generation wireless products that
achieve higher performance levels at lower cost. Fur-
thermore, the crowded conditions that exist in the tradi-
tional VHF frequency bands and the need for additional
bandwidth to support advanced products using digital
modulation techniques have motivated product design-
ers to develop products that operate in spectrum allo-
cated at UHF, such as the U.S. 902MHz to 928MHz
ISM band. To reduce product development cycles in
these next generation products three new monolithic
RF transceiver products (RF2905/2915/2925) have
been introduced by RF Micro Devices that provide
product designers with "Antenna-to-Bits and Back"
solutions for their wireless applications. Target wireless
applications for the RF2905 transceiver family include
utility meter reading, security systems, bar code scan-
ners, cordless telephones, and RFID tags just to name
a few.
All three transceiver products were designed to
achieve high performance levels while minimizing DC
power consumption. Integral to the design process
was the use of Optimum Technology MatchingTM to
select the appropriate IC technology to meet the prod-
uct objective. Optimum Technology MatchingTM means
that we consider all RFIC technologies at our disposal
for new designs to develop the lowest cost solution that
meets the technical requirements of a given applica-
tion. In the case of the RF2905 transceiver family, the
primary applications for these devices are in battery
powered applications, such as those found in wireless
utility meter reading applications where the primary
battery may be expected to last for 10 years or more. In
the case of the RF2905 transceiver family, a 15GHz sil-
icon bipolar process technology was deemed as an
Optimum Technology Match for this application. The
silicon bipolar process we selected provides a mini-
mum geometry device that exhibits a 15GHz FT under
3V and 0.5mA DC operation. Of the GaAs and silicon
IC processes we considered, this process allows us to
provide a monolithic, low-power, UHF transceiver IC
that sells for less than $4.00 in volume.
Copyright 1997-2000 RF Micro Devices, Inc.
13-131
13




TA0028 pdf, 반도체, 판매, 대치품
TA0028
block) and provide a 50interface without external
matching components in the transmit and receive oper-
ating modes. The RF2905/2925 provide an on-chip
PLL to lock the VCO to one of the reference crystal
oscillators. The RF2915 is designed to interface the
on-chip VCO to an external PLL frequency synthesizer.
Selection of the transceiver appropriate for a given
application begins with consideration of the number of
RF channels required by the system. Tables 2a & 2b
provide a selection guide for the transceivers based on
the number of channels required in transceiver or sim-
plex (Rx or Tx only) applications.
Product Part #
RF2925
RF2905
RF2915
# of RF Channels in
European 433MHz
ISM band
1
1
10
# of RF Channels in
European 869MHz
ISM band**
1
1
10
Table 2a. RF Channels* Supported in Two-Way Transceiver Operation
# of RF Channels in
U.S. 915MHz ISM
band
1
2
171
Product Part #
RF2925
RF2905
RF2915
# of RF Channels in
European 433MHz
ISM band
1
2
10
# of RF Channels in
European 869MHz
ISM band**
1
2
10
# of RF Channels in
U.S. 915MHz ISM
band
1
4
171
Table 2b. RF Channels Supported in One-Way Simplex Operation
* 150kHz Channel Bandwidth
** Operation in bandwidth allocated for wideband use in non-specific applications within the 868-870MHz
Frequency band
13
While only single-channel transceiver operation is sup-
ported in the 433MHz to 869MHz European ISM
bands using the RF2905/2925, the RF2915 provides
ten (10) 150kHz channels in 433MHz European ISM
band when used in conjunction with an external PLL
frequency synthesizer. As shown in Table 2b, the
RF2905 can be used to provide a four-channel receiver
or transmitter in the U.S. 915MHz ISM band for simplex
applications. Four channel operation is accomplished
with the RF2905 by independent control of the two ref-
erence oscillators (OSCSLT) and the prescaler modu-
lus control input (MODCTL). Using the RF2905, a two-
channel receiver or transmitter in the 433MHz to
869MHz bands is also supported without the need for
an external PLL. For applications requiring more chan-
nels, the RF2915 should be used to interface to an
external PLL frequency synthesizer. The RF2915
includes all the functionality of the RF2905 minus the
on-chip PLL.
By design, all three transceivers are intended to oper-
ate half-duplex. Therefore, an additional consideration
in selection of the transceiver product is the Rx/Tx turn-
around time. The RF2905 provides independent con-
trol (ON/OFF) of the on-chip crystal reference oscilla-
tors to provide multiple channels, but minimum Tx/Rx
(or Rx/Tx) turn-around times are typically on the order
of milliseconds due to the start-up time required by the
crystal reference oscillators. In general, applications
requiring turn-around times less than 1msec, such as
those found in systems using TDD, should utilize the
RF2915 or RF2925. Using the RF2925, both on-chip
reference oscillators remain on in both transmit and
receive modes and the device is capable of providing
turn-around times less than 150µsec. The RF2915
relies on an external reference oscillator circuit, thus
the device is applicable to both TDD and FDMA system
applications.
13-134
Copyright 1997-2000 RF Micro Devices, Inc.

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TA0028 전자부품, 판매, 대치품
VCO, the PLL bandwidth should also be set much
lower than the data bandwidth to avoid distortion of the
transmitted FSK signal. As a competing consideration,
the PLL bandwidth (in conjunction with the start-up
time of the crystal reference oscillators) also effects the
Rx/Tx turn-around time (Rx/Tx switching time) of the
transceiver. Therefore, a trade-off exists in selecting
the appropriate PLL bandwidth that allows the VCO to
be directly modulated at the data rate while achieving
the desired Rx/Tx turn-around time.
As the primary consideration, the modulation rate
needs to be much higher than the PLL bandwidth to
prevent the PLL from tracking out the modulation. In
application, long strings of "1's" or "0's" can also be
tracked out by the PLL and can produce bit errors.
Therefore, the lowest frequency data input to the mod-
ulator should always be greater than the PLL band-
width. For these reasons, Manchester encoding is
recommended when using the FSK modulator. When
Manchester encoding is used, two symbols are
required to transmit a single bit of information. Using
Manchester encoding, a binary one is represented by a
positive half-bit period pulse followed by a negative
half-bit period pulse. A binary zero is represented by a
negative half-bit period pulse followed by a positive
half-bit period pulse. Since a Manchester encoded
data stream has zero DC value on a bit-by-bit basis,
long strings of "1's" or "0's" will not be distorted by the
PLL. The PLL bandwidth is set externally to the device
using a passive second-order loop filter that uses two
capacitors and a resistor in a shunt configuration (see
Figure 4). Experimentally, we have determined that the
PLL bandwidth should be a least five to ten times lower
than the minimum data rate input to the modulator. In
our 28.8Kb/s example, the 1kHz PLL bandwidth main-
Reference
Oscillator
Phase/Freq.
Detector
TA0028
tains data symbol integrity and provides a 6msec Rx/
Tx turn-around time. The peak frequency deviation
provided by the modulator varies with the amplitude of
the modulation input. Figure 5 provides a graph of the
FM deviation characteristics of the open loop VCO as a
function of the modulation input (MODIN). Once a PLL
bandwidth consistent with the data rate has been
established, the PLL lock time can be predicted using
second-order PLL loop equations. Figure 6 provides a
plot of the RF2905 PLL lock time as a function of the
PLL loop bandwidth. To determine the Rx/Tx turn-
around time of the transceiver, the start-up time of the
crystal reference oscillator should be added to the PLL
lock times in Figure 6. The start-up time of the crystal
reference oscillators can also be influenced by the
choice of external circuitry and the PLL configuration.
The Rx/Tx turn-around time includes both the start-up
time of the crystal oscillator and the PLL lock time. For
all practical purposes, the start-up time of the crystal
oscillator sets a lower bound on the Rx/Tx turn-around
time that can be achieved in operation. In other words,
if the PLL takes zero seconds to lock, the Rx/Tx turn-
around time is the time it takes the crystal reference
oscillator to start. The RF2905 includes a dual-modu-
lus/dual-divide prescaler that provides 64/65 and 128/
129 divisors. Thus, for operation in the 915MHz ISM
band, either 7MHz or 14MHz crystals can be used in
the reference oscillator circuitry. Since the start-up time
of an oscillator is inversely proportional to the band-
width of the crystal, a 14MHz crystal oscillator will start
twice as fast as a 7MHz crystal oscillator (assuming a
constant crystal "Q"). Therefore, by using a 14MHz
crystal and the 64/65 prescaler, the start-up time of the
crystal oscillator can be cut in half over that achieved
by the 7MHz crystal oscillator circuit. Experimentally,
PLL Loop
Filter
VCO
Output = 916.4 MHz
13
MODCTL
DIVCTL
Figure 4. RF2905 PLL Block Diagram
128/129
(or 64/65)
Dual Modulus/Dual Divide
Prescalar
Copyright 1997-2000 RF Micro Devices, Inc.
13-137

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