A P P L I C AT I O N S
R A D I O S YS T E M I C s
Transmitter and receiver ICs for SRD applications
Lower component count,
higher performance
The author:
JOHANNES
GERSTLAUER,
Dr.-Ing., is a consultant with the
Siemens Semiconductor Group
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The TDA 5100 (transmitter) and
TDA 5200 (receiver) components were developed as inexpensive alternatives to the
SAW-based systems for short
range devices currently in use.
Thanks to the selected architecture, circuits containing a minimum number of components
can be implemented without
major outlay on development.
Despite their low cost, these solutions enable very good electrical values to be attained.
The new system components for
short-range radio devices can be
used universally in both the relevant
UHF bands at 434 and 869 MHz.
The transmitter can be operated with
both amplitude and frequency modulation, whereas the receiver can only process amplitude-modulated signals.
The market
The market for radio systems is
characterized by above-average
growth rates worldwide. This includes the booming communica-
tions market and – somewhat less
obviously – the market for short
range devices (SRDs).
This latter market was initially driven principally by applications in
the automotive sector in the form of
remote keyless entry (RKE) systems. Today, a much larger home
market is on the horizon, thanks to
the radio networking of numerous
functions in and around the home.
These include security, detection,
recording, data transmission and
control applications.
The 433.05 - 434.79 MHz and
868.00 - 870.00 MHz bands are of
particular interest for short range devices. The 869 MHz band was released as recently as 1998 and has a
number of outstanding properties.
Although its free space attenuation
is 6 dB higher than at 434 MHz, this
drawback is more than counterbalanced by the higher efficiency of the
antennas with their small dimensions. Whereas the unregulated allocation of the 434 MHz band has led
to a high interference potential there,
a very effective distribution has been
specified for the 869 MHz band. The
division into subsections with different maximum permissible duty
cycles of 100%, 10%, 1% and 0.1%
makes operation much less liable to
interference. A study has shown that
its interference immunity is at least
two orders of magnitude greater than
in the 434 MHz band, which is partly occupied by “continuous carriers”.
The maximum level of radiated
transmission power permissible in
the critical frequency ranges is approximately 10 mW. In practice, and
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A P P L I C AT I O N S
R A D I O S YS T E M I C s
with the use of sensitive receivers,
transmitter powers of this order of
magnitude even allow sufficient
ranges to be attained in built-up areas which normally represent very
difficult conditions for radio reception.
The large numbers of SRD components required in the RKE and home
sectors have opened up a mass market for the UHF technology used in
the low-cost sector. This highly
price-sensitive market obliges manufacturers to exploit the technology
to its utmost in order to develop inexpensive and easily-fitted subassemblies. This target can only be
achieved with a high level of integration.
SAW-stabilized systems in discrete
or semi-integrated circuit technology have hitherto been the preferred
solution for the SRD sector. This applies particularly to the 434 MHz
range which has been used up to
now. The new highly integrated circuit technology permits the application of more sophisticated principles
for frequency generation. Thus, the
HF PLL principle makes use of a
crystal resonator as a frequency reference. An automatically controlled
oscillator (VCO) locks on to a multiple of this reference frequency. It
offers a frequency stability that is almost ten times better than that of
SAW resonators, and at lower cost.
This higher frequency stability allows the use of narrowband receivers: a considerable asset in view
of the need to minimize external interference.
• Output power +5 dBm
(434 MHz), -1 dBm (869 MHz)
• Power down mode 80 nA typically
• Low power detect 2.15 V
• Response time ≤ 1 ms
• Integrated VCO and PLL synthesizers
• Frequency range 433-435 and
868-870 MHz
• ASK/FSK modulation, 0 - 100/
20 kHz
• Reference frequency 6.8 or 13.6
MHz selectable
• Programmable clock output for
the controller: 3.39 MHz/847.5 kHz
The operations performed inside the
transmitter are shown in Fig. 1. They
reveal some very interesting details.
An LC circuit with a coil integrated
on the silicon is used as the frequency-determining element in the VCO.
A similarly integrated capacitance
diode performs the tuning. For operation in the 869 MHz range, the VCO
signal is used via a driver stage to
control the final transmit stage directly. When the transmitter operates
in the 434 MHz range, the VCO frequency remains in the 868 MHz
range. The final stage is controlled by a
driver and pulse-former stage after frequency division by a factor of two. In
class-C operation, the final stage attains a
high efficiency with a current flow angle
of less than 180°. The collector efficiency reaches values of approximately 45%.
Powerful transmitter for a modest
outlay
In amplitude modulation, amplitude
shift keying (ASK) is applied to the final stage transistor, whereas frequency
modulation involves the application of
frequency shift keying (FSK) to the reference oscillator. A pull capacitor is connected in series to the quartz in accordance with the modulation sequence.
Because of the large bandwidth of the
(integrated) loop filter of around 150
kHz, the VCO can also be modulated by
high modulation frequencies. The application circuit in Fig. 1 shows how little
expenditure is needed in order to implement a powerful transmitter. The external components required for the onechannel transmitter are limited to the
reference quartz, eight chip components
and, of course, the encoder. The trans-
Complete circuit diagram of the TDA 5100, FSK 869 MHz transmitter
The TDA 5100 transmitter
The transmitter IC was developed as
a component for universal applications. It can be operated alternatively with ASK or FSK modulation in
both the 434 MHz and 869 MHz
bands. A single lithium cell provides
the power supply.
Properties
• Power supply 2.1 V - 4.0 V,
6.9 mA typically
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Fig. 1 A powerful transmitter can be implemented for a modest outlay: the external components
required for the one-channel transmitter are limited to the reference quartz, eight chip components
and the encoder.
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A P P L I C AT I O N S
R A D I O S YS T E M I C s
SUMMARY
mitter requires no tuning. A loop configuration acts as the antenna.
The antenna and the circuit matching it to the
power amplifier are designed so that an electric field component is
radiated in addition to
the magnetic component. This produces a measurable improvement in
the antenna efficiency. The photograph on page 26 illustrates the implementation of the application circuit on a test board. The board can be
operated at both 434 and 869 MHz
with appropriate dimensioning of
the frequency-determining components. The following values were
measured on this board:
The TDA 5100 transmit component and the TDA 5200 receive
component were developed for
use in the cost-sensitive SRD
systems market. They combine
such properties as maximum
integration, universal applicability, high efficiency and an attractive price.
Board size
30 mm x 35 mm
Power supply
Lithium cell, CR 3032
2.6 V/6.9 mA
Emitted RF power
434 MHz
869 MHz
-10 dBm
-5 dBm
Noise spectrum
Harmonics
≤ -40 dBm
Spurious emissions ≤ -65 dBm
The TDA 5200 receiver
The TDA 5200 receiver component was developed to match the
TDA 5100 transmitter component.
However, whereas the transmitter is
designed for both ASK and FSK
modulation, the receiver can only
process ASK signals. The universal
functionality in both the 434 MHz
and 869 MHz bands was retained.
Properties
• Power supply 5 V ± 10%, 4.7 mA
typically
• Power down mode 50 nA typically
• Wakeup time ≤ 1ms
• Integrated VCO and PLL synthesizers
• Frequency range 433-435 MHz
and 868-870 MHz
• Reference frequency 6.7 MHz or
13.4 MHz selectable
• Intermediate frequency 10.7 MHz
(5-20 MHz)
• Data filter and pulse-former stage
adjustable externally to the baud rate
• Sensitivity -112 dBm (434 MHz)
or -108 dBm (869 MHz) at 4.8 kbit/s
The TDA 5200 is a simple superhet
receiver with an intermediate frequency of nominal 10.7 MHz, as
shown in Fig. 2. This concept again
allowed a maximum level of integration to be reached. All the circuits
required for generating the oscillator
signal for the mixer were integrated
on the silicon. Only the frequencydetermining quartz still needs to be
connected externally. A very inexpensive ceramic filter can be used
for selection at the intermediate frequency. Some external circuits are
required in connection with the lownoise amplifier (LNA). A simple LC
network is added at both its input
and output for matching and RF selection. The receiver requires no tuning if this network is suitably dimensioned.
The data filter and the pulse-former
stage in the baseband are optimally
matched to the relevant data signal
by their external circuits. If a code
containing a DC component is present, the pulse-former stage is operated in conjunction with a clamping
circuit.
Outlook
Complete circuit diagram of the TDA 5200 receiver
Fig. 2 All the circuits required for generating the mixer oscillator signal on the TDA 5200 simple
superhet receiver with an IF of 10.7 MHz were integrated on silicon.
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The TDA 5100 and TDA 5200 components, with their new, highly integrated architecture allow simple and
inexpensive radio subassemblies
with very good technical data to be
implemented. They represent the
first members of a range of cost-optimized universal RF components
designed for use in the growth-intensive market for short range devices.
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