One of the factors that will undoubtedly influence the choice of modulation format for mobile satellites, is the availability of cheap, power-efficient, linear amplifiers for mobile terminal equipment operating in the 1.5-1.7 GHz band. Transmitter linearity is not easily achieved at these frequencies, although high power (20W) class A/AB devices are becoming available. However, these components are expensive and require careful design to achieve a modest degree of linearity. In this paper an alternative approach to radio frequency (RF) power amplifier design for mobile satellite (MSAT) terminals using readily-available, power-efficient, and cheap class C devices in a feedback amplifier architecture is presented

Bateman, Andrew

Beach, Mark

Macleod, John

Wilkinson, Ross

English

NASA Technical Reports Server

Linear Transmitter Design for
MSAT Terminals
Ross Wilkinson, John MacLeod*, Mark Beach, and Andrew Bateman,
Centre for Communications Research,
University of Bristol,
Queen's Building, University Walk,
Bristol, BSg 1TR, England.
Tel: +44 272 303726, Fax: +44 272 255265
ABSTRACT
One of the factors that will undoubtedly
influence the choice of modulation format for
mobile satellite, is the availability of cheap,
power-efficient, linear amplifiers for mobile
terminal equipment operating in the 1.5-1.7GHz
band. Transmitter linearity is not easily
achieved at these frequencies, although high
power (20W) class A/AB devices are becoming
available. However, these components are
expensive and require careful design to achieve a
modest degree of linearity. In this paper an
alternative approach to RF power amplifier
design for MSAT terminals using readily-
available, power-efficient, and cheap class C
devices in a feedback amplifier architecture is
presented.
INTRODUCTION
Global mobile communication services such
as maritime, aeronautical and land mobile are
most effectively provided by using satellite
transponders. The long range and limited
radiated power of the satellites, together with
the modest allocation of radio spectrum[l] given
to these services necessitates the efficient use of
both bandwidth and power, if a service is to be
provided for a large user population. The
modulation format selected must therefore be
tolerant to the restrictive propagation
characteristics of the mobile radio channel, if
intelligible and reliable communications are to
be provided.
*On sabbatical leave from Footscray Institute of
Technology, PO Box 64, Footscray, Victoria,
Australia.
Tel: +61 3 6884422, Fax: +61 3 6884806.
Significant advances in both analogue and
digital modulation techniques have been made
for both this and the closely related field of
terrestrial mobile communications. The majority
of modulation techniques currently used for
analogue and digital radio are in existence not
because of their spectral efficiency, but rather
because of their compatibility with non-linear
RF amplifier technology. Examples are
analogue frequency modulation and constant-
envelope data modulation formats (MSK,
GMSK, TFM, etc.). However, techniques which
combine both amplitude and phase modulation,
such as analogue linear modulation (single
sideband)[2] and M-ary QAM[3], have
significant advantages in terms of power drain
and spectral efficiency, and thus are well suited
to mobile satellite applications[4].
Numerous proposals have been made
regarding the choice of modulation format for
future generation mobile satellite systems.
These include the use of amplitude-companded
single sideband with a transparent tone-in-band
(ACSSB-TTIB) by both the Australians[5] and
Canadians[6], and heavily filtered Quadrature
Phase Shift Keying by Inmarsat[7] for their
Standard-M service. In order to obtain the
necessary adjacent channel performance in these
systems, it is essential that a linear channel is
employed. This requirement dictates the use of
either linear, or linearised RF power amplifiers.
This paper describes a simple technique for
the linearisation of high efficiency class C
amplifier stages using feedback compensation.
New results obtained for a linear transmitter
system operating at 1.7GHz are presented here,
and compared with results previously obtained at
900MHz[8],[9].
International Mobile Satellite Conference, Ottawa, 1990
297
https://ntrs.nasa.gov/search.jsp?R=19920014876 2020-03-17T12:29:23+00:00Z
LINEAR POWER AMPLIFIERS
The requirements of efficient spectrum
utilisation, good power-efficiency, and
compatibility with any modulation scheme can
only be met by a completely linear transmitter.
To date, all linear transmitters have utilised
devices operating in classes A or AB, where
their characteristics are approximately linear.
These transmitters, with their poor efficiency,
and large heatsinks, have not been attractive in
small battery-powered equipment.
The use of a high-gain, narrow-band
feedback loop allows amplifiers whose open-
loop characteristics are far from linear to be
employed[10]. A class C RF amplifier, for
instance, would normally only be suited to
constant-envelope signals, but the prototype
amplifiers here described have achieved
excellent performance with a two-tone test
signal, the most rigourous test of linearity. The
advantages of class C amplifiers over their more
linear counterparts are increased efficiency (a
most important factor in battery-powered
systems), and reduced size (since they require a
smaller heatsink). In addition, they do not suffer
from the gradual deterioration in performance
due to drift in the bias point, etc.
IMPLEMENTATION OF THE CARTESIAN
LOOP TRANSMITTER
Essentially, the transmitter is a direct
conversion design: the baseband message
information, in quadrature components, is
modulated directly onto the local oscillator,
without the use of any intermediate frequency
translation. The quadrature components of the
modulation are generated by digital signal
processing, and hence the channel filtering, and
modulation scheme are controlled in software,
and may easily be reconfigured.
Any modulation which can be represented in
the form of two quadrature channels, whose
bandwidth does not exceed the transmitter's
linearised bandwidth, may thus be upconverted
and amplified without distortion.
A schematic of the transmitter is shown in
figure 1. The I and Q components of the
modulation are supplied by the DSP, and
modulated onto the LO via a phasing/Weaver
method upconverter. A series of amplifiers
preomps driver PA coupler
otlenuo_or
_] combiner
,-........................ I/\1
', dlff, l bosebond '
'r LO generotor
phase
adjust
Fig.l Schematic of the cartesian loop transmitter.
bring the low-level output from the mixers up
to the required output power. The amplifiers in
this chain are progressively more non-lilaear,
with the final stage being in class C. After the
amplifiers, a directional coupler samples the
output signal, which is then attenuated and fed
into a quadrature demodulator, akin to the
upconverter. A pair of identical differential
amplifiers, one in each quadrature arm, derive
error signals between the required modulation
(from the DSP) and the downconverted output.
It is these error signals which drive the
upconverter. Figure 2 shows an ideal two-tone
test signal, and the error signal generated in the
feedback loop around a class C amplifier: the
distortion in this signal is exactly complementary
to that in the amplifiers and upconverter, and
thus the output is undistorted. The plot shows
the predistortion to be most evident near the
minimum of the modulation envelope, where the
class C stage is in its most non-linear region.
The feedback loop also includes phase
compensation, here shown in the local oscillator
driving the upconverter. This is required to
cancel the phase shift through the amplifiers,
coupler and attenuator, so that the
downconverted signals are in phase with their
respective upconverted counterparts. If this
phase was not equalised, then positive feedback
around the loop could occur, resulting in
wideband spurious outputs. Since the phase
shift through the transmitter will vary with
operating frequency, this phase equalisation
must be altered as the channel is changed.
298
International Mobile Satellite Conference, Ottawa, 1990
!Fig.2 A two-tone test signal and predistorted
error signal.
The loop dynamics are controlled by the
differential amplifiers: the aim is the greatest
suppression of distortion, whilst maintaining
overall stability. The gain and bandwidth of the
feedback loop must be designed accordingly.
DC offsets in the differential amplifiers and
the mixers in the up- and down-converters will
result in an unwanted RF output at the local
oscillator frequency. This will change with
operating frequency, time, temperature, etc, and
so steps must be taken to null out this residual
DC. The use of a TTIB-based system will relax
considerably the need for this suppression, since
the unwanted DC will be insignificant compared
with the level of the pilot tone.
RESULTS FROM PROTOTYPE SYSTEMS AT
1.7GHZ AND 900MHZ
Two prototype systems have been
constructed, both employing diode-ring mixers
in the up- and down-converters, and using the
same differential amplifier configuration,
consisting of a first-order loop with a
bandwidth of 10kHz (note that this is effectively
10kHz on either side of the local oscillator, i.e. a
20kHz correction bandwidth).
The 1.7GHz system included an ACR 2001
transistor as the final class C amplifier, and
figure 3 shows the results achieved with
feedback, compared with the open-loop
performance. The tones are spaced at 4.2kHz,
and the output power is 400roW PEP. Although
the device is capable of more output, initial tests
showed that it was not possible to achieve stable
closed-loop operation at higher power levels.
Examination of the gain/phase characteristics of
the amplifier chain over the top 10dB of the
output range, figure 4, shows an abrupt change
of over 35dB and 120": much more than can be
accommodated by the gain and phase margin of
the loop.
1
I
i
CENTER _ . 6[SBOOOOOSHz SP/._.I .C_O . OOkHz
Fig.3 Transmitter for 1.7GHz open- and closed-
loop.
r.... i _- / l i ,
i i i i I / / '
, I _ ! T 1 i : :
i i
t ! " : l '
Fig.4 Gain/phase characteristic for 1.7GHz
amplifier.
International Mobile Satellite Conference, Ottawa, 1990
299
STA¢_T -6 o (_Bm CW 900 000 (}00 MH: SIO/s ,I 0 (IBm
Fig.5 Gain/phase characteristic for 900MHz
amplifier.
For comparison, figure 5 shows the same
measurement for the 900MHz system: here the
change is more gradual, 15dB and 60 °, but this
is still a class C amplifier, (here an NE080191
transistor). The open- and closed-loop plots at
an output of IW PEP (full power) are shown in
figure 6. This illustrates what may be achieved
with an efficient linearised class C transmitter.
I ! I [ I | _ i
 ]:22A- I:
I I......W] ] I t J.... .......
CENTE_ _9_ 70OmMHZ SP_N _00 0 I,<, U Z
_BV4 300_4z 'v' _5 W 300HZ _/_ 2 O_C
Fig .6 Transmitter for 900MHz open- and
closed-loop.
CONCLUSIONS
It has been shown, using the cartesian
feedback technique at 900MHz, that excellent
intermodulation suppression (>60dB for a two-
tone test signal) and good power efficiency (55-
65%) can be obtained. Initial results at I.TGHz
show that although the feedback loop performs
just as well at this frequency, the amplifier
itself requires careful design. The use of a RF
non-linear simulation software package would
facilitate the design of such an amplifier, and
work in this field should soon be underway at
Bristol. Some advantage may also be gained by
using higher-order feedback loops, to more
carefully tailor the gain and phase margin of the
system.
Hence, linear power amplifiers for future
generation mobile satellite terminal equipment
can be designed using a linearised class C
architecture, which has better linearity and
power efficiency performance then its class A
counterpart. Besides mobile satellite
applications, another major driving force behind
the development of linear transmitter
architectures at 1.7GHz is the personal
communication networks (PCN's), which is also
being addressed at Bristol. In addition, other
techniques for linearising amplifiers, offering
wider channel bandwidths and power efficiency
are also being considered. These include
adaptive predistortion and LINC[I 1].
ACKNOWLEDGEMENT
The authors wish to thank David Bennet for
his invaluable help with the fabrication of the
up- and down-converters and the loop phase
shifter for the 1.7GHz system.
300
International Mobile Satellite Conference, Ottawa, 1990
REFERENCES
[1] M.Goddard, 'Frequency Allocations for
Mobile Satellite Systems and Satellite
Navigation,' IEE 4th Int. Conf. on Satellite
Systems for Mobile Communications and
Navigation, CP294, Oct 1988, ppl0-14.
[2] A.Bateman & J.P.McGeehan, 'A Survey of
Potential Narrowband Modulation Techniques
for use in Satellite and Cellular System,' Second
Nordic Seminar on Digital Land Mobile Radio
Communications, Sweden, 1986.
[3] K.Feher, 'Digital Communications,' Prentice-
Hall, 1983.
[4] W.J.Whitmarsh, A.Bateman &
J.P.McGeehan, 'Analogue and Digital Linear
Modulation Techniques for Mobile Satellite,'
Second International Mobile Satellite
Conference, Ottawa, Canada, June 1990.
[5] 'Trials Report on the Subjective Performance
of Modulation Techniques for Mobile Satellite,'
Australian Telecom Internal Report, 1989.
[6] J.S.Butterworth, 'Satellite Communications
Experiment for the Ontario Air Ambulance
Service,' ibid [1], pp83-86.
[7] 'On the Subjective Voice Quality Test of
ACSSB Modulators and Digital Vocoders,'
Inmarsat Internal Document, 1989.
[8] A.Bateman & R.J.Wilkinson, 'Linearisation
of Class C Amplifiers using Cartesian Feedback,'
IEEE Workshop on Mobile and Cordless
Telephone Commuications, London, Sept 1989,
pp 62-65.
[9] A.Bateman, D.M.Haines & R.J.Wilkinson,
'Direct Conversion Linear Transceiver Design,'
IEE 5th International Conf. Mobile Radio &
Personal Communications, Warwick, Dec. 1989,
pp 53-56.
[10] V.Petrovie, 'Reduction of Spurious Emission
from Radio Transmission by means of
Modulation Feedback,' IEE Conf. Radio
Spectrum Conservation Techniques, Sept. 1983,
pp 44-49.
[11] A.Bateman, R.J.Wilkinson & J.D.Marvili,
'The Application of DSP to Transmitter
Linearisation,' 8th Euro. Conf. Electrotechnics,
Stockholm, June 1988.
International Mobile Satellite Conference, Ottawa, 1990
301


Linear transmitter design for MSAT terminals

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