A New Generation Cartesian Loop Transmitter for Flexible Radio Solutions and Software Defined Radio
By C.N. Wilson and J.M. Gibbins
The concept of ‘Software Defined Radio’ (SDR) is much discussed, but in practice
can mean many different things to different people and organizations. In essence,
the
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Figure 1. Cartesian Loop |
concept is simply the ability of a radio solution to select multiple modes of
transmission. More rigorously it might be argued that the same hardware must be
used, and software used to generate and demodulate the waveforms. In this context,
a great deal of attention is paid to SDR receivers and their ability to deal with
differing regulatory requirements, e.g. modulation, bit rate, signal bandwidth,
adjacent channel rejection, etc.
Transmitters face similar problems, although varying signal bandwidths are generally
less of an issue as long as the digital-to-analog function has suitable performance,
but differing modulations and a wide operating frequency can present a greater
challenge. A key problem facing the designer occurs if the system must support
linear modulations schemes, i.e. modulations which have significant amplitude
content. Such modulations present a challenge as either the Power Amplifier (PA)
must be inherently very linear, which generally involves very high power consumption;
or some form of linearization scheme needs to be used. The challenge then becomes
making the linearization scheme work for all the required operating parameters.
The situation is further complicated by another requirement often found in modern
radio designs, which is that organizations are frequently looking to maximize
the use of a
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Figure 2. Cartesian Loop Linearization using the CMX998, open loop (left)
with ?/4-DQPSK Modulation, 390 MHz and 1 W output. |
particular solution across a range of designs. This means that one solution often
has to meet the requirements of hand portables, mobiles, and, in some cases fixed
stations as well. Further, in markets like specialized / professional radio systems
or military systems a product can span operating frequencies from, 150 MHz to
1 GHz. This is not necessarily the case in one product, but often build variants
must support such a range.
Various linearization schemes come to mind to meet this requirement, but the focus
of this article is on Cartesian feedback. It will be seen that with a modern integrated
solution this scheme can offer a genuine answer to all the challenges discussed
above.
The Cartesian Loop
The Cartesian Feed-back Loop (CFBL), first developed in the 1980’s [1,2], is now
well established as a solution for highly-efficient linear transmitters using
modulation such as ?/4-DQPSK, 8PSK, QAM etc. The scheme has been universally utilized
in products implementing the TETRA standard [3], and also widely used in Japanese
digital
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Figure 2. Cartesian Loop Linearization using the CMX998, closed loop (right) with ?/4-DQPSK Modulation, 390 MHz and 1 W output. |
technology. The Cartesian loop has the advantage of offering a large degree of
linearization improvement; gains of over 30 dB are not untypical. It is most suited
to channel spacing up to about 200 kHz, beyond this, keeping the loop stable becomes
a compromise with linearity improvement.
Integrated solutions have been in use for a number of years, but the requirements
of low noise and high linearity offer a number of challenges to IC designers.
Now, with modern IC processes, solutions offer performance and functionality that
makes the adoption of the technique over more product areas increasingly attractive,
opening the door for SDR transmitters.
The Cartesian Loop works to improve the linearity of a power amplifier device
by the action of feedback. A block diagram of the scheme is shown in Figure 1.
The input signal is required in in-phase and quadrature (I/Q) format. This is
applied to a summing amplifier (usually know as the ‘error amplifier’) where it
is compared to the feedback signal. The output of the ‘error amplifier’ is applied
to an up-converter to generate an RF signal that is then amplifier by a power
amplified. A sample output of the amplifier is taken, generally using a directional
coupler; this is down-converted and applied to the error amplifier as described
earlier. This forms the closed loop system, such that as
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Figure 3. Cartesian Loop Linearization using the CMX998, open loop (red) and closed loop (green) with TETRA ?/4-DQPSK modulation, 800 MHz and 1 W output. |
long as the feedback path does not introduce distortion, the system will attempt
to correct the signal at the output to match the I/Q input signal applied to the
error amplifier. The effectiveness of the solution is demonstrated in the results
shown in Figure 2 where the upper trace is the PA operated without the feedback,
and the lower trace is with feedback applied, in both cases the output power is
approximately the same. It is obvious that the closed loop spectrum is a very
substantial improvement on the non-corrected performance of the PA.
To achieve these results necessary to ensure the feedback phase is correct. This
can be done by including a variable phase shifter in the LO path, typically to
the down-converter, so that the relative phases of up and down conversion can
be adjusted. This allows the desired feedback phase to be set. Also, it is necessary
to include a means of constraining the loop bandwidth to ensure stability; hence
a filter is included either around the ‘error-amplifier’ or immediately after
it.
The Challenge
The need for the feed-back path to not introduce distortion places demanding requirements
on the design in terms of linearity and noise. The need for low wide-band noise
also makes the design of the up-converter challenging. Further, the need for excellent
isolation between up-converter and down-converter (as well as I and Q) makes the
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Figure 4. Cartesian Loop Linearization using the CMX998, open loop (left) with two-tone modulation, 800 MHz. |
Cartesian loop a testing challenge for IC designers.
Traditional markets using CFBL solutions have been 25 kHz-channeled radios at
300 to 500 MHz. For some time manufactures have been demanding solutions operating
up to 1 GHz while running from 3 V, taking less current than before and with no
compromise on performance. One of the features of the specialized / professional
radio markets is the diverse nature of the products and technologies employed,
and while some manufactures are moving higher in frequency, many application are
still operating at VHF leading to a demand for operation down to the 100 MHz.
To examine what can be achieved with a modern CFBL design we will use the CMX998
IC (ref [4]) as an example. The CMX998 represents a state-of-the-art solution
for
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Figure 4. Cartesian Loop Linearization using the CMX998, closed loop (right) with two-tone modulation, 800 MHz. |
a flexible integrated CFBL design and is believed to be the first open-market
IC solution to achieve all the objectives discussed above.
Typical Performance
The TETRA standard is a good benchmark for CFBL transmitters as it requires good
linearity to achieve the required adjacent channel power levels of up to –60 dBc.
Today the majority of TETRA terminals operate in the 300 to 500 MHz frequency
range, but an increasing market is growing in frequencies around 800 to 900 MHz.
Although Cartesian loop technology has no problems operating at these frequencies,
the availability of integrated solutions has been a major constraint on manufactures
until now. One of the key advantages of the CMX998 is that it has taken the technology
forward, so 900 MHz operation is now as easy as 400 MHz. This is demonstrated
in Figure 3 and Figure 4. The plot in Figure 4 is a particularly impressive demonstration
of the performance with adjacent channel powers of 㫗 dB / 㫘 dB representing
a 28 dB improvement on the open-loop situation.
Wideband Noise
A key performance requirement for PMR standards such as TETRA is the wideband
noise generated by the transmitter. This critically affects the ability of multiple
terminals to operate in close proximity.
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Figure 5. Wideband Noise Performance of 1 W (mean power) Transmitter using CMX998 |
The TETRA standard states that transmitters must meet a noise mask that is evaluated
using the TETRA channel bandwidth (equivalent to 18 kHz). The mask is more stringent
for higher output powers to protect users on adjacent channels. The Cartesian
loop is like any closed loop system in that noise inside the loop bandwidth behaves
differently to noise outside the loop. Wideband noise, for example at a 5 MHz
offset, is typically outside the loop bandwidth and is generally dominated by
noise from the up-converter section. Closer to the carrier, e.g. at 100 kHz, noise
is generated within the loop bandwidth, which is often dominated by the noise
figure of the down-converter. The effect can be seen in Figure 5 where noise is
relatively flat to about 500 kHz then rolls off quickly as the edge of the loop
bandwidth is reached. By 5 MHz the noise is dominated by the broadband noise floor
of the up-converter. Also shown on the graph are the TETRA 1 W and 3 W requirements,
and it will be observed that the CMX998 based solution easily meets these requirements?
DC Calibration
The Cartesian loop is a DC coupled system, so any DC offset generated by the various
amplifiers and mixers will have the effect of causing a DC error in the modulated
baseband signal at the output of the error amplifiers. This, when applied to the
up-conversion mixers, results in an increased carrier component in the RF signal
(the so called
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Figure 6. DC calibration measurement and correction system |
‘carrier leak’). If excessive, this will cause the intended modulation to be distorted
so a method of removing the error is required.
Various solutions to the DC problem have been used over the years, but a simple and effective solution is a software controlled DC calibration loop. Figure 6 shows a test system used with the CMX998 which uses the DCMEAS output on the CMX998 to provide an error signal which is then sampled by the auxiliary ADC on the CMX981[5]. From the measured value a correction is computed, in the test system a Texas Instruments C55x DSP was used, and then the correction is loaded in the offset correction registers associated with the main Tx DACs in the CMX981. This results in a correction being applied to the I/Q inputs of the CMX998.
Adjusting the DC offset on the modulating IQ input signals can compensate for
the cumulative error and thus the carrier can be nulled to better than 㫊
dBc if required. Calibration time depends on the accuracy of the required calibration,
but typical numbers from the test system are around 300’s. Although a powerful
DSP was used for convenience, the
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Figure 7. MX998 and S-AV35 operation at 10 W (mean power), upper trace is open loop, lower trace is closed loop, ?/4-DQPSK modulation. |
processing requirement of the correction algorithm is low, most of the time being
spent managing communications to ADC and DACs.
Versatility
Today, manufactures are looking for versatile ICs with a common design that can
be used in multiple products. An example of the versatility of the CMX998 CFBL
solution is shown in Figure 7 where operation is at 150 MHz and the output power
is 10 W mean, 30 W peak. The power amplifier used in these tests is a Toshiba
PA module designed for FM operation (type: S-AV35,) but the CMX998 has no problem
producing a clean transmit spectrum as can be seen in the lower trace on the plot.
It has already been stated that the Cartesian loop can support almost any modulation
type. The loop is an analog system, it will reproduce whatever modulation is placed
on the I/Q inputs, such as QPSK, GMSK, QAM, Analog FM, OFDM etc. One point worthy
of note is that it is not easy to use the Cartesian Loop for peak limiting a signal.
If Peak to Average Power Ratio (PAPR) reduction is required, this should be done
prior to the CFBL. The reason is that the analog loop will try to reproduce the
input precisely. This works well up until the maximum power the PA stage can deliver,
at which point the CFBL will still try to increase the output power causing a
limiting effect which leads to a broadening of the signal. As a result, limiting
in the CFBL should be avoided.
A tough test of the flexibility of a linearization system is a two-tone test.
Although the PAPR of a two-tone signal is only 3 dB, the fact the modulation has
a zero (effectively goes through the origin in I/Q format) makes it a challenge
for some linearization schemes, such as polar loop. As can be seen in Figure 4
the Cartesian loop deals with two-tone
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Figure 8. Loop Configuration for TETRA 2 testing |
modulation very effectively.
Another aspect of the professional market is tough environmental requirements;
to this end the CMX998 operates over a temperature range of 㫀°C to +85°C.
The thermal environment for the CFBL IC can be severe, as it is often located
close to the power amplifier that generates significant heat during transmission.
With this in mind, the CMX998 uses a small QFN package, featuring excellent thermal
properties due to the large ground pad specially added in the base. This helps
keep the die temperature as low as possible benefiting performance. The QFN package
also has the benefit of being physically small and has low-bond inductances, helping
to enhance RF performance.
Future Technology – Broadband / Multicarrier / QAM
With the move to higher user data requirements, there is a general move to higher
on-air bit rates. Often, this means broader bandwidth and higher-level modulations.
The TETRA community is addressing this with the TETRA Release 2 which uses multi-carrier
QAM modulation to achieve user bit rates of up to 500 kb/s. The TETRA 2 system
can operate
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Figure 8. Loop Configuration for TETRA 2 testing |
on up to 150 kHz channels, but the transmitter must still meet the adjacent channel
levels associated with a 25 kHz transmission. Wideband noise also must not be
degraded.
To test performance for higher bit rate systems evaluation has been carried out
using 16 QAM modulation, with a square root raised cosine filter, Bt = 0.2. For
example, in a 50 kHz channel a QAM symbol rate of 38.4 kS/s can be used and in
a 150 kHz channel a QAM symbol rate of 115.2 kS/s. These modulations are representative
of broadband
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Figure 9. 150 kHz QAM Open (upper) and Closed (lower) Loop Modulation on
1 MHz Span |
modulation, including a typically high peak-to-average power ratio of approximately
8 dB.
The results show the CMX998 can linearize systems with 150 kHz bandwidth signals very well, approximately 25 dB linearization can be observed in the plots. Results show that for both 50 kHz and 150 kHz QAM a 1st Adjacent Channel requirement of 55 dBc can easily be met.
To further demonstrate the flexibility of the scheme tests with GSM EDGE modulation,
(8PSK) have shown excellent linearization. One of the main alternatives to the
Cartesian scheme is ‘Polar Loop’, which has the advantage that low-noise phase
modulators can be used, however the polar scheme is not as straightforward to
implement as it first appears, and typical linearization gains are relativity
small. For example, typical improvements in the adjacent spectrum may be only
6 dB to 10 dB. This compares with the 25 dB demonstrated in Figure 9 at 100 kHz
offsets and beyond.
Cartesian loop can also be employed at higher frequencies by using up and down
mixing to a suitable IF. This scheme is a little cumbersome and does suffer from
some
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Figure 10. 150 kHz QAM Eye & Constellation Diagrams |
performance compromises; however, it is still an attractive option for some systems,
such as portable satellite terminal transmitters that use QAM modulation. It has
been found that mix-up / mix-down is often preferable to using discrete components
to implement the loop directly at the operating frequency. This is because the
CMX998 implements a whole range of functions: up-conversion, down-conversion,
phase shifting, baseband amplification, gain control etc. A discrete solution
may therefore require as many as six ICs to replicate the basic functions of the
CMX998. This multiplicity of ICs often leads to layout and coupling problems to
which the Cartesian Loop is quite susceptible; however, these can be largely avoided
using an integrated solution. For example, the authors were involved in a discrete
Cartesian loop design some years ago
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Figure 10. 150 kHz QAM Eye & Constellation Diagrams |
that took 3 engineers upwards of 6 months to design and optimize. The re-design
of this circuit with an IC solution took one person two weeks.
Conclusion
We have seen that the CMX998 demonstrates a solution that fulfills all the key
requirements of an SDR transmitter. The solution can be used over a wide- operating
frequency range. Typically a product design is constrained by broadband local
oscillator design and the availability of a suitable broadband power amplifier
not the CFBL. Multiple modulation formats, excellent modulation accuracy and flexible
configuration are all achieved.
References
1. Petrovic, V., "Reduction of Spurious Emissions from Radio Transmitters by means
of Modulation Feedback", IEE Conference on Radio Spectrum Conservation Techniques,
1983.
2. Petrovic, V., "VHF SSB Transmitter Employing Cartesian Feedback", IEE Conference
on Telecommunications, Radio and Information Technology, 1984.
3. ETSI EN 300 392-2 Terrestrial Trunked Radio(TETRA) voice and data. Part 2:
Air Interface V2.4.2 (2004-02)
4. CMX998 Datatsheet, available from www.cmlmicro.com
5. CMX981 Datasheet, available from www.cmlmicro.com
About the Author
C.N. Wilson title is ‘Director — Applied Technology Ltd’ J.M. Gibbins title is
‘Senior RF Engineer’
CML Microcircuits
Oval, Park, Langford
Maldon, Essex
© 2008 Advantage Business Media
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