IN MY OPINION

Interference: The Creeping Menace
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Interference has been the bane of communication system designers since the earliest days of radio. At least back then, there were fewer sources to contend with but also far fewer remedies at their disposal.
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FROM WHERE WE SIT

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A federal agency like DARPA is a sitting duck for politicians and assorted other critics. It has come up with some truly bizarre programs over years that ultimately either delivered no tangible results, were canceled before they could cause any damage, or attempted to answer questions that nobody was asking or needed answers to. Read More...


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Group Delay Equalized Bandpass Filter
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January 2008

Comparison of High Power Amplifier Technologies:
TWTAs vs SSPAs
By Howard Hausman, Vice President, Engineering, MITEQ, Inc

Designers of high power communication systems are constantly evaluating and debating the merits of using Traveling Wave Tube Amplifiers (TWTA) and Solid State Power Amplifiers (SSPA) in their transmitting systems. Most decisions are made on personal preferences for one technology over another. The reality is that both technologies have advantages and disadvantages, with higher power levels and higher frequencies leaning toward the TWTA as a better choice.

Full Disclosure
There have been many articles written on the subject of TWTAs versus SSPAs, usually by manufacturers of one technology or the other. These articles invariably tend to favor the author’s sponsored technology. In the interest of full disclosure, MCL, a manufacturer of TWTAs, is a wholly-owned subsidiary of MITEQ, Inc. MITEQ is currently a manufacturer SSPAs and plans on expanding this effort in the near future.

The reader is encouraged to weigh all of the facts and make an unbiased decision regarding which technology is best suited for his or her respective communication project.

Traveling Wave Tube Amplifier (TWTA)
The concept: the Traveling Wave Tube (TWT) amplifies microwave energy through the interaction of an electron beam and a slow wave structure. As the electron beam travels down the slow wave structure, an energy exchange takes place between the particles and the RF wave, effectively amplifying the RF signal.

The basic components of the TWT are the electron gun, the slow wave structure, and the collector; see Figure 1.

The source of the electrons is known as the cathode, which is heated to approximately 1,000 degrees Celsius. With an application of a high voltage bias (cathode voltage, on the order of 10,000 volts) the electrons are drawn down the tube. The cathode has a finite source of available electrons, which limits the operating life of the TWT. Well-designed TWTs have been known to supply a beam of electrons in excess of one hundred thousand (100,000) hours of continuous operation.

Solid State Power Amplifier (SSPA)
Solid state power amplifiers are usually divided into low power driver sections and high power output stages. Gallium Arsenide Field Effect Transistors (GaAs FET) are used for power amplification. To obtain high powers, many stages are fed in parallel from a medium to high power amplifier and combined at the output; see Figure 2.

The power combiners, especially at higher frequencies can have considerable loss (0.5 dB to 1 dB plus the VSWR uncertainty factor), thereby limiting the number of parallel stages to four (eight parallel stages are sometimes used but the amplifier efficiency can be severely degraded). As an example, combining four amplifiers in parallel theoretically increases the output power four times, but because of the coupler losses, the output power only increases about three times. The primary power increases four times, lowering the overall amplifier efficiency about 30%. In reality, the amplifier efficiency is even lower when considering the fact that higher power driver amplifiers are necessary to power multiple parallel output stages.

One advantage claimed by Solid State Amplifier manufacturers is the fact that when an output stage fails, the amplifier is still operational. This is readily seen from the block diagram, but in reality this is an oversimplification of the failure effect on system performance. A sophisticated power maintenance technology must quickly restore the power level. If a drive amplifier stage fails, a catastrophic failure results and the output signal is lost.

Comparing the TWTA and SSPA
TWTAs and SSPAs have both been proven to be very reliable alternatives that provide high power microwave signals for satellite communications. They are very different technologies and have very distinct advantages and disadvantages, which makes each more applicable to some applications more than others.

Typically, SSPAs are rated in terms of their 1 dB compression points and TWTAs are rated in terms of their saturated output power. Since most communication applications are concerned with linearity, i.e. intermodulation distortion and spectral re-growth, the technology comparison focuses on comparing units with equivalent performance in these areas. This comparison focuses on power amplifiers with output levels that produce third order intermodulation distortion less than -25 dBc and spectral re-growth less than -30 dB. At these levels, the individual system designer must consider which parameters are most important in the respective system before making a final decision on the preferred technology.

Linearity, Intermodulation Distortion and Available Output Power
Linear performance is rated in terms of two tone intermodulation distortion for multiple carriers and spectral re-growth for single carriers. These two are related in that the primary distortion is caused by third order and fifth order intermodulation distortion, where third order intermodulation is usually the dominant effect. Most amplifiers are characterized by their output power and by inference at their third order intermodulation intercept point (the imaginary point used to calculate third order intermodulation interference).

SSPAs are rated in terms of their 1 dB compression point, the output level where the amplifier gain is decreased by 1 dB with respect to the small signal gain. TWTAs are rated by their saturated power level, a point where the output level doesn’t change when the input level is increased; see Figure 3. Saturation of an SSPA is generally 0.5 dB to 1 dB above the 1 dB compression point, which leads to an understanding of “Rated Power” as the approximate maximum available power from the device. This is in general not a useful operating level, but many times considered for emergency operation. Figure 3 compares rated characteristics and equivalent linearity levels.

Useful levels of operation, in terms of spectral re-growth and multi-carrier interference, are determined by the Intermodulation Intercept Points, of which the third order intermodulation intercept point (IP3) is usually the most dominant in determining interference levels. The general rule of thumb for SSPAs and TWTAs is:

1. IP3 for a SSPA is 7 dB to 10 dB above the 1dB compression point
2. IP3 for a TWTA is 2 dB to 5 dB above Psat

The higher IP3 is a typical value but, when it comes to committing to a minimum number, the lower IP3 value is usually quoted.

Using this as a measure of comparing amplifier performance, it is obvious that a TWTA with an equivalent linearity of an SSPA must be rated at three times the power, i.e. the TWTA rated power must be 4.8 dB higher than the SSPA rated power (e.g. a 133 watt SSPA has the same linearity performance as a 400 watt TWTA). On the surface, this looks like a very dismal scenario for the TWTA, a fact which is exploited by many SSPA manufacturers. When all of the facts are presented, the advantage is actually on the side of the TWTA.

As an example to prove this point, a 700 watt TWTA with 665 watts saturated power at the flange will be compared to a leading SSPA manufacturer’s 200 watt power amplifier. The 665 watt TWTA with 10% more power than three times the 200 watt SSPA still has a clear advantage over the SSPA. In addition, it must be noted that the linearity performance will be equivalent, but the TWTA has about 2½ times more the available power than the SSPA. Figure 4 is information from the MCL MT4000 data sheet. Note the output power, the back-off intermodulation characteristics, and the required AC power.

Efficiency of Operation, i.e. AC Power Consumption
One of the decided advantages of TWTAs over SSPAs is efficiency of operation. TWTAs operating at rated power have a 30% to 40% efficiency compared to SSPA efficiencies, typically less than 10%. Immediately it is obvious that TWTAs with three times the rated power compare favorably if not advantageously with SSPAs at one third the rated power and equivalent IP3s. In the area of efficiency, TWTAs actually have a decided advantage over SSPAs because most applications, for linearity reasons, are conducted at a considerable amount of back-off from rated power (typically 7 dB for TWTAs and 3 dB for SSPAs). Under these conditions, both technologies operate less efficiently but the SSPA primary power remains approximately the same, whereas the TWTA primary power decreases. The TWTA rated at three times the SSPA actually consumes about 30% less power than a linearity equivalent SSPA under actual operating conditions.

Consuming AC power is an important consideration in its own right, but the added benefits are lower heat generation. Heat is well known to be exponentially related to reliability of the power amplifier and all of the ancillary components in close proximity to the heat source. Added to this TWTA advantage is the ability of the TWTA to operate at much higher temperatures than the typical SSPA.

Figure 5 is a compilation of specifications from the data sheet of a major SSPA manufacturer. The Ku Band unit outlined in red is compared to an MCL MT4000 TWTA amplifier with a Psat of 665 watts at the flange. Figure 6 compares the efficiency of this 200 watt SSPA to the MCL 665 watt TWTA. The SSPA consumes 2800 watts of AC power and the TWTA consumes 2400 watts of AC power at Psat, 16% less than the SSPA. Most of the time, HPAs are used in a back-off mode; the out signal is significantly below the maximum operating level. The SSPA power consumption is independent of signal level, whereas the TWTA operates with significantly less AC in the back-off mode. The MCL MT4000 HPA consumes less than 1650 watts in the back-off mode. Under these conditions, i.e. linear operation, the TWTA uses 40% less power than an SSPA with the same linear characteristics.

• 200 Watt SSPA consumes 2800 Watts of AC Power
• 665 Watt Psat TWTA, 2400 Watts, AC Power at Psat (27.8% Efficient)
• 1650 Watts in back-off using 40% less power than the SSPA

Typical power consumption of a 200 watt SSPA, a 665 watt saturated TWTA and a 360 watt TWTA with linearizer that have the same linearity performance characteristics is plotted in Figure 6. The SSPA is backed-off 3 dB to 100 watts, the 665 watt TWTA is backed-off 7 dB to 135 watts and the 360 watt TWTA with a linearizer is backed-off 4 dB to 145 watts; all attain the same linear performance characteristics. The TWTA with and without linearizer consume significantly less power than the 200 watt equivalent SSPA.

Size, Weight and Heat
The basic solid state power amplifier module is much smaller and weighs less than the basic Traveling Wave Tube (TWT), so conceptually the size and weight of the SSPA should be much smaller and lighter than the equivalent TWTA at three times the rated power of the SSPA. In reality this is not the case; the heat generated in the SSPA is from a small constellation of point sources which, due to the relatively low absolute maximum junction temperatures, must be quickly dissipated. Aggravating this problem is that SSPA power is achieved by combining multi-output devices (all point sources) that must be in close proximity to affect low loss power combining, all generating a considerable amount of heat. The heat generated must be dissipated quickly to prevent thermal transients from destroying the RF modules. The required heat sinks and cooling systems not only totally negate the size and weight advantage that the individual solid state power device has over the TWT, but because of the lower efficiency of operation, in most cases the overall size and weight of the TWTA is less than the SSPA.

An example of this is given by comparing MCL’s MT4000 665 watt power amplifier with a competitor’s 200 watt SSPA. Both units are mounted in a 19 inch rack taking up 7 inches of rack height, but the TWT is four inches shorter in depth, equating to 15% less volume used by the TWT.

MCL MT4000
Ku-Band 665 Watt TWTA

Physical Specifications Dimensions:
7.00" H (178.00 mm)
19.00" W (483.00 mm)
24.00" L (610.00 mm)

Ku-Band 200 Watt SSPA
• Chassis:
19.0 x 7.0 x 28.0 in.
483 x 178 x 711 mm.

Reliability
Tubes are known to have a limited life and transistors do not. This, on the surface, suggests that the reliability of solid state amplifiers should far exceed the reliability of tube amplifiers. That would be true if the RF amplifier were the most failure prone component, which it is not.

The following is a quote from a manufacturer of solid state power amplifiers:

“The reliability of the microwave power Gallium Arsenide Field-Effect Transistors (GaAs FETs) used in today’s solid state power amplifiers is so good that their mean time between failure (MTBF) is measured in millions of hours. Generally, other common electronics components are more likely to fail than the output devices.”

In another article on TWTAs and SSPAs, the following statement was made:

“Reliability is also a major concern for users of power amplifiers. The largest study performed on the relative MTBFs for the two types of amplifiers has been done on amplifiers used in all the Intelsat satellites. The failure rate of the SSPA population was higher than the TWTA amplifier population by about 15%. Therefore the user should consider similar sparing philosophies for amplifiers when developing maintenance plans for Satcom systems.”4

Most high power amplifier failures occur in the power supply system, whether it is high voltage (TWTA) or high current (SSPA). With proper heat sinking and power handling, the TWTA typical MTBF around 70,000 hours (including the tube) should be exceeded by Solid State Power Amplifiers. In reality, most SSPA systems are redundant, with many advertising redundant power supplies that are “hot swappable” for user convenience when they fail. This is a good feature that suggests that this is a major problem for the ultimate user.

• Most failures in TWTAs and SSPAs are not in the RF devices
• Heat kills components
• Heat in TWTAs is less than or equal to the heat in SSPAs

SSPA Soft Failure Mode
Soft failures occur when one stage of an array of parallel power amplifier stages fails. The other stages continue to operate and the power, instead of completely dropping out, decreases from a few dB to about 6 dB (much worse for some of the more remote failure modes). The worst case soft failure mode is when two stages are combined and one stage fails. This leads to a 6 dB decrease in amplitude. When four stages are combined at the output (see Figure 7) failure of a single stage (no signal) causes the output level to drop 3 dB; see Figure 8. When the failed stage passes some signal and the phase of that signal is 180 degrees out of phase with the other parallel signals, the output can decrease anywhere from 3 dB to 6 dB. Figures 9 and 10 show some possible soft fail scenarios for four and eight stages in parallel, respectively. All of these possible scenarios result in a decrease in power that is unacceptable for earth station operation.

Satellite earth stations must maintain power at the satellite within +/- 0.5 dB, of their pre-assigned level (under clear sky conditions). The soft failure mode of a solid state amplifier is only a good feature when the system is equipped with an automatic failure recovery system, one that makes up for the typical 3 to 6 dB power loss of a single amplifier stage. The most prevalent failure recovery systems used by most earth stations in mission critical situations are 1:1, 1:2, or 1:N redundancy configurations, where a spare HPA is automatically switched into the main communications channel. The redundancy topology is used in both SSPA and TWTA HPA systems. Failure in the driver stage of an SSPA results in a catastrophic failure.

Sources of Supply
The primary output amplifier in a TWT is obviously a Traveling Wave Tube. MCL designs its TWTA with sufficient flexibility to be able to use tubes from multiple sources or vendors without reconfiguring the HPA. There are usually multiple domestic or international vendors that have designs or design their tubes specifically for MCL such that they readily drop into MCL HPA sockets.

Field Effect Transistors (FET) used in the output stage of higher power SSPAs are mostly foreign manufactured from a single source (some lower power, higher frequency devices are manufactured domestically). Designs are based on solid state devices that are unique to the product. Drop-in replacement devices rarely exist and usually require a redesign of the amplifier, causing major problems for the manufacturer and its customers.

Conclusion
SSPA and TWTA are both viable alternatives to consider for power amplification in a satellite earth station, with SSPA the more advantageous at lower power, i.e. less than 20 watts. At greater power levels, size, weight, and efficiency become important characteristics that skew the preferred choice to the TWTA. The following table summarizes the comparison of a 200 watt SSPA and a 665 watt TWTA, but this comparison typically can be scaled to other power levels in the range of 50 watts to 2.5k watts. The TWTA advantage increases at higher power levels and higher frequencies, where SSPA efficiency decreases.

200 Watt SSPA vs. MCL MT4000 700 Watt TWTA

SSPA
• 200 Watt P1dB (250 Watts Psat)
• 2800 Watts AC Power
• 7.1% Efficient

MCL MT4000
• 665 Watt Psat (Equivalent to SSPA 222 Watts)
• 2400 Watts AC Power at Psat (27.8% Efficient)
• 28% Efficient at Psat
• Back-Off (Usually Mode of Operation)
• 1650 Watts TWT vs. 2800 Watts SSPA
• TWTA uses 40% less power

TWT vs. SSPA Comparison
• MT4000 is 10% higher equivalent power
• Typically uses 41% less power
• 266% more available power
• Smaller size (24" vs. 28" length)

References
1. “Microwave PA Thermal Design for SATCOM Systems,” Stephen D. Turner, and Ahmed M. Zaghlol, Jan 1, 2003.
2. “Power Amplifiers,” Robert A. Nelson, Via Satellite supplement, December 2004.
3. “A Comparison of SSPA and TWTA Amplifier Systems / Advantech” Application Note.
4. “Traveling Wave Tube vs. Solid State Amplifiers,” Stephan Van Fleteren Communications and Power Industries (CPI, formerly Varian Associates).


MITEQ
www.miteq.com
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