Welcome

Welcome to RF Fact Check, a site compiled by employees of CPI. In this site we attempt to dispel some myths, add some facts, and provide some insight into how and why various technologies are selected for different applications and needs. We then provide some comparison charts on product efficiency. Please note that CPI designs and manufactures traveling wave tubes, TWTAs, klystrons and KPAs, as well as both GaAs and GaN-based solid state amplifiers (SSPAs). Thus CPI's interest is that you find the best technology for your application, regardless of type. Your comments are welcome, especially those that can be backed up with data or studies.

FAQ

Q: Are traveling wave tubes (TWT) equivalent to glass TV tubes?

A: No.

Some solid state amplifier manufacturers would have you believe that traveling wave tubes are equivalent to glass TV tubes that were in use 30 years ago.

The Journal of Electronic Defense demystifies the Traveling Wave Tube. It is NOT a glass TV tube. 1

"TWT manufacturers spend considerable sums every year to advance TWT technology and manufacturers are refining TWT and power supply integration to increase frequency coverage, efficiency and reliability and reduce TWT assembly size, weight and cost. This is not to say that TWTs have short operating lifetimes; many can operate for 100,000 hours (more than 11 years) of continuous service at their rated RF output powers. While not a patch for conservatively operated solid-state power amplifiers, this is well-matched to the lifetime of satellite communications transponders, as well as radar and EW systems that can remain in service for decades."

Q: Are traveling wave tubes (TWT) old technology compared to solid state?

A: No.

Some Solid State amplifier manufacturers would have you believe that solid state is the "new" technology while traveling wave tubes are the "old" technology.

TWTAs were invented in the 1940s. 2 SSPAs were invented in the 1970s. 3 Both are over 50 years old. We therefore suggest that if you're looking for something fashionable and chic, you find another industry. The good news is that both technologies still have much potential and both continue to be improved upon. At CPI, we've created an entire line of SuperLinear® TWTAs that lead the industry in prime to output power efficiency. CPI also offers GaN-based SSPAs up to 100 W in C and X-band, and 80 W in Ku-band.

What is Memory Effect and How Does it Occur?

The end result of memory effect is that it impairs the expected intermodulation performance of the HPA, sometimes by as much as 10 dB. This requires the user to back off the output power by more than the expected amount, negatively impacting the link budget and/or the overall performance of the earth station. Memory effect is not necessarily limited to GaN devices. It can also affect GaAs based SSPAs. However, GaAs SSPAs are rarely used in applications where the effect manifests itself, since they are predominately used in single-carrier operations.

The issue has never manifested itself in TWTAs, at least to a significant or measurable level.

The severity of memory effect is determined by the manufacturing and biasing methods selected during the design and production of solid state devices/MMICs, and its interaction with the carrier spacing utilized in actual transmission. Carrier spacing has a significant effect on the level of the intermodulation distortion (IMD) when the bias decoupling networks are not sufficiently broadband. In addition, the addition of more carriers makes the problem worse. The real world impact is that when reviewing an IMD specification on a product datasheet, the general statement of compliance to “below -25 dBc” up to the specified Plinear operating point appears to show compliance because it assumes a very “coarse” IMD product. However, during these resonance events, the increase in IMD is typically sharper, thus ending up in poor linearity performance, requiring further backoff to compensate. The parasitics in the capacitors, resistors and inductors used in the bias decoupling network can cause series and parallel self-resonant frequencies where filtering/bypassing is inadequate. Most of our Ka-band SSPAs are mitigated against memory effect. If you have questions about Memory Effect Mitigation, please contact us. No matter whose HPA you want to use, be sure that the Memory Effect Phenomenon won’t impact your RF Uplink in a negative manner.

The Truth About Solid State and TWT Reliability

The only studies ever done comparing solid state and TWT reliability were done in satellite fleets, and found that if anything, TWTs are more reliable (the latest was done by Boeing Corporation in 2006 4). But there really isn't a significant difference. The vast majority of TWTAs and SSPAs live up to and past the planned lifetimes of the systems they inhabit. Cooling fans and power supplies of both systems are likely to fail first. While GaN technology is inherently more reliable than GaAs technology due to better efficiency, most manufacturers have traded the improved thermal margin for smaller enclosures, thus largely negating the advantage. Still, all technologies discussed here, including TWTAs and klystrons, are quite reliable and improvements are being made all the time.

The Truth About SSPA and TWTA Efficiency

Until the linearizer was invented a couple decades ago, solid state amplifiers were more efficient than TWTAs. Then TWTAs took a step forward with multi-stage depressed collectors and with the linearizer. Finally, CPI came up with SuperLinear TWTs, which have put TWTAs even further ahead of traditional SSPAs. Some SSPA manufacturers have since made use of some breakthroughs in device spacing and combining, marginally increasing the efficiency of some SSPAs. And now, GaN devices present a promising opportunity for SSPAs to close the efficiency gap with SuperLinear TWTAs, as well as in achieving higher power.

The Truth About Linear Output Power

Required linear output power is typically specified in one of three ways: 1) Intermodulation Products (IM3); 2) Spectral Regrowth; or 3) Noise Power Ratio. HPA manufacturers typically provide an IM3 specification on their data sheets, and may or may not provide the others. Furthermore, there are different ways of calculating IM3, and data sheets are typically vague about which method has been used.

United States military applications usually call for a spectral regrowth specification, but when they use IM3 as a measure, it is typically given as: the output power level where -25 dBc is achieved with regard to the sum of two equal carriers. When the rest of the world calls out an IM3 specification, it is typically given as: the output power level where -25 dBc is achieved with regard to each of two equal carriers (single carrier method). The resulting output power level is typically 1.5 dB lower than the result from the "sum of two carriers" method (1).

Nevertheless, most solid state amplifier manufacturers state IM3 on their data sheets using the U.S. military "sum of two carriers" method, despite that the U.S. military typically uses spectral regrowth, and despite that the rest of the world mostly uses the IM3 single carrier method. As stated, it is often difficult to tell which method each HPA manufacturer has used on their data sheet. If you are unsure of the method used to calculate the spec on any amplifier, do not hesitate to contact CPI for assistance.

It is important to keep in mind that all HPAs under consideration be compared for linear power capabilities using the same method of measurement. As long as the same method is used, the resulting relative power efficiency will then be accurately revealed.

  1. This figure is for when linearizers are not involved. Output linear power for linearized HPAs drops at a lower rate when converting from the "sum of two carriers" method to the "single carrier" method, typically only 1.0 dB as opposed to 1.5 dB.

Why SSPA, TWTA, or KPA?

When would you want to use an SSPA?

When choosing amplifier technology, the typical factors which are considered are price for the power level desired, efficiency, operating costs, weight, size and bandwidth.

If your application is not wide band, and is low power in a commercially common band, then Solid State will likely be the best solution for your application. Another consideration is training and experience. SSPAs do not utilize high voltage, and thus may not require the staff expertise that TWTAs do.

When would you want to use a TWTA?

TWTAs are going to be a more viable solution in medium to high level power requirements, across both commercially common bands and "not so common" frequency bands. The main reason for this is that the technology to meet these relatively higher power levels with a higher level of efficiency and cost effectiveness is mature. The market has pushed the technology to a point where the people still making these products are the best in the industry and the others have long since left the business. The TWTAs will give you the widest bandwidth, with the best power consumption and reliability (in these higher power levels) at a cost effective price when compared to other technologies.

Why would you want to use a KPA?

KPAs are the preferred solution for "very high" power, narrow bandwidth applications. The most popular application for KPAs today in the commercial arena is Direct-to-Home (DTH) Television. The applications demand the requirement to transmit at high power (even if the user is not always transmitting at that level; but they need the capability in an emergency). The KPAs are narrow bandwidth devices (usually less than 100 MHz) with multiple channels to enhance flexibility. KPAs would be chosen over the other technologies primarily because, if using other RF technologies, the products that would be produced to reach such high power levels would be large, inefficient, and expensive.

Compare Efficiency

Since most satcom applications require amplifiers to operate in a linear fashion, it makes sense to compare their efficiency at linear output operating levels.

Compare Ka-Band Amplifiers

Specification T05KO 500 W CW TWTA w/ lin TL06KO 650 W Peak Power TWTA w/ lin 325 W CPI SSPA w/lin
(CPI product under development)
Linear output power 165 W 215 W 165 W
Typical power consumption at linear output power 1.2 kW 1.2 kW 1.6 kW
Power efficiency 13.75% 17.9% 10.3%
Power costs running at 24/7/365 at Plin @ $0.25 /kWh $2628 $2628 $3504
Cost difference, annually 0 0 +$876
Cost per linear watt produced annually @ $0.25 per kWh $17.47 $9.90 $21.24
Weight 65 lbs 65 lbs 90 lbs
Volume 2394 sq in 2394 sq in 4151 sq. in.

Compare Ku-Band Amplifiers

Specification T07UO 750 W TWTA TL07UO 750 W SuperLinear TWTA w/lin. TL12UO 1.25 kW SuperLinear TWTA w/lin 800 W Competing SSPA
Linear output power 288.4 331.1 540 398.0
Typical power consumption at linear output power 2.3 kW 1.5 kW 2.2 kW 5.0 kW
Power efficiency 12.50% 22.07% 24.54% 8.00%
Power costs running at 24/7/365 at Plin @ $0.25 /kWh $5,037 $3,285 $4,818 $10,950
Cost difference annually +$1,752 - +$1,533 +$7,665
Cost per linear watt produced annually @ $0.25 /kWH $17.47 $9.90 $8.92 $27.51
Weight 79 lbs 55 lbs 80 lbs 176 lbs
Volume 3262 sq. in. 2206 sq. in 3262 sq. in 8730 sq. in.

References

  1. Manz, B. (2009). Advancing TWTs: The Traveling Wave Tube Lives On... And On. Advancing Technology TWT, 32 (7), 28-30.
    < www.manzcomm.com/page/PDFs/Byline/JED%20TWTs%20709.PDF >

  2. "Traveling-wave Tube." Wikipedia. Wikimedia Foundation, 06 Dec. 2012.
    < en.wikipedia.org/wiki/Traveling-wave_tube >

  3. "Microwave Hall of Fame Part III." Microwave Hall of Fame. N.p., n.d.
    < http://www.microwaves101.com/encyclopedia/halloffame3.cfm >

  4. TWTA versus SSPA: A New Look at Boeing Fleet On-Orbit Reliability Data and Comparison Factors
    E.F. Nicol, B.J. Mangus, M.K. De Pano

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