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TWTAs as compared to SSPAs

Started by Lucky_12 5 years ago5 replieslatest reply 5 years ago421 views

I have a few ( I assume very basic) questions about high frequency amplifiers in satellite communication. 

So, the traveling-wave tube is an amplifier which can be used from 3 GHz upwards. It covers a bandwidth of about 0.8 GHz. Well, what does this mean? When I consider a sine wave as input signal, does this mean, only sine-waves of frequencies not further apart than 0.8 GHz can  be amplified? And why can a TWT only amplify signals of frequencies greater than 3 GHz ? 

Also, solid-state amplifiers are common amplifiers in satellite communication as well. In short, what are the main advantages/disadvantages of a TWT as compared to the SSPA ? 

And last, what possibilites are there to analyze the output power of a TWTA ? I mean, measurements, obviously. But what are the most common simulation tools used ? 

I hope anybody knows some good answers. I also hope this is the correct forum to post these questions. 

cheers, 

Lucky_12

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Reply by drmikeJanuary 29, 2017

Traveling wave tubes have been around for a very long time.  They stay around because they can get to very high power levels.  Here is a recent article you might find interesting from L-com. Here is another article about a GaN replacement for a TWT.  It takes a lot of amplifiers in parallel to do what 1 TWT can do.  Think about jitter and cross talk - what kinds of problems will you have there?  A TWT is a vacuum device - how do you maintain the vacuum (if you are not half way to the moon)?  There are always advantages and problems to everything, each application needs to figure out what is most important.  You can not say a TWT is always better for some given problem - it really depends on the environment, duty cycle and lots of other things.

Books on TWT's are older than I am, and you can probably find some for 50 cents.  It would be worth your time to read up on the details - the basics have not changed in 70 years.  The frequencies and power levels have, but that is why they are still in use.  Besides, it is only 1 step away from a particle accelerator, and you can't do that with solid state!

Dr. mike

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Reply by soferJanuary 29, 2017

So, let me try to explain:

1. TWTA - Traveling-Wave Tube Amplifier is usually built using Microwave frequency generating tubes and can range in frequency from a few GHz up to 100 GHz and more, depending on the required output power. Those at higher frequencies, like X-band (~10 GHz) are more common than those at lower frequencies. TWTAs can have an operating frequency range of a few GHz and not as you described. TWTAs of "regular" 10% bandwidth exist like 10-12 GHz and also wide-band amplifiers like 6-18 GHz also exist. As a general rule the wider the bandwidth the lower is the output power.

2. SSPA - Solid-State power amplifier is usually built using RF/Microwave transistors that use semiconductor technologies like GaAs, GaN for high power (>30 dBm) devices and Si, SiGe and other semiconductors for low power (<20 dBm).

Some basic comparison between the two technologies:

* TWTAs require a high-voltage power supply, making it very dangerous. Thus, the PSU needs to be sealed so as not to allow accidental operator contact. These PSUs are specialized and not many people know how to make them. SSPAs usually operate on few common voltages that are typical of RF power transistors like 28V, 50V.

* For high power TWTAs usually employ one tube. SSPAs have to combine many RF power transistors to reach very high-power. As the frequency rises the RF power of the basic transistor building block is lower. In S-band you can find RF transistors of 200 W and higher while in X-band you will find only devices of 10-40W and at Ka-band you will find only devices of a few Watts.

* TWTAs are less reliable because you have to operate them periodically to ensure they still work and it is likely slowly burning a match. A high-power microwave tube has a short and limited life span and every time you operate it it gets shorter and shorter until it fails. Operating life is hundreds of hours, depending on the exact operating conditions. SSPAs on the other hand, use semiconductor devices that have very high MTBF in the millions of hours and can operate for many years without the need to be turned on periodically.

* Production: With the exception of tubes made for microwave ovens, where millions are sold each year and a big effort has been made to produce them using mass-production techniques and very low cost, high-frequency microwave tubes are somewhat custom, low-volume products which are very labor intensive. Very few companies have the knowledge and expertise to still make them. SSPAs are using solid state transistors. Many companies that make semiconductors have the knowledge to manufacture these devices and they are easily mass-produced like any other semiconductor.

* Power: If you need 1MW transmitter your only option will be to use TWTA, like for FM radio, some types of radars, etc. It would be impractical to combine so many RF transistors to reach these power levels. The loss on the combining network alone will make this exercise futile.

There are many more things to consider but this is it in a nutshell.

You also asked about parameters to be measured: Both types of amplifiers are compared using similar parameters like: Output power in terms of P1dB. For satellite communication linearity is also important so you would measure IP3 or in more complex systems you would use a modulated signal and measure EVM (which would better represent the actual expected performance). There is much more.

If you would be more specific maybe I can provide more detailed info. 

Why are you interested?

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Reply by shafie7January 29, 2017

Hello Lucky_12,

Sounds like everyone else has already answered your questions.

Just wanted to add that by BW, that means the amplifier (or any other device) meets the required performance for that frequency band.  For instance, in your case 3GHz+/- 400MHz or 3GHz to 3.8GHz.

By required performance, that depends on the user or whomever defined the BW.  Some typical and important parameters could be, Gain, Output Power, Adjacent Channel Power Rejection, input and output return loss, and so on.  There are large number of parameters, and the user specify the requirements for each parameter over the band of interest.  In other words, the amplifier that has 0.8GHz bandwidth to you, may be 0.4GHz bandwidth to me, it all depends on our requirements for that device.

Best regards,

Shahram

https://ortenga.com/

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Reply by Tim WescottJanuary 29, 2017

Folks have answered your questions about the technologies fairly well.

On bandwidth, an amplifier is usually specified for it's useful bandwidth, so yes, if it's got a bandwidth of 1GHz centered on 10GHz, then you can send frequencies from 9.5 to 10.5GHz.  Such specifications are always a bit fuzzy at the edges -- you can usually push the bandwidth out with reduced performance at the band edges, or get better performance by only using the stuff close to the center.  Plus, bandwidth is what the manufacturer defines as bandwidth, so they can start playing specsmanship games.

Note that you aren't limited to sine waves: if you have a 1GHz bandwidth, you can send a single 1GHz wide signal through it (assuming that specsmanship games aren't being played too hard).

The reason that TWT amps below 3GHz aren't available is because the size of the tube scales with wavelength.  You could, in theory, have a 3MHz TWT -- but if it was made in the typical way you'd need a mile-long building to keep it in.

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Reply by SlartibartfastJanuary 29, 2017

One point that hasn't yet been mentioned (aside from the excellent comments so far), is that TWTAs have historically often been run in saturation for efficiency, which requires constant-envelope signals.   In communication systems, e.g., a small terminal, this means that constant-envelope signalling such as FM or MSK should be used for maximum power efficiency.  Likewise radar systems often used constant-envelope waveforms with TWTs.

While SSPAs often also behave like Class C amplifiers, they are generally more amenable to Class A operation as well, at least more so than TWTs.

Amplifier technology has changed quite a bit over the last decade or two, and many more options are available now than when saturated TWTs ruled the world.