Amplifier Specification Guide
The amplifier is one of many important components in your satellite link. Due to differences in measurement techniques, it is important to ensure that you when you are comparing two amplifiers, you are comparing them on the right data. Amplifiers are commonly measured in watts, but there are various ways to measure the watts, and the watts that you can use, or the Usable Linear Power (ULP) is a not a simple number.
Where to Start
Output Power, Frequency Output and Input Range, Power Consumption, Weight, Size and Price. You can get most of this information from our website, and the accompanying datasheets. For most amplifiers, the manufacturers datasheets provided details on the amplifier specifications and this is where it gets confusing. In the following paragraphs, we're going to describe what all that data means and how to interpret it.
The Performance that Matters : Usable Linear Power (ULP)
The output power that you can use is typically defined as the linear power. Linear Power is a fundamental characteristic of an amplifier where Pin (Power In) = Pout (Power Out). Unfortunately, this is not always listed.
Measuring the linear power is complicated and like most things, the result you get depends on how you measure it. So when I told one smart engineer that linear power was the important metric, his response was "OK, define linear."
Consequences of Non-Linear operation
When we get out of the "linear" range of amplifier, we run into a couple of problems: Splatter and Distortion.
Also known as Adjacent Channel Interference, the result of splatter is that you are jamming the users channels on either side of you. You will likely get a phone call from someone (the satellite operator) because someone (the user whose channel you are jamming) called them. The green and red carriers below represent varying degrees of splatter, the blue carrier is non-distorted.
When an amplifier saturates, it will generate Intermodulation Distortion. One result of this can be seen in the constellation graph below. Turning up the signal more way will not help your receiver lock, you need to turn it down. The problem isn’t the signal, it’s the noise. The constellation represented below is a four phase signal (QPSK) and it is easy to imagine how linearity becomes increasingly important as we get to higher order modulations like 16, 32, and 64 phase and beyond.
Two Tone Amplifier Measurement
Everyone understands that there are different amplifier rating techniques and then talks about linear power as the holy grail of measurement but the next question to ask is “Define Linear.” Next you may hear something like our amplifiers meet MIL-STD-188-164B which sounds great since it is a Military specification but this is exactly where you get into trouble. Unfortunately, comparing linear power is complicated since different people measure it different ways.
In the 1960s, engineers were concerned with the linearity in radio broadcast applications and employed a method of measuring against a single carrier in an equal two-carrier configuration. As the satellite communications industry developed, the methods evolved to evaluate TWTA and Klystron Power Amplifiers, and this method continues to be used today.
In this method, two tones are injected into an amplifier and then the tester measures the distortion products the amplifier generates. The difference between the two tones, or the IMD is the result : "X dBc", or -24 dBc in the case of Intelsat Specs
This method is defined as:
-23 dBc or better with two equal carriers at total output power of 7 dB below rated saturated carrier output.
Anywhere from -18 dBc to -28 dBc is typically provided as the testing point and the actual power in watts or dBm is often used in place of how much backoff is required.
Over time, the military developed separate standards which were formalized in 1989. MIL-STD-188-164 defines linearity as meeting both a spectral regrowth and intermodulation specification as follows:
The maximum combined transmit power of two equal amplitude continuous wave (CW) carriers, when the third order intermodulation product power of -25 dB relative to the power of the two CW carriers.
For Spectral Regrowth
The single carrier maximum-linear power equals the carrier power when the power spectral density in the modulated carrier sidelobes, 1.5 times symbol rate removed from the carrier center frequency, is -30 dB relative to the power spectral density at the carrier center frequency.
The military standard for intermodulation in the United States is measured against the sum of two carriers, rather than against a single carrier and hence the military and commercial comparisons are different. The same amplifier that meets -25 dBc at a certain power level with regards to two carriers (the military method) typically only meets -22 dBc with regard to either of two equal carriers (the commercial method). In order for this amplifier to meet -25 dBc with regard to each of two carriers, output power would have to be lowered approximately 1.5 dB since there is roughly 1:2 ratio of increase in power vs. increase in intermodulation products.
Starting in the late 1980’s, SSPA manufacturers started to introduce HPAs to compete against already established TWTAs for total power output and adopted the military intermodulation standard. An SSPA datasheet will typically read something like :
-25 dBc or better at 3 dB output backoff from P1dB with two carriers
SSPA manufacturers were then claiming in some cases that linear power was achieved in their products at a much higher power level than TWTA’s could provide. While technically correct, it is also a bit misleading. For example: a 400W TWTA provides 350W of CW output power at the flange (55.44 dBm). Using the traditional commercial method of stating intermodulation, this amplifier meets the -23 dBc threshold for output linearity at 78 watts. An SSPA manufacturer could point to this and claim that their 200W (Psat) rated amplifier achieves linearity at 78 watts. But in reality, if you measure the TWTA using the military method of measuring intermodulation, the TWTA produces 95 watts and if you add a linearizer, it produces 190 watts.
Fortunately, most amplifier manufacturers have begun to make themselves more consistent and clear in how they measure their amps and what their measurements mean. Although some are stating that their products meet the MIL-STD-188-164B standard at a specified output power, others are spelling it out more clearly stating something like :
PLinear is the power at which the IMD=-25dBc for two CW signals 5 MHz apart and the spectral regrowth is <-30 dBc @ 1.0 x symbol rate for a single QPSK/OQPSK/8PSK signal.
So now this increase in the verbage and technical terms should sound straightforward put you at ease. However, it is really just a spectral regrowth measurement at -30 dBc and since the spectral regrowth value is almost always more difficult to meet than the -25 dBc intermodulation specification, the diligent engineer now has to go through an extra calculation to figure out at what power the HPA achieves linearity in terms of intermodulation products. Typically this can be estimated by either adding 0.5 dB to this number to achieve the linearity when defined with the regard to the “sum of two carriers”, or by decreasing the output power by 1.0 dB to achieve linearity with regard to “of two carriers”.
Commercial vs. MIL-STD-188-164 Two Tone Measurement
The comparison below is a great example of the confusion surrounding amplifier measurement. In the olden days, we just had to compare PSAT vs. P1dB but the introduction of the Military Standard in comparison to the commercial standard is misleading and by rating your amplifiers in this way, you could be thinking you have more power than you actually do. Chances are that you amplifier is oversized anyways so you may not operate at that maximum output power very often but when you do, you may get into a distortion situation, or as your network grows, your amp may not be able to keep up.
The standard commercial convention is to compare the level of one individual intermodulation product to the level of one of the two CW Carriers as shown below:
MIL-STD-188-164 compares the level of one of the individual intermodulation product to the combined power, or 3 dB above a single carrier, of the two CW Carriers as shown below.
Thus a -25 dBc MIL-STD-188-164 Linear Power Level would be more like a -22 dBc standard intermodulation definition.
Noise Power Ratio Measurement
Used frequently by broadcasters, Noise Power Ratio is considered the ultimate measure of an amplifier's ability to handle multi-carrier signals.
How it Works
White Noise is injected into the amplifier emulating the presences of many carriers of random amplitude and phase. A deep notch is created in the center of the noise pedestal before the noise enters the amplifier. The Intermod created fills up the notch and the remaining notch depth is the Noise Power Ratio (NPR) of the amplifier.
Measured NPR of a 2nd Generation 200W Ku-Band SSPA using a 500 MHz whide Noise Spectrum.
Noise Power Ratio is a much more accurate characteristic of many carriers spread across the entire 500 MHz bandwidth of the amplifier.