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In general, RF amplifiers demonstrate a characteristic that their gain decreases with frequency as shown in Graph 1 below. Likewise, RF Cables and Passive components have a characteristic that involves insertion loss and frequency, but their insertion loss increases with frequency as shown in Graph 2 below. Those characteristics can pose significant risk when used without compensation in Satellite Communication Systems.
[Graph 1] Example of gain characteristic of RF amplifiers (reference: https://d2lxe0fofddnat.cloudfront.net/BT09AG.pdf)
[Graph 2] Example of insertion loss characteristic of Passive Components (Directional Coupler)
(reference: Mini-Circuits Directional Coupler)
Additionally, you can observe that the communication between systems gets worse when the gain slope in those systems are tilted more heavily by large amounts of amplifiers like the curve shown in Graph 3. The communication also loses quality as a wider operating frequency bandwidth is used in systems.
[Graph 3] Example of gain characteristic of a RF equipment whose amplifiers are lined up in cascade
2. Types of Gain Equalizers
RF Gain Equalizers can be categorized into several types, for example, by operating frequency range, power supply requirement, or the shape of gain slope.
2-1. Categorizing Gain Equalizers by Operating Frequency Range
Gain Equalizers can be typically categorized into L-Band Gain Equalizers and Microwave Gain Equalizers by the frequency range they operate in. The L-Band Gain Equalizers are typically used in 950 MHz – 2.15 GHz frequency range, and they are typically implemented using lumped elements and have a Low Order BPF (Band Pass Filter) gain characteristic as shown in Graph 4.
[Graph 4] Example of gain characteristic of a L-Band Gain Equalizer
On the other hand, Microwave Gain Equalizers are typically used in 2GHz – 20GHz frequency range, and they are implemented using various transmission lines and have a Low Order BPF (Band Pass Filter) gain characteristic as shown in Graph 5.
[Graph 5] Example of gain characteristic of a Microwave Gain Equalizer (reference: Gain Equalizers from Knowles)
2-2. Categorizing Gain Equalizers by Power Supply Requirement
Gain Equalizers can be categorized into Active Gain Equalizers and Passive Gain Equalizers based on whether they require a power supply. For example, Gain Equalizers that require an external power source are called Active Gain Equalizers. Those that do not require an external power supply are called passive gain equalizers.
Active Gain Equalizers produce a positive gain characteristic because they use an amplifier to amplify the signals it receives. On the other hand, Passive Gain Equalizers produce an insertion loss characteristic because they do not use an amplifier.
2-3. Categorizing Gain Equalizers by the Shape of their Gain Slope
Gain Equalizers can be categorized into Positive Slope Gain Equalizers, Negative Slope Gain Equalizers, Positive Parabolic Gain Equalizers, Negative Parabolic Gain Equalizers by the shape of their gain slope.
3. Examples of Gain Equalizer Applications
Illustration 1 shows an example of a SATCOM Earth Station where Negative Slope Passive Gain Equalizers are applied.
Transmit Path: The gain slope that became tilted more heavily downward while the RF signal was traversing the Up Converter (2) and HPA (High Power Amplifier 3) gets compensated by the Negative Gain Equalizer (1) between the MODEM and Up Converter to achieve a flat gain slope.
Receive Path: The gain slope that became tilted more heavily downward while the RF signal was traversing the LNA (Low Noise Amplifier 5) and the Down Converter (6) gets compensated by the Negative Gain Equalizer (7) between the Down Converter and the MODEM to achieve a flat gain slope.
OMT: Ortho-Mode Transducer, HPA: High Power Amplifier, LNA: Low Noise Amplifier
[Illustration 1] Example of Negative Gain Equalizer Applications in SATCOM Earth Station
4. Performance Evaluation Criteria of Gain Equalizers
Four parameters exist that are mainly considered to evaluate the performance of Gain Equalizers: Slope Linearity, VSWR, Insertion Loss, and Slope Compensation Range.
[Graph 6] Evaluation parameters of Gain Equalizers (reference: http://rf.mrcy.com/RF_Components/linear_equalizers.html)
Slope Linearity: Slope Linearity is defined as the deviation in dB between the measured curve and the Best Fit Straight Line as shown in Graph 6. Gain Equalizers whose measured curve deviates little from the Best Fit Straight Line are evaluated as superior. Gain Equalizers designed by professionals will provide linearity within ±0.75 dB max. (±0.5dB typ.).
VSWR: VSWR is defined as the 50 Ohms matching characteristic of Gain Equalizers. Gain Equalizers designed by professionals will provide VSWR within 1.67:1 max. (1.5:1 typ.).
Insertion Loss: Gain Equalizers with little insertion loss at the highest frequency within their operating frequency range are evaluated as superior. Gain Equalizers designed by professionals will provide insertion loss within 1.75dB max. (1.0dB typ.).
Slope Compensation Range: Slope Compensation Range is defined as the range in dB in which Gain Equalizers compensate for gain characteristics. Gain Equalizers with more inclusive range are evaluated as better than others. Gain Equalizers designed by professionals will provide slope compensation range between 1 dB to 12 dB.
5. More Applications of L-Band Gain Equalizers
L-Band Gain Equalizers are used in the following applications:
A gain equalizer is simply a circuit that “equalizes’, or flattens, the sloping gain of a circuit. There are a variety of different types of gain equalizers, a common circuit type is a frequency variable attenuator, which equalizes gain by having a slope which complements the sloping gain of the original circuit.
For example, a RF or microwave amplifier will often have a gain versus frequency response that drops toward higher frequencies. This is especially true for wideband amplifiers with high gain, such as traveling wave tube amplifiers (TWTA) and high powered solid state amplifiers. If additional gain is needed, often two or more RF/microwave amplifiers will be cascaded, which may further deepen the undesirable slope in the gain versus frequency response.
A simple way of correcting this is to use an attenuator with a frequency response that slopes downward toward lower frequencies and presents less attenuation at higher frequencies. If the attenuation slope compliments the slope of the amplifier(s), then the overall frequency response of the combined circuits will be relatively flat. Some gain will be sacrificed to achieve flatness, so this method only works with applications that can afford the tradeoff of gain for gain flatness. There are also active gain equalizer circuits. However, active gain equalizers tend to be larger, more complex, and more expensive circuits that passive gain equalizers, which can be purchased in surface mount technology (SMT) packages similar in scale to SMT capacitors or inductors.
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