Why Use Harmonic Balance Analysis?

A) Time-Domain/Transient Analysis main advantage is its ability to handle very strong non-linearities in large circuits. Its robustness results in part from the fact that small time steps can be used in the time-domain integration. As long as the non-linearities are continuous, the time steps can always be made short enough so that the circuit voltages and currents change very little between steps.

B) Harmonic Balance Analysis used to complement conventional Transient Analysis Methods especially in:

1. Matching circuits contain elements such as dispersive transmission lines, transmission-line discontinuities, and multiport subnetworks described by S or Y parameters. These are difficult to analyze using the transient analysis. 
2. The circuit’s time constants may be large compared to the period of the fundamental excitation frequency. When long time constants exist, it becomes necessary to continue the numerical integration of the equations through many of excitation cycles (i.e. thousands), until the transient part of the response has decayed and only the steady-state part remains. The long integration will consume a large amount of time; furthermore numerical truncation errors in the long integration may become large and reduce the accuracy of the solution. Although algorithms exist to improve this difficulty but it is still complicated. 
3. Third, each linear or nonlinear reactive element in the circuit adds a differential equation to the set of equations that describes the circuit. A large circuit can have many reactive elements, so the set of equations that must be solved may be very large. For this reason, time-domain analysis is notoriously slow.

Push Pull Amplifier

Introduction

A push-pull amplifier consists of an input 0-180-degree power splitter driving two identical devices in antiphase and a 0-180-degree output power combiner adding the output power of the two devices in the amplifier load. This type of splitter and combiner, which are the key elements of the amplifier, are called baluns (BALanced UNbalanced). They transform a balanced system that is symmetrical with respect to ground to an unbalanced system with one side grounded. Note that the microwave push-pull amplifier is two independent devices each amplifying an individual signal of half the total power.

Typical Block Diagram:

Advantages:

1. Four times higher device impedance  (Zin Gate-to-Gate & Zout Drain-to-Drain) in comparison of a single-ended device impedances with the same output power. Thus it is easier to match.
2. Virtual ground, which can be used for more compact and simpler matching structures.
3. Cancellation of even products and harmonics, such as f2 - f1, 2f1, 2f2, f1 + f2, etc.
   

Disadvantages:

1. Poor input and output external match due to the fact that the baluns used for push-pull amplifiers do not eliminate the input and output power reflected by the device.
2. With conventional baluns, isolation between the two sides of the part is theoretically only 6dB; this poor interdevice isolation can cause instability problems.
3. Use of baluns: manually made coaxial baluns are simple to make for lab use but in production they require labor that makes mass production difficult. SMT baluns are available but add cost and tend to occupy more real estate than equivalent quadrature couplers.

 Comparison versus Balanced Configuration

1. Single tone performance should be equivalent. The external elements matching circuits and the splitters and combiners have similar loss. Note that both styles have different advantages in multi-octave amplifiers that do not come into play in this discussion of narrow band (5-10 %) commercial amplifiers. Due to the fact that even products and harmonics are out of the pass band of the matching circuits (internal and external) and the baluns, the cancellation of these products doesn’t exist and consequently for narrow band applications the push-pull configuration is not more efficient.
2. Linearity is the same for amplifiers with less than an octave in bandwidth. Matching circuits (internal and external) are filtering the even products and harmonics for the push-pull configuration and the products and harmonics attenuated or cancelled by the balanced configuration before they reach the output combiner. The output balun and quadrature coupler have limited band and in general cannot cancel these products and harmonics that are out of their pass band.
3. Push-pull configuration has an impedance transformation ratio advantage of two for conventional baluns. This can make design easier, depending on the impedance to be matched.
4. Balanced amplifiers have a significant external match advantage.
5. Balanced amplifiers are more stable due to the good isolation between the two device sides.
6. The virtual ground present for the push-pull configuration can be used to advantage with lumped tuning capacitors between the two sides to make for fast tuning.
7. Both configurations can be tuned using open stubs, which are preferred in production to lumped capacitors for their lower cost (free and no assembly), lower loss, ease to model and their power handling capability.

How WiMAX System Change the Modulation Techniques?

WiMAX system adjust or change the modulation used based on the radio link quality/signal strength; in other words based on SNR:

1. When the quality of radio link is very good, the highest modulation scheme (the most complex) is used, which translates into greater bandwidth capacity to the system.
2. In contras, when the signal quality is degrade, the system will shift to a lower order of modulation scheme in order to retain the connection quality instead of the data bandwidth.
   

for example:

1. 22 dB of SNR = 64QAM modulation is used.
   
2. 16 db of SNR = 16QAM modulation is used.
3. 9dB of SNR = QPSK modulation is used.
4. 6dB of SNR = BPSK modulation is used.

Semiconductor Material & Fabrication Process for RF Transistors

Most of new RF & Microwave Engineer still confused with RF Semiconductor Materials & Process Technology and can't really differentiates them well.

Semiconductor Materials for example:

1. Silicon (Si)
   
2. Silicon Germanium (SiGe)
3. Gallium Arsenide (GaAs)
4. Gallium Nitride (GaN)
5. Silicon Carbide (SiC)

Transistor Fabrication Process for example:

1. Metal-Semiconductor FET (MESFET)
2. Bipolar Junction Transistor (BJT)
   
3. Heterojunction Bipolar Transistor (HBT)
4. Laterally Diffused Metal-Oxide Semiconductor (LDMOS)
5. High Electron Mobility Transistor (HEMT)
6. Pseudomorphic HEMT (pHEMT)
7. Enhanced-Mode pHEMT (E-pHEMT)
   

Five Common Methods for LNA Stabilization

1. Input resistive loading: One major and obvious drawback - degrades NF of the LNA and is almost never used.
2. Output resistive loading: Should be carefully used because the effects are lower Gain and lower OP1dB.
3. RLC feedback at the Collector to Base: This is to lower the gain at the lower frequencies and therefore improve the stability.
4. Filter matching network at the transistor's output: The objective is to decrease the gain at a specific frequency or narrowband. This method is frequently used for eliminating gain at high frequencies, far above the operating band. Short circuit quarter wave lines designed for problematic frequencies, or simple capacitors with the same resonant frequency as the frequency of oscillation (or excessive gain) can be used to stabilize the circuit.
5. Emitter feedback inductor: A small inductor/inductance can stabilize the circuit at higher frequencies. However, excessive source inductance will cause the K-factor at higher frequencies falls bellow unity. This effect limits the amount of source inductance that can safely be used.

Precautions When Measuring IMD Products

Background:

1. Whenever measuring distortion products, remember that the instrumentation contains analog components.
2. This analog elements are subject to produce the exact same symptoms that we are trying to measure (harmonics and IMD products) when they are driven into their nonlinear regions of operation.
   
3. The IMD generated by the instruments will add to the measured response from our device and give us an incorrect reading.

A simple step to check whether the instrument is being overdriven are:

1. Manually increase the input attenuation and observe the levels of the intermodulation products.
   
2. If the intermodulation products do not change, then the instrument was not being overdriven and we can restore the attenuation to the previous setting.
   
3. If the instrument was generating intermodulation products before we raised the attenuation, then we would observe that the intermodulation products are reduced after we change the attenuation.
   
4. Measurements should be taken with the attenuation set so that the intermodulation products do not change when we increase it.
   

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