Designing For Minimal Noise

Keeping noise levels low in an active RF/microwave circuit requires a great deal of diligence, not to mention careful choices in transistors, circuit-board materials, and even in the placement of the circuit within the system.

Minimizing noise in an active circuit, such as an amplifier or an oscillator, is an art. Of course, it helps to start with a low-noise active device, such as a discrete transistor or monolithic-microwave-integrated-circuit (MMIC) device. But achieving a low noise figure for a target set of frequencies involves much more than just inserting such a device into a circuit. The matching input and output impedances to the active device are critical for low noise.

Noise is usually more critical at the receiver end of a system, where signals are lower and the noise is more likely to mask the desired signal. Noise can be generated by a number of sources, including the receiver's own RF/microwave components. Once it has blended with the desired signals, especially if the noise is broadband in nature, it is difficult (if not impossible) to remove from the receiver's signal-processing chain. An analog receiver's sensitivity is specified in terms of the signal-to-noise ratio (SNR) that it can process. For receivers that rely on digital signal processing (DSP), any noise present with the desired signal will be captured and digitized by an analog-to-digital converter (ADC), and the receiver's bit-error-rate (BER) performance will suffer.

In addition to an active device in a circuit, everything connected to it and the ways in which the connections are made must be considered when minimizing noise. The excellent application note (57-1) from Agilent Technologies, "Fundamentals of RF and Microwave Noise Figure Measurements," details some of the forms of noise and their origins, such as shot and thermal noise. Such forms of noise are dependent not only on operating conditions, but also on material characteristics. For example, when designing a low-noise amplifier (LNA), the choice of active device is critical. But almost as important is the selection of the printed-circuit-board (PCB) material, since a low-cost material like FR-4 may suffer high circuit losses and contribute to a higher noise figure for the LNA. In addition, the PCB's conductive layer should be specified for lowest possible loss performance and highest conductivity.

Ideally, an active circuit designed for low noise would begin with the lowest-noise transistor, but that choice is often steered by other factors. For example, designers of YIG oscillators have long achieved extremely low phase noise at frequencies to about 20 GHz by using silicon bipolar transistors as the active devices. For higher-frequency operation, however, signals from a YIG resonator are usually boosted by a GaAs field-effect transistor (FET), with phase-noise performance that is typically 10 dB or worse than that possible with a silicon bipolar.

The quests for low noise figures in LNAs and low phase noise in RF and microwave oscillators are ongoing. They have been buttressed by continuing improvements in computer-aided-engineer (CAE) programs, which allow engineers to simulate amplifier and oscillator performance under different sets of theoretical matching circuits. For a transistor oscillator to achieve low phase noise, for example, the maximum amount of energy from the resonator or resonant circuit must be coupled to the input port of the transistor under optimum impedance conditions for the desired range of frequencies. The wider that range of frequencies, the more difficult it becomes to achieve impedance conditions that are optimum at all frequencies; typically, some compromise is necessary. Even then, to maintain low phase noise, the energy at the output of the oscillator transistor must then be optimally impedance matched to the next component in the signal chain, which quite often is a microwave mixer's local oscillator (LO) port in the case of a high-frequency receiver.

For this example of the oscillator, it is clear that achieving active RF/microwave components takes careful design and painstaking effort. Even when evaluating an LNA for noise with a noise-figure meter, or an oscillator for phase noise with a phase-noise analyzer or spectrum analyzer, the impedance match between the device under test (DUT), if not optimized, can affect the accuracy of the measured results. Care is required at all stages of design of a low-noise active component.

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