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RF and Microwave Amplifiers

Technical Overviews

Microwave broadband signal amplification

  • Broadband performance up to 50 GHz, replacing several narrow band amplifiers, simplifies test setup and optimizes the operating range of your test systems
  • Excellent noise figure and high gain, significantly reduces overall test system noise figure
  • High output power, boosts available power for measurements

Introduction

The Keysight Technologies, Inc. 83006/017/018/020 /050 /051A and N4985A test system amplifiers offer ultra broadband performance up to 50 GHz. With excellent noise figure relative to their broad bandwidth and high gain, these products can be used to significantly reduce test system noise figure. By replacing several amplifiers with a single broadband product, test setups can be greatly simplified. You can place this amplification power where you need it by using remotely-locatable Keysight power supplies. In addition, the Keysight 87415A provides octave band performance from 2 to 8 GHz.

The Keysight 87405B/C and N4985A-S30/S50 low noise preamplifiers provide exceptional gain and flatness. The 87405B/C preamplifiers are very portable and come with a convenient probe-power bias connection which eliminates the need for an additional DC power supply, making them an ideal front-end preamplifier for a variety of Keysight instruments.

The N4985A-S30/50 system amplifiers are a high-performance broadband amplifier featuring baseband RF (> 100 kHz) through millimeter wave (> 30 GHz) frequency coverage. These amplifiers are designed to be a multi-use laboratory RF amplifier as a gain block for frequency domain applications, or as a time domain pulse amplifier. Its small size and versatile performance make it an excellent choice for general purpose gain block with moderate power output in a single package, potentially replacing two or three narrower-band amplifiers.

What Selection Criteria Do I Consider?

Today’s engineers are constantly seeking for amplifiers of exceptional gain and power performance over a broad bandwidth. There exists a very large number of potential electrical specifications that can be applied to a microwave power amplifier selection. These elements are defined by the following characteristics:

Frequency range

RF and microwave applications range in frequency from 100 MHz for semiconductor to 60 GHz for satellite communications. Broadband accessories increase test system flexibility by extending frequency coverage. However, frequency is always application dependent and a broad operating frequency may need to be sacrificed to meet other critical parameters.

Noise figure

Noise figure is the primary specification for a typical microwave power amplifier selection. The noise figure is defined as the ratio of the signal-to-noise power ratio at the input to the signal-to-noise power ratio at the output. The noise factor is thus the ratio of actual output noise to that which would remain if the device itself did not introduce noise, or the ratio of input SNR to output SNR.

Low noise amplifiers are always preferred as the noise figure of the system is dominated by the noise figure of the preamplifier. By adding a preamplifier to noise figure measurement systems, the total system noise figure can also be reduced.

For systems with a single preamplifier, where the gain of the preamplifier is greater than or equal to the spectrum analyzer noise figure, the system noise figure is approximately equal to the noise figure of the preamplifier.

Output power (Psat & P1dB)

Among the key specifications for microwave amplifiers are their power output specifications. Output power at Psat refers to the saturated output power, or maximum output power from the amplifier. This is the output power where the Pin/Pout curve slope goes to zero. Output power at P1dB refers to the output power during 1 dB compression point. Unlike the gain specification, implicitly it is assumed that the specification is at an operating point where the amplifier is exhibiting some degree of non-linear behavior. With an inherently broadband amplifier, power output as a function of power input does not vary discontinuously as a function of frequency. Typically, a wideband microwave power amplifier that could deliver in excess of several watts required a solution where numerous narrowband amplifiers were either multiplexed or switched; often introducing undesired issues, such as power curve discontinuities, at frequency cross-over points.

Gain

Gain usually is specified within the context of power output. Often, if no context for power output is given, then this is assumed to be small signal gain. Conditions for small signals at the input and output are usually easy to reproduce and verify, whereas gain and gain flatness can vary significantly when an amplifier approaches compression. Gain flatness for an amplifier with a significant frequency range is often specified over subsets of the entire frequency range. Gain and gain flatness typically include an implicit assumption that the reverse gain from the output to the input is negligible; i.e. the amplifier is unilateral.

Typically, gain flatness could only be achieved over narrow bandwidths with classic reactive matching techniques, such as those used for internally matched devices. Attempts to broaden the gain bandwidth of a high-power microwave amplifier requires trade-offs with resistive matching, or feedback techniques that take power output. The spatially combined topology overcomes these limitations.

Input and output return loss (VSWR)

The standing wave ratio, often referred to interchangeably as VSWR, is the result of wave interference. Peaks and troughs in a given field pattern remain in a static position as long as the sources of interference do not change with respect to each other. Return loss, expressed in dB, is a measure of voltage standing wave ratio (VSWR). Return loss is caused by impedance mismatch between circuits. At microwave frequencies, the material properties as well as the dimensions of a network element play a significant role in determining the impedance match or mismatch caused by the distributed effect. Keysight amplifiers guarantee excellent return loss performance by incorporating appropriate matching circuits to ensure optimum power transfer through the amplifier and the entire network.

Application Examples

Adding preamplifiers to measurement systems as shown in Figure 2 can improve sensitivity and reduce the noise floor when measuring low-level signals. By adding a preamplifier to noise figure measurement systems, the total system noise figure can also be reduced. The noise figure of the system is dominated by the noise figure of the preamplifier. For systems with a single preamplifier, where the gain of the preamplifier is greater than or equal to the spectrum analyzer noise figure, the system noise figure is approximately equal to the noise figure of the preamplifier.

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