RF预算分析的目的是检查限幅放大器中不同测试点的宽带频率响应和RF功率水平。 The analysis must be completed to correct for the worst-case operating temperature, gain slope, and wide RF input power range.必须完成分析，以纠正最坏情况下的工作温度，增益斜率和较宽的RF输入功率范围。
那么，谁知道什么是RF预算分析？
The basic layout of a limiting amplifier with a 40 dB limiting dynamic range is a cascade of four gain block amplifiers or LNAs.极限动态范围为1 dB的极限放大器的基本布局是四个增益块放大器或LNA的级联。 The ideal design uses only one or two dedicated amplifier devices to reduce power variation at different frequencies and minimize thermal/slope compensation requirements.理想的设计仅使用一个或两个专用放大器设备，以减少不同频率下的功率变化，并使热/斜率补偿要求降至最低。 Figure XNUMX shows the block diagram of the first initial limiting amplifiers before temperature correction and slope compensation.图XNUMX显示了温度校正和斜率补偿之前的第一个初始限幅放大器的框图。
图1.初步设计的框图
先来一个小的好处，推荐一种技术来完成宽带限幅放大器的设计：
1.管理极限功率动态范围并消除RF过驱动条件
2.在温度范围内优化性能
3.最后，校正电源衰减并展平小信号增益
4.可能需要进行最后的次要校正，即在将频率均衡功能纳入设计后，重新考虑温度补偿
功率限制
The main problem with the preliminary design shown in Figure 1 is that as the RF input power increases, RF overdrive is likely to occur at the output gain stage.图XNUMX所示的初步设计的主要问题在于，随着RF输入功率的增加，在输出增益级可能会发生RF过驱动。 When the saturated output power of any gain stage exceeds the absolute maximum input of the next amplifier in the queue, RF overdrive will occur.当任何增益级的饱和输出功率超过队列中下一个放大器的绝对最大输入时，就会发生RF过驱动。 In addition, the design is prone to VSWR-related ripples, and oscillations are likely to occur due to the high undamped gain in the small RF package.此外，该设计易于产生与VSWR相关的纹波，并且由于小型RF封装中的高无衰减增益，很可能发生振荡。
In order to prevent RF overdrive, eliminate VSWR effects and reduce the risk of oscillation, a fixed attenuator can be added between each gain stage to reduce power and gain.为了防止RF过驱动，消除VSWR效应并降低发生振荡的风险，可以在每个增益级之间添加一个固定的衰减器以降低功率和增益。 An RF absorber may also be required on the RF cover to eliminate oscillations.在RF盖上可能还需要一个RF吸收器以消除振荡。 Sufficient attenuation is needed to reduce the maximum input power of each gain stage below the rated input power level of the MMIC.需要足够的衰减以将每个增益级的最大输入功率减小到MMIC的额定输入功率水平以下。 Sufficient attenuation must be included to accommodate the top input power margin, to accommodate temperature changes and differences between devices.必须包括足够的衰减以适应最高输入功率裕度，以适应温度变化和器件之间的差异。 Figure 2 shows where the RF attenuator is needed in the limiting amplifier chain.图XNUMX显示了限制放大器链中需要RF衰减器的位置。
图2. RF过驱动校正框图
ADI的宽带限制放大器HMC7891使用四个HMC462增益级，以使工作范围达到10 dBm。 The absolute maximum input power is 15 dBm.绝对最大输入功率为18 dBm。 Each gain stage can tolerate a maximum RF input of 17 dBm.每个增益级可以承受3 dBm的最大RF输入。 Following the design steps outlined in the previous paragraph, an attenuator has been added between the two gain stages to ensure that the maximum amplifier input power level does not exceed XNUMX dBm.按照上一段概述的设计步骤，在两个增益级之间添加了一个衰减器，以确保最大放大器输入功率电平不超过XNUMX dBm。 Figure XNUMX shows the maximum power level at the input of each gain stage when a fixed attenuator is added to the design.图XNUMX显示了在设计中添加固定衰减器后每个增益级输入端的最大功率电平。
图6. HMC7891在整个温度范围内模拟的小信号增益
频率均衡
This compensates for the natural gain roll-off in most broadband amplifiers.这补偿了大多数宽带放大器中的自然增益衰减。 There are various equalizer designs, including passive GaAs MMIC chips.有各种均衡器设计，包括无源GaAs MMIC芯片。 Passive MMIC equalizers are small in size and have no DC and control signal requirements, so they are very suitable for limiting amplifier design.无源MMIC均衡器体积小，没有直流和控制信号的要求，因此非常适合于限制放大器的设计。 The number of frequency equalizers required depends on the uncompensated gain slope of the limiting amplifier and the response of the selected equalizer.所需的频率均衡器数量取决于限幅放大器的未补偿增益斜率和所选均衡器的响应。 A design recommendation is to slightly overcompensate the frequency response to offset transmission line loss and connector loss, as well as package parasitics that have a greater impact on gain at higher frequencies.设计建议是对补偿传输线损耗和连接器损耗以及对较高频率下的增益有较大影响的封装寄生效应的频率响应稍有过度补偿。 Figure 7 shows the test results of the custom ADI GaAs frequency equalizer.图XNUMX显示了定制ADI GaAs频率均衡器的测试结果。
图7.测得的频率均衡器损耗
ADI的HMC7891限幅放大器需要三个频率均衡器来校正热补偿的小信号响应。 Figure 8 shows the simulation results of HMC7891 after thermal compensation and frequency equalization.图XNUMX显示了热补偿和频率均衡后的HMCXNUMX的仿真结果。 Deciding where to insert the equalizer is critical to a successful design.决定在哪里插入均衡器对于成功设计至关重要。 Before adding any equalizers, remember that an ideal limiting amplifier should evenly distribute the maximum amplifier compression between all gain stages to avoid excessive saturation.在添加任何均衡器之前，请记住，理想的限幅放大器应在所有增益级之间平均分配最大的放大器压缩，以避免过度饱和。 In other words, in the worst case, each MMIC should compress equally.换句话说，在最坏的情况下，每个MMIC应该均等压缩。
图8. HMC7891仿真频率均衡在整个温度范围内的小信号增益
In the current design stage shown in Figure 5, an equalizer connected in series with the Thermopad attenuator can be added at the input of the device to replace the fixed attenuator at the output of the device.在图XNUMX所示的当前设计阶段，可以在设备的输入端添加与Thermopad衰减器串联的均衡器，以替换设备的输出端的固定衰减器。 Why did you do this?你为什么这么做？ Four reasons四个原因
1. Adding an equalizer to the input of the limiting amplifier will reduce the power of the first gain stage.将一个均衡器添加到限幅放大器的输入将降低第一增益级的功率。 Therefore, the compression of level 1 is reduced.因此，降低了级别XNUMX的压缩。 The reduction in gain stage compression is equivalent to the reduction in limiting dynamic range.增益级压缩的减少等同于极限动态范围的减少。 In addition, due to the attenuation slope of the equalizer, the limiting dynamic range is dispersed in the frequency range.另外，由于均衡器的衰减斜率，限制动态范围分散在频率范围内。 The lower the frequency, the more the dynamic range is reduced.频率越低，动态范围越减少。 To compensate for the reduced limiting dynamic range, the RF input power must be increased.为了补偿减小的极限动态范围，必须增加RF输入功率。 However, due to the slope of the equalizer, an uneven increase in input power will increase the risk of overdrive of the amplifier gain stage.但是，由于均衡器的斜率，输入功率的不均匀增加将增加放大器增益级过驱动的风险。 It is possible to add an equalizer to the input of the device, but this is not the ideal location.可以在设备的输入端添加一个均衡器，但这不是理想的位置。
2.添加与Thermopad串联连接的均衡器将减少后续放大器的压缩。 This will result in uneven distribution of amplifier compression between gain stages, reducing the overall limiting dynamic range.这将导致增益级之间放大器压缩的分布不均匀，从而减小了整体极限动态范围。 It is not recommended to connect the equalizer in series with the Thermopad attenuator.不建议将均衡器与Thermopad衰减器串联。
3.使用一个或多个均衡器代替固定衰减器只会改变输出级放大器的压缩水平。 To minimize this variation and avoid RF overdrive, the equalizer loss should be roughly equal to the fixed attenuation value removed from the system.为了最大程度地减少这种变化并避免RF过驱动，均衡器损耗应大致等于从系统中去除的固定衰减值。 In addition, as mentioned above, adding an equalizer before the gain stage will result in a dispersion of the limiting dynamic range and frequency.另外，如上所述，在增益级之前添加均衡器将导致极限动态范围和频率的分散。 To minimize this effect, replace as few equalizers as possible.为了最大程度地减小这种影响，请更换尽可能少的均衡器。
4.均衡器可以添加到设备的输出中。 Output equalization will reduce the output power, but will not produce limiting dynamic range dispersion.输出均衡将降低输出功率，但不会产生限制的动态范围色散。 Output equalization produces a slightly positive output power slope, but this slope is offset by high-frequency packaging and connector losses.输出均衡会产生一个略为正的输出功率斜率，但该斜率会被高频封装和连接器损耗所抵消。
完成的四级限幅放大器布局如图9所示。
图9.频率均衡框图
Figure 10 shows the output power and temperature simulation results of ADI HMC7891.图40显示了ADI HMC3的输出功率和温度仿真结果。 The final design achieved a limiting dynamic range of XNUMX dB.最终设计实现了XNUMX dB的极限动态范围。 Under all operating conditions, the simulated worst-case output power change was XNUMX dB.在所有工作条件下，模拟的最坏情况下的输出功率变化为XNUMX dB。
