Correctly set the spectrum analyzer bandwidth

In measuring some CATV system indicators, spectrum analyzers are often used. In order to make the measurement results accurate, the use of spectrum analyzers often involves the problem of setting the resolution bandwidth. To understand this problem, you need to know some basic principles of spectrum analyzers. Figure 1 is a basic principle block diagram of a spectrum analyzer. The intermediate frequency in the figure (the input signal is generated by the sum frequency or difference frequency with the local oscillator signal) and the local oscillator are controlled by the ramp generator. Under the control of the ramp generator, the local oscillator frequency will change linearly from low to high. In this way, when displaying, the ramp voltage generated by the ramp generator is added to the X-axis of the display, and the detector output is connected to the Y-axis after passing through the low-pass filter. When the ramp generator scans the local oscillator frequency, the spectrum of the input signal will be automatically drawn on the display. The low-pass filter at the output of the detector is called a video filter, which is used to smooth the response during analysis scanning.

1. Resolution bandwidth

In spectrum analyzers, frequency resolution is a very important concept. It is determined by the bandwidth of the intermediate frequency filter, which determines the resolution bandwidth of the instrument. For example, if the bandwidth of the filter is 100KHZ, then the frequency of the spectrum line will have an uncertainty of 100KHZ, that is, if two spectrum lines appear within the bandwidth frequency range of a filter, the instrument cannot detect these two spectrum lines, but only displays one spectrum line. At this time, the spectrum line level (power) reflected by the instrument is the superposition of the level power of these two spectrum lines. Therefore, measurement errors will occur. Therefore, for two closely related spectrum lines, their resolution depends on the bandwidth of the filter.

We take the measurement of carrier level as an example to compare the resolution bandwidth settings of the instrument. Figure 2 shows the spectrum curves of resolution bandwidths of 30KHZ, 300KHZ, and 3MHZ (from bottom to top) (the input is a single carrier signal). When setting the resolution bandwidth, we consider whether the instrument has enough bandwidth to fully respond to the input signal. The correct method is to widen the bandwidth of the filter. When the signal carrier amplitude is no longer increased on the screen, it means that the intermediate frequency filter has enough bandwidth to respond to the input signal. In the figure, we can see that when the resolution bandwidth changes from 300KHZ to 3MHZ, the displayed signal amplitude does not change, which means that the 300KHZ bandwidth is sufficient. In addition, when the resolution bandwidth is set between 300KHZ and 3MHZ, the signal amplitude does not change for a single carrier. However, when actually measuring the image carrier level of the CATV system, the resolution bandwidth cannot be set to 3MHZ. This is because in reality there are sound carriers of adjacent channels near the image carrier (1.5MHZ apart), and the 3MHZ bandwidth cannot filter out the energy of the adjacent sound carrier. In this way, the energy of the adjacent sound carrier will be added to the image carrier being measured, making the measured level higher than the actual value.

2. Video Filters

The filter after the detector in Figure 1 is called a detector filter or video filter. It is a low-pass filter that reduces noise variations in the detector output, reveals some signals that have been masked and are close to the background noise, and helps stabilize the measurement if the noise power is being measured.

检波器输出端往往存在直流分量和交流分量，直流分量代表着中频带宽内存在的能量，所以通过视频滤波器可达到提取直流分量去除一些交流分量，这样能给出更稳定的无噪声输出。图3是不同视频带宽下，检波器输出的信号图，图3a采用宽带视频滤波器，图3b采用窄带视频滤波器，由图中可看出，采用宽带滤波器时噪声的波动较大，采用窄带滤波器时波动显著减少，两者的噪声平均值却一样，也就是说滤波器不会降低平均噪声电平，但能减少噪声的峰值电平。因而能暴露出用较宽视频滤波器不能看到的低电平信号。但在某些情况下，如分析一些特殊的噪声信号时，我们则需要较宽的视频滤波器带宽，以便观察和分析，所以我们可根据不同的情况来设置视频滤波器的带宽。

The relationship between the bandwidth of the video filter and the resolution bandwidth is: the noise before detection can be reduced by a narrower resolution bandwidth, thereby reducing the noise output level of the detector; the noise after detection is smoothed and reduced by a narrowband video filter to reduce noise fluctuations, but the average power level of the noise cannot be reduced.