The most commonly used spectrum analyzer is a scan-tuned spectrum analyzer. The variable-variable local oscillator generates a linearly varying oscillation frequency over time via a scan generator synchronized with the CRT, which is mixed with the input signal by the mixer. After the frequency, the intermediate frequency signal (IF) is amplified, filtered and detected and transmitted to the vertical direction board of the CRT. Therefore, the vertical axis of the CRT shows the relationship between the amplitude of the signal and the frequency. The signal flow architecture is shown in Figure 1.3.
An important part of the signal response is the filter bandwidth. The filter is characterized by a Gaussian-Shaped Filter. The function of the influence is the resolution bandwidth (RBW) commonly used in measurement. RBW represents the lowest bandwidth difference that can be clearly distinguished by two different frequency signals. The signal bandwidth of two different frequencies is lower than the RBW of the spectrum analyzer. At this time, the two signals will overlap, which is difficult to distinguish and lower. The RBW is helpful for the resolution and measurement of different frequency signals. The low RBW will filter out the higher frequency signal components, resulting in distortion when the signal is displayed. The distortion value is closely related to the set RBW. The higher RBW is of course It helps to detect the broadband signal, which will increase the Noise Floor and reduce the measurement sensitivity. It is easy to detect the low-intensity signal. Therefore, the proper RBW width is an important concept for the correct use of the spectrum analyzer.
The additional video bandwidth (VBW, Video Bandwidth) represents the minimum bandwidth required for a single signal to be displayed on the screen. As explained above, when the signal is measured, the video bandwidth is too low or too low, which will cause measurement troubles. How to adjust must be understood. Generally, the bandwidth of the RBW is greater than or equal to VBW, and the RBW is adjusted without a significant change in the signal amplitude. At this time, the RBW bandwidth can be used. When measuring the RF video carrier, the signal is down-converted by the internal mixer of the device and then amplified, filtered (RBW decision) and detected. If the scan is too fast, the RBW filter will not be fully charged to the amplitude peak of the signal. Therefore, sufficient scanning time must be maintained, and the width of the RBW is interactive with the scanning time. The RBW is larger and the scanning time is faster, and vice versa. The selection of the appropriate width of the RBW thus shows its importance. A wider RBW is more responsive to the waveform and amplitude of the input signal, but a lower RBW will be able to distinguish signals at different frequencies. For example, for measurement of 6MHz bandwidth video channel, it is known that when the RBW is 300kHz and 3MHz, the peak amplitude of the carrier does not change significantly. The video signal of 6MHz is usually selected with RBW of 300kHz to reduce noise. When the antenna signal is measured, the spectrum analyzer's spread spectrum (Span) uses 100MHz to obtain a wider signal spectrum requirement, and the RBW uses 3MHz. These measurement parameters are not static and will be adjusted based on site conditions and past measurement experience.