Analysis of the Specifications of Digital IF and Analog IF Spectrum Analyzers

 

    1. The use of all-digital IF technology can measure smaller signals: By implementing an IF filter with a smaller bandwidth, the displayed average noise level is greatly reduced. The DSA1030A achieves a minimum RBW of 10Hz, and it is technically difficult to implement an analog filter with such a small bandwidth. When a smaller RBW is set, the background noise of the spectrum analyzer will also be reduced accordingly (VBW is selected as 10Hz, and the sweep time is selected as automatic). The reduction can be explained by formula (1). If the RBW is changed from 10kHz to 1kHz, the background noise will drop by 10dB. After the internal standard preamplifier of the DSA1030A is turned on, a 10Hz RBW is selected to measure signals up to -148dBm.

 

 

    Where: ΔdB is the change in low noise, in dB; BW1 is the resolution bandwidth value before modification, in Hz; BW2 is the resolution bandwidth value after modification, in Hz.

 

    2. The use of all-digital IF technology can distinguish closer signals: By implementing an IF filter with a smaller bandwidth, two signals with a frequency difference of only 10Hz can be distinguished. The minimum resolution bandwidth is an indicator used to illustrate the ability of the spectrum analyzer to distinguish two signals with similar amplitudes but a small frequency difference. The smaller the resolution bandwidth, the closer the signals can be distinguished. For example, if a two-tone signal with a frequency difference of 150Hz is input, it can be easily distinguished using a 30Hz RBW, but it cannot be done with a 3kHz RBW. The resolution bandwidth refers to the bandwidth at which the IF filter drops 3dB.

 

    In addition, the form factor of the digital filter can be made smaller, so that small signals that were previously buried under the skirt of large signals can be highlighted by using a more selective digital filter. Selectivity is the ratio of 60dB bandwidth to 3dB bandwidth. The DSA1030A's IF filter selectivity is 5, which is much better than the analog filter's 15. The DSA1030A with a digital filter can more easily distinguish two signals 9kHz apart and 30dB apart in amplitude.

 

    3. The use of all-digital IF technology can obtain higher-precision amplitude indicators: the amplitude error caused by many factors such as IF filter switching error, reference level uncertainty, scale distortion, amplitude log-linear switching error, etc. in traditional analog IF is almost eliminated, thereby obtaining higher full-amplitude accuracy. Modifying the reference level in the analog IF spectrum analyzer is achieved by adjusting the analog IF amplifier, and the scale fidelity is also limited by the logarithmic amplifier that acts as an envelope detector. In addition, the switching of the IF filter is achieved by selecting different analog filters. Since analog devices are affected by ambient temperature and other factors, amplitude errors will be caused in each link. However, digital IF greatly reduces and eliminates these error effects. The full-amplitude error of DSA1030A is less than 1dB.

 

    4. Using a fully digital IF can obtain a wider dynamic range and display range: After the digital IF spectrum analyzer converts the IF signal into a digital signal, the dynamic range is determined by the word length in the digital fixed-point processing process. As long as there are sufficient processing resources, a very high dynamic range can be obtained. The analog IF is restricted by the dynamic range of devices such as logarithmic amplifiers. Therefore, the analog IF spectrum analyzer usually only displays signals within the range of 80dB, while the digital IF spectrum analyzer can obtain a much larger measurement range and display range. The DSA1030A can set a display range of 200dB, and the measurement range reaches 130dB under the same setting.

 

    5. The use of all-digital intermediate frequency technology can achieve more stable performance: Compared with traditional analog intermediate frequency, it greatly reduces the use of analog devices, reduces the complexity of the hardware system, and also reduces the system instability caused by channel aging, temperature sensitivity, and device failure. The more complex the system, the more unstable it is. Hardware designers know that the simpler the board, the more stable it must be, because the fewer devices, the lower the probability of error, and the system stability depends on the stability of each device. With digital intermediate frequency, the complex device combination is programmed into the code solidified in the chip. After repeated testing and debugging, the program can work according to the designed program without worrying that a section of the code suddenly does not work properly.

 

    6. The use of all-digital intermediate frequency technology can achieve faster measurement speed: The use of digital intermediate frequency filter technology greatly improves the bandwidth accuracy and selectivity of the filter, reduces the response time, thereby greatly reducing the scanning time and improving the measurement speed (see formula 2).

 

 

    (2) Where: Span—the span of the current measurement setting, in Hz; RBW—the resolution bandwidth of the current setting, in Hz; VBW—the video bandwidth of the current setting, in Hz; KRBW—the proportionality factor of the transient response time of the current RBW; KVBW—the proportionality factor of the transient response time of the current VBW;

 

    The proportionality coefficient of the transient response time of RBW is related to the implementation of the spectrum analyzer's intermediate frequency filter. For analog intermediate frequency spectrum analyzers, the proportionality coefficient K is generally around 3 due to the slow response of crystals and LC filters. However, digital filters that use digital intermediate frequency solutions like DSA1030A can obtain a smaller proportionality coefficient K, and the proportionality coefficient of DSA1030A reaches 1, as shown in Figure 9. For example, if the spectrum analyzer span is set to 10MHz, RBW is set to 1kHz, and VBW is greater than 1kHz, the corresponding K value of DSA1030A is 1, and the scanning time is 10s; if the K value in the analog intermediate frequency is 3, it will take 30s.