When measuring high frequency signal, the heterodyne spectrum analyzer can obtain higher sensitivity because of amplification after mixing, and can easily change the resolution of frequency by changing the frequency bandwidth of the IF filter. However, because the superheterodyne spectrum analyzer scans in the frequency band, it is not necessary to scan. The time tends to zero and the real-time response of the input signal can not be obtained. Therefore, in order to obtain a superheterodyne spectrum analyzer with the same performance as the real-time analyzer, the scanning speed is very fast. If the scanning time is smaller than the time constant of the IF filter, the correct vibration of the signal can not be obtained. Amplitude, therefore, in order to improve the frequency resolution of the spectrum analyzer, and to obtain accurate response, it is necessary to have an appropriate scanning speed. From the above description, we can see that the superheterodyne spectrum analyzer can not analyze the spectrum of transient signal or impulse signal, and its main application is to test the spectrum of periodic signal and other stray signal. The characteristics of the internal and panel display of the spectrum analyzer system, as described in Appendix 1, will be helpful to the operation and use of the spectrum analyzer. I n general, the frequency of the output signal of the local oscillator is higher than that of the intermediate frequency signal. The frequency of the output signal of the local oscillator can be adjusted to the frequency of harmonics, that is, in = n?? LO *? I F n = 1,2,3....
It is known from the model (2) that the signal measurement range of the spectrum analyzer has been broadened invisibly, and the input signal which is lower or higher than the local oscillator or other harmonic frequencies can be mixed to produce intermediate frequency. The mixing principle of extending the input signal frequency, in which the longitudinal axis represents the input signal (? IN), the horizontal axis represents the local oscillation frequency (? LO), and the positive and negative integers in the figure represent the positive and negative sign corresponding to the IF amplifier.
The principle of spectrum analyzer extending input signal frequency by local oscillating harmonic signal can be realized. However, it may correspond to multiple input signal frequencies. To eliminate this phenomenon, a frequency preselector is added in front of the attenuator to enhance the dynamic range of the spectrum analyzer, while the output results can eliminate other unnecessary frequencies and truly reflect the frequency of the input signal.