Those who have worked as receivers and spectrum analyzers should know zero frequency suppression. In fact, many users should have seen the phenomenon of poor zero frequency suppression, but they may not know what it is. When using the spectrum analyzer, set the starting frequency to 0 or the lowest frequency point of the instrument and the ending frequency to 100kHz or a relatively small value after startup. There will be a wave peak falling phenomenon at the very low frequency position as shown in the figure below, which is zero frequency amplitude. Many spectrum analyzers use swept superheterodyne architecture, which is bound to produce zero frequency.

In short, zero frequency is the false response signal displayed at the 0Hz frequency point on the spectrum display window of the spectrometer. If the amplitude of the false response signal is very high, it will have a serious impact on the dynamic range of the test low-frequency signal and the lower frequency limit that can be tested by the spectrum analyzer. It can be said that every spectrum analyzer has this phenomenon, but it is just to say how much range the amplitude value will be controlled within. The probable cause is caused by the LO leakage of the receiving mixer in the spectrum analyzer. The leakage determines the size of its zero frequency signal. The greater the leakage, the smaller the measurement dynamic range of the low-frequency signal of the spectrum analyzer, so the zero frequency must be suppressed.

Basic modules of spectrum analyzer

The basic modules of the spectrum analyzer are as follows: firstly, the spectrum analyzer is generally divided into two (or more) high and low frequency bands, which are divided according to the functional modules, mainly including RF front-end, mixing, first local oscillator, second local oscillator, third local oscillator, self calibration signal, reference internal and external synchronization.

Taking the low-frequency band as an example, when the low-frequency signal starts to work through the first mixer, for example, when the signal below 100MHz is input to the equipment, due to the limited isolation of the mixer, the corresponding first LO signal will leak into the first if channel, and the first IF filter cannot filter the LO signal. In order to avoid the impact of LO leakage signal on subsequent circuits, reduce the zero frequency, Therefore, it will be coupled from the local oscillator to the first intermediate frequency, and its phase will be adjusted to offset the local oscillator signal in the first intermediate frequency. Roughly adjust the amplitude and phase of the lo coupled signal, so that the amplitude of the signal is the same as that of the cancelled signal, and the phase is opposite, so as to achieve the purpose of local oscillator suppression.

After RF and lo mixing, it is called the first intermediate frequency. The phase control unit is realized by phase shifter, most of which are varactor diodes.

The first local oscillator is generated by VCO, keeps the phase consistent through coupling and phase discrimination, and is sent to the first mixer as the local oscillator signal.

Editor in charge: PJ

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