SPECTRUM ANALYZER; RF attenuator, Low-pass filter or preselector

RF attenuator
The first part of our analyzer is the RF input attenuator. Its purpose is to ensure the signal enters the mixer at the optimum level to prevent overload, gain compression, and distortion. Because attenuation is a protective circuit for the analyzer, it is usually set automatically, based on the reference level.However, manual selection of attenuation is also available in steps of 10, 5, 2, or even 1 dB. The diagram below is an example of an attenuator circuit with a maximum attenuation of 70 dB in increments of 2 dB. The blocking capacitor is used to prevent the analyzer from being damaged by a DC signal or a DC offset of the signal. Unfortunately, it also attenuates low frequency signals and increases the minimum useable start frequency of the analyzer to 100 Hz for some analyzers, 9 kHz for others.

In some analyzers, an amplitude reference signal can be connected as shown in Figure 2-3. It provides a precise frequency and amplitude signal, used by the analyzer to periodically self-calibrate.

Figure 2-3. RF input attenuator circuitry

Low-pass filter or preselector

The low-pass filter blocks high frequency signals from reaching the mix r. This prevents out-of-band signals from mixing with the local oscillator and creating unwanted responses at the IF. Microwave spectrum analyzers replace the low-pass filter with a preselector, which is a tunable filter that rejects all frequencies except those that we currently wish to view. In Chapter 7, we will go into more detail about the operation and purpose of filtering the input.