Specifications

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.

Low-pass filter or preselector

The low-pass filter blocks high frequency signals from reaching the mixer.

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.

Tuning the analyzer

We need to know how to tune our spectrum analyzer to the desired frequency

range. Tuning is a function of the center frequency of the IF filter, the

frequency range of the LO, and the range of frequencies allowed to reach

the mixer from the outside world (allowed to pass through the low-pass filter).

Of all the mixing products emerging from the mixer, the two with the greatest

amplitudes, and therefore the most desirable, are those created from the sum

of the LO and input signal and from the difference between the LO and input

signal. If we can arrange things so that the signal we wish to examine is either

above or below the LO frequency by the IF, then one of the desired mixing

products will fall within the pass-band of the IF filter and be detected to

create an amplitude response on the display.

RF input

Amplitude

reference

signal

0 to 70 dB, 2 dB steps

Figure 2-3. RF input attenuator circuitry