Specifications

Following the input filter are the mixer and the local oscillator, both of

which add to the frequency response uncertainty. Figure 4-2 illustrates what

the frequency response might look like in one frequency band. Frequency

response is usually specified as ± x dB relative to the midpoint between the

extremes. The frequency response of a spectrum analyzer represents the

overall system performance resulting from the flatness characteristics and

interactions of individual components in the signal path up to and including

the first mixer. Microwave spectrum analyzers use more than one frequency

band to go above 3 GHz. This is done by using a higher harmonic of the local

oscillator, which will be discussed in detail in Chapter 7. When making

relative measurements between signals in different frequency bands, you

must add the frequency response of each band to determine the overall

frequency response uncertainty. In addition, some spectrum analyzers

have a band switching uncertainty which must be added to the overall

measurement uncertainty.

After the input signal is converted to an IF, it passes through the IF gain

amplifier and IF attenuator which are adjusted to compensate for changes in

the RF attenuator setting and mixer conversion loss. Input signal amplitudes

are thus referenced to the top line of the graticule on the display, known as the

reference level. The IF amplifier and attenuator only work at one frequency

and, therefore, do not contribute to frequency response. However, there is

always some amplitude uncertainty introduced by how accurately they can be

set to a desired value. This uncertainty is known as reference level accuracy.

Another parameter that we might change during the course of a measurement

is resolution bandwidth. Different filters have different insertion losses.

Generally, we see the greatest difference when switching between LC filters

(typically used for the wider resolution bandwidths) and crystal filters

(used for narrow bandwidths). This results in resolution bandwidth

switching uncertainty.

The most common way to display signals on a spectrum analyzer is to use a

logarithmic amplitude scale, such as 10 dB per div or 1 dB per div. Therefore,

the IF signal usually passes through a log amplifier. The gain characteristic

of the log amplifier approximates a logarithmic curve. So any deviation from

a perfect logarithmic response adds to the amplitude uncertainty. Similarly,

when the spectrum analyzer is in linear mode, the linear amplifiers do not

have a perfect linear response. This type of uncertainty is called display

scale fidelity.

Frequency response

- 0.5 dB

+0.5 dB

BAND 1

Specification: ±0.5 dB

Signals in the same harmonic band

Figure 4-2. Relative frequency response in a single band