Additional Mixing Stages

To separate closely spaced signals (see Resolving signals later in this
chapter), some spectrum analyzers have IF bandwidths as narrow as 1 kHz;
others, 10 Hz; still others, 1 Hz. Such narrow filters are difficult to achieve
at a center frequency of 3.9 GHz. So we must add additional mixing stages,
typically two to four stages, to down-convert from the first to the final IF.
Figure 2-5 shows a possible IF chain based on the architecture of a typical
spectrum analyzer. The full tuning equation for this analyzer is:

Figure 2-5. Most spectrum analyzers use two to four mixing steps to reach the final IF

So simplifying the tuning equation by using just the first IF leads us to the
same answers. Although only passive filters are shown in the diagram, the
actual implementation includes amplification in the narrower IF stages.
The final IF section contains additional components, such as logarithmic
amplifiers or analog to digital converters, depending on the design of the
particular analyzer.

Most RF spectrum analyzers allow an LO frequency as low as, and even below,
the first IF. Because there is finite isolation between the LO and IF ports of
the mixer, the LO appears at the mixer output. When the LO equals the IF,
the LO signal itself is processed by the system and appears as a response
on the display, as if it were an input signal at 0 Hz. This response, called LO
feedthrough, can mask very low frequency signals, so not all analyzers allow
the display range to include 0 Hz.