Digital IF

Chapter 3
Digital IF Overview

Since the 1980's, one of the most profound areas of change in spectrum
analysis has been the application of digital technology to replace portions
of the instrument that had previously been implemented as analog circuits.
With the availability of high-performance analog-to-digital converters, the
latest spectrum analyzers digitize incoming signals much earlier in the signal
path compared to spectrum analyzer designs of just a few years ago. The
change has been most dramatic in the IF section of the spectrum analyzer.
Digital IFs 1 have had a great impact on spectrum analyzer performance, with
significant improvements in speed, accuracy, and the ability to measure
complex signals through the use of advanced DSP techniques.

Digital filters
A partial implementation of digital IF circuitry is implemented in the Agilent
ESA-E Series spectrum analyzers. While the 1 kHz and wider RBWs are
implemented with traditional analog LC and crystal filters, the narrowest
bandwidths (1 Hz to 300 Hz) are realized using digital techniques. As shown
in Figure 3-1, the linear analog signal is mixed down to an 8.5 kHz IF and
passed through a bandpass filter only 1 kHz wide. This IF signal is amplified,
then sampled at an 11.3 kHz rate and digitized.

Figure 3-1. Digital implementation of 1, 3, 10, 30, 100, and 300 Hz resolution filters in ESA-E Series

Once in digital form, the signal is put through a fast Fourier transform
algorithm. To transform the appropriate signal, the analyzer must be fixed-
tuned (not sweeping). That is, the transform must be done on a time-domain
signal. Thus the ESA-E Series analyzers step in 900 Hz increments, instead
of sweeping continuously, when we select one of the digital resolution
bandwidths. This stepped tuning can be seen on the display, which is
updated in 900 Hz increments as the digital processing is completed.

As we shall see in a moment, other spectrum analyzers, such as the PSA Series,
use an all-digital IF, implementing all resolution bandwidth filters digitally.

A key benefit of the digital processing done in these analyzers is a bandwidth
selectivity of about 4: 1. This selectivity is available on the narrowest filters,
the ones we would be choosing to separate the most closely spaced signals.

1. Strictly speaking, once a signal has been digitized,
it is no longer at an intermediate frequency, or IF.
At that point, the signal is represented by digital
data values. However, we use the term digital IF
to describe the digital processing that replaces the
analog IF processing found in traditional spectrum
analyzers.