Agilent Spectrum Analysis Basics - Agilent Technologies

Chapter 7
Extending the
Frequency Range
As more wireless services continue to be introduced and deployed, the
available spectrum becomes more and more crowded. Therefore, there has
been an ongoing trend toward developing new products and services at higher
frequencies. In addition, new microwave technologies continue to evolve,
driving the need for more measurement capability in the microwave bands.
Spectrum analyzer designers have responded by developing instruments
capable of directly tuning up to 50 GHz using a coaxial input. Even higher
frequencies can be measured using external mixing techniques. This chapter
describes the techniques used to enable tuning the spectrum analyzer to
such high frequencies.
Internal harmonic mixing
In Chapter 2, we described a single-range spectrum analyzer that tunes
to 3 GHz. Now we wish to tune higher in frequency. The most practical way
to achieve such an extended range is to use harmonic mixing.
But let us take one step at a time. In developing our tuning equation in
Chapter 2, we found that we needed the low-pass filter of Figure 2-1 to
prevent higher-frequency signals from reaching the mixer. The result was
a uniquely responding, single band analyzer that tuned to 3 GHz. Now we
wish to observe and measure higher-frequency signals, so we must remove
the low-pass filter.
Other factors that we explored in developing the tuning equation were the
choice of LO and intermediate frequencies. We decided that the IF should
not be within the band of interest because it created a hole in our tuning
range in which we could not make measurements. So we chose 3.9 GHz,
moving the IF above the highest tuning frequency of interest (3 GHz). Since
our new tuning range will be above 3 GHz, it seems logical to move the new IF to
a frequency below 3 GHz. A typical first IF for these higher frequency ranges
in Agilent spectrum analyzers is 321.4 MHz. We shall use this frequency in
our examples. In summary, for the low band, up to 3 GHz, our first IF is
3.9 GHz. For the upper frequency bands, we switch to a first IF of 321.4 MHz.
Note that in Figure 7-1 the second IF is already 321.4 MHz, so all we need to
do when we wish to tune to the higher ranges is bypass the first IF.
Atten
Low band
path
3 GHz
3.9214 GHz
321.4 MHz 21.4 MHz
Analog or
Digital IF
Input
signal
High
band path
3 - 7 GHz
To
external
mixer
3.6 GHz
300 MHz
321.4 MHz
Preselector
Sweep generator
Display
Figure 7-1. Switching arrangement for low band and high bands
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