Specifications

96
External harmonic mixing
We have discussed tuning to higher frequencies within the spectrum analyzer.
For internal harmonic mixing, the ESA and PSA spectrum analyzers use the
second harmonic (N=2
–
) to tune to 13.2 GHz, and the fourth harmonic (N=4
–
)
to tune to 26.5 GHz. However, what if you want to test outside the upper
frequency range of the spectrum analyzer? Some spectrum analyzers provide
the ability to bypass the internal first mixer and preselector and use an
external mixer to enable the spectrum analyzer to make high frequency
measurements
2
. For external mixing we can use higher harmonics of the
1
st
LO. Typically, a spectrum analyzer that supports external mixing has two
additional connectors on the front panel. An LO OUT port routes the internal
first LO signal to the external mixer, which uses the higher harmonics to mix
with the high frequency signals. The external mixer’s IF output connects to
the analyzer’s IF IN port. As long as the external mixer uses the same
IF frequency as the spectrum analyzer, the signal can be processed and
displayed internally, just like any signal that came from the internal first
mixer. Figure 7-13 illustrates the block diagram of an external mixer used
in conjunction with a spectrum analyzer.
3 GHz
3 - 7 GHz
Sweep
generator
Display
3.9214 GHz
Analog or
digital IF
321.4 MHz 21.4 MHz
3.6 GHz
321.4 MHz
Input
signal
External harmonic mixer
Preselector
300 MHz
Atten
Input
signal
Figure 7-13. Spectrum analyzer and external mixer block diagram
2. For more information on external mixing, see
Agilent Application Note 1485, External Waveguide
Mixing and Millimeter Wave Measurements with
Agilent PSA Spectrum Analyzers, literature number
5988-9414EN.