Harmonic Mixing Modes with External Mixers

Table 7-1 shows the harmonic mixing modes used by the ESA and PSA at
various millimeter wave bands. You choose the mixer depending on the
frequency range you need. Typically, these are standard waveguide bands.
There are two kinds of external harmonic mixers; those with preselection
and those without. Agilent offers unpreselected mixers in six frequency bands:
18 to 26.5 GHz, 26.5 to 40 GHz, 33 to 50 GHz, 40 to 60 GHz, 50 to 75 GHz,
and 75 to 110 GHz. Agilent also offers four preselected mixers up to 75 GHz.
Above 110 GHz, mixers are available from other commercial manufacturers
for operation up to 325 GHz.

Some external mixers from other manufacturers require a bias current to set
the mixer diodes to the proper operating point. The ESA and PSA spectrum
analyzers can provide up to Â± 10 mA of DC current through the IF OUT port
to provide this bias and keep the measurement setup as simple as possible.

Table 7-1. Harmonic mixing modes used by ESA-E and PSA Series with external mixers
Band Harmonic mixing mode

Whether performing harmonic mixing with an internal or an external mixer,
the issues are similar. The LO and its harmonics mix not only with the RF
input signal, but any other signal that may be present at the input as well.
This produces mixing products that can be processed through the IF just
like any other valid signals. There are two ways to deal with these unwanted
signals. A preselector designed into the external mixer will offer you the same
type of tunable filter, as in the spectrum analyzer, for the frequency band of
interest. Figure 7-14 shows a spectrum analyzer and an external mixer with
internal preselection. The benefits and drawbacks of a preselected external
mixer are very similar to those for the preselector inside the spectrum
analyzer. The most significant drawback of preselected mixers is the
increased insertion loss due to the filter, resulting in lower sensitivity for
the measurement. Preselected mixers are also significantly more expensive
than unpreselected mixers. For these reasons, another way to deal with
these unwanted signals has been designed into the spectrum analyzer. This
function is called signal identification.