Signal Identification

Figure 7-14. Block diagram of spectrum analyzer and external mixer with built-in preselector

Signal identification
Even when using an unpreselected mixer in a controlled situation, there
are times when we must contend with unknown signals. In such cases, it is
quite possible that the particular response we have tuned onto the display
has been generated on an LO harmonic or mixing mode other than the one
for which the display is calibrated. So our analyzer must have some way
to tell us whether or not the display is calibrated for the signal response in
question. For the purposes of this example, assume that we are using an
Agilent 11970V 50 to 75 GHz unpreselected mixer, which uses the 14 mixing
mode. A portion of this millimeter band can be seen in Figure 7-15.

The Agilent E4407B ESA-E spectrum analyzer offers two different
identification methods: Image shift and Image suppress . We shall first
consider the image shift method. Looking at Figure 7-16, let's assume that
we have tuned the analyzer to a frequency of 58.5 GHz. The 14th harmonic
of the LO produces a pair of responses, where the 14 mixing product appears
on screen at the correct frequency of 58.5 GHz, while the 14 + mixing product
produces a response with an indicated frequency of 57.8572 GHz, which is
2 times f IF below the real response. Since the ESA has an IF frequency of
321.4 MHz, the pair of responses is separated by 642.8 MHz.