Out-of-band Multiple Responses

Other situations can create out-of-band multiple responses. For example,
suppose we are looking at a 5 GHz signal in band 1 that has a significant third
harmonic at 15 GHz (band 3). In addition to the expected multiple pair caused
by the 5 GHz signal on the 1 + and 1 tuning curves, we also get responses
generated by the 15 GHz signal on the 4 + , 4 - , 3 + , and 3 - tuning curves. Since
these responses occur when the LO is tuned to 3.675, 3.825, 4.9, and 5.1 GHz
respectively, the display will show signals that appear to be located at 3.375,
3.525, 4.6, and 4.8 GHz. This is shown in Figure 7-6.

Figure 7-6. Out-of-band multiple responses in band 1 as a result of a signal in band 3

Multiple responses generally always come in pairs 1, with a plus mixing
product and a minus mixing product. When we use the correct harmonic
mixing number for a given tuning band, the responses will be separated
by 2 times f IF . Because the slope of each pair of tuning curves increases
linearly with the harmonic number N, the multiple pairs caused by any
other harmonic mixing number appear to be separated by:

1. Often referred to as an image pair. This is
inaccurate terminology, since images are actually
two or more real signals present at the spectrum
analyzer input that produce an IF response at the
same LO frequency.