Agilent Spectrum Analysis Basics - Agilent Technologies
Agilent Spectrum Analysis Basics - Agilent Technologies
Agilent Spectrum Analysis Basics - Agilent Technologies
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Other situations can create out-of-band multiple responses. For example,<br />
suppose we are looking at a 5 GHz signal in band 1 that has a significant third<br />
harmonic at 15 GHz (band 3). In addition to the expected multiple pair caused<br />
by the 5 GHz signal on the 1 + and 1 – tuning curves, we also get responses<br />
generated by the 15 GHz signal on the 4 + , 4 – , 3 + ,and 3 – tuning curves. Since<br />
these responses occur when the LO is tuned to 3.675, 3.825, 4.9, and 5.1 GHz<br />
respectively, the display will show signals that appear to be located at 3.375,<br />
3.525, 4.6, and 4.8 GHz. This is shown in Figure 7-6.<br />
30<br />
4+<br />
4–<br />
25<br />
Band 4<br />
3+<br />
Signal frequency (GHz)<br />
20<br />
15<br />
Out-of-band<br />
multiple responses<br />
3–<br />
2+<br />
2–<br />
Band 3<br />
10<br />
Band 2<br />
1+<br />
5<br />
1–<br />
Band 1<br />
0<br />
3<br />
3.675 3.825 4.7 4.9 5.1 5.3<br />
4<br />
5<br />
LO frequency (GHz)<br />
6<br />
7<br />
Band 0<br />
(lowband)<br />
Figure 7-6. Out-of-band multiple responses in band 1 as a result of a signal in band 3<br />
Multiple responses generally always come in pairs 1 , with a “plus” mixing<br />
product and a “minus” mixing product. When we use the correct harmonic<br />
mixing number for a given tuning band, the responses will be separated<br />
by 2 times f IF . Because the slope of each pair of tuning curves increases<br />
linearly with the harmonic number N, the multiple pairs caused by any<br />
other harmonic mixing number appear to be separated by:<br />
1. Often referred to as an “image pair.” This is<br />
inaccurate terminology, since images are actually<br />
two or more real signals present at the spectrum<br />
analyzer input that produce an IF response at the<br />
same LO frequency.<br />
where<br />
2f IF (N c /N A )<br />
N c = the correct harmonic number for the desired tuning band<br />
N A = the actual harmonic number generating the multiple pair<br />
88