Rohde & Schwarz R&S FSL3 - Page 28

R&S FSL Measurement of Harmonics
1300.2519.12 2.3 E-11
As shown in Fig. 2-1, the level of the 2
n
d
harmonic drops by 20 dB if the level of the fundamental wave
i
s reduced by 10 dB.
-50
-30
-20 0-10 10 20 30 40 50
-20
-10
0
10
1
30
40
50
RF level
/ dBm
-40
-60
-70
-80
Level display
/
dBm
2nd harmonic
intercept point /
dBm
-30
1st harmonic
2nd harmonic
1
1
2
Fig. 2-1 Extrapolation of the 1st and 2nd harmonics to the 2nd harmonic intercept at 40 dBm
The following formula for the obtainable harmonic distortion d
2
in dB is derived from the straight–line
equations and the given intercept point:
d
2
= S.H.I – P
I
(1)
d
2
= harmonic distortion
P
I
= mixer level/dBm
S.H.I. = second harmonic intercept
Note: The mixer level is the RF level applied to the RF input minus the set RF attenuation.
The formula for the internally generated level P
1
at the 2
nd
harmonic in dBm is:
P
1
= 2  P
I
– S.H.I. (2)
The lower measurement limit for the harmonic is the noise floor of the spectrum analyzer. The harmonic
of the measured DUT should – if sufficiently averaged by means of a video filter – be at least 4 dB
above the noise floor so that the measurement error due to the input noise is less than 1 dB.
The following rules for measuring high harmonic ratios can be derived:
 Select the smallest possible IF bandwidth for a minimal noise floor.
 Select an RF attenuation which is high enough to just measure the harmonic ratio.
The maximum harmonic distortion is obtained if the level of the harmonic equals the intrinsic noise level
of the receiver. The level applied to the mixer, according to (2), is:
2
2/ IPdBmP
P
noise
I
+
=
(3)
At a resolution bandwidth of 10 Hz (noise level –143 dBm, S.H.I. = 40 dBm), the optimum mixer level is
– 51.5 dBm. According to (1) a maximum measurable harmonic distortion of 91.5 dB minus a minimum
S/N ratio of 4 dB is obtained.