Instrument Functions
R&S
®
FSVA/FSV
413Operating Manual 1176.7510.02 ─ 09
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)
where:
d2 = harmonic distortion
P
I
= mixer level/dBm
S.H.I. = second harmonic intercept
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 signal 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:
At a resolution bandwidth of 10 Hz (noise level -143 dBm, S.H.I. = 40 dBm), the opti-
mum mixer level is – 51.5 dBm. According to (1) a maximum measurable harmonic dis-
tortion of 91.5 dB minus a minimum S/N ratio of 4 dB is obtained.
If the harmonic emerges from noise sufficiently (approx. >15 dB), it is easy to check (by
changing the RF attenuation) whether the harmonics originate from the DUT or are
generated internally by the signal analyzer. If a harmonic originates from the DUT, its
level remains constant if the RF attenuation is increased by 10 dB. Only the displayed
noise is increased by 10 dB due to the additional attenuation. If the harmonic is exclu-
sively generated by the signal analyzer, the level of the harmonic is reduced by 20 dB
or is lost in noise. If both – the DUT and the signal analyzer – contribute to the har-
monic, the reduction in the harmonic level is correspondingly smaller.
Measurement Functions
To Next Page
Loading...
