Stanford Research Systems DS360 Operating Manual And Programming Reference page 99

Distortion Measurements
The notch filters described here are used with a spectrum analyzer for verifying the distortion
performance of the DS360. Since there are few, if any, spectrum analyzers with a -120 dB
distortion floor, a pre-filter is necessary to attenuate the fundamental frequency and increase the
sensitivity to distortion signals. Since the pre-filter attenuates the fundamental component far
more than any of the harmonics, the distortion products can be viewed using a spectrum analyzer
with a more reasonable -80 dB distortion floor. The figure below shows a "twin-tee" notch filter.
Filter Center Frequency
1.0 kHz
11.0 kHz
31.0 kHz
It is important to use high quality resistors and capacitors in the twin-tee notch filter, or distortion
measurements will be limited by its performance. Thin film resistors and npo ceramic, or
polystyrene capacitors should be used.
The "twin-tee" filter attenuates harmonic components as well as the fundamental, although not
nearly as much. The harmonics are attenuated by:
To obtain the true harmonic distortion, these values must be added to the readings obtained from
the spectrum analyzer. In addition, the amplitude of the fundamental must be taken into account.
For example, if the 2nd harmonic of a 10 V
of the 2nd harmonic would be -123 dB (112 dB + 20 dB - 9 dB).
2R
C
2C
Figure 6-1 Twin-Tee Notch Filter
Filter Component Values
Resistor "R" Value
7.87 K ± 1%
7.15 K ± 1%
2.55 K ± 1%
Harmonic Attenuation
2nd
3rd
4th
5th
>5th
signal is measured at -112 dBV
RMS
DS360 Ultra Low Distortion Function Generator
Performance Tests
2R
C
R
Capacitor "C" Value
0.01 µF ± 1%
0.001 µF ± 1%
0.001 µF ± 1%
-9 dB
-5 dB
-3 dB
-2 dB
∼0 dB
6-5
, the true value
RMS