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Appendix III
The notch filter must be very deep and narrow. Notch depth assures that no energy at the fundamental
frequency "leaks" to the meter. Narrow notch is required to leave noise and harmonics intact. How steep
should the notch be? For theoretical tones one may strive for the steepest notch possible. Real
applications require full attenuation over a slightly wider frequency range to accommodate possible small
jitter of both sampling clock and the tone itself. Model 3000S provides sufficient attenuation over about
.1% (allowing about 20 nsec jitter).
Additional 20 Hz high pass filtering ensures that very low frequency components (such as DC and other
low frequency inaudible energy) does not alter the outcome.
Model 3000S begins its measurement by locking to the test tone frequency. There are 2 modes (user
selectable) for achieving lock:
• Auto mode: Proper locking in the presence of noise and distortion requires "reasonable" signal to noise
and distortion ratio. When using Auto mode we recommend starting the test with a large enough signal
(full-scale signals are ideal, but locking will take place at 40 dB over the noise and distortions floor).
Pressing the "Go" button sets the notch in place, allowing THD+N measurements at any level.
• Normal mode: when using Model 3000S tone generator, the notch frequency follows the tone frequency
settings. Normal mode frees the lock mechanism from any signal to noise restrictions. This mode requires
the tone sample rate and incoming signal sample rate to match within +/- .1% of each other (testing
Sample rate converters requires Auto mode).
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LE3000S
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