The filter tuning circuit also has hardware nulling which will slow down
dramatically for inputs below about 25 mV, regardless of the notch
steering mode.
GEN-TRACK mode is generally faster than AUTO-TUNE, especially
during sweeps. It can be used to tune to very low-amplitude or very
distorted signals. However, it always tunes to the current generator
frequency, so it can only be used on signals that originate from the
local generator.
FIX-TUNE mode can be used if only a single-frequency measurement
is required. This is generally used to test single-tone quantization
distortion from a CD player, or in conjunction with optional bandpass
filters for second and third harmonic distortion measurements.
The THD+N is measured using true RMS detection except in a few of
the filtering modes described below. Several types of filters may also
be applied to the measurement.
The filter and detector combination is selected by the lower left soft
key. Three configurations are available:
.
UN-WTD – The ‘unweighted’ selection provides no filtering
except selectable bandwidth-limiting:
.
The lower center soft key selects the lower band limit (<10
Hz, 22 Hz or 400 Hz).
.
The lower right soft key selects the upper band limit (>300
kHz, 22 kHz, 22 kHz-QPK, 30 kHz, 80 kHz). The 22
kHz-QPK selection uses the quasi-peak detector instead of
the true RMS detector in accordance with CCIR-468. The
22 Hz lower limit is only available if the upper limit is 22
kHz; the <10 Hz lower limit is only available with other
upper limits.
.
AVERAGE - The ‘Average’ selection allows the same
band-limiting filters as UN-WTD. Instead of displaying each
reading as it is obtained, the average of the eight most recent
readings is displayed. This is used to reduce digit bobble,
especially when testing distortion in the presence of noise and
flutter from analog tape recorders.
& The shapes of the
bandwidth-limiting filters are
shown in Appendix E.
4
Operation
Controlling the Analyzer : THD+N (Total Harmonic Distortion plus Noise) Operation
Portable One Plus Access User's Manual 4-27
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