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68
Operation
from 1 to 4 (6 to 24 dB). Another press and hold of the [Slope/adv] key reverts the filters
back to RC filters and turns off the
For two filters with the same noise bandwidth, i.e. whose outputs would be equally noisy
if the input was white noise, the advanced filters have faster transient response, less
overshoot, and higher stopband attenuation to ensure greater suppression of out-of-band
spurs.
When the time constant is 3 s or faster an advanced FIR filter pole of equivalent noise
bandwidth (ENBW) is substituted for each RC filter pole.
For time constants of 10 s through 1000 s, the advanced filter is a Linear Phase (LP) filter
with same attenuation slope as the RC filters of the same time constant and number of
poles. In other words, the Linear Phase filter's stop band is aligned with the
corresponding RC filter.
For time constants greater than 1000 s, the advanced filter is the same as the RC filter.
In an actual experimental situation, the signal is seldom simple. If the goal is to read a
static output value, then longer time constants will achieve that. If the output value is
changing because of parameter sweeping or a signal turning on and off, then the
advanced filters can be of great help. In practice it is simple to try these filters, at various
time constants and number of poles, in comparison with RC filters. They often yield
better results in less time.
See Appendix A for more information about the advanced filters.
Filter [Sync]
Pressing this key turns synchronous filtering on or off.
Synchronous filtering removes outputs at harmonics of the reference frequency, most
commonly 2×f
would require very long time constants to remove. The synchronous filter does not
attenuate broadband noise very well. The low pass filters should be used to remove
outputs due to noise and other non-harmonic interfering signals.
The synchronous filter computes moving averages of the X and Y outputs over a
complete reference period, 1/f
zero and are removed. Thus frequency components in the output at n×f
The synchronous filter follows the normal low pass filters (time constant) and is applied
at the outputs.
Sensitivity
In order for the synchronous filter to perform accurately, the sensitivity must be set
appropriately. This is because the synchronous filter acts on the output scaled values of
X and Y. If the sensitivity is set too high, the values of X and Y (in the numeric displays)
will lack the necessary resolution. If the sensitivity is set too low, the values will
overload. In general, setting the sensitivity to display a reasonable amount of bar graph is
sufficient for accurate synchronous filtering. If the synchronous filter overloads because
of the sensitivity, the Sync warning is displayed with displays of X, Y, R or θ.
SR865A DSP Lock-in Amplifier
Advanced
. This is very effective at low reference frequencies since 2×f
ref
. In this way, outputs with periods 1/(n×f
ref
LED.
Chapter 3
outputs
ref
) average to
ref
are all removed.
ref
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