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Chapter 3
Input Overload
The
Overload
inputs greater than the voltage input range (unless removed by ac coupling) or current
inputs exceeding ±4 µA (1 µA range) or ±40 nA (10 nA range).
An input overload compromises the measurement of X, Y, R and θ. Any display of these
quantities will be accompanied by an Ovld indication.
Either increase the input range or reduce the input signals to remove the overload.
Slew Rate
Overloads can also be caused by high slew rate voltage signals. These are large signals
with fast rise or fall times. This can be a square wave (at any frequency) with fast
transitions, or simply a large amplitude high frequency sine wave. In these cases, the
amplifier may become slew rate limited and unable to accurately amplify other
components of the signal. When this occurs, the
the signal strength indicator is not at its maximum. Once again, increase the input range
or reduce the input signal.
[Time Constant]
The time constant may be set from 1 µs to 30 ks. The time constant is indicated by 1 or 3
times 1, 10 or 100 with the appropriate units.
This time constant sets the bandwidth of the low pass filter after the phase sensitive
detectors for X and Y. This is the filter that removes signals at frequencies other than f
In general, longer time constants provide more noise filtering and quieter measurements
but longer response times.
The time constant also determines the equivalent noise bandwidth (ENBW) of the low
pass filter. This is the measurement bandwidth for X and Y noise and depends upon the
time constant and filter slope. (See the Noise discussion in the Basics section.)
In some experiments, output latency (delay from signal input to analog output) at short
time constants is important. Use the rear panel BlazeX output for the lowest latency
analog X output. Otherwise, the front panel outputs CH1 and CH2 as well as the rear
panel X and Y outputs have sufficient bandwidth for all time constants.
Filter [Slope/adv]
This key selects the number of poles in the low pass filter. Choosing 6, 12, 18 or 24
dB(/oct) selects 1, 2, 3 or 4 poles. Using more poles can decrease the required time
constant and make a measurement faster.
The normal low pass filter is an RC filter. This is equivalent to the traditional filter found
in analog lock-ins.
To use advanced filters in place of the RC filters press and hold the [Slope/adv] key until
the
Advanced
LED in this section indicates an input overload. This occurs for voltage
LED turns on. Brief presses of [Slope/adv] cycles the number of poles
Operation
Overload
LED may light even though
SR865A DSP Lock-in Amplifier
67
.
ref
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