Stanford Research Systems SR844, User Manual

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3-12 Time Constants
SR844 RF Lock-In Amplifier
NO FILTER
To choose no output filtering, press the
UP
key until the
NO FILTER
indicator is on.
The time constant indicators turn off in this case.
No Filter bypasses the IF and output filters to provide the fastest response time of the
instrument. This mode should be used with caution because these bypassed filters
provide most of the instrument’s spurious frequency rejection. In No Filter mode, the
SR844 is acting more like a tuned receiver than a lock-in amplifier. No Filter mode is
provided for those users who need the faster response time and are not concerned with
limiting the detection bandwidth.
Important!
The update rate for the X and Y analog outputs is 48-96 kHz, depending upon the
reference frequency. The update rate is fastest at the upper end of each octave, where the
data is sampled at about 10
µ
s per point. The e
-1
response time of the instrument is
approximately 2 sample periods, or about 20
µ
s best case. The update rate for R and
θ
remains 12-24 kHz even in the No Filter mode.
In addition to the update rate, the instrument has a latency of 3 sample periods.
The recommended operating frequencies for the fastest response time are shown in the
following table.
46 kHz 92 kHz 180 kHz 370 kHz 740 kHz 1.4 MHz 2.9 MHz
5.9 MHz 11 MHz 23 MHz 47 MHz 95 MHz 190MHz
Settle...
[Shift–Time
ConstUp]
This key sequence shows the elapsed time (in units of the current Time Constant) in the
Reference Display. The display increments from the time the key sequence is pressed. It
is a useful aid in making measurements with very long time constants where the user can
wait a specified number of time constants before recording a measurement. Elapsed
times are displayed from 0.01 to 99.99 time constants.
FILTER
OVLD The FILTER OVLD indicator shows that an overload has occurred in the DSP output
filters. Try increasing the time constant or filter slope. Another solution is to use a larger
full scale sensitivity.