Stanford Research Systems SR844, User Manual

Page: 69 / 246

Time Constants 3-11
SR844 RF Lock-In Amplifier
Time Constants
The keys in this section operate on the signal output of the instrument, after
the signal is mixed down to DC. Refer to Chapter 2,
Inside the DSP , for
more information.
Time
Constant The output low-pass filter directly determines the bandwidth of the lock-in amplifier. The
relationship between the filter time constant,
τ
, and the low-pass filter bandwidth,
f
–3 dB
is shown in the table below.
A lock-in output signal at f
o
is due to an interfering input signal at f
ref
±
f
o
. This output
signal is attenuated by the output low-pass filter. Signals whose f
o
is greater than
∆
F
LP
are attenuated while signals closer than
∆
F
LP
to the reference frequency will appear at the
output (and obscure the output from the actual signal).
Time
Constant
Up/Down
The left hand
UP/DOWN
keys in this section select the output filter time constant. The
time constant of the SR844 may be set from 100
µ
s to 30 ks in 1-3-10 steps. The time
constant is indicated by a set of indicators, ( 1x , 3x ), ( 1 , 10 , 100 ), and (
µµµµ
s , ms , s , ks ).
Filter Slope 0 to 4 stages of output low-pass filtering may be selected. These provide up to
24 dB/octave of attenuation for AC signals at the output.
Each filter stage contributes 6 dB/octave of roll-off in the output filter response. Using a
higher slope can decrease the required time constant and make a measurement faster.
Note that the frequency response of a
single filter stage is such that the -3 dB point is at
an output frequency f
o
= 1/2
πτ
, where
τ
is the time constant. With 3 poles, the filter
response at f
o
= 1/2
πτ
will be –9 dB. The correct -3 dB points for more than one filter
stage are given in the following table
Slope Poles f
–3 dB
f
–3dB
[
ττττ
=1 ms] f
–3dB
[
ττττ
=3 ms]
6 dB/oct 1 1.000/2
πτ
159 Hz 53.1 Hz
12 dB/oct 2 0.644/2
πτ
102 Hz 34.2 Hz
18 dB/oct 3 0.510/2
πτ
81.2 Hz 27.1 Hz
24 dB/oct 4 0.435/2
πτ
69.2 Hz 23.1 Hz
Important!
The filter slope also determines the output update rate for the X (CH1) and Y (CH2)
analog outputs. X and Y update at 48-96 kHz with 6 or 12 dB/oct slope and 12-24 kHz
with 18 or 24 dB/oct slope. The update rate for R and
θ
remains 12-24 kHz regardless of
filter slope.
Slope
Up/Down
The right hand
UP/DOWN
keys in this section select the output low-pass filter slope
(number of poles) in the time constant filter.