Time Constants - Stanford Research Systems SR844 User Manual

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 The output low-pass filter directly determines the bandwidth of the lock-in amplifier. The
relationship between the filter time constant,
Constant
f is shown in the table below.
–3 dB
A lock-in output signal at f
signal is attenuated by the output low-pass filter. Signals whose f
are attenuated while signals closer than
output (and obscure the output from the actual signal).
Time The left hand UP/DOWN keys in this section select the output filter time constant. The
Constant
time constant of the SR844 may be set from 100
Up/Down
constant is indicated by a set of indicators, (1x, 3x), (1, 10, 100), and (
0 to 4 stages of output low-pass filtering may be selected. These provide up to
Filter Slope
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
response at f
stage are given in the following table
Slope
6 dB/oct
12 dB/oct
18 dB/oct
24 dB/oct
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
filter slope.
Slope The right hand UP/DOWN keys in this section select the output low-pass filter slope
Up/Down
(number of poles) in the time constant filter.
is due to an interfering input signal at f
o
πτ
= 1/2 , where
o
πτ
= 1/2 will be –9 dB. The correct -3 dB points for more than one filter
o
Poles f f
–3 dB –3dB
πτ
1 1.000/2
πτ
2 0.644/2
πτ
3 0.510/2
πτ
4 0.435/2

Time Constants 3-11

τ
, and the low-pass filter bandwidth,
∆
F to the reference frequency will appear at the
LP
µ
s to 30 ks in 1-3-10 steps. The time
τ
is the time constant. With 3 poles, the filter
[τ τ τ τ =1 ms] [τ τ τ τ =3 ms]
f
–3dB
159 Hz 53.1 Hz
102 Hz 34.2 Hz
81.2 Hz 27.1 Hz
69.2 Hz 23.1 Hz
θ
remains 12-24 kHz regardless of
SR844 RF Lock-In Amplifier
± f
. This output
ref o
∆
is greater than F
o LP
µ µ µ µ
s, ms, s, ks).