2 – 10
Introduction
2.2.4
Output
The DC portion of the SR124, beginning with the output of the phase-
sensitive detector, is collectively referred to as the output section.
2.2.4.1
Output filter
The output of the phase sensitive detector contains many signals.
Most of the output signals are at the sum or di
ff
erence frequency
between an input signal frequency and the reference frequency. Only
the component of the input signal whose frequency is exactly equal
to the reference frequency will result in a DC output. To remove all
the unwanted AC signals, both the “2
×
f” (sum of the signal and
reference) as well as the noise components, a configurable low-pass
filter is used.
The output low pass filter follows the mixer, and comes before the
final output DC gain is applied. This filter is key to the performance
of the lock-in amplifier, as it selects the DC output of the mixer while
rejecting the high frequency ripple artifacts naturally generated by
the mixing process. The filter can be configured as either 1 pole or 2
poles, with a time constant between 1 ms and 300 s settable in 1–3–10
steps.
The time constant is related to the
−
3 dB frequency of the filter by the
relation
f
-3dB
=
1
/
(2
π
TC), where TC is the time constant in seconds.
The low pass filters are simple 6 dB
/
octave roll o
ff
, RC-type filters. A
1 second time constant refers to a filter whose -3 dB point occurs at
0.16 Hz. In the SR124 the user can select one or two successive stages
of output filter, so that the overall filter can roll o
ff
is either 6 dB or
12 dB per octave. The time constant refers to the
−
3 dB point of each
filter stage alone (and not the combined filter).
The time constant also determines the equivalent noise bandwidth
(ENBW) for the measurement. The ENBW is not the filter
−
3 dB
bandwidth; rather it is the e
ff
ective bandwidth for Gaussian dis-
tributed white noise. When set to 6 dB
/
octave, the ENBW is 1
/
(4
×
TC);
when set to 12 dB
/
octave, ENBW
=
1
/
(8
×
TC).
It can be useful to consider the frequency domain, in which the
output filter defines the width of the passband for detection at the
reference frequency. By mixing with the lock-in amplifier, this AC
band is mixed down to DC for final output. The
−
3 dB bandwidth
for signal selection at the input is simply 1
/
(2
π
×
TC). To e
ff
ectively
reject the mixer AC artifacts, the output filter should be set so that
TC
>
(2 – 10)
×
(1
/
f), where f is the reference oscillator frequency.
SR124
Analog Lock-In Amplifier
