Stanford Research Systems SR810 Manual page 15

Use the knob to adjust the frequency back
to 1 kHz.
5. Press [Ampl]
Use the knob to adjust the amplitude to
0.01 V.
6. Press [Auto Gain]
7. Press [Sensitivity Up] to select 50 mV full
scale.
Change the sensitivity back to 20 mV.
8. Press [Time Constant Down] to change the
time constant to 300 µs.
Press [Time Constant Up] to change the
time constant to 3 ms.
9. Press the [Slope/Oct] key until 6 dB/oct is
selected.
Press [Slope/Oct] again to select 12 dB/oct.
Press [Slope/Oct] twice to select 24 db/oct.
Press [Slope/Oct] again to select 6 db/oct.
10. Press [Freq]
Use the knob to adjust the frequency to
55.0 Hz.
The Basic Lock-in
The internal oscillator is crystal synthesized with 25
ppm of frequency error. The frequency can be set with
4 1/2 digit or 0.1 mHz resolution, whichever is greater.
Show the sine output amplitude in the Reference
display.
As the amplitude is changed, the measured value of X
should equal the sine output amplitude. The sine
amplitude can be set from 4 mV to 5 V rms into high
impedance (half the amplitude into a 50 Ω load).
The Auto Gain function will adjust the sensitivity so that
the measured magnitude (R) is a sizable percentage of
full scale. Watch the sensitivity indicators change.
Parameters which have many options, such as
sensitivity and time constant, are changed with up and
down keys. The sensitivity and time constant are
indicated by leds.
The value of X becomes noisy. This is because the 2f
component of the output (at 2 kHz) is no longer
attenuated completely by the low pass filters.
Let's leave the time constant short and change the filter
slope.
Parameters which have only a few values, such as
filter slope, have only a single key which cycles through
all available options. Press the corresponding key until
the desired option is indicated by an led.
The X output is somewhat noisy at this short time
constant and only 1 pole of low pass filtering.
The output is less noisy with 2 poles of filtering.
With 4 poles of low pass filtering, even this short time
constant attenuates the 2f component reasonably well
and provides steady readings.
Let's leave the filtering short and the outputs noisy for
now.
Show the internal reference frequency on the
Reference display.
At a reference frequency of 55 Hz and a 6 db/oct, 3 ms
time constant, the output is totally dominated by the 2f
component at 110 Hz.
2-3