Stanford Research Systems SR844, User Manual

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1-12 Outputs, Offsets and Expands
SR844 RF Lock-In Amplifier
Outputs, Offsets and Expands
This measurement is designed to use the internal oscillator to explore some of the basic lock-in outputs.
You will need BNC cables and a digital voltmeter (DVM).
Specifically, you will measure the amplitude of the reference oscillator and provide analog outputs
proportional to the measurement. The effect of offsets and expands on the displayed values and the analog
outputs will be explored.
1 Disconnect all cables from the Lock-In.
If the power is off, turn it on. Wait for self-
tests to complete.
Turn on the unit.
2 Press Shift then Recall (PRESET) to restore
factory presets.
The factory preset configuration is:
1 Vrms sensitivity.
100 ms, 12 dB/oct time constant.
Internal Reference at 1.00 MHz.
Signal Input 50
Ω
.
3 Connect REF OUT on the front panel to the
SIGNAL IN with the BNC cable. The SR844 reference output (1.0 Vpp nominal
square wave into 50
Ω
) is within the unit’s
measurement range (1 Vrms) so we can connect it
directly to the input. The SR844 input impedance is
set to 50
Ω
(shown by the 50
Ω
Ω Ω
Ω
indicator) which is
appropriate for the REF OUT.
The CH1 Display is set to X and should show
something close to 0.54 V. The CH2 display is set
to Y and should show less than
±
0.05 V.
Remember, the signal is a 0.5 Vpk (1.0 Vpp) square
wave. A square wave is composed of signals at all
odd harmonics. The SR844 is a square wave
detecting lock-in and detects all of the odd
harmonics of the fundamental. The amplitude of the
fundamental (at 1.00 MHz) is 4/
π
x Vpk. The
contribution from all odd harmonics is
1 + (1/3)
2
+ (1/5)
2
+ (1/7)
2
+ ...
≈
1.19
The detected amplitude is 4/
π
x
0.5 Vpk
x
1.19 or
0.759 Vpk. The SR844 reads the signal in units of
Vrms (0.707
x
Vpk) or 0.537 Vrms.