Stanford Research Systems SR844, User Manual

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X, Y, R,
θθθθ
and dBm 1-9
SR844 RF Lock-In Amplifier
X, Y, R,
θθθθ
and dBm
This measurement is designed to use the internal oscillator and an external signal source to explore some
of the signal types. You will need a synthesized signal generator cable of providing 200 mVrms (0 dBm)
sine waves at 100 kHz into a 50
Ω
load (the DS335 from Stanford Research Systems will suffice), and
BNC cables.
Specifically you will display the lock-in outputs when measuring a signal that has a frequency close to,
but not equal to, the internal reference frequency.
Note : The last few items in this section require that the signal generator have a Sync output; if you are
using a signal generator that has a single output only, you can split the output using a BNC Tee (or a
power splitter or a directional coupler).
1 Disconnect all cables from the SR844.
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 Use the
knob
to adjust the SR844 reference
frequency to 100 kHz.
We are using a low reference frequency so that the
intrinsic frequency difference between the SR844
and the signal generator has a smaller absolute
value.
4 Press Sensitivity Down .
Press
Time Constant Down
twice until the
time constant is 10 ms.
The SR844 sensitivity should now be 300 mVrms.
We need a shorter time constant to measure the
output signal.
5 Turn on the external signal generator and set
the frequency to 100 kHz exactly, and the
amplitude to 200 mVrms, 0 dBm, or
600 mVpp into 50
Ω
. The exact value doesn’t
really matter. Low-frequency signal generators
may have waveform selection (select sine
wave) and DC offset (set it to zero). If the
signal generator offers modulation, make sure
it’s off.
While not phase-locked, the signal generator and
SR844 should be at very nearly the same frequency;
the slight frequency difference will be manifested as
a changing relative phase.