Stanford Research Systems SR865A, Operation Manual

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Appendix B The FFT Display 163
SR865A DSP Lock-in Amplifier
Using the FFT Display
This example is designed to use the SR865A internal oscillator and an external signal source to explore
some the FFT display. You will need a synthesized function generator capable of providing a 100 mVrms
sine wave at 100.000 kHz, BNC cables and a terminator appropriate for the generator function output.
You will display the FFT when measuring a signal close to, but not equal to, the internal reference
frequency. This setup will il
lustrate the input and output FFT’s .
Do This Explanation
1. Disconnect all cables from the lock-in. Turn the
power on while holding down the [Local] key.
The power switch is on the power entry module
on the rear panel. When the power is turned on with [Local]
pressed, the lock-in returns to its standard
default settings. See the Standard Settings list in
the Operation section for a complete listing of
the settings.
2. Turn on the function generator, set the frequency
to 100.000 kHz (exactly) and the amplitude to
100 mVrms.
Connect the function output (sine wave) from
the synthesized function generator to the A input
using a BNC cable and appropriate terminator.
The input impedance of the lock- in is 10 MΩ.
The generator may require a terminator. Many
generators have either a 50Ω or 600Ω output
impedance. Use the appropriate feedthrough or
T termination if necessary. In general, not using
a terminator means that the function output
amplitude will not agree with the generator
setting.
The SR865A is set to an internal reference of
100.000 kHz. This should be very close to the
external generator frequency. The X (green)
and Y (blue) displays should read values which
change slowly. The lock-in and the generator
are not phase locked but they are at the same
frequency with a
slowly changing θ (orange).
The signal magnitude R (yellow) is phase
independent and does not change.
You can adjust the generator frequency slightly
to get closer to the SR865A.
15. Press [Screen Layout] multiple times to change
the display to the full screen FFT.
The FFT of the signal input is displayed. There
is only a single quantity shown.
The left and right edge frequencies are labelled
at the bottom of the graph. The cursor readout is
at the right.
The display is adjusted with the tiles across the
bottom. {Src} selects the source data for the
FFT. {dB} and {Hz} adjust the vertical and
horizontal scales. {Avgs} sets the amount of
averages and {Live} toggles to {Paused}.