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166
The FFT Display
6.
Touch the {Src} tile along the bottom to display
the source up/down buttons.
Touch
repeatedly to change the source to
Post Filter.
7.
Touch the {Hz/div} tile along the bottom to
display the span up/down buttons.
Use the
and
305.2 Hz.The total span is about 1 kHz.
8.
Increase the external generator frequency to
101.000 kHz. Move the cursor to the peak.
Decrease the external generator frequency to
99.000 kHz.
9.
Leave the external generator at 99.000 kHz.
Move the cursor to the signal peak.
SR865A DSP Lock-in Amplifier
buttons to set the Hz/div to
because the PSD output is frequency limited by
the time constant filters.
The FFT of the PSD filtered output is
displayed. This display is presented as a
zoomed spectrum (the same as a spectrum
analyzer). The center of the span is the lock-in
reference frequency. Signals to the right of
center are above the f
ref
below f
. The center frequency is the signal at
ref
f
.
ref
Note that this is not the same as connecting the
lock-in output BNC to a spectrum analyzer.
At a 1 ms time constant the maximum span is
about 50 kHz (4.883 kHz/div). At a 1 s time
constant, the maximum span is 1.6 kHz
(152.6 Hz/div).
The signal peak moves to the right of center and
the cursor reads 101.00 kHz.
The signal peak moves to the left of center and
the cursor reads 99.00 kHz.
Both of these signal frequencies will appear at
the lock-in output at 1.00 kHz (|f
signal at f
appears at the lock-in output at dc.
ref
On the FFT display, these signals are shown at
their true input frequencies.
Note that the peak amplitude is diminished
from its value at 100.000 kHz. The readout is
about −36 dB. This is because the lock-in time
Appendix B
, signals to the left are
− f
|). Any
sig
ref
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