Stanford Research Systems SR860, Руководство по эксплуатации

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166 The FFT Display Appendix B
SR860 DSP Lock-in Amplifier
the time constant filters.
6. Touch the {Src} tile along the bottom to display
the source up/down buttons.
Touch repeatedly to change the source to
Post Filter.
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
, signals to the left are
below f
ref
. The center frequency is the signal at
f
ref
.
Note that this is not the same as connecting the
lock-in output BNC to a spectrum analyzer.
7. Touch the {Hz/div} tile along the bottom to
display the span up/down buttons.
Use the and buttons to set the Hz/div to
305.2 Hz.The total span is about 1 kHz.
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).
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.
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
sig
−
f
ref
|). Any
signal at f
ref
appears at the lock-in output at dc.
On the FFT display, these signals are shown at
their true input frequencies.
9. Leave the external generator at 99.000 kHz.
Move the cursor to the signal peak. 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
constant acts like a bandpass filter in the
spectrum centered at f
ref
(center).