Stanford Research Systems SR865A Operation Manual page 180

162 The FFT Display
to be dc. The output spectrum thus represents the frequency range from dc to 128 kHz
with points every 250 Hz.
Advantages and limitations
The advantage of this technique is its speed. The entire spectrum takes only 4 ms to
measure. The limitation of this measurement is its resolution. Because the time record is
only 4 ms long, the frequency resolution is only 250 Hz. Suppose the signal has a
frequency component at 260 Hz. The FFT spectrum will detect this signal but place part
of it in the 250 Hz point and part in the 500 Hz point. In fact, the way to measure the
signal accurately is to lengthen the time record and change the resolution of the spectrum.
Of course eventually you end up with a very long measurement of a very narrow
spectrum — much like a lock-in amplifier. At this point, you lose the FFT advantange
and should indeed rely on the lock-in measurement.
FFT's in the SR865A
The SR865A FFT display is not a replacement for a full FFT spectrum analyzer
instrument. It is intended to give the user an overview of the spectrum of the input signal
or the output of the PSD and time constant filters.
The length of the time record is always 1024 points, the maximum span of an output FFT
is determined by the time constant, the window is Kaiser and the averaging is rms. The
amplitude is always dB (Vrms) and is uncalibrated (only accurate to a few dB). None of
this can be changed.
Important Facts About SR865A FFT's
1. The acquisition time of a spectrum (averaging off) is equal to the time record.
Narrow frequency spans means long time records. The time record is about
500/span (Hz). Changing the measurement in narrow spans means waiting for at least
a time record for a spectrum.
2. The lock-in time constant determines the maximum span for post filter FFT's. Using
a long time constant means having a long FFT time record.
3. The frequency scale is labelled with respect to the signal's frequency at the input
BNC. Thus a signal at f
as f
4. Amplitudes are uncalibrated. Amplitude measurements are generally accurate to a
few dB.
SR865A DSP Lock-in Amplifier
, which appears at dc at the lock-in output, will be labelled
ref
in the FFT.
ref
Appendix B