Lambda SR1, Operation Manual, Page: 178 / 317

SR1 Operation Manual 178
© 2014 Stanford Research Systems
complex FFT points) and averaging that power into the power computed for previous FFTs. This type of
averaging does not reduce the noise floor of the spectrum but it does reduce the variation of the noise
floor making it easier to see spectral details on the order of the noise amplitude. The Jitter Analyzer also
computes the
Linear Spectrum . The Linear Spectrum is computed by averaging the real and
imaginary parts of each jitter FFT separately. The average of the real and imaginary parts are then used
to compute the Linear Spectrum amplitude and phase. In the Linear Spectrum, unlike the Power
Spectrum, noise that is uncorrelated to the signal is actually reduced by further averaging. Because of
this, use of the Linear Spectrum, unlike the Power Spectrum, requires that the jitter time record be
triggered so that the signal waveform will have the same phase relative to the beginning of the time
record for each averaged FFT. As an example, consider the spectra below, both of the same 48k digital
audio carrier with 200 mUI of added sine jitter at 10 kHz.
The top trace shows the averaged (N=100) Power Spectrum. The bottom trace is the (N=100) averaged
Linear Spectrum (taken with the trigger enabled.) Note the substantial improvement in the noise floor and
the additional spectral detail revealed in the linear spectrum.
Highpass, lowpass, and weighting filter selections are available in Frequency Domain analysis but these
filters are applied in the frequency domain, after the spectra are computed.
Jitter Analyzer Measurements
Measurement Description
Time Domain
Amplitude The amplitude of the jitter signal calculated from an RMS sum of the jitter input
samples. (Though the amplitude is calculated with an RMS sum it is expressed
in equivalent
peak units of sec or UI.)
Physical Sample
Rate The effective sampling frequency of the input digital audio signal, or in the case of
a square wave input, the square wave frequency.
Frequency Domain
Time Record The amplitude vs. time record of the jitter signal. When displayed on a graph this
produces an "Oscilloscope" type display.
Power Spectrum The power-averaged FFT of the jitter signal.
Linear Magnitude The amplitude of the synchronously averaged jitter spectrum. The amplitude of
signals that are synchronous with the time record is preserved, other