Spectrum Analyzer Module

In LabOne Releases prior to 17.12, the Spectrum Analyzer Module allows access to the functionality available in the Spectrum Analyzer Tab of the LabOne User Interface from an API. It’s a measurement tool that performs Fast Fourier Transforms (FFT) on demodulator output.

Future Deprecation Warning

In LabOne Release 17.12 and subsequent releases the Spectrum Analyzer (zoomFFT) Module has been superceded by the Data Acquisition Module. We strongly recommend using the Data Acquisition Module instead of the Recorder Module for frequency (and time) domain data acquisition. In particular, existing users of the Recorder module can use the guide Migrating a zoomFFT program to the DAQ Module.

To get started please refer to the Spectrum Analyzer Tab in the relevant instrument-specific User Manual. An example is available in the each of the LabVIEW, Matlab, Python or .NET APIs.

See Table 1 for the input parameters to configure the Spectrum Module and Table 2 for a description of the Spectrum Module outputs.

example spectrum ziDAQ 8004
Figure 1. The FFT result from the LabOne Matlab API Spectrum Module example.
Table 1. Spectrum Module Input Parameters
Setting/Path Type Unit Description

device

string

-

The device ID that is being used for the measurement; the demodulator data is taken from this device. For example, dev123 (compulsory parameter).

absolute

bool

-

When enabled, shifts the frequencies in the output grid so that the center frequency corresponds to the demodulation frequency. If not enabled, the center frequency is 0 Hz.

bit

uint64

-

Number of lines of the FFT spectrum (powers of 2). A higher value increases the frequency resolution of the spectrum.

endless

bool

-

Enable Endless mode; Run the FFT spectrum analysis continuously.

loopcount

uint64

-

The number of FFTs to perform if not running in Endless mode.

mode

uint64

-

Select the source signal for the FFT.

0=FFT(x+iy)

Complex FFT of the demodulator result.

1=FFT(R)

FFT of the demodulator amplitude sqrt(x² + y²). The FFT is single-sided as performed on real data.

2=FFT(phase)

FFT of the demodulator phase atan2(y, x). The FFT is single-sided as performed on real data.

3=FFT(f)

FFT of the oscillator frequency of the selected demodulator. This mode is only interesting if the oscillator is controlled by a PID / PLL controller. The FFT is single-sided as performed on real data.

4=FFT(dθ/dt)/(2π)

FFT of the demodulator phase derivative. This value is equivalent to the frequency noise observed on the demodulated signal. The FFT is single-sided as performed on real data.

overlap

double

-

Overlap of the demodulator data used for the FFT. Use 0 for no overlap and 0.99 for maximal overlap.

settling/tc

double

TC

Minimum wait time in factors of the demodulator time constant (TC) before starting the measurement. The maximum of this value and settling/time is taken as the effective settling time.

settling/time

double

s

Minimum wait time in seconds before starting the measurement. The maximum of this value and settling/tc is taken as effective settling time.

window

uint64

-

The type of FFT window to use.
0=Rectangular, 1=Hann, 2=Hamming, 3=Blackman Harris.

Table 2. Spectrum Module Output Values
Name Type Unit Description

x

double

VoltsRMS

The real part, x, of the complex FFT result.

y

double

VoltsRMS

The imaginary part, y, of the complex FFT result.

r

double

VoltsRMS

The absolute value, R=Abs(X+iY), of the complex FFT result.

timestamp

uint64

Ticks

Demodulator timestamp of the measurement (divide by the device’s clockbase to obtain seconds).

center

double

Hz

The center frequency (corresponds to the demodulation frequency) of the spectrum.

rate

double

1/s

Sampling rate of the demodulator.

filter

double

-

The absolute values of the demodulator’s low-pass filter transfer function for each grid point. The FFT result, x, y or R, may be divided by this value to obtain a filter compensated spectrum.

bandwidth

double

Hz

The noise-equivalent power bandwidth of the demodulator

grid

double

Hz

The frequency grid.

nenbw

double

-

The normalized equivalent noise bandwidth. If calculating spectral densities multiply your spectrum by this value to correct for windowing effects.

resolution

double

Hz

FFT resolution: Spectral resolution defined by the reciprocal acquisition time (sampling rate/number of samples recorded).

aliasingreject

double

dB

The damping of the demodulator present at the border of the spectrum.