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Description |
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The GoLogic™ FFT Plug-In performs a Fast Fourier
Transform on an Analog-to-Digital Converter's output data. The resulting
frequency spectrum is graphed in the Plug-In window. Clicking and dragging
the left and right mouse buttons on the graph moves two independent
measurement markers. Useful analysis values are displayed in the corner of
the graph. The following decibel
reference units are used in the FFT Plug-In…
dBFS is the "Full Scale" reference.
dBc is decibels referenced to the carrier signal.
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Options |
The Toolbar area contains setup buttons 
(Analysis setup)
(Grid setup) plus various configuration parameters...
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Analysis setup button |
The
button displays the Analysis setup dialog box...
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- Show N harmonic frequencies - Enter the
number of harmonic frequencies to display. Entering "0" hides all harmonic
frequencies. The maximum that can be displayed is 30.
- Fundamental frequency - Check this box to
show the fundamental frequency value.
- SNR (signal to noise ratio) - Check this
box to show the signal to noise ratio....
- Exclude N harmonics in SNR calculation -
This value is used to exclude harmonics from the SNR calculation.
- SINAD (signal to noise and distortion) -
Check this box to show the signal to noise and distortion...
- ENOB (effective number of bits) - Check
this box to show the effective number of bits...
- THD (total harmonic distortion) - Check
this box to show the total harmonic distortion...
- Use N harmonics in THD calculation - This
value defines the number of harmonics to use in the THD calculation.
- SFDR (spurious free dynamic range) - Check
this box to show the spurious free dynamic range. This is the ratio
expressed in decibels of the RMS amplitude of the fundamental (maximum
signal component) to the RMS value of the next-largest spurious component,
excluding DC offset.
- Analysis values position - The analysis
values can be displayed in any corner of the graph area.
- Exclude N 'bins' above DC - This value is
used to exclude noise from the analysis calculations.
- Include N 'bins' on either side of
fundamental - This value is used to improve the fundamental frequency
analysis by including more frequency or fewer 'bins' in the calculation.
- Include N 'bins' on either side of each
harmonic - This value is used to improve the fundamental frequency
analysis by including more frequency or fewer 'bins' in the calculation.
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Grip setup button |
The
button displays the Grid setup dialog box...
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- Auto-scale (X or Y axis) - The X and Y
axis can scale automatically or manually. Check this option to auto-scale
the associated axis. When this value is unchecked, enter the axis maximum
and minimum values to define the scale manually.
- Show N divisions (X or Y axis) - Enter the
number of division lines to show on the associated axis.
- Scale (Y-axis only) - The Y-axis
(frequency magnitude) can be displayed using a logarithmic or linear
scale.
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Toolbar controls |
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Source |
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Select
the source for the
ADC data values...
- Raw trace -
This option extracts the values from the raw trace data.
When this option is active, the "Channels"
option (below) defines the channels which contain the ADC values.
- Converted serial data - To reduce pins,
many ADC chips output their 16 or 24-bit data values on a standard
serial bus rather than in parallel form. This
option extracts the data values from an SPI, I2C, or UART serial bus.
The "Serial bus" option (below) defines which serial bus is used.
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Channels |
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Enter
(or select)
the channels
connected to the ADC's output pins. At least 2
channels must be used. |
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Converted serial data |
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Select
the serial bus which
contains the ADC's output data. Serial buses are
defined in the Setup window's Sampling tab. The bus must be defined
correctly so the GoLogi can capture the raw signals and convert them to
parallel form. The raw signals and translated bus states can be viewed
in the WaveForm window. The parallel data values from the bus are then
passed to the FFT plugin for analysis. |
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Interpreted as... |
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Select the data type
interpretation of the ADC data...
- Two's Compliment - This is the most
common method of representing signed integers. Negative numbers are
represented by inverting all bits and adding 1.
- Sign Magnitude - The most significant
bit in the channel group is the sign bit while the remaining bits
define the magnitude. If the sign bit is "1", the value is negative.
- Binary Offset - The channel group's
middle value is zero. Values less than the middle are negative while
values greater than the middle are positive. For example, an 8-bit
channel group has 256 values. 0x80 (128) represents zero, 0xFF
represents "+127" which is the largest positive value, while 0x00
represents "-128" which is the largest negative value.
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Windowing |
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The most popular FFT
windowing functions are supported. These functions are "generalized
cosine windows" consisting of a DC term (the amount the cosine is
raised), the cosine fundamental term, and usually one or more harmonics
of that fundamental. |
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- Rectangular - The FFT is calculated with
no windowing function.
- Hamming - The coefficients of a Hamming
window are computed from the following equation…
where…
- Hanning (also known as Hann) - The
coefficients of a Hanning window are computed from the following equation…
where…
- Blackman/Harris- The equation for
computing the coefficients of a minimum 4-term Blackman/Harris window is…
where…
and the coefficients are…
a0 = 0.35875
a1 = 0.48829
a2 = 0.14128
a3 = 0.01168
- Flat-top - The coefficients of a flat-top
window are computed from the following equation…
where…
and...
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Size |
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The number of samples in the
FFT can be between 512 and the GoLogic's maximum memory depth. |
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Zoom |
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- Fit to window - This option sizes the
FFT data inside the window so that no horizontal scroll bar is
required. When fitting the data to the window size, a custom x-axis
scale can be defined using the "Setup grid…" button.
- X bins-per-pixel - Each FFT frequency
"bin" uses the number horizontal pixels requested. This zooms in on
the FFT data so details are easier to view. If the FFT size exceeds
the window width, then a horizontal scroll bar is provided to view the
data. The graph is always scaled to match the full frequency spectrum
of the input data. The custom x-axis scale defined with the "Setup
grid…" button is ignored when zooming in on the graph.
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ADC samples per value |
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This option is only
available when the "Raw trace" source is active. Many ADC chips can output between
on and four values per system clock. The FFT Plug-In expects one large
channel group to represent the ADC values. These combined ADC values are
then de-multiplexed by the plugIn before performing an FFT analysis. |
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If the ADC outputs two values per system clock,
the FFT PlugIn
de-multiplexes the captured data as follows…
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If the ADC outputs four values per system clock,
the FFT PlugIn
de-multiplexes the captured data as follows…
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Start FFT at... |
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- Trace trigger - The FFT uses 'N'
samples starting at the trigger sample.
- Trace start - The FFT uses the first
'N' samples in the active data.
- Trace end - The FFT uses the last 'N'
samples in the active data.
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- Custom position - Enter the sample
number to start the FFT. This must be between zero and the total
samples captured.
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Display |
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- Trace only - Only the trace data is
graphed using bright yellow.
- Reference only - Only the reference
data is graphed using bright green.
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- Trace over Reference - The trace data
is graphed on top of the reference data. The trace data is bright
yellow while the reference data is dark green.
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- Reference over Trace - The reference
data is graphed on top of the trace data. The reference data is bright
green while the trace data is dark yellow.
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Mouse actions |
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Moving markers |
| Click the left mouse button
to place the red marker on the FFT graph and view the exact frequency
and magnitude at that point.
Dragging the left mouse button across the graph forces the red marker to
follow the FFT data without skipping frequency "bins". This is similar
to "nudging" the red marker with the left and right keys. |
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Likewise, click the right mouse button to place the blue marker on the
FFT graph and view the exact frequency and magnitude at that point.
Dragging the right
mouse button across the graph forces the blue marker to follow the FFT
data without skipping frequency "bins". This is similar to "nudging" the
blue marker with the shift+left and shift+right keys. |
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Hiding markers |
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Click the left or right
mouse button off the FFT graph to hide the marker. |
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Scrolling |
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The wheel-mouse and
up/down/pageup/pagedown keys scroll the window horizontally when zoomed in
on the graph. |
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Keyboard actions |
- The left and right keys nudge the red
marker one frequency "bin" left or right.
- The shift+left and shift+right keys nudge
the blue marker one frequency "bin" left or right.
- The ctrl+shift+left and ctrl+shift+right
keys move the red marker to the next/previous harmonic frequency. The
first ctrl+shift key moves the red marker to the nearest harmonic in the
indicated direction. Later ctrl+shift moves jump to the next/previous
harmonic in the series. This feature is disabled when no harmonics are
displayed. No similar key operation is provided for the blue marker.
- The ctrl+Left and ctrl+right keys scroll
the graph left and right when zoomed in.
- The home and end keys scroll the graph to
the far left and right when zoomed in.
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Copyright and trademark information |
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